A study of some complexometric titrations in nonaqueous

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Masters Theses Student Theses and Dissertations

1972

A study of some complexometric titrations in nonaqueous A study of some complexometric titrations in nonaqueous

solvents solvents

Ngo The Hung

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A STUDY OF SOME COMPLEXOMETRIC TITRATIONS IN

NONAQUEOUS SOLVENTS

BY

NGO THE HUNG, 1940-

A THESIS

Presented to the Faculty of the Graduate School of the

UNIVERSITY OF MISSOURI-ROLLA

In Partial Fulfillment of the Requirements for the Degree

MASTER OF SCIENCE IN CHEMISTRY

1972

Approved by

T2732 92 pages c.l

To my Parents

ii

ABSTRACT

Complexometric titrations of calcium, zinc and lead with polyamino

carboxylic acids: ethylenediaminetetraacetic acid (EDTA), 1,2-diamino

cyclohexanetetraacetic acid (DCTA), ethyleneglycol-bis(2-aminoethylether)

tetraacetic acid (EGTA) and tetraethylenepentamine (tetren) have been

investigated and compared in the following organic solvents: methanol,

dimethyl formamide, dimethyl sulfoxide and methyl ethyl ketone.

Various end point detection methods have been used: direct visual

titration with metallochromic indicators and instrumental detection by

photometry, potentiometry (mercury electrode and lead ion selective

electrode) and amperometry.

Calcium, zinc and lead can be determined up to trace levels (ppm)

under specific conditions.

A concrete application of this thesis is the determination of zinc

or calcium in a sample of polystyrene within a few minutes while the

classical methods will necessitate several hours.

iii

ACKNOWLEDGMENT

The author wishes to extend his appreciation to his advisor, Dr.

William R. Carroll, Professor of Analytical Chemistry, for suggesting

the topic of this research and for introducing him to the fascinating

field of titrations in nonaqueous media. Without his expert guidance

this thesis could not have been led to completion.

He also wishes to thank Mr. Edward W. Hennecke for diligent help in

some glass blowing situations.

He is particularly grateful to the United States Government (Agency

for International Development) and to the Vietnamese Government for

financing his stay in the United States.

iv

TABLE OF CONTENTS

Page

ABSTRACT . •.•.•...•....•...••.•..•.•.•..•...•..•..•...•...•..••..... ii

ACI<NO\Vl..EDGE:t-fENT • .................................................... iii

TABLE OF CONTENTS • ••.•••••••..•••••••..•••.•••.•.•.••••..•.•..•••• iv

LIST OF FIGURES . .................................................. . vii

LIST OF TABLES ..................................................... viii

ABBREVIATIONS. . • . • • • . • • . • . . • . . . • . . . . . • • . . • . • . . . . . . . . • . . . . . . . . . . . . . ix

I. INTRODUCTION • •••••••••••••••••••••••••••••••.••••••••••• 1

II. REVIEW OF LITERATURE • ..................................... 2

A. Studies with a Mixed Solvent System .•••••.•.• 2

B. Studies with Pure Anhydrous Organic Solvents ••••••.•• 5

c. Conclusion . ......................................... . 6

III. EXPERIMENTAL SECTION ••••••••••••.••••••••••••••••••••.•.• 8

A. Apparatus . ............................................. . 8

B. Reagents and Solutions •••••••••••.••••••••••.••.••... 10

1. Chelan So 1 utions . ................................ . 10

2. Standard Metal Solutions •••.••••••.•••...•••..•.• 13

3. Indicator Solutions •••••••...•.•.••••..•.•.•...•• 14

c. Procedure . ........................................... . 16

1. Direct visual Titrations with Metal1ochromic Indica tors .. ................................. . 16

a} Titrations of zinc with EDTA ••••••••••..•••• 16

b) Titrations of zinc with DCTA ••••••••••••••••• 17

c) Titrations of zinc with EGTA •••••.•••••••.••• 17

d) Titrations of zinc with tetren ••••••••••••••• 17

Table of Contents (continued)

Page

e) Titrations of calcium with EDTA11 ••••• - ••••••••• 18

f) Titrations of calcium with DCTA .. ................ 18

g) Titrations of calcium with EGTA • •.••••••....•.• 18

h) Titrations of calcium + zinc with DCTA •..•..... 19

i) Titrations of calcium + zinc with EGTA • ••••.••. 19

j) Analysis of calcium and zinc in a sample of polystyrene.......................... 19

2. Photometric Titrations............................. 20

3. Potentiometric Titrations.......................... 22

a) Titration of mercury with the mercury elec trade. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

b) Titrations of zinc with the mercury electrode...................................... 22

c) Titration of calcium with the mercury elect rode . ..................................... 22

d) Titration of lead with the lead elec trade .•...• 23

e) Titration of zinc with the lead electrode •..... 23

4. Anlperometric Titrations .••••••.•.•••...••••..•.••.•. 24

IV. RESULTS AND DISCUSSION.. . • • • • • . . . . • • • . . . • • . . . • • • . • . . • . • • • . • 25

A. Direct Visual Titrations with Metallochromic In-die a tors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1. General connn.ents .................................... 25

2. Titrations of Zinc ................................... 26

3. Titrations of Calcium . ............................. 39

4. Titrations of Calcium + Zinc . ...................... 45

5. Analysis of Calcium and Zinc in a Sample of Polystyrene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

vi

Table of Contents (continued)

Page 6. Conclusion...................................... 48

B. Photometric Titrations ••••••••••••••••••••••••••••• 51

c. Potentiometric Titrations •••••••••••••••••••••••••• 59

D. Amperometric Titrations •••••••••••••••••••••••••••• 73

v. CONCLUSION • •••••••••••••••••••••••••••••••••••••••••.•. 75

REFERENCES. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • . • • • • • . 77

VITA. • • • • • • • • . • • . . • • . • . . . • • . . • . . • . • . • . • . • • • . . . . • • . • • • . • . • • . . • . . . . 82

vii

LIST OF FIGURES

Figure Page

1. Absorbance Curves in Dimethyl Sulfoxide .........•.........•. 52

2. Photometric Titrations of Zinc in Dimethyl Sulfoxide .•...... 54

3. Photometric Titration of Zinc in Dimethyl Formamide ...•..... 56

4. Potentiometric Titrations of Calcium and Zinc with EDTA ..•.. 61

5. Potentiometric Titrations of Lead with EDTA and EGTA .....•.. 66

6. Potentiometric Titrations of Zinc with EDTA, DCTA and EGTA .. 69

viii

LIST OF TABLES

Table Page

I. Results of Titrations of Zinc with EDTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 27

II. Results of Titrations of Zinc with DCTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 31

III. Results of Titrations of Zinc with EGTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 34

IV. Results of Titrations of Zinc with Tetren Using Metallochromic Indicators ••••••••••••••••••••••••••••• 36

V. Results of Titrations of Calcium with EDTA in Methanol Using Methylthymol Blue ••••••••••••••••••••••••• 40

VI. Results of Titrations of Calcium with DCTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 41

VII. Results of Titrations of Calcium with EGTA Using the System Zincon + Zinc-EGTA ••••••••••••••••••••••••• 43

VIII. Results of Titrations of Calcium + Zinc with DCTA Using Methylthymol Blue ••••••••••••••••••••••••••••••••••••• 46

IX. Results of Titrations of Calcium + Zinc with EGTA Using Metallochromic Indicators ••••••••••••••••••••••••••••• 47

X. Results of Titration of Zinc and Calcium in Polystyrene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 9

XI. Results of Photometric Titrations of Zinc •••••••••••.••••••• 58

XII. Results of Potentiometric Titrations of Calcium and Zinc with EDTA ••••••••••••••••.••••••••••••••••• 60

XIII. Results of Potentiometric Titrations of Lead with EDTA arid EGTA. . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . 65

XIV. Results of Potentiometric Titrations of Zinc With EDTA, DCTA and EGTA. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 68

ix

ABBREVIATIONS

DCTA: 1,2-diamino£Yclohexanetetr~cetic acid

DMSO: Di..!!!ethyl _!Ulfoxide

EDTA: Ethylenediamine.E_etra.!_cetic acid

EGTA: Ethylenealycol-bis(2-aminoethylether)-tetra~cetic acid

S.C.E.: saturated calomel electrode

Tetren: Tetraethylenepentamine

1

I. INTRODUCTION

Complexometric titrations of metal ions with chelating agents like

polyamines or polyaminocarboxylic acids have been extensively studied

for two decades.(l-6) All these studies, though, deal with aqueous

titrations and the field of complexation in non-aqueous media has been

relatively uninvestigated.

When metals in organic products are determined by classical aqueous

complexometry, it is required to destroy the organic matrix by an ashing

procedure followed by a dissolution of the residue. This method is time

comsuming and the ashing procedure can lead to some loss of material.

If the sample can be dissolved in an organic solvent and a direct

complexometric titration be performed in this medium, any loss of

material inherent to the ashing procedure will be prevented and the time

required for the analysis will be tremendously shortened.

It is the purpose of this thesis to investigate some complexometric

titrations in various organic s6lvents and to find a rapid method of

determination of calcium and zinc in polystyrene.

II. REVIEW OF LITERATURE

Studies of complexometric titrations in non-aqueous media using

polyaminocarboxylic acids as chelating agents are met with two main

difficulties: 1) the insolubility of the chelating agent in the

organic solvents and 2) the lack of knowledge concerning the forma

tion of the metal-chelate in these media.

2

These studies can be classified into two divisions: those which

involve a mixed solvent system (organic solvent + water) and those

which deal with pure organic anhydrous solvents.

A. Studies with a Mixed Solvent System

The metals to be titrated are dissolved in a mixed solvent system

and the chelating agent is always dissolved in water. The role of the

organic solvent is to dissolve the sample and/or to prevent the

blocking of the metallochromic indicator.

Schwarzenbach and Ackerman (7) found that the stability constants

of some EDTA-alkaline earth chelates are 100 times greater in a 48%

alcoholic medium than in water.

Gotschalk (8) discovered the same property in titrating aluminum

with EDTA in a 50% ethanolic medium and using dithizone as indicator.

Potentiometric titrations of several metal ions with EDTA, nitrilo

triacetic acid (NTA) and N,N-Di(-hydroxyethyl)glycine in a water +

pyridine mixture have been investigated by Siggia (9) who used various

electrode systems as Pt vs. calomel, Hg on Pt vs. calomel or Ag vs.

calomel.

3

A lead ion-selective membrane electrode was used by Rechnitz and

Kenny (10) to determine lead by a differential measuring technique in

various concentrations of water and methanol, dimethyl sulfoxide, 1,4-

dioxane and acetonitrile.

The determination of metal additives in petroleum products has

been often investigated in various water + organic solvent mixtures.

Gerhardt and Hartmann (11) determined calcium or zinc in lubricating

oils and concentrates by dissolving the samples in acetone, complexing

the metals with an excess of aqueous standard solution of EDTA and then

back titrating the excess EDTA with an a~ueous solution of magnesium

chloride using Eriochrome Black T as indicator.

Fisher (12) used an extraction procedure to separate barium,

calcium and zinc in oil samples but this method could not avoid excessive

emulsion formation and sometimes incomplete extraction.

A toluene + isopropyl alcohol mixture was used by Crump (13) to

dissolve lubricating oils. The additive metals (barium, calcium, lead

and zinc) were complexed with an excess of standard EDTA which was then

back titrated with a standard magnesium iodate solution using Eriochrome

Black T as indicator. The method was rapid but the solution was

cloudy due to the presence of a suspension which tended to obscure some

end-points. Instead of using EDTA Hooks and Noar (14) used

4

diethylenetriaminepentaacetic acid (DTPA) and dithizone as indicator to

determine zinc directly while the other metal additives were complexed

with an excess of DTPA. This excess DTPA was back titrated with a

standard magnesium solution.

Combining their methods Crump and Noar (15) found them having an

accuracy comparable to classical "wet" methods of the ASTI1 and the

Institute of Petroleum. An analysis took 30 minutes.

A double extraction procedure was used by Hypta (16) to determine

barium and calcium in lubricating oils. An analysis took 3 hours and

calcium could be determined down to 0.06% and barium down to 0.02%.

Kiss (17) investigated various water + organic solvent mixtures to

avoid the blocking of Eriochrome Black T and found that ethanol was

the best organic solvent.

Methylthyrnol Blue and Methylthyrnol Blue + Fluorexone have been

used by Kiss, Gaal, Suranyi and Zsigrai (18) to determine consecutively

calcium and magnesium with EDTA in a 60% ethanolic medium.

Micro amounts of magnesium (0.0243 to 12.16 mg) have been titrated

by Kiss and Suranyi (19) with EDTA and Methylthyrnol Blue in a 60%

ethanolic medium.

Magnesium sterate dissolved readily in 96% ethanolic medium and

magnesium could be titrated with EDTA and Methylthyrnol Blue by Kiss

and Knezevic (20).

A high frequency technique to determine bivalent metal ions in

5

water + ethanol and water + ethanol + benzene mixtures was investigated

by Casassas and Fernandex (21) who used 1,2-diaminocyclohexanetetraacetic

acid (DCTA) as chelating agent. They found that the DCTA-metal complexes

are more stable than the EDTA-metal complexes and the high frequency

titration could be used in non-buffered media.

B. Studies with Pure Anhydrous Organic Solvents

Yoshimura and Tamura {22) used a conductometric method· to analyze

antimony (III) and (V) in dimethyl formamide with EDTA. They found

that the end point corresponds to molar ratio EDTA:Sb equal to 4:1.

An amperometric end point detection was used by Arthur and Hunt

(23) to analyze lead, cadmium and zinc in petroleum products. They

dissolved the sample in 2-propanol and the chelating agent was 1,2-

diaminocyclohexanetetraacetic acid (DCTA) dissolved in 95% ethanol made

0.33 M in ethanolamine.

Micro titrations being possible with a high frequency technique

have been successfully investigated by Fernandez, Casassas and Garcia

-5 MOntelongo (24) who studied the complexation between 4.10 M EDTA in

ethanol and cadmium, mercury, zinc, copper and lead solutions in

-4 -5 concentration range from 3.3 x 10 M to 2.5 x 10 M.

A complexation reaction is always followed by a release of protons,

which can be titrated by a high frequency technique. Fernandez and

Casassas (25) applied this method when they studied the reaction be-

tween 1,2-diaminocyclohexanetetraacetic acid (DCTA) and some bivalent

6

Metals in methanol +benzene (1:4 and 4:1) mixtures.

Acetone, acetonitrile and methanol were the media used by Rechnitz

and Kenny (26) when they titrated potentiometrically cupric ions with

EDTA, ethylenediamine, tetraethylenepentamine and 5,6-dimethyl-1,10-

phenanthroline and a solid membrane cupric ion electrode.

Direct visual titrations of zinc, cadmium and magnesium in water

insoluble stearates with EDTA, nitrilotriacetic acid (NTA), 1,2-diamino

cyclohexanetetraacetic acid (DCTA) and N-(2-hydroxyethyl)-ethylene

diamine-NNN'-triacetic acid (HEDTA) have been performed by Kiss,

Zsigrai, Cirin and Krizsan (27) in formamide and dtmethyl sulfoxide with

Eriochrome Black T and PAN as indicators. Magnesium and zinc in micro

amounts from 1 to 12 pg in a 0.5 ml sample could be successfully deter

mined.

A thermometric end point detection was used by Kiss (28) in the

direct titration of EDTA, 1,2-diaminocyclohexanetetraacetic acid (DCTA)

and nitrilotriacetic acid (NTA) with manganese (II) nitrate in dimethyl

sulfoxide and in the indirect titration of some bivalent and tervalent

metals with EDTA or DCTA in dimethyl sulfoxide.

C. Conclusion

From the survey of literature the following conclusions can be

drawn:

1. Complexometric titrations in non-aqueous media are possible.

7

2. The theory of complexometric titrations in aqueous medium is

mainly centered around the notion of pH. In a non-aqueous medium the

pH is not well defined and this explains why all the investigations up

to now have been highly empirical and are still in a stage of trial and

error experiments.

8

III. EXPERIMENTAL SECTION

A. Apparatus

1. Potentiometric studies were performed with a Sargent recording

titrator, model D; a Beckman Zeromatic SS 3 pH meter and a Beckman Ex-

pandomatic pH meter. The following electrodes have been used:

a) Glass electrodes: Beckman No. 39301 on the Expandomatic

pH meter, and Beckman No. 41263 on the SS 3 pH meter.

b) Modified calomel reference electrode made from a Beckman

quartz fiber standard calomel electrode with saturated KCl - methanol

electrolyte, and a Standard calomel electrode, Beckman No. 39402.

c) Silver billet indicator electrode, Bec~n No. 39261. The

electrode was cleaned periodically by light buffing with steel wool.

d) Mercury electrode, J - type, Sargent-Welch No. S-30438.

Before use the mercury electrode cup was cleaned with 5 N nitric acid,

rinsed with distilled water followed by deionized water and dried in

0 an oven at 105 C.

e) Lead specific ion electrode, Orion model 94-82. Before

use the electrode was polished with the abrasive plastic supplied with

the electrode by the manufacturer and soaked in the same lead standar-

dizing solution for five minutes.

2. Photometric instruments included a Beckman DK-2A ratio re-

cording spectrophotometer used for running absorption curves and a

Bausch and Lomb Spectronic 20 spectrophotometer equipped with a 55

milliliter, one inch diameter test tube containing a micro Teflon

coated stirring bar used for measuring absorbance during titrations.

9

The test tube was surrounded by a tube opaque to light and closed by a

rubber stopper with a hole in the middle to let the tip of the buret

dip into the tube. A magnetic stirrer was placed under the instrument,

under the bottom of the test tube. The stirrer was stopped for 15

seconds before any absorbance reading and set into motion for 30 seconds

after each increment of titrant.

3. Amperometric studies were performed with a Sargent Polarograph

model XV. Three different electrolysis vessels have been used:

- polarographic, H form, with stopcock, Sargent No. S-29401

- polarographic, Heyrovsky, 10 ml volume, Sargent No. S-29370.

- 50 ml Pyrex glass beaker.

The reference electrodes were a mercury pool used with the Heyrovsky

polarographic electrolysis vessel, or a standard calomel reference

electrode prepared according to Hume and Harris (29) with a flexible

salt bridge containing a saturated KCl solution and terminated by a

6 mm diameter glass tubing containing an agar KCl gel dipping into a

10 mm diameter glass fritted tube containing a minimum amount of solvent

and an appropriate electrolyte for contact. The glass fritted tube was

fitted in a rubber stopper and plunged into the measuring solution the

level of which was 1 or 2 millimeters higher than the level of the

solution in the tube.(30)

10

The working electrodes used were:

- a capillary dropping mercury electrode, 6 em length, Sar-

gent No. S-29417.

- a silver-silver chloride electrode, Beckman No. 39261.

4. All glassware used was Pyrex glass. 10 milliliter burets with

straight or curved delivery tips were graduated in 0.02 milliliter

divisions.

B. Reagents and Solutions

1. Chelan Solutions.

a) Ethylenediaminetetraacetic acid (EDTA)(Matheson Coleman

& Bell) 99% purity was used as received. Solutions were prepared by

weighting an approximate amount required for 0.01 M solution and after

dissolution in the solvent were standardized against a 0.01 M calcium

chloride standard aqueous solution using Calcon as metallochromic in-

dicator. The pH of the solution was maintained around 13 by the use

of diethyl amine. EDTA dissolved in methanol made 1 M in ethanolamine.

EDTA dissolved in dimethyl formamide upon heating near 148°C. The so-

lution was stable for less than six hours and a voluminous white pre-

cipitate appeared after that period of time. This precipitate could

be redissolved by heating the solution again but the normality had to

be redetermined. EDTA dissolved in dimethyl sulfoxide upon heating

0 near 106 C. The solution was very stable over months.

11

EDTA did not dissolve in the following organic solvents: methanol,

iso propanol, acetonitrile, propylene carbonate, 1,4-dioxane, benzene,

methyl ethyl ketone, methyl isobutyl ketone.

Aqueous 0.01 M standard EDTA solutions used to determine the nor

mality of metal solutions were prepared by dissolving an approximate

amount of the disodium salt (Fisher Scientific Co.) in water and the

titre was checked against the 0.01 M primary standard calcium solution.

b) 1,2-diaminocyclohexanetetranaacetic acid (DCTA) (Ciba

Geigy, Chel CD) was used as received.

A 0.01 M solution of DCTA in methanol, made 0.5 M in ethanolamine

was prepared by weighing an approximate amount of DCTA. The titre

was determined against the 0.01 M primary standard calcium solution

using Calcon as metal indicator and diethyl amine to adjust the pH to

13. A standard zinc nitrate solution can also be used with Eriochrome

Black T as indicator at a pH of 10 obtained with a buffer recommended

by Schwarzenbach (1) (dissolve 70 g ammonium chloride in 570 ml ammonia

of density 0.90, dilute to one liter with demineralized water.)

DCTA solutions in dimethyl formamide or dimethyl sulfoxide were

obtained by dissolving the appropriate amount of reagent in the organic

solvent under heating. Standardization was done with calcium or zinc

standard solutions as with the case of methanolic solutions.

c) Ethylene glycol-bis(2-aminoethylether)-tetraacetic acid

(EGTA)(Ciba-Geigy, Chel DE) was used as received. Solutions were

12

prepared by dissolving an approximate amount of reagent in the organic

solvent and were standardized against a standard zinc aqueous solution

with Zincon as indicator at pH around 9.5 according to Ringbom (31) or

with Eriochrome Black T as indicator at a pH of 10. EGTA dissolved

with heating in dimethyl sulfoxide but dissolved in methanol made 0.25

M in ethanolamine at room temperature.

d) Tetraethylenepentamine (tetren)(Eastman Kodak, technical)

was purified as the pentahydronitrate according to Reilley and Holloway

(32): dissolve 75 g of tetren in 150 ml of ethanol (solution A), cool

0 near 0 C. Prepare a mixture of 150 ml of water and 150 m1 of ethanol

and 2.5 moles (approximately 140 ml of nitric acid, density 1.42) of

nitric acid (solution B) and cool near 0°C. Add slowly with stirring

0 solution B to solution A, maintaining the temperature under 10 C.

Filter the precipitate by suction. Recrystallize five times from a-

queous 5% (by volume) nitric acid solution. The white precipitate is

washed with acetone and dried at 50°C.

To prepare a 0.01 M solution of tetren, weigh approximatly 0.6 g of

the salt and dissolve in 100 ml of organic solvent. Tetren being a

primary amine reacts with ketones to form condensation products

(Schiff bases) (33), therefore it was not prepared in these solvents.

A 0.0129 M tetren solution in methyl ethyl ketone was found to have a

concentration of 0.0041 M one month later. Tetren salt dissolves in

methanol made 1 M in ethanolamine and in dimethyl sulfoxide without

13

ethanolamine with slight heating. Due to the fact that nitrates are

decomposed in dimethyl fomamide (34) no experiments have been attempted

with tetren solutions in this solvent.

Tetren solutions were standardized according to Reilley and Schmid

(35) against a standard zinc nitrate solution with Eriochrome Black T

as indicator and triethanolamine maintaining a pH of 7.8.

2. Standard metal solutions.

a) A one liter 0.01 M calcium standard solution was prepared

by dissolving 1.000 g of calcium carbonate (primary standard, Thorn

Smith Co.) in 20 m1 water and 2 ml concentrated nitric acid, followed

by dilution with demineralized distilled water to volume.

b) A 0.01 M zinc standard solution was prepared by dissolving

0.8137 g of zinc oxide (Mallinckrodt, analytical reagent) in 20 ml water

and 2 ml concentrated nitric acid, followed by dilution to one liter

with demineralized distilled water.

c) Calcium, zinc and lead perchlorates were prepared by

dissolving calcium carbonate (primary standard), zinc oxide and lead

carbonate (Mallinckrodt, analytical reagent) in the minimum amount of

0.2 ~ perchloric acid solution under heating to expel the carbon

dioxide and heating to dryness on the sand bath. To prepare 100 ml

of 0.01 ~ solutions dissolve 0.3, 0.4 and 0.5 g of calcium, zinc or

lead perchlorate respectively in the appropriate solvent. Completely

anhydrous solutions of these metals have been prepared according to

14

Erley (36) and Starke (37) by dissolving the above mentioned amounts of

salts in 20 m1 of organic solvent and 2 m1 of 2,2-dimethoxypropane

followed by 30 minutes of stirring on a combination magnetic stirrer -

hot plate. Toward the end of that period the solution was heated to

expel the last trace of methanol and acetone and diluted to 100 m1 with

the organic solvent. No better results have been obtained with these

completely anhydrous metal solutions.

3. Indicator solutions.

a) Eriochrome Black T (Fisher Scientific Co.) was prepared

in 0.1% solution by dissolving 20 mg in 15 ml triethanolamine and 5 m1

absolute alcohol or 25 mg in 25 ml dimethyl formamide or dimethyl sul

foxide. The solutions were stable for months.

b) Calcon (Fisher Scientific Co.) solutions were prepared in

0.05% solution in methanol (12.5 mg in 25 ml methanol) and 0.1% solution

in dimethyl sulfoxide (25 mg in 25 ml dimethyl sulfoxide). The solutions

were stable for months.

c) Zincon (Aldrich Chemical Co.) was prepared 0.1% in methanol

or dimethyl sulfoxide and 0.065% in dimethyl formamide. The indicator

dissolves in methyl ethyl ketone only when tetrabutylammonium hydroxide

(25% in methanol) (Eastman Kodak) is present: (13 mg of Zincon in 20

ml of methyl ethyl ketone+ 0.5 m1 of tetrabutylammonium hydroxide).

All Zincon solutions were stable for one week only.

d) Methylthymol Blue (pentasodium salt) (Eastman Kodak) is

15

not soluble in alcohol (38), dimethyl sulfoxide or dimethyl formamide.

A 0.1% solution of Methylthymol Blue was prepared by dissolving

20 mg of the indicator in 2 ml of water followed by dilution with 18

m1 of methanol.

e) Aqueous Mercury-EDTA indicator solution (Anderson Labora

tories, Inc.) was used as received. The concentration of the complex

was determined according to Starostin (39) by back titration of an ex

cess of standard bismuth nitrate solution with 0.01 M EDTA using Xylenol

Orange as indicator and found to be 0.020 M.

4. 1,4-Dioxane (City Chemical Co., New York) was purified according

to Huber (40) by filtering through a chromatographic column containing a

strong acid cation exchanger, Amberlite IR-120 (Mallinckrodt). Other re

agents, analytical grade, were used as received. Methanol and methyl

isobutyl ketone were from Matheson Coleman & Bell, dimethyl formamide

dimethyl sulfoxide and methyl ethyl ketone from Fisher Scientific Co.

To adjust the "pH" of the solution during titrations, 0.1 M, 1 M

or concentrated solution of monoethanolamine, diethylamine, triethyl

amine, triethanolamine, tetrabutylammonium hydroxide, tetramethylam

monium hydroxide and perchloric acid were prepared in the solvent used

for titration. Perchloric acid, giving an explosive reaction with di

methyl sulfoxide (40), was first diluted with 1,4-dioxane. MOnoethanol

amine being a primary amine reacts with ketones to form condensation pro

ducts (Schiff bases) (33), therefore it was not used with these solvents.

16

C. Procedure

1. Direct visual titrations with metallochromic indicators.

Titrations were performed in a 50 ml beaker, at room temperature,

without using a dry box. The metal solution, one to five milliliters,

was pipeted into 25 ml of solvent followed by two to five drops of

metallochromic indicator. The acidity of the solution was brought to

the desired value, read on the millivolt scale of the pH meter equiped

with glass electrode and modified S.C.E. A magnetic stirrer was used

continuously during the analysis.

a) Titrations of zinc with EDTA

Zinc was titrated in methanol with Eriochrome Black T, Methylthy

mol Blue or Zincon as indicator. The acidity was maintained around

-300 mV with ethanolamine (around -200 mV for the last indicator). The

color changes were respectively from pink to blue for Eriochrome Black

T, blue to yellow for Methylthymol Blue and blue to pink for Zincon.

In dimethyl formamide the potential was maintained around -250 mV by

use of diethylamine (Eriochrome Black T) or ethanolamine (Zincon). The

color changes were respectively from pink to blue and blue to pink. In

dimethyl sulfoxide the same indicators were used at a potential around

-350 mV.

When methyl ethyl ketone was used as solvent, the titrant was a

solution of EDTA first dissolved in dimethyl sulfoxide then diluted

with methyl ethyl ketone in a 1 to 3 volume ratio. Tetrabutyl ammonium

hydroxide (20% in methanol) diluted 1:10 with the solvent was used to

bring the potential around -400 mV with the occasional addition of

perchloric acid (0.1 Min the solvent). The indicator Eriochrome Black

17

T changed color from pink-violet to blue.

b) Titrations of zinc with DCTA

In methanol as solvent, Eriochrome Black T, Methylthymol Blue and

Zincon required respectively the potential values: -350 mV, -300 mV

and -275 mV, which were obtained with 1 ml of ethanolamine, 1 ml of

diethylamine and 0.25 ml of ethanolamine in each particular case, for a

volume of 25 m1 of solution. They changed color respectively from pink

to blue, violet to canary yellow and blue to pink orange. The reaction

with Eriochrome Black T was very slow at room temperature but increased

noticeably when the solution was heated to 60°C. Micro titrations of

zinc in dimethyl sulfoxide were performed with Zincon as indicator at a

potential around -350 mV, obtained with the addition of ethanolamine.

The color change was from blue to pink orange. When methyl ethyl ketone

0 was used, the solution had to be heated at 60 C, and Methylthymol Blue

changed from blue to yellow at the end of the reaction. Two milliliters

of diethylamine added to 25 ml of solvent gave a potential around -300

mV.

c) Titrations of zinc with EGTA

Zincon used in methanolic medium buffered with ethanolamine around

-275 mV gave a very sharp color change from blue to pink orange. In

dimethyl sulfoxide it reacted slowly with the potential set around

-350 mV with the same buffer. Eriochrome Black T, on the contrary,

gave a very sharp change from violet pink to blue in the same experi-

mental conditions.

d) Titrations of zinc with tetren

The potential of the titrating solution was around -350 mV when

18

dimethyl sulfoxide served as solvent. Zincon changed color as in the

other media. In methyl ethyl ketone the solution was buffered with

triethanolamine, one molar in the same solvent. The ionic strength was

kept at 0.1 by the addition of lithium or sodium perchlorate. Zincon

was used as indicator and gave a very sharp end point.

e) Titrations of calcium with EDTA

In methanol calcium was determined with EDTA using Methylthymol Blue.

The potential was around -275 mV and was attained by the use of 1 ml of

ethanolamine for 25 ml of solvent. The indicator changed rapidly from

blue to yellow with a brief transition of grey before the end point.

f) Titrations of calcium with DCTA

Calcon was used for the titration of calcium in methanol. Two

milliliters of diethylamine added to the solution gave a potential

around -300 mV. The indicator changed from pink to violet.

With the same experimental conditions, Methylthymol Blue gave a

sharper end point from blue to yellow in passing through a grey tran

sition just before the appearance of the yellow color.

In methyl ethyl ketone, with the same experimental procedure as

above, the potential was -330 mV and Methylthymol Blue changed from

blue to canary yellow with a transition of greenish yellow which was

rather persistent.

g) Titrations of calcium with EGTA

In methanol calcium was titrated with the indicator system Zincon

+ Zn-EGTA. To 25 ml of methanol were added 1 ml of ethanolamine, 5

drops of Zincon, 2 drops of 0.005 M Zn-EGTA solution prepared by

mixing equal parts of 0.01 M zinc perchlorate and 0.01 M EGTA in

19

methanol, and the calcium solution. The solution was titrated slowly

and changed color from blue to orange.

In dimethyl sulfoxide the same procedure was used but the Zn-EGTA

solution was prepared in this solvent. The color change was from blue

to pink. The potential was around -400 mV.

If, instead of Zincon, Eriochrome Black Twas used, the color change

was even sharper from pink to blue. Quantitative results were obtained

if the ionic strength of the solution was maintained at 0.1 with potas

sium perchlorate.

h) Titrations of calcium + zinc with DCTA

In methyl ethyl ketone with Methylthymol Blue as indicator and

DCTA in methanol as titrant, the sum of calcium + zinc was determined

with the solution heated to 60° C and buffered around -330 mV with 2

ml of diethylamine. The color change was from blue to yellow.

i) Titrations of calcium + zinc with EGTA

The sum calcium + zinc was determined in dimethyl sulfoxide, using

Eriochrome Black T as indicator and ethanolamine as buffer at -300 mV.

The ionic strength was maintained at 0.1 with potassium perchlorate.

In methanol the indicator used was Zincon and the procedure was iden

tical with the ones concerning the titration of calcium or zinc

separately, but no need for the addition of the zinc-EGTA complex.

j) Analysis of calcium and zinc in a sample of polystyrene

One gram of polystyrene was dissolved in 25 ml of methyl ethyl

ketone and 10 drops of 0.1 M perchloric acid in the same solvent. In

the hot solution were added 2 ml of diethylamine and 5 drops of Methyl

thymol Blue. The solution assumed a blue color and was titrated

20

immediately and slowly with a 0.001 M solution of DCTA in methanol until

a yellow color was reached. The volume of titrant gave the sum of

calcium + zinc.

To titrate zinc, another aliquot was dissolved in the same way with

the addition of 0.25 g of lithium perchlorate. To the resulting solution

cooled to room temperature, were added 1 ml of 1 M triethanolamine in

methyl ethyl ketone, and 5 drops of zincon, (0.1 %) in dimethyl sulfoxide.

The solution assumed a blue color, with the potential around -110 mV.

Zinc was then titrated with a 0.001 M solution of tetren in dimethyl

sulfoxide.until the appearance of the pink color.

2. Photometric titrations.

a) The absorbance curves of the indicators and of the metal-

indicator complexes were recorded on the Beckman DK-2A instrument. The

concentration of the indicator was around 10-6 M, the metal concentration

about one hundred times larger.

Although Eriochrome Black T, Calcon, Methylthymol Blue were also

studied, only Zincon appeared to give the best resolved curves. Ac-

cording to Rush and Yoe (41) a 0.13 % solution of Zincon has a con-

centration of 0.002 M, therefore a 0.065 % or 0.1 % solution of the

indicator has a concentration of 0.001 M or 0.0015 M respectively. The

0.1% soltuion in dimethyl sulfoxide was diluted 50 times with the

solvent and 0.50 ml of the resulting solution was further diluted to

-6 25 m1 with the same solvent, giving a 6 x 10 molar solution. One

tenth milliliter of ethanolamine was added to provide a basic medium

having a concentration of about 0.06 M in ethanolamine. The newly pre-

pared solution was run on the Beckman DK-2A with dimethyl sulfoxide

21

serving as the blank solution. The zinc-Zincon solution was prepared

by the same procedure but one milliliter of 0.01195 M solution of zinc

perchlorate in dimethyl sulfoxide was added before diluting to 25 m1

with the solvent. The concentration of zinc was calculated to be

-4 4.78 x 10 M.

b) Photometric titrations were performed only for the deter-

mination of zinc with Zincon as indicator.

To 25 m1 of dimethyl sulfoxide were added 1 ml of 0.00119 M zinc

perchlorate and 5 drops of 0.1 % Zincon in the same solvent. A paten-

tial of the glass electrode vs. the modified S.C.E. around -350 mV

was obtained by adjusting the acidity with ethanolamine diluted ten

times with the solvent. The absorbance was measured at 650 nm while

titrating with a 0.00103 M solution of DCTA in dimethyl sulfoxide. The

concentration of Zincon was calculated to be 1.44 x 10-5 M in the titrating

solution.

With other titrants the same experimental conditions were followed

except that 20 ml of solvent were used instead of 25, thus giving a

-5 more concentrated Zincon solution (1.78 x 10 M). One milliliter

aliquots of 0.00119 M, 0.00179 M and 0.00171 ~ zinc perchlorate solution

in dimethyl sulfoxide were titrated respectively with 0.00103 M EGTA,

0.00104 M EDTA and 0.00102 M tetren, all dissolved in the same solvent.

For the titration of zinc in dimethyl formamide, 1 m1 of 0.01085 M

zinc perchlorate in dimethyl formamide was added to 25 ml of the solvent

followed by 1 m1 of 0.065 % Zincon solution in the same solvent. The

concentration of Zincon was calculated to be 3.7 x 10-5 M. The titrant

was 0.0070 M EDTA in dimethyl formamide. When the absorbance curves of

22

Zincon and zinc-Zincon in dimethyl formamide were recorded they appeared

to have the same shape as in the case of dimethyl sulfoxide but the

maximum difference in the absorbances was at 625 nm instead of 650 nm.

During the photometric titration the instrument was set at this wave

length of 625 nm.

3. Potentiometric Titrations

Two kindsofmetal indicator electrodes have been successfully used:

the mercury electrode and the lead ion selective electrode.

a) Titration of mercury with the mercury electrode

To 40 ml of dimethyl sulfoxide were added 4 m1 of 1 M potassium

perchlorate, 5 drops of ethanolamine diluted ten times with the same

solvent and 5 drops of 0.01 M mercuric acetate, all dissolved in the

same solvent. The potential indicated by the glass electrode was

around -380 mV. The solution was titrated with 0.01046 M EDTA in di

methyl sulfoxide and the potential of the mercury electrode recorded.

b) Titration of zinc with the mercury electrode

The former titration was stopped after the addition of 0.50 ml of

titrant, during which period the reaction was already completed, 1 m1 of

0.01364 M zinc perchlorate in dimethyl sulfoxide was then added and the

titration started again until a potential break was obtained.

c) Titration of calcium with the mercury electrode

To 40 ml of dimethyl sulfoxide were added 4 m1 of 1 M potassium per

chlorate, one drop of 0.001 M mercuric acetate and 1 m1 of 0.00914 M

calcium perchlorate, all dissolved in the same solvent. Ethanolamine

diluted ten times with dimethyl sulfoxide was used to bring the potential

of the glass electrode around -400 mV. The titration was performed with

23

0.01055 M EDTA in dimethyl sulfoxide.

When methanol was used as solvent, to 25 ml of solution were added

two drops of 0.020 M aqueous mercury-EDTA indicator solution and one

milliliter of 0.01344 M calcium perchlorate in methanol. The titration

was performed with 0.0100 M EDTA in methanol.

d) Titration of lead with the lead electrode

To 25 ml of dimethyl sulfoxide, made 0.1 ~with potassium per

chlorate, were added ethanolamine, diluted ten times with the same

solvent in order to have a potential at the glass electrode around -400

mV, and one milliliter of 0.0125 M lead perchlorate in dimethyl sulfoxide.

The potential at the lead electrode was recorded during the titration

with 0.01055 M EDTA in dimethyl sulfoxide.

e) Titration of zinc with the lead electrode

To 25 ml of dimethyl sulfoxide made 0.1 M with potassium perchlorate

were added ethanolamine, diluted ten times with the same solvent in

order to have a potential at the glass electrode around -400 mV., one

drop of 0.0125 M lead perchlorate and one milliliter of 0.01364 M zinc

perchlorate, both in dimethyl sulfoxide. The titration was performed

with 0.01055 M EDTA in the same solvent.

When 0.00845 M DCTA in dimethyl sulfoxide was used as titrant the

same experimental conditions were followed except for the drop of lead

perchlorate which had a concentration of 0.00125 M.

With 0.0101 M EGTA in dimethyl sulfoxide as titrant the zinc solu

tion had a concentration of 0.0171 M and the same experimental procedure

was used with one drop of 0.001 M lead perchlorate.

24

4. Amperometric Titrations

Before attempting an amperometric titration, different polarograms

for dilute solutions of zinc or calcium (10-4 M) were run in order to

determine the potentials of the plateaus. The amperometric titration

was then performed with the voltage set to the value of the beginning

of the plateau. A wide variety of experimental comditions were tried

but no quantitative results were obtained.

25

IV. RESULTS AND DISCUSSION

A. Direct Visual Titrations with Metallochromic Indicators

1. General comments.

The results of titrations of zinc, calcium and their sum are listed

in Tables I - IX. In performing these titrations the main problem was

the search for suitable "buffer substances" which could be used in the

solvents studied. Since there is no universal pH scale in organic

media the only logical way of recording the acidity of the solution was

to use the millivolt scale on the pH meter, equiped with the electrode

assembly: glass electrode vs. modified S.C.E. This system has been

investigated by Rorabacher et al. (42) in methanol, McClure and Reddy

(43) in dimethyl formamide and by Reddy (44) in di~thyl sulfoxide who

found it to be reliable. A comparison of the relative scales of

acidity of methanol, dimethyl formamide, dimethyl sulfoxide and methyl

ethyl ketone published by Bykova and Petrov (45) and by Kreshkov (46)

show that the pK values for ionization of these solvents are respective

ly 16.7, 24, 31 and 31. These figures prove that the "pH range" in

these media vary widely but according to Petrov and Bykova (47) these

values are valid only for pure anhydrous solvents and if traces of

water are present, which is mostly the case in practical analytical

determinations, then these figures change drastically. Bates (48) and

Reddy (44) have studied some buffer mixtures in methanol and dimethyl

sulfoxide respectively. Some of these mxitures (benzoic acid + sodium

benzoate) have been used ( in dimethyl sulfoxide ) but did not give

any good results.

26

The three metallochromic indicators mostly used, Eriochrome Black T,

Methylthymol Blue and Zincon change color in basic aqueous media. Var

ious amines have been tried to control the basicity of the solution

during the titrations. The potential values listed in each determination

were determined by two factors: 1) they must keep the basicity of the

solution within a reasonable narrow range, 2) the indicator must change

color at these conditions. The first factor was determined experimental

ly by titrating a small amount of amine intended to be used as buffer,

with a standard acid solution dissolved in the same solvent. The

titration curve, titrant added vs. potential at the glass electrode,

was drawn and the potential at the half neutralization point was re

corded. As in the case of aqueous acidimetric titrations the neutral

ization curve shows at the half neutralization point an almost flat

portion in which the potentials do not vary very much. The possibility

of a reaction between the amine buffer and the solvent was another

problem, eg. ethanolamine cannot be used in methyl ethyl ketone (33).

2 Titrations of zinc.

The results are condensed in Tables I - IV.

With EDTA as titrant, Eriochrome Black T seemed to be the best in

dicator in all the media studied. When methyl ethyl ketone was the

solvent, EDTA was first dissolved in dimethyl sulfoxide then diluted

(1:3) with the solvent. This experiment was performed in the hope to

use it for the determination of zinc in polystyrene, since polystyrene

does not dissolve in dimethyl sulfoxide but dissolves in methyl ethyl

ketone or in a mixture of this latter solvent with a small amount of

dimethyl sulfoxide added. Attempts to titrate zinc in dimethyl

Table I. Results of Titrations of Zinc with EDTA Using Metallochromic Indicators

A. Solvent: Methanol

1. Indicator: Eriochrome Black T (Buffer: ethanolamine)

Taken mg.

0.635

1.270

1.905

2.540

3. 085

Found mg.

0. 647

1.273

1.906

2.520

3.085

Number of Determinations

4

4

4

4

4

% Relative Standard Deviation

2.68

0.57

0.43

0.69

0.00

2. Indicator: Zincon (Buffer: ethanolamine)

Taken Found Number of % Relative Standard mg. mg. Determinations Deviation

0.635 0.645 4 2.31

1.270 1.262 4 0.86

1905 1.901 4 0.41

2.540 2.518 4 1.02

27

% Error

+1.90

+0.24

+0.05

-0.80

0.00

% Error

+1.60

-0.63

-0.20

-0.86

28

Table I (continued)

3. Indicator: Methylthymol Blue (Buffer: ethanolamine)

Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation

0.588 0. 540 3 10.00 -8.16

2. 940 2.964 3 0.97 +0.80

3.085 3.087 3 0.16 +0.06

B. Solvent: Dimethyl formamide

1. Indicator: Eriochrome Black T (Buffer: diethylamine)


0.709 0.723 1 +1.97

1.418 1.395 1 -1.62

2.127 2.094 1 -1.55

2.836 2.796 1 -1.41

29

Table I (continued)



0.709 0.753 1 +6.20

1.418 1.477 1 +4.16

2.127 2.109 1 -0.85

2.836 2.823 1 -0.46

C. Solvent: Dimethyl sulfoxide


Taken mg.

0.781

1.562

2.343

3.124

3.905

Found mg.

o. 779

1.561

2.338

3.127

3.913


3

3

3

3

3


0.32

0.32

0.37

0.19

0.28

% Error

-0.26

-0.06

-0.20

+0.96

+0. 20

30

Table I (continued)

2. Indica tor: Zincon (Buffer: ethanolamine)


0.078 0.078 3 1. 74 0.00

0.156 0.157 3 0.86 +0.64

0.234 0.235 3 1. 60 +0.43

0.312 0.311 3 1.38 +0.32

0.391 0.380 3 3.54 -2.81

0. 781 0.784 3 1.12 +0.38

1.562 1.568 3 0.56 +0.38

2.343 2.336 3 0.39 -0.30

3.124 3.102 3 0.92 -0.70

3.905 3.865 3 1. 25 -1.02

D. Solvent: Methyl ethyl ketone

Titrant: EDTA dissolved in DMSO +methyl ethyl ketone (1:3)

Indicator: Eriochrome Black T

(Buffer: tetrabutylammonium hydroxide + perchloric acid)


0.891 0.926 1 +3.93

1. 782 1.819 1 +2.07

2.673 2. 729 1 +2.09

Table II. Results of Titrations of Zinc with DCTA Using Metallochromic Indicators

31




0.635 o. 634 3 1.03 -0.16

1.270 1.292 3 3.23 +1. 73

1.905 1.920 3 1.53 +0.80

2.540 2.525 3 1.62 -0.60

3.175 3.105 3 2. 74 -2.20

2. Indicator: Methylthymol Blue (Buffer: diethylamine)


0.634 0.641 3 1.35 +1.10

1.268 1. 266 3 0.33 -0.16

1.902 1.902 3 0.61 0.00

2.536 2.512 3 1.16 -0.95

32

Table II (continued)


Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviations

0.634 0.629 3 1.30 -0.79

1.268 1.255 3 1.45 -1.02

1.902 1.873 3 1.77 -1.52

2.538 2.509 3 1.26 -1.14

B. Solvent: Dimethyl sulfoxide

Indicator: Zincon (Buffer: ethanolamine)

.Taken Found Number of % Relative Standard % Error mg. mg. Determinations Deviation

0.078 0.078 2 o.oo 0.00

0.156 0.158 2 1.43 +1.28

0.234 0.235 2 0.85 +0.49

0.312 0.314 2 1.28 +0.64

0.391 0.391 2 0.25 0.00

Table II (continued)

C. Sol vent: Methyl ethyl ketone

Titrant: DCTA in methanol

Indicator: Methylthymol Blue (Buffer: diethylamine)

Taken mg.

0.065

0.130 .

0.195

0.260

Found mg.

0.064

0.129

0.191

0.257


3

3

3

3


1.08

0.54

0.36

0.62

33

% Error

-1.54

-0.77

-2.05

-1.15

Table III. Results of Titrations of Zinc with EGTA Using Metallochromic Indicators




0.635 0.639 4 1.23

1.270 1.268 4 0.86

1.905 1.914 4 0.92

2.540 2.543 4 0.69


34

% Error

+0.63

-0.16

+0.47

+0.12



0.892 0.922 2 5.0 +3.36

1.784 1.844 2 5.0 +3.36

2. 676 2.783 2 5.7 +4.00

3.568 3.701 2 5.4 +3.72

4.460 4.646 2 5.9 +4.17

35

Table III (continued)

2. Indicator: Zinc on (Buffer ethanolamine)


0.892 0.878 4 2.21 -1.57

1.784 1. 742 4 2.77 -2.35

2.676 2.592 4 3. 71 -3.14

3.568 3.559 4 1.50 -0.25

4.460 4.447 4 1. 72 -0.29

Table IV. Results of Titrations of Zinc with Tetren Using Matallochromic Indicators.

A. Solvent: Dimethyl sulfoxide



0.112 0.115 1

0.224 0.230 1

0.335 0.345 1

0.447 0.467 1

0.559 0.573 1

1.118 1.127 2 1.44

2.236 2. 241 2 1. 73

3.354 3.368 2 1.52

4.472 4.501 2 1.38

5.590 5.595 2 0.20

36

% Error

+2.68

+2.68

+2.98

+4 .47

+2.50

+0.81

+0.22

+0.41

+0.65

+0.09

37

Table IV (continued)

2. Indicator: Zincon (tetraethylammonium perchlorate +

ethanolamine)


0.112 0.113 2 1. 78 +0.89

0.224 0.233 2 5.35 +4.01

0.335 0.342 2 2.82 +2.09

0.447 0.453 2 1.89 +1.34

0.559 0.568 2 2.15 +1.61

1.118 1.134 1 +1.43

2.236 2. 240 1 +0.18

3.354 3.347 1 -0.21

4.472 4.461 1 -0.25

5.590 5.574 1 -0.29

38

Table IV (continued)

B. Solvent: Methyl ethyl ketone

Titrant: Tetren in DMSO

1. Indicator: Zincon (sodium perchlorate+ triethanolamine)

Taken mg.

0.784

1.568

2.352

3.136

Taken mg.

0.784

1.568

2.352

3.136

Found Number of % Relative Standard % Error mg. Determinations Deviation

0.800 2 2.97 +2.04

1.562 2 1.46 +0.38

2.336 2 0.96 -0.68

3.113 2 1.05 -0.73

2. Indicator: Zincon (lithium perchlorate + triethanolamine)

Found mg.

o. 777

1.555

2.328

3.109


2

2

2

2


1.45

1.39

1. 52

1. 22

% Error

-0.89

-0.83

-1.02

-0.86

39

formamide with Eriochrome Black T buffered by ethanolamine or triethanol

amine, with Methylthymol Blue and ethanolamine, with Calcon and diethyl

amine have failed. In methyl ethyl ketone, Zincon buffered with tri

ethanolamine, Calcon with triethylamine, Eriochrome Black T with trieth

anolamine or the mixture tetramethylammonium hydroxide + tetraethyl

ammonium perchlorate did not give any result. In dioxane, which

dissolves polystyrene, Eriochrome Black T buffered by ethanolamine or

triethylamine did not give any quantitative result when EDTA dissolved

in dimethyl sulfoxide was used as titrant.

With DCTA as titrant (Table II), Methylthymol Blue appeared to be

the best indicator, particularly in the solvent methyl ethyl ketone and

diethylamine as buffer the figures suggest a possibility of determination

of zinc in polystyrene. With EGTA {Table III), Zincon proved to be

superior to Eriochrome Black T.

Tetren dissolved in dimethyl sulfoxide could be used to titrate

zinc in methyl: ethyl ketone. It was noticed that lithium perchlorate

used to maintain the ionic strength at 0.1 was much better than sodium

perchlorate since the color change was very rapid and sharp.

3. Titrations of calcium.

Direct titrations of calcium have been less successful than the case

of zinc. To the indicators used for zinc was also added Calcon.

With EDTA (Table V), only Methylthymol Blue gave some results in

methanol. Calcon did not behave quantitatively in all the solvents

and buffers {even tetrabutylammonium hydroxide, one of the most basic

buffers) tried.

DCTA appeared to be more effective than EDTA since results were

Taken mg.

0.397

0. 794

1.191

1.588

Table V. Results of Titrations of Calcium with EDTA in Methanol Using Methylthymol Blue

(Buffer: ethanolamine)

Found Number of % Relative Standard mg. Determinations Deviation

0.404 3 2.36

0.806 3 2.04

1.206 3 1.80

1.608 3 1.68

40

% Error

+1.76

+1.51

+1.26

+1.26

Table VI. Results of Titrations of Calcium with DCTA Using Metallochromic Indicators


Taken mg.

0.842

1.263

1.684

2.105

Taken mg.

0.421

0.842

1.263

1.684

2.105

1. Indicator: Calcon {Buffer: diethylamine)

Found Number of % Relative Standard mg. Determinations Deviation

0.850 4 1.55

1.276 4 1.40

1.695 4 1.12

2.120 4 0.90

2. Indicator: Methylthymol Blue (Buffer: diethylamine)

Found mg.

0.427

0.852

1.280

1. 707

2.128


4

4

4

4

4


1.90

1.45

1.58

1.67

1.28

41

% Error

+0.95

+1.02

+0.65

+0.71

% Error

+1.42

+1.18

+1.34

+1.36

+1.09

42

Table VI (continued)

B. Solvent: Methyl ethyl ketone


Indicator: Methylthymol Blue (Buffer: diethylamine)


0.057 0.058 3 2.14 +1.75

0.114 0.115 3 1.52 +0.87

0.171 0.172 3 1.01 +0.58

0.228 0.228 3 0.70 0.00

0.285 0.285 3 0.43 0.00

0.421 0.429 3 2.85 +1.90

0.842 0.851 3 1.39 +1.06

1.263 1. 276 3 1.31 +1.02

1.684 1. 714 3 2.16 +1. 78

2.105 2.135 3 1. 74 +1.42

Table VII. Results of Titrations of Calcium with EGTA Using the System Zincon + Zinc-EGTA


Indicator: Zincon + Zn-EGTA (Buffer: ethanolamine)


0.397 0.401 3 1. 22

0. 794 0.805 3 1. 74

1.191 1. 212 3 2.20

1.588 1.618 3 2.39


1. Indicator: Zincon + Zn-EGTA (Buffer: ethanolamine)


0.366 0.367 4 2.24

0.040 0.044 1

0.080 0.089 1

0.120 0.134 1

0.160 0.173 1

43

% Error

+1.00

+1.38

+1. 76

+1.88

% Error

+0.27

+10.00

+11.25

+11.66

+8.13

Table VII. (continued)

2. Indicator: Eriochrome Black T + Zn-EGTA

~uffer: ethanolamine)

Taken Found Number of % Relative Standard mg.

0.040

0.080

0.120

0.160

Taken mg.

0.040

0.080

0.120

mg. Determinations Deviation

0.042 1

0.083 1

0.124 1

0.164 1

3. Indicator: Eriochrome Black T + Zn-EGTA (+KC104)

~uffer: ethanolamine)

Found mg.

0. 040

0.080

0.120


1

1

1


44

% Error

+5.00

+3 0 75

+3.34

+2.50

% Error

0.00

0.00

0.00

45

obtained with Calcon and Methylthymol Blue in methanol and with Methyl

thymol Blue in methyl ethyl ketone. This latter case suggested a pos

sibility of determination of calcium in polystyrene with zinc. Calcon

was once more inefficient in all the other media tried.

With EGTA (Table VII.) none of the indicators used gave quantitative

results but if trace quantities of zinc-EGTA complex were added then a

substitution reaction took place and the titration was possible. This

kind of reaction was investigated by Ringbom, Pensar and·Wanninen (31),

who used the system Zincon + zinc-EGTA. In methanol only this latter

reaction was studied, but in dimethyl sulfoxide the system Eriochrome

Black T + zinc-EGTA was also tried and proved to be superior to the

system with Zincon, in particular if the ionic strength was kept at 0.1

with potassium perchlorate, then quantitative results seemed to be

attained.

To sum up, in comparison with EDTA and EGTA, DCTA proved to be the

best titrant for calcium. This result does not contradict the findings

of Holloway and Reilley (49) when they studied the metal stability

constants of various aminopolycarboxylate ligands in water with their

mercury electrode.

4. Titrations of calcium+ zinc.

Calcium + zinc have been determined in methyl ethyl ketone with

DCTA dissolved in methanol and Methylthymol Blue as indicator in an

anticipated try to use the method for the analysis of polystyrene. The

sum of these two metals has also been determined with EGTA, but the

addition of the complex zinc-EGTA was not necessary. EGTA titrant

was much better than DCTA but unfortunately EGTA could not be used

Table VIII. Results of Titrations of Calcium + Zinc with DCTA Using Methylthymol Blue

Solvent: Methyl ethyl ketone


(Buffer: diethylamine)

Taken: J.l-mole Ca • ••••••••••••••••• 1.01 1.01

ll-mole Zn . ••.••.••.••.•.... 0.99 1.98

Total JJ-mole Ca+Zn taken . .......... 2.00 2.99

Total ).!-mole Ca+Zn found . .......... 2.00 3.00

Number of determinations •••••••.••• 4 4

% Relative standard deviation ••.... 1.04 1.43

% Error . ........................... 0.00 +0.33

46

1.01

2.97

3.98

4.01

4

1.65

+0.75

Table IX. Results of Titrations of Calcium + Zinc With EGTA Using Metallochromic Indicators


Indicator: Eriochrome Black T (Buffer: ethanolamine)

Taken: ~-mole Ca •••••••••••••• 0.91

p-mole Zn .... ......... . 1.71

Total ~-mole Ca + Zn taken ••••• 2.62

Total ~-mole Ca + Zn found ••••• 2.63

Number of determinations •••.••• 5

%Relative Standard Deviation •• 0.02

% Error ....................... . +0.38

B. Solvent: Methanol


Taken: ~-mole Ca • ••••••••••••• 9.9 19.8

~-mole Zn • ••••••••••••• 9.7 9.7

Total ~-mole Ca + Zn taken ••••• 19.6 29.5

Total ~-mole Ca + Zn found ••••• 19.9 30.0

Number of determinations ••••.•• 3 3

% Relative Standard deviation •• 0.16 0.21

% Error ......•........ · · · · · · · • · +1.53 +1.69

47

48

with methyl ethyl ketone since an immediate precipitate formed when they

were brought together.

5. Analysis of calcium and zinc in a sample of polystyrene. The

polystyrene sample was dissolved by pouring it into the solution already

stirred on the magnetic stirrer-hot plate; this prevented it from

forming a big coagulate which sticked to the stirring bar and stopped its

motion. The dissolution was complete within ten minutes and the titra

tion performed with the hot solution required less than three minutes

thus the total analysis time was less than 15 minutes. Two samples

were analyzed; one with an assay labled Zn = 100-115 ppm, Ca = 5 ppm,

the other Ca = 100 ppm, Zn ~ 1 ppm,(?), phosphate (?). The sample with

out phosphate gave after analysis of zinc a concentration of calcium

equal to 18 ppm. The sample with phosphate gave a very low result for

calcium and the color of the complex was not well developed. This

interference due to the phosphate has been reported by Garcia Montelongo,

Herrera and Arias (50) who studied the colorimetric determination of

calcium with Methylthymol Blue in aqueous medium. It appeared that this

interference was also valid in methyl ethyl ketone.

6. Conclusion

Direct visual titrations of zinc and calcium with metallochromic

indicators in non aqueous media are possible, although more limited

than in aqeuous solution. Eriochrome Black T and Zincon gave good re

sults for the analysis of zinc, Methylthymol Blue for the analysis of

calcium. This latter indicator has been extensively studied in aqueous

medium by Korbl and Pribil (38), Buben, Korbl and Pribil (51), Korbl

and Kakac (52), Janousek and Studlar (53), Chalmers and Miller (54) and

Table X. Results of Titration of Zinc and Calcium in Polystyrene

Origin of sample: Dow expandable polystyrene (Bendix Co.)

Product: SD - 572

Lot No.: OH - 1701

Assay: Zinc = 100 to 115 ppm

Calcium 5 ppm

A. Titration of Zinc

Solvent methyl ethyl ketone

Titrant 0.001 M tetren in dtmethyl sulfoxide

Indicator Zincon

Buffer Triethanolamine

Weight of sample Weight of zinc found

1.000 g 109 ppm

112

106

98

114

106

Average • •••••..•••• • • • • • • • • • • • • • • 108 ppm

Standard deviation............... 6

%Relative standard deviation.... 6

Confidence ltmit at 90 % Probability level •••••••••••••••• + S

Result: 108 ± 5 ppm Zinc

49

50

Table X. (continued)

B. Microtitration of zinc

Same experimental conditions as in A except titrant is 0.0001 M and

weight of sample is 0.100 g.

Weight of zinc found

100 ppm Average . ..........•............. . 103 ppm

110 Standard deviation. . • • • • • • • • • • • • 10

110 %Relative standard deviation •••• 10

90 Confidence limit at 90 %

100 probability level. • • • • • • • • • • • • •. + 8

110 Result: 103 ± 8 ppm Zinc

C. Titration of calcium+ zinc.

Solvent: methyl ethyl ketone

Titrant: 0.001 M DCTA in methanol

Indicator: Methylthymol Blue

Buffer: diethylamine

Weight of sample

DCTA }l-mole

* Zinc p-mole

*

1.000' g 2.10 2.08 2.16 2.07 2.12 2.12

Value taken from A

1.65

Average for calcium •••• 18 ppm

Standard deviation ••••• 1

Calcium J..l-mole

0.45 0.43 0.51 0.42 0.47 0.47

% Relative standard

Calcium ppm

18 17 20 17 19 19

deviation ................ 7

Confidence limit at 90% probability level •.••••• ± 1

Result: 18 + 1 ppm Calcium

51

Yoshino et al. (55). Since it shows two ranges of pH color changes it

could be used to titrate zinc buffered by urotropin at pH = 6.5 and the

sum zinc + calcium buffered by annnonia. Its behavior in "acidic" medium

in methyl ethyl ketone for the analysis of zinc should be an interesting

investigation.

B. Photometric Titrations

1. The absorbance curves in Figure 1 show a maximum difference be

tween the indicator and the metal-indicator complex at 650 nm. Due to

the fact that the total volume of titrant added represented less than

8 % of the total volume of the solution, a dilution factor was not

necessary in constructing the titration curve.

2. The titration curves in dimethyl sulfoxide as solvent are shown

on Figure 2. DCTA appears to be the best titrant for zinc: the curve

drops sharply and the relative error is +0.84 %. No extrapolation is

needed to locate the end point which lies on the abscissa axis.

For the case of EGTA, EDTA and tetren, the end point has to be extra

polated by the interesection of two straight lines.

Concerning their effectiveness the titrants can be ordered in the

following decreasing order according to Table XI:

DCTA > Tetren > EDTA > EGTA

This compares favorably with the direct visual titration which gives the

following order:

DCTA > EDTA > Tetren > EGTA

52

Figure 1. Absorbance curves in dimethyl sulfoxide

(A) -6 -4 6.0 x 10 M Zincon, 4.78 x 10 M Zinc perchlorate

0.06 M ethanolamine in dimethyl sulfoxide

(B) -6 6.0 x 10 M Zincon, 0.06 M ethanolamine in

dimethyl sulfoxide

0 0 U)

0 I() &

()

0 0 &()

&()

"'" v 0 &()

v &()

~------~------~------~------~----~N

0 v 0

0 0

0 v

ro N

0

0 0

3:>Ntt8~0S8tt

53

E

c: .. :I: ..._ <.!)

z w

...J .

w

r-l

~

Q)

.... ;j

;: 00

..-I ~

54

Figure 2. Photometric titrations of zinc in dimethyl sulfoxide

Wavelength: 650 nm; Indicator: Zincon

(A) Titrations of 1.19 ~-mole of zinc perchlorate in DMSO

Titrant: 0.00103 M DCTA in DMSO

Zincon: 1.44 x 10-5 M in DMSO

(B) Titration of 1.19 ~-mole of zinc perchlorate in DMSO

Titrant: 0.00102 M EGTA in DMSO

Zincon: 1.78 x 10 -S Min DMSO

(C) Titration of 1.79 v-mole of zinc perchlorate in DMSO

Titrant:

Zincon:

0.00104 M EDTA in DMSO

1.78 x 10-5 M in DMSO

(D) Titration of 1.71 ~-mole of zinc perchlorate in DMSO

Titrant: 0.00102 M tetren in DMSO

Zincon: 1.78 x 10-S Min DMSO

55

0.30 r---T1----.,.1----,1---,--1 ---.

0.20 ~ A -

0.10- -

I I I I

lJJ 0.20 u z ~ CD a: 0.10 0 (/) CD <t

0.30

0.20 D

0.10

0 0.5 1.0 1.5 2.0

TITRANT ADDED (MICROMOLES) Figure 2.

56

Figure 3. Photometric titration of zinc in dimethyl formamide

Wavelength: 625 nm; Indicator: Zincon

Titration of 10.85 ~mole of zinc perchlorate in dimethyl formamide

Titrant:

Zincon:

0.0070 ~ EDTA in dimethyl formamide

-5 3.7 x 10 Min dimethyl formamide

57

. 0.20~--------

L&J 0.15 u z <( m a:: 0 0.10 (/) m <(

0.05

0 I 2 3 4 5 6 7 8 9 10 II 12 13

TITRANT ADDED (MICROMOLES)

Figure 3.

Table XI. Results of Photometric Titrations of Zinc



Zinc Taken Zinc Found Number of % Error ll-mole ll-mole Determination

1.19 1.20 1

1.19 1.23 1

1. 79 1.84 1

1. 71 1.68 1

B. Solvent: Dimethyl formamide


Zinc Taken v-mole

10.85

Zinc Found ll-mole

10.95


1

+0.84

+3.36

+2.79

-1.75

% Error

+0.92

58

Titrant

DCTA

EGTA

EDTA

Tetren

Titrant

EDTA

59

In fact the relative errors in the two methods are of the same order

of magnitude and the photometric titration does not give better results

in comparison with the direct visual titration. One further disadvan

tage of the photometric method is the fact that it is time consuming and

the end point must be determined on a graph. A direct visual titration

takes about five minutes while a photometric titration requires twenty

to thirty minutes. The photometric method will only be useful in the

case where impurities in the solution might block the indicator or when

a recording automatic phototitrator is available.

3. In dimethyl formamide the titration curve of zinc at 625 nm in

Figure 3. shows a sharp end point. The relative error of less than 1 %

is much better than the direct visual titration which has a error of 6 %.

Due to the lack of time no investigation have been made in methanol

as solvent.

4. The absorbance curves run for different indicators Calcon, Methyl-

thymol Blue, Eriochrome Black T and Zinconc and their complexes with

calcium or zinc showed that Zincon (Figure 1), gave two best resolved

curves in which the maximum absorbance of one curve occured at a wave

length corresponding to the minimum absorbance of the other, and vice

versa. For this reason, only some photometric titrations of zinc with

Zincon as indicator have been studied. Photometric titrations being an

extension of the direct visual method of titration should of course

give positive results for cases which work in the latter method.

C. Potentiometric Titrations

1. Table XII and Figure 4 show the results of potentiometric

titrations of calcium and zinc with EDTA in dimethyl sulfoxide and in

Metal

Hg

Zn

Ca

Ca

Ca

Ca

Zn

Zn

Taken ll-rnole

2.50

13.64

9.14

18.28

13.44

33.60

13.64

27.28

Table XII. Results of Potentiometric Titrations of Calcium and Zinc with EDTA

Found ll-mole

2.55

13.90

9.35

17.65

13.80

33.50

13.37

27.37

% Error

+2.00

+1.90

+2.30

-2.90

+2.68

-0.30

-1.98

+0.33

Electrode Solvent

Mercury DMSO

Mercury DMSO

Mercury DMSO

Mercury DMSO

Mercury Methanol

Mercury Methanol

Lead DMSO

Lead DMSO

Amplitude of Potential Break

rnV

25

75

25

60

90

96

55

130

Curve in Figure 4.

A

B

c

Not Drawn

D

Not Drawn

E

Not Drawn

0\ 0

61

Figure 4. Potentiometric titrations of calcium and zinc with EDTA

(A) Titration of 2.5 ~-moles of mercuric acetate in DMSO

Titrant: 0.01046 M EDTA in DMSO

Mercury electrode vs. modified S.C.E.

(B) Titration of 13.64 ~-moles of zinc perchlorate in DMSO



(C) Titration of 9.14 ~-moles of calcium perchlorate in DMSO



(D) Titration of 13.44 ~-moles of calcium perchlorate in DMSO

Titrant: 0.0100 M EDTA in methanol


(E) Titration of 13.64 ~-moles of zinc perchlorate in DMSO


Lead ion selective electrode vs. modified S.C.E.

000

. IJJ . 0 . C/)

~ -3.00 IJ.. -0 0 :E

. -400 tn >

> E ..

...J <( -1-

-500

z IJJ -600 1-0 a..

-700

0

62

E

5 10 15 20

TITRANT ADDED (MICROMOLES)

Figure 4.

63

methanol.

a) Volume determinations.

Except for curve A and D where no volume correction is needed since

in A mercury was titrated directly with the mercury electrode and in

D 2 drops of Hg-EDTA indicator solution were used, all the volumes read

on the end point of other curves must be corrected for the indicator

blank. These blank volumes are respectively 2.5 ~-moles for curve B,

0.05 ml x 0.001 M = 0.00005 mM = 0.05 ~-mole for curve C, and 0.05 ml

x 0.00125 M = 0.000625 mM = 0.63 ~-mole for curve E, since one drop of

the indicator has a volume of 0.05 ml.

b) Comments on the results.

The amplitude of the potential break will be defined by the dif

ference in potential between the two points on the curve, one on each

side of the end point, separated by an increment of one micromole.

The titration of zinc in dimethyl sulfoxide with the mercury elec

trode gives a sharper potential break than with the lead electrode (75

mV vs. 55 mV). The volume of indicator ion used was respectively 2.5

~-moles of mercuric ions and 0.63 ~-mole of lead (II) ions. Figure 6

shows that when the lead electrode is used the amplitude of the poten

tial break increases with an increase of the concentration of zinc.

The experiments with the mercury electrode show the opposite.

It has not been possible to determine calcium with the lead elec

trode. When the mercury electrode was used the volume of indicator

was respectively 2 x 0.05 ml x 0.020 M = 0.002 mM = 2 ~-moles of Hg (II)

ions in methanol and 0.05 ~-mole of Hg (II) ions in dimethyl sulfoxide

(see part a) above). In methanol the mercuric ions were introduced

64

in the form of an aqueous solution. The presence of these 2 drops of

water in 45 ml of solution gives a water concentration of 0.22 % and

might explain why the amplitude of the potential break is larger in

methanol than in dimethyl sulfoxide (90 mV vs. 25 mV) (curves C and D).

Other experiments performed at different concentrations of calcium show

that in methanol the amplitude of the potential break increases slightly

with an increase of concentration of calcium while in dimethyl sulfoxide

this trend is more accentuated.

Figure 1 indicates also that when the mercury electrode is used, the

titration curve for zinc appears in a higher potential region than the

curve for calcium. This result in dimethyl sulfoxide is also found in

aqueous medium (56).

A comparison between curves B and C shows that in the titrations of

zinc and calcium with the mercury electrode the potential break is larger

for zinc than for calcium (75 mV vs. 25 mV). This leads to the assumption

that in dimethyl sulfoxide, as in water, the stability constant of the

zinc-EDTA complex is larger than the stability of the calcium-EDTA com

plex.

2. Table XIII and Figure 5 give the results of the potentiometric

titration of lead in dimethyl sulfoxide with EDTA and EGTA using the lead

ion selective electrode.

Since the lead ion is determined with the lead electrode no volume

correction is necessary.

EDTA appears to be a better titrant for lead than EGTA. For almost

the same quantity of lead the potential break with EDTA is 140 mV, with

Table XIII. Results of Potentiometric Titrations of Lead with EDTA and EGTA


Solvent: Dimethyl sulfoxide

Taken Found % Error Titrant Amplitude of 11-mole 11-mole Potential Break

mV

12.46 12.66 +1.60 EDTA 140

24.92 25.32 +1.60 EDTA 100

12.83 12.73 -0.78 EGTA 44

25.66 25.76 +0.40 EGTA 26

65

Curve in Figure 5

A

B

c

D

66

Figure 5. Potentiometric titrations of lead with EDTA and EGTA


(A) Titration of 12.5 u-moles of lead perchlorate in DMSO


(B) Titration of 25.0 umoles of lead perchlorate in DMSO


(C) Titration of 12.8 u-moles of lead perchlorate in DMSO

Titrant: 0.0100 ~ EGTA in DMSO

(D) Titration of 25.6 u-moles of lead perchlorate in DMSO


0 It) ,.._ I

·3·~·5 031-:IIO

OW

"S

t\ A

W

'1\1

11

N3

l0d

0 ~

It) (\J

0 (\J

!Q

0

67

- (/) UJ

__. 0 :E 0 a:: u -:E -0

. 1.()

UJ

al

0 ~

&, 0

......

<X: I'Ll

..... z <X: a:: ..... -.....

Table XIV. Results of Potentiometric Titrations of Zinc with EDTA, DCTA and EGTA.



Taken Found % Error Titrant Amplitude of Jl-mole Jl-mole Potential Break

mV

13.64 13.37 -1.98 EDTA 55

27.28 27.37 +0.33 EDTA 130

13.64 13.35 -2.12 DCTA 120

27.28 26.99 -1.06 DCTA 90

17.10 17.80 +4 .09 EGTA 60

34.20 25.80 +4.67 EGTA 35

68

Curve in Figure 6

A

B

c

not drawn

D

E

69

Figure 6. Potentiometric Titrations of Zinc with EDTA, DCTA and EGTA



(A) Titration of 13.64 ~-moles of zinc perchlorate in DMSO


(B) Titration of 27.28 u-moles of zinc perchlorate in DMSO


(C) Titration of 13.64 ~-moles of zinc perchlorate in DMSO

Titrant: 0.00845 M DCTA in DMSO

(D) Titration of 17.10 ~-moles of zinc perchlorate in DMSO


(E) Titration of 34.20u-moles of zinc perchlorate in DMSO


0 0 ~ I

0 0 10

I

0 0 U)

I

0 0 .-... I

0 0 (X)

I

CD

0 0 en I

0 !'()

It) (\J

0 (\J

It)

0 It)

·3·:> ·s

031:JIOO

W

·sA

J\W

'l"llN

3lO

d

70

-(/) LLJ _

J

0 :E 0 a: u -~

-0 . \0

LLJ Q

)

0 ~

;::l

0 bO

•.-l

<X ~

t-z <X a: ~

-.....

71

EGTA it is only 44 mV. This might tell that the stability of the lead

EDTA complex in dimethyl sulfoxide, as in water, is greater than the

stability of the lead-EGTA complex.

Here again the potential break decreases noticeably with an increase

in concentration of lead ions. This phenomenon might be explained by the

fact that at higher lead ion concentrations other reactions, possibly ion

association, compete with the complexation reaction.

3. Figure 6 and Table XIV give the results of potentiometric deter

minations of zinc with the lead ion selective electrode in dimethyl

sulfoxide using different titrants.

All the volumes read on the curves must be corrected for the blank

volume due to the drop of lead perchlorate added before the titration:

the quantity of lead introduced as indicator was respectively 0.63, 0.06

and 0.05 ~-mole in the case of EDTA, DCTA and EGTA used as titrant. EDTA

in contrast with DCTA and EGTA gives a sharper break with an increase in

concentration of zinc. The three titrants can be listed in the following

decreasing effectiveness in their reaction with zinc:

EDTA > DCTA > EGTA

This result differs slightly with the one obtained in the photometric

titration (with zinc solutions ten times more dilute):

DCTA > EDTA > EGTA

One possible explanation of this result may be the fact that the mole

cules of titrant are more complex from EDTA ·to DCTA and EGTA. This

complexity of the molecule is expected to give a slower chelation

72

reaction with zinc ions. In the photometric method the solution was

stirred for 30 seconds after each addition of titrant and let stand for

15 seconds before any absorbance reading, thus the complex formation has

more time to be effective in contrast with the potentiometric method

where the potentials were read continuously with the addition of titrant.

According to the manufacturer (57) the following solvents attack

the epoxy used to seal the sensiting crystal into the body of the elec

trode: acetonitrile, methyl ethyl ketone, dimethyl formamide and methyl

isobutyl ketone. For this reason no attempts have been made to use the

lead electrode in these media.

The following potentiometric titrations have been attempted but did

not give any quantitative results:

Potentiometry with the mer~ury·electrode:

- Titration of zinc with EDTA in m~thanol and ethanolamine as buffer.

The potential at the mercury electrode was not stable.

- Titration of zinc with DCTA in methanol and ethanolamine as buffer:

The potential break was too small.

- Titration of zinc with EDTA in a solvent mixture methanol +methyl

ethyl ketone (1:1), and ethanolamine as buffer: No potential break.

- Titration of mercury with EDTA, both dissolved in dimethyl sulfoxide,

in methyl ethyl ketone with a mixture of tetramethyl ammonium hydroxide

and tetraethylammonium perchlorate as buffer: The potential at the

mercury electrode was not stable.

- Titration of mercury with DCTA in dimethyl formamide: Erratic poten-

tials. Potentiometry with the lead ion selective electrode:

73

- Titration of zinc with EDTA in methanol: No potential break.

- Titration of calcium with EDTA in dimethyl sulfoxide with ethanolamine

as buffer: No potential break.

- Same titration of calcium but use tetrabutylammonium hydroxide as basic

medium: No potential break.

- Titration of calcium with EGTA in dimethyl sulfoxide with ethanolamine

as buffer: Potential not stable.

Potentiometry with the silver electrode.

The silver electrode has been extensively used by Strafelda (58-60)

and by Fritz and Garralda (61) in aqueous medium buffered with borax.

This substance was tried in the titration of calcium with DCTA in di

methyl formamide but did not dissolve in the solvent.

Zinc and calcium could not be determined with EDTA in methanol

buffered with ethanolamine. One explanation might be that the buffer

complexes all the silver ions added to serve as indicator. To avoid

the blocking of these indicator ions it might be interesting to investi

gate the behavior of the ion selective electrodes of the first kind in

organic media, particularly the calcium ion electrode since it has been

successfully: used in many cases in aqueous solution (62-63).

D. Amperometric Titrations

Since polarography in organic solvents, particularly in dimethyl

formamide and dimethyl sulfoxide, has been extensively studied within

the last decade (44, 64-76). It was thought that amperometric titrations

in these two media could be performed as in alcohol (23). All the at

tempts have been unsuccessful despite the fact that almost all the

74

combinations of electrode systems have been tried: mercury pool vs. DME,

S.C.E. vs. DME, Rotating platinum electrode vs. S.C.E.

Polarographic curves for zinc in particular have been recorded but

the amperometric titration curves did not give quantitative results.

75

V. CONCLUSION

Some complexometric titrations in nonaqueous media have been studied.

The principal difficulty encountered was the problem of dissolving the

chelating agents in these solvents. Polyamine carboxylic acids dissolved

in alcohols with the presence of ethanolamine, in dimethyl sulfoxide and

dimethyl formamide upon heating and without any addition of other sub

stances. Their solutions in alcohols and dimethyl sulfoxide were very

stable, in dimethyl formamide they reprecipitated after a period of

several hours.

Direct visual titrations of calcium and zinc have been investigated

in the solvents methanol, dimethyl formamide, dimethyl sulfoxide and

methyl ethyl ketone. The metallochromic indicators which were used

appeared to work well in these media: Eriochrome Black T, Methylthymol

Blue and Zincon proved to be satisfactory for the determination of zinc,

Calcon, Methylthymol Blue, the systems Zincon + zinc-EGTA and Eriochrome

Black T + zinc-EGTA for the determination of calcium.

Photometric titrations of zinc were also investigated with Zincon

as indicator. Microgram quantities of zinc could be determined by this

method.

Potentiometric analysis with the mercury and lead electrodes gave

some results, particularly for the determination of zinc. The use of

the lead ion selective electrode in nonaqueous media was restricted by

the nature of the solvent which must be chosen to be inert to the resin

which composed the body of the electrode.

Amperometric titrations have been investigated without success.

76

A practical application of these experiments was found in the

analysis of trace quantities of zinc and calcium in a sample of poly

styrene dissolved in methyl ethyl ketone. Provided no phosphate is

present in the sample the sum calcium + zinc can be determined with

DCTA dissolved in methanol and Methylthymol Blue as indicator. Zinc is

determined separately with tetren, dissolved in dimethyl sulfoxide, and

Zincon as indicator.

A large number of experiments which did not work do not prove

though that they are impossible under other experimental conditions.

This research has been performed in a one year period, therefore it

does not pretend to be exhaustive. The following topics of investigation

would bring more light to the field of complexometry in nonaqueous media:

research on the problem of dissolving the chelating agents, the determin

ation of metal-chelate stability constants, a comparative study of metal

chelates positively, negatively or not charged, a systematic study on

buffer compositions in nonaqueous media, research on stepwise titrations

by the use of masking agents, a wide investigation to devise ion selec

tive electrodes with materials compatible with organic solvents. If

this last problem could be solved, complexometry in nonaqueous media

would be an extremely powerful, versatile and simple method in the

hands of the analytical chemist.

77

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82

VITA

Ngo The Hung was born on November 18, 1940 in Hanoi, Vietnam. He

received his primary, elementary and almost all his secondary education

in Paris, France. After finishing high school at the lycee Jean-Jacques

Rousseau in Saigon he studied physics and chemistry at the faculty of

science of the university of Saigon. He obtained the licence in Physical

Sciences in 1965. Since 1965 he has been a member of the faculty at the

National Technical Center in Saigon, the only university level engineer

ing college in his country.

In August 1970 his government sent him under the auspices of the

Agency for International Development to the United States for advanced

studies in analytical chemistry at the University of Missouri - Rolla

Documents

A study of some complexometric titrations in nonaqueous