Titrimetry

(Volumetric Methods)

OCN 633

Fall 2013

Titrimetric Methods of Analysis

• Some of the oldest classical wet methods…

• High accuracy and precision

• Analyte reacts with solution of known

composition

• Applicable to a wide variety of analytes

• Originally manual, now automated

Titration

• The controlled addition of a measured

volume of reagent of exactly known

concentration to a known volume of a

solution of analyte whose concentration is

unknown

Requirements

• Known reaction stoichiometry

• Rapid reaction

• No side reactions

• Large change in some solution property at

equivalence point

• Coincidence of equivalence and end points

• Quantitative reaction

Standards in Titrimetry

• Primary Standards

- materials that can be weighed out exactly

• Secondary Standards

- standardized against primary standards

• Standard Solutions

- made up approximately, then standardized

Requirements of Primary Standard

• Highest purity…

• High molecular/formula weight

• Stable to drying

• Readily available (and cheaply)

• Not deliquescent (Phenomenon of a substance

absorbing so much moisture from the air that it ultimately

dissolves in it to form a solution) or hygroscopic

Types of Titrimetric Methods

and oceanographic applications

• Acid-base alkalinity

• Precipitation Chloride/Chlorinity

• Complexometric Ca, Ca + Mg + Sr

• Redox Winkler/DO

General Helpful Hints

• Need to know molarity and normality

-concept of equivalence

-all reactions occur on an equivalent basis!

• Use factor label system in all equations and

calculations

• Review an “Analytical Chemistry” text if

necessary…

Normality/Equivalents

• Concentration based on idea that all substances

react on an equivalency basis

• Definition of an equivalent dependent on type

of reaction - acid/base produces/reacts with one mole of protons

- precipitation …one mole of univalent ion

- redox …one mole of electrons

- complexometric one mole of substance forming complex

• Normality of a solution is the # eq/L

Examples of Normality

• HNO3 H+ + NO3- 1M = 1N

• H2SO4 2H+ + SO42- 1M = 2N

• KMnO4… must define the reaction (i.e., how many

electrons involved in the redox)

• MnO4-(aq)+ 8H+ + 5e- Mn2+(aq) + 4H2O 1M = 5N

• MnO4-(aq) + 4H+ + 3e- MnO2 (s) + 2H2O 1M = 3N

• AgNO3 + NaCl AgCl + NaNO3 1M = 1N

• 2AgNO3 + K2Cr2O7 Ag2Cr2O7 + 2KNO3 1M = 2N

Equivalent Weight

• Mass in grams of one equivalent of a

substance

• Equals molecular/formula weight divided

by # eq/mole

• Equivalent weight may be less, equal to or

greater (latter is rare) than molecular/formula

weight

Volumetric Relationships

• For any reaction, at equivalence…

V1N1 = V2 N2

• This is the basis of all calculations!

• For titrations we adapt…

VstdNstd = Vunknown Nunknown

Acid-Base Titrations

• Plot pH versus volume of titrant (base) added

• Near equivalence pH changes rapidly with small additions of base

• Equivalence point at pH = 7

• End point should be nearest as possible to equivalence point…

Weak Acid-Base Titrations

• Plot pH versus volume

of titrant (base) added

• Note slope of pH

change in first half of

titration

• Have situation when

part of titration curve

involves a “buffer”

• Equivalence point at

pH > 7

Effect of Ka on Titration

• Strength of acid

impacts titration curve

• Weaker acids more

difficult (buffer effect)

• Hard to “see” end

point as most

indicators are weak

acids(bases) too…

End Points

• The “flag” that goes up to indicate that

equivalence has been reached…

• Should coincide with the equivalence point

• Usually triggered by a (relatively) large

change in a property of the solution - change in pH of solution

- Demf or Dcurrent flow in a solution

- refractive index changes

• Usually “enhanced” by use of an indicator

Types of Indicators

• Acid/base: weak acid/base organic dyes

• Precipitation: other ion that forms insoluble

substance with titrant

• Complexometric: other substance that forms

complex with titrant (and is colored)

• Redox: other substance that undergoes

redox reaction with titrant (or use

electrometric detection)

Phenolphthalein

• Weak organic acid (C20H14O4)

• Used as an acid-base indicator.

• Colorless in acid/pinkish in base

• Transition occurs around pH 9

• Does not dissolve very well in water,

• Usually prepared in alcohol solution.

Transitions of Phenolphthalein

• Acid-base transition(s)

• Involves cleavage of ring,

dissociation of two -OH

groups, then hydroxylation

• Activates/triggers

chromophore

Phenolphthalein End Point…

• Weak acid/base

• Transition near pH 9

End point

Eriochrome Black T

• Complexes Mg2+ (pink)

• When all free Mg2+ has been titrated, Mg2+ EBT complex dissociates to allow formation of stronger Mg2+EDTA complex

• Free EBT indicator is light blue…

3-Hydroxy-4-[(1-hydroxy-2-naphthalenyl)azo]-7-

nitro-1-naphthalene-sulfonic Acid, Na

Reaction of Halides with Ag+

AgNO3 + NaCl AgCl + Na+ + NO3-

• Basis of Mohr method for detn. of CL in seawater

CL = [Cl-] + [Br-] + [I-]

• Use standardized solution of AgNO3 as titrant

• Dissolved halides form insoluble compounds with Ag+

• Reactions are quantitative (because of low Ksp of AgCl: 1.82 X 10-10, AgBr: 5.2X 10-13, and AgI: 8.3 X 10-17)

Mohr Titration

• Visual e.p.: K2Cr2O7/K2CrO4 indicator …forms brown Ag2CrO4 precipitate

• Accuracy and Precision ~0.5%

• Includes Cl- (558 mM in standard seawater)

• …Br- (0.86 mM in standard seawater)

• …and I- (at most 0.2 µM in standard seawater).

• In extreme cases, Br- and I- can rise to 2 mM each

• Determination of Cl- should be corrected for Br- and I- (on a practical basis the corrections will always be less than 1%, or approximately twice the level of accuracy

achievable by this method).

Mohr Titration

• Mohr titration with AgNO3 using the is the

preferred analytical method for CL, unless

equivalent but slightly more precise Mohr

titration with electrometric e.p. is used.

• Electrometric e.p. uses Ag+ ISE

• E.p. detected on basis of Ag half reaction:

Ag+ + e- = Ago

• Electronic e.p. ~0.1% accuracy-precision

Silver ion electrode

Ag+ + 1e- Ago

E = Eo – (0.059/n)log{ [products] /[reactants]}

E = Eo - 0.059 log 1/[Ag+]

E = Eo + 0.059 log [Ag+]

Upon addition of excess AgNO3, E rises sharply…

Complexometric Titrations

• Derived from inorganic chemistry

• Basis is sharing of electron pair between

metal and ligand complex formation

• Ligand(s) is(are) electron rich species…

• Complex formation is a multi-step process

Ni(H2O)42+ + 4 Cl- <===> NiCl4

2- + 4H2O

Step-wise Complexation

Ni2+ + Cl- NiCl+

NiCl+ + Cl- NiCl2

NiCl2 + Cl- NiCl3-

NiCl3- + Cl-

NiCl4-2

• Each step is described by a Kf

• Overall K is product of individual K’s

• ß1= K1 ß2= K1K2 ß3= K1K2K3 ß4=K1K2K3K4

Complexometric Titrations

• Based on complexation of analyte with large

multidentate molecule rather than step wise

complexation with individual ligands

• Common functional groups that serve as

individual ligands are e- rich

• Caboxylic acids, amines, imides, thio,

sulfate…

Stability of Chelates

• Complexes of multidentate ligands are more stable due to the chelate effect.

• Rings with five- or six-fold complexation are best.

• Entropy effects enhance stability

M(H2O)6 + 6 L ML6 + 6H2O

7 7

M(H2O)6 + L ML + 6H2O

2 7

• Greater entropy change for the latter reaction.

Electron Pair Sharing

Carbamic or Aminoformic acid, NH2COOH

Amino Polycarboxylic Acids

• EDTA - Ethylenediamine tetraacetic acid

• Has 6 Lewis base sites (4 oxygen and 2

amine electron pairs)

• It is often designated as H4Y and its anion

as Y4-

• All six complexation sites are able to bind

to a single metal ion for many metals.

EDTA

• Hexadentate complex

• Two amine groups

• Four acetate groups

• Very strong K’s with most metal ions (esp. high + valency ions)

• Binding constants are f(pH)

Changes in EDTA Speciation as f(pH)

Formation Constants with EDTA

Cation KMY Cation KMY

K+ 6.31 Ce3+ 9.5x1015

Na+ 45.7 Al3+ 1.3x1016

Li+ 6.17x102 Co2+ 2.0x1016

Tl+ 3.5x106 Cd2+ 2.9x1016

Ra2+ 1.3x107 Zn2+ 3.2x1016

Ag+ 2.1x107 Gd3+ 2.3x1017

Ba2+ 5.8x107 Pb2+ 1.1x1018

Sr2+ 4.3x108 Y3+ 1.2x1018

Mg2+ 4.9x108 Sn2+ 2.0x1018

Be2+ 1.6x109 Pd2+ 3.2x1018

Ca2+ 5.0x1010 Ni2+ 4.2x1018

V2+ 5.0x1012 Cu2+ 6.3x1018

Cr2+ 4.0x1013 Hg2+ 6.3x1021

Mn2+ 6.2x1013 Th4+ 1.6x1023

Fe2+ 2.1x1014 Fe3+ 1.3x1025

La+ 3.2x1015 V3+ 7.9x1025

VO2+ 3.5x1015 Co3+ 2.5x1041

Ca & Mg Titration with EDTA

Cd Titration with EDTA

• Plot p(M) vs volume of

EDTA (titrant)

• Use colorimetric e.p.

• Use Cd2+ ISE

• E.p. is where pCd

changes sharply

Other Amino Polycarboxylic Acids

• NTA - Nitrilotriacetic acid. Tetradentate (best

suited for small metals)

• DETPA - Diethylenetriamine pentacetic acid.

(works best for 8 coordinate metals such as the

3rd transition series and lanthanides)

• TTHA - Triethylenetetramine hexacetic acid.

(for 10 coordinate metals such as the actinides)

Complexometric Titration of Ca (old color end point method)

• Ca2+ is determined using a modification of the

method by Tsunogai, Nishimura, and Nakaya

(1968), Talanta, 15, p385-390.

• Uses ethylene bis(oxyethylene-nitrilo)-tetra acetic

acid (EGTA) as titrant

• Uses 2,2'ethane-diylidine-dinitrilo-diphenol

(GHA) as indicator

• The Ca(GHA) complex is quantitatively extracted

into a layer of n-C4H9OH (pink color).

New Ca titration with EGTA

• Same as old method wrt basis of method

• End point measured with Ca ISE

• Large change in pCa leads to large E

change

• Precision of method vastly improved

Autotitrators you will use…