1 Complexation Titration

8/12/2019 1 Complexation Titration

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Complexation Titration


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Forming Complexes

Most metals ions react with electron-pairdonors to form coordination compounds or

complexes.

The donor species (ligand) must have atleast one pair of unshared electrons

available for bond formation.

Cu(H2O)42+, Cu(NH3)42+, Cu(NH2CH2COO)2

CuCl42-


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Ligands

A ligands is a neutral molecule or ion having alone pair that can be used to form a bond to a

metal ion. Chelating agents: unidentate, bidentate,

tridentate, tetradentate,pentadentate,

hexadentate


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Gramicidin A antibiotic ion channel


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Nonact informs a complex with the K + ion; the

coordination occurs through the 8 O atoms.


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Chelation in Biochemistry

Chelating ligands can form

complex ions with metals

through multiple ligands.This is important in many

areas, especially

biochemistry.


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Box 13-1 Chelation Therapy & Thalassemia

A successful drug for iron excretion


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The ions of alkali metals cam form complexes with crown

ether and cryptand

D. J. Cram, C. J. Pedersen and J.-M. Lehn Nobel prize inChemistry in 1987


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Crown Ether Complex of Potassium

Potassium permanganate is dissolved in

benzene by addition of 18-crown-6.


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Coordination Compounds

colored & paramagnetic (often)consists of a complex ion

(1) Coordination compounds are neutral

species in which a small number ofmolecules or ions surround a central

metal atom or ion.

ex.[Co(NH3)5Cl]Cl2

complex ion : [Co(NH3)5Cl]2+


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Coordination Compounds

coordinate covalent bond

Complex ion = metal cation + ligands

e acceptor e donor

center (one) surrounding (

2 )transion metal

Lewis acid Lewis base

[ Co(NH3)5Cl ]Cl2

H2O , NH3 , :Cl

..

.... ..

..

ionic force

counter ionscentral metal ligands

complex ion


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20.3 Coordination Compounds

(2) Coordination number :

The # of donor atoms surrounding the central

metal

The most common : 4 or6

(3) Ligands :

A neutral molecule or ion having a line pair that

can be used to from a bond to a metal ion.

monodentate : H2O , NH3bidentate : en , ox

polydentate : EDTA

Chelating agents
http://localhost/var/www/apps/conversion/tmp/scratch_10/f20.6.ppthttp://localhost/var/www/apps/conversion/tmp/scratch_10/f20.7-8.ppthttp://localhost/var/www/apps/conversion/tmp/scratch_10/f20.7-8.ppthttp://localhost/var/www/apps/conversion/tmp/scratch_10/f20.6.ppt


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13-1 Metal-Chelate Complexes

EDTA forms strong 1:1 complexes with most metal ions

As a metal-binding agent


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Useful chelating agents


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13-2 EDTA(ethylenediaminetetraacetic acid, a hexadentate)

(1) The most widely used chelating agent in titration

(2) Forms strong 1:1 complexes regardless of the charge onthe cation


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Acidic Properties of EDTA

C CN N

CH2-COOH

CH2-COOH

HOOC-H2C

HOOC-H2C

H H

HH

C CN N

CH2-COOH

CH2-COO-

-OOC-H2C

HOOC-H2C

H H

HH

H++

H

H4Y


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C CN NCH2-COOH

CH2-COO-

-OOC-H2C

-OOC-H2C

H H

HH

H++

H

C CN N

CH2-COO-

CH2-COO-

-OOC-H2C

-OOC-H2C

H H

HH

H++H

H3Y-K1=1.0210

-2

H2Y-2K2=2.1410

-3


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C CN N

CH2-COO-

CH2-COO-

-

OOC-H2C

-OOC-H2C

H H

HH

H+

C CN N

CH2-COO-

CH2-COO-

-OOC-H2C

-OOC-H2C

H H

HH

HY-3K3=6.9210

-7

Y-4K4=5.510

-11


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The Nature of EDTA Complexes

with Metal Ions

The reagent combines with metal ions in a 1:1 ratioregardless of the charge on the cation.

M+n+Y-4 MY+(n-4)

]][[

][4

)4(

YM

MY

K n

n

MY


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(1) Multidentate chelating agents form strongercomplexes (Kf) with metal ions than bidentateor monodentate ligands.

(2) Neutral EDTA is a tetrabasicacid

(3) Metal-EDTA complex is unstable at both low pH& high pH.

At low pH H+& M n+

At high pH OH-& EDTA

For EDTA

][Y][M

][MY

KMYYM 4

4

f

44

n

nnn


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Pb2+as example:

At pH 10, tartrate is present

to prevent Pb(OH)2

Pb-tartrate complex must be

less stable than Pb-EDTA

(4) Auxiliary complexing agents:

prevent metal ions from

precipitating.


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13-3 Metal Ion Indicators

Metal ion indicator: a compound whose color changes whenit binds to a metal ion.

For an useful indicator, it must bind metal less strongly thanEDTA does.the indicator must release its metal to EDTA

Example: MgIn + EDTAMgEDTA + In Indicator is pH dependent.

If metal blockthe indicator, use back titration.

NH3

NH3

[H3N:Cu:NH3]2+Cu2+ + 4:NH3

pale bule deep bule

:

:


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Demonstration 13-1 Metal Ion IndicatorColor Changes

P.294


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COLOR PLATE 8

Titration of Mg2+by EDTA, Using Eriochrome Black T Indicator

(a) Before (left), near (center), and after (right) equivalence point.

(b) Same titration with methyl red added as inert dye to alter

colors.

Demonstration 13-1 Metal Ion Indicator Color Changes


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Indicator for EDTA Titrations

Ertichrome Black T

H2O+H2In-

HIn-2

+H3O+

H2O+HIn-2

In-3

+H3O+

O2N

SO3-

OH

N

N

OH

K1=510-7

K2=2.810-12

red blue

blue orange

MIn-+HY

-3 HIn-2

+MY-2

red blue


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At the end point:

MgIn + EDTA MgEDTA + In-

(red) (colourless) (colourless) (Blue)

Before Titration:

Mg2+ + In- MgIn(colourless) (blue) (red)

During Titration: Before the end point

Mg2+ + EDTA MgEDTA

(free Mg2+

ions) (Solution red due to MgIn complex)

Compounds changing colour when binding

to metal ion.

Kffor Metal-In-< Kffor Metal-EDTA.


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Requirements for Indicator

Metal-indicator complex must be less stablethan the metal-EDTA complex.

Binding between metal and indicator must notbe too weak. It has to avoid EDTA replacing at

the beginning of the titration.

In general, the metal-indicator complex shouldbe 10 to 100 timesless stable than the metal-

titrant complex.


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Eriochrome Black T is blue, but turns red in the presence of metals
http://upload.wikimedia.org/wikipedia/en/2/2b/Erichrome.JPG


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13-4 EDTA Titration Techniquesare useful for the determination of [metal]

Direct titration Titrate with EDTA Buffered to an appropriate pH Color distinct indicator Auxiliary complexing agent

Back titration Excess EDTA, & titrate with metal ion For analyte

ppt in the absence of EDTA :

Ex: (Al3+-EDTA)at pH 7, indicator Calmagite) backtitration with Zn2+react slowly with EDTA

block the indicator


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Displacement titration No satisfactory indicator Ex1: Hg2++ MgY2-HgY2-+ Mg2+ KfHgY

2- >MgY2-

Ex2: 2Ag++ Ni(CN)42-2Ag(CN)2+ Ni2+ , Ni2+is titrated with EDTA

Indirect titration Determine [Anion] that precipitate metal ions: CO3

2-, CrO42-S2-SO4

2-

Ex: SO42-+ Ba2+BaSO4(s) at pH 1

filter BaSO4(s) and boil with excess EDTA at pH 10

Ba(EDTA)2-and excess EDTA is back titration with Mg2+

Masking

Masking prevents one element from interfering in the analysis ofanother element.

Ex: Al3++ Mg2++ F-AlF63++ Mg2+then only Mg2+can be react with

EDTA masking Al3+with F- Masking agent: CN- , F- (using with pH control to avoid HCN & HF)


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In general, the metal-indicator complex should

be 10 to 100 times less stable than the metal-

titrant complex

Expt:

The formation constants of the EDTA complexes

of Ca2+

and Mg2+

are too close to differentiatebetween them in an EDTA titration, so they will

titrate together. Ca2+can actually be titrated in the

presence of Mg2+by raising the pH to 12 with

strong alkali; Mg(OH)2precipitates and does nottitrate.

E ilib i C l l ti


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Equilibrium Calculations

Involving EDTA

EDTA titrations are always performed insolutions that are buffered to a known pH to

avoid interferences by other cations or toensure satisfactory indicator behavior.


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Conditional Formation Constant

.applicableisfor whichpHat theonlyconstantaisK

][

][

][

][

]][[

][

][][][][][

][][][][

4

'

MY

)4(

4

'

4

)4(

4

)4(

43

2

2

34

4321321

2

21

3

1

4

43214

Tn

n

MYMY

T

n

n

MYn

n

MY

T

CM

MY

KK

CM

MYK

YM

MYK

YHYHYHHYYC

KKKKHKKKHKKHKH

KKKK


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D

HKKK

DHKK

D

HK

D

H

KKKKHKKKHKKHKHD

][

][

][

][

][][][][

3213

2

212

3

11

4

0

4321321

2

21

3

1

4


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13-5 The pH-dependent Metal-EDTA

Equilibrium

Since the anion Y4-is the ligand species in complexformation, the complexation equilibria are affected

markedly by the pH

Fraction Composition of EDTA Solutions

.....KKK

][H

KK

][H

K

][H1

1

][Y

[EDTA]

KKKKK][HKKKKK][HKKKK][HKKK][HKK][HK][H

KKKKKK

][EDTAY[EDTA]

][Y

][Y][HY]Y[H]Y[HY][H]Y[H]Y[H

][Y

654

3

65

2

6Y

4

5432154321

2

4321

3

321

4

21

5

1

6

654321

Y

Y

4

4

Y

4322345

26

4

Y

4

4

44

4


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15D-3 Equilibrium: pH dependent M-Y

YH4

4

a4a3a2a1a3a2a1

2

a2a1

3

a1

4

a4a3a2a14

a4a3a2a1

4

a4a3a2

3

a4a3

2

a444

YH

4

4

CY

KKKK][HKKK][HKK][HK][H

KKKK

KKKK

][H

KKK

][H

KK

][H

K

][H1

1

][Y

C


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Species EDTA as a function of pH

Y4- complexes with metal ions and so the complexation equilibria are very pH dependent


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Fig. 9.1. Fraction of EDTA species as a function of pH.

Y complexes with metal ions, and so the complexation equilibria are very pH dependent.

Only the strongest complexes form in acid solution, e.g., HgY2-; CaY2-forms in alkaline solution.

Gary Christian,

Analytical Chemistry,

6th Ed. (Wiley)


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C l l ti f th C ti


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Calculation of the Cation

Concentration in EDTA Solutions

Calculate 4

Calculate conditional formation constants K

Ni+2+Y-4 NiY -2

184

)4(

102.4]][[

][

YNi

NiYK n

n

MY

0.015M-xx x

Calculate equilibrium [Ni+2] at pH=3 and pH=8


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pH 4 pH 4

2 3.710-14 8 5.410-3

3 2.510-11

9 5.210-2

4 3.610-9 10 3.510-1

5 3.510-7 11 8.510-1

6 2.210-5 12 9.810-1

7 4.810-4

Value for 4 for EDTA at selected pH value


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)(101.8][Ni1027.2015.0

1027.2102.4104.5K8pH

)(102.1][Ni1005.1015.0

1005.1102.4105.2K3pH

KK

CY][H]Y[H]Y[H][HY][Y][Ni

10216

2

16183'

NiY

528

2

81811'

NiY

NiY4

'

NiY

T43

2

2

342

Mxx

x

Mxx

x


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C diti l F ti C t t


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Conditional Formation Constant

Most of the EDTA is not Y4-

belowpH=pK6=10.37. The species HY3-, H2Y

2-, and soon, predominate at lower pH.

It is convenient to express the fraction of freeEDTA in the form Y4-

P.300


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The number Ktf =Y4- ,Kfis called the conditional formationconstantor the effective formation constant.

P.300

(1) We can use Kfto calculate the equilibrium concentrationsof the different species at a given pH.

(2) Kf : HgY-2PbY-2CaY-2KfpHKf

pH

pH9.0Kf

EDTA(pH9.0)


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Complex-Formation Titrations


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Complex-Formation TitrationsFormation Constants for EDTA Complexes

Cation KMY Log KMY Cation KMY Log KMYAg+ 2.1 x 107 7.32 Cu2+ 6.3 x 1018 18.80

Mg2+ 4.9 x 108 8.69 Zn2+ 3.2 x 1016 16.50

Ca2+

5.0 x 1010

10.70 Cd2+

2.9 x 1016

16.46Sr2+ 4.3 x 108 8.63 Hg2+ 6.3 x 1021 21.80

Ba2+ 5.8 x 107 7.76 Pb2+ 1.1 x 1018 18.04

Mn2+

6.2 x 1013

13.79 Al3+

1.3 x 1016

16.13Fe2+ 2.1 x 1014 14.33 Fe3+ 1.3 x 1025 25.1

Co2+ 2.0 x 1016 16.31 V3+ 7.9 x 1025 25.9

Ni2+ 4.2 x 1018 18.62 Th4+ 1.6 x 1023 23.2

K = conditional formation constant = K


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Fig. 9.2. Effect of pH on Kf values for EDTA chelates.

Kf= conditional formation constant = Kf4.

It is used at a fixed pH for equilibrium calculations (but varies with pH since 4does).

Gary Christian,


6th Ed. (Wiley)

As the pH increases the equilibrium shifts to the right


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Fig. 9.3. Titration curves for 100 mL 0.1 M Ca2+

versus 0.1 M Na2EDTA at pH 7 and 10.

As the pH increases, the equilibrium shifts to the right.

Gary Christian,


6th Ed. (Wiley)

E l


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Example atp . 277

pH affects the titration of Ca2+with EDTA

K,

f

is smaller at lower pH.


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470

Figure 17-7Influence of pH on the

titration of 0.0100 M

Ca2+with 0.0100 M

EDTA. Note that the

end point becomes lesssharp as the pH

decreases because the

complex formation

reaction is lesscomplete under these

circumstances.


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471

Figure 17-8

Titration curves for 50.0 mL of 0.0100 M solutions of

various cations at pH 6.0.

The points represent the pH at which the conditional formation


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Fig. 9.4. Minimum pH for effective titrations

of various metal ions with EDTA.

constant, Kf', for each metal is 106, needed for a sharp end point.

Gary Christian,


6th Ed. (Wiley)

The Effect of Other Complexing


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472

Figure 17-10

Influence of ammoniaconcentration on the end

point for the titration of 50.0

mL of 0.00500 M Zn2+.

Solutions are buffered to pH

9.00. The shaded region

shows the transition range

for Eriochrome Black T.

Note that ammonia

decreases the change in

pZn in the equivalence-

point region.

p gAgents on EDTA Titration Curves

Formation constants are the equilibrium


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Formation constants are the equilibrium

constants for complex ion formation one

ligand at a time.

The overall or cumulative formation of the complex ion at

any given step in this process is denoted as beta, .For step 2: the cumulative equation is the sum ofequations 1 and 2 in the formation process.


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12.5 Auxiliary Complexing Agents


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12.5 Auxiliary Complexing Agents

Metal-ligand Equilibria; Fractions of metal ion in

solution: General formula to find fraction of metal ion

in solution with all of its complexed ions:

Problem: Ammonia complexes of zinc: Zn2+and NH3form four complexes. If the concentration of free,

unprotonated NH3is 0.10 M, find the fraction of zinc in

the form Zn2+.

Problem: Find the fraction of thallium present whenthallium complexes with 0.20 M CN. Thallium forms

four complexes with cyanide ion: log K1= 13.21, log K2

= 13.29, log K3= 8.67, log K4= 7.44.

13 6 EDTA Tit ti C


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13-6 EDTA Titration Curves

The end point break depends upon1) [Mn+]

2) [L1]

3) [pH] selectivity

4) Kf

The smaller Kf, the more alkaline the solution must

be to obtain a kfof 106.


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12-3 EDTA Titration Curves

The titration curve is a graph of pMversus the volume of added EDTA.

The right side figure illustrates forreaction of 50.0 mL of 0.050 0 M

Mn+with 0.050 0 M EDTA,assuming Kf

= 1.15 1016,

where the concentration of free

Mn+decreases as the titration

proceeds.

There are three regions in anEDTA titration curve:

(a) Before the equivalence point.

(b) At the equivalence point.

c After the e uivalence oint.

EDTA Titration Curve


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EDTA Titration CurveRegion 1

Excess Mn+left after each addition

of EDTA. Conc. of free metal

equal to conc. of unreacted Mn+.

Region 2

Equivalence point:[Mn+] = [EDTA]

Some free Mn+generated by

MYn-4 Mn++ EDTA

Region 3Excess EDTA. Virtually all metal

in MYn-4form.


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Since at equivalence point,there is exactly as muchEDTA as metal in the

solution, we can treat thesolution as if it had beenmade by dissolving pureMYn4. Some free Mn+isgenerated by the slightdissociation of MYn4:

The Ca2+end point is moredistinct than the Sr2+end

point because theconditional formationconstant, for CaY2is greaterthan that of SrY2


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470

Figure 17-6

EDTA titration curves for

50.0 mL of 0.00500 MCa2+(KCaY=1.7510

10) and

Mg2+(KMgY=1.72108) at pH

10.0. Note that because of the

larger formation constant, the

reaction of calcium ion withEDTA is more complete, and a

larger change occurs in the

equivalence-point region. The

shaded areas show the

transition range for the

indicator Eriochrome Black T.

The titration rxn:


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Mn++ EDTAMYn-4

Kf= 4Kf

Three regions:(1) Before equivalence

point : excess Mn+(2) At equivalence point

[Mn+]= [EDTA]

(3) After equivalence point :

excess EDTA

Example at p.302

f C 2 2


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Titration of Ca+2and Mg+2

The K of EDTA of Ca+2and Mg+2are tooclose to differentiate between them in an

EDTA titration.

Generally, they will titrate together. This titration is used to determine totalhardness of water.

Ti i f C 2


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Titration of Ca+2

EB-T cannot be used to indicate the directtitration of Ca+2in the absence of Mg+2with

EDTA.

The indicator forms too weak a complex withCa+2to give a sharp end point.

Example: calculate the pZn of solutions


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Example: calculate the pZn of solutions

prepared by adding 20.0, 25.0, and 30.0 mL of

0.0100M EDTA to 50.0 mL of 0.00500M Zn2+

.Assume that both the Zn2+and EDTA solutions

are 0.100M in NH3and 0.175M NH4Cl to provide

a constant pH of 9.0

pH9.0(NH3+NH4Cl)0.0100M

EDTA50.00mL 0.00500M Zn2+pZn

EDTA(a) 20.0 mL (b)25.0 mL (c) 30.0 mL

KZnY:pHNH3

K"ZnY= M4KZnY =[ZnY2-]

CTCM


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476

Figure 17-11

Structure andmolecular model of

Eriochrome Black

T. The compound

contains a sulfonicacid group that

completely

dissociates in

water and two

phenolic groupsthat only partially

dissociate.


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478

Figure 17-12

Structural formulaand molecular

model of

Calmagite. Note the

similarity toEriochrome Black T

(see Figure 17-11).

Example 17 5


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Example 17-5

Determine the transition ranges for Eriochrome

Black T in titrations of Mg2+and Ca2+at pH 10.0,given that (a) the second acid dissociation

constant for the indicator is

(b) The formation constant for MgIn-is

(c) Ca2+ Kf= 2.5x105

H2O + HIn2- In3- + H3O+ K2= 2.8 x 10-12

Mg2+ + In3- MgIn- Kf= 1.0 x 107

pH10.0EBTCa2+

Mg2+EBT

R l ti


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Resolution

A small measured amount of Mg+2is added tothe Ca+2solution.

Ca+2gives more stable K than Mg+2.

A correction is made for the amount of EDTAused for titration of the Mg+2.


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At the end point:MgIn + EDTA MgEDTA + In-

(red) (colourless) (colourless) (Blue)

Before Titration:

Ca+2+EDTA+Mg2++In- MgIn+CaEDTA

During Titration: Before the end point Ca2+ + EDTA CaEDTA

Kffor Mg+2-EDTA < Kffor Ca

+2-EDTA.

EDTA tit ti l l ti


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EDTA titration calculations

Problem 12-7: Consider the titration of 25.0 mL of 0.0200M MnSO4with 0.0100 M EDTA in a solution buffered to

pH 8.00. Calculate pMn2+at the following volumes of

added EDTA and sketch the titration curve: (a) 0 ml, (b)

20.0 ml, (c) 40.0 ml, (d) 49.0 ml, (e) 49.9 ml, (f) 50.0 ml,

(g) 50.1 ml, (h) 55.0 ml, (i) 60.0 ml.

Solution: from Table 12.2, we get log(Kf) = 13.89. At pH= 8.00, (Y4-) = 4.2 x10-3.

The above calculation will be carried out based on

whether the titration is before or after the equivalence

point. Here Ve*0.0100 M = 25.0*0.0200 M; Ve = 50.0 ml


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(a) to (e) are before the equivalence point and will follow the samecalculation equation:

[Mn2+] * (0.025 + V) = 0.025 *0.020.01*V

(f) this is the equivalence pointKf = 4.2 x10-3x 1013.89= [MnY2-]/([Mn2+][EDTA])

= (0.02*0.025/0.075)/ ([Mn2+])2

[Mn2+] =8.179 x10-7.

pMn2+=

Calculations (g) to (i) are beyond the equivalence point, where[EDTA] can be obtained from the following

[EDTA]*(0.025 + V) = 0.01*V0.02*0.025

then Kf = 4.2 x10-3x 1013.89= [MnY2-]/([Mn2+][EDTA])

A beaker containing 50.0 mL of 0.300 M Ca2+ at pH 9 is titrated


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g pwith 0.150 M EDTA.The pCa at the equivalence point is:(a) 4.97.

(b) 5.13.(c) 5.84.

For Ca2+ Kf= 1010.65 ; = 0.041 at pH = 9

2

' 9

21.8 10f

CaYK

Ca EDTA

22

2

9 9

0.1

1.8 10 1.8 10

CaYCa

2 67.45 10Ca

pCa= 5.13

At the equivalent point, all Ca2+ions have formed complex

and free Ca2+come from the dissociation of complex:

CaY2- Ca2+ + EDTA


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"50 Mg2+ 0.050M " EDTA 0.050M . pH .10.0

Mg2+ ? K=2.02 x 108

EDTA ,+Mg2 :. Y4- - Mg2+ 1:1 MgY2-.

[MgY2-]50.0ml x 0.05M

100ml= 0.025M

Mg2+ + Y4- MgY2-

K =[MgY2-]

[Mg2+] [Y4-]= 2.02 x 108

0.025M

[Mg2+]= 2.02 x 108

[Mg2+][Mg2+] = 1.1 x 10-5M


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OCH3

OH

+HN

CO2-

CO2-

CH3

NH

+

-O2C

-O2C

SO3-

Xylenol Orange

M-In + EDTA M-EDTA + In

' '

M-In+ EDTA M-EDTA + In

OH

-

O3S

O2N

N N

OH

Eriochrome black T

pK1=6.3

pK1=11.6

+Zn2 EDTA


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EDTA 0.01M

Zn-EDTA

n(EDTA)=n(Zn2+)

Erio T

EDTA 0.01M

Zn2+, 20ml

Buffer pH=10

Erio T-Zn

Zn2+ + EDTA Zn-EDTA

+Ca2 EDTA


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Ca2+ + EDTA Ca-EDTA

(x mole)Ca

2+

+(y mole)

EDTA-Mg

(y mole)Ca-EDTA+ (y mole)Mg2++ (x-ymole)Ca2+

x mole> y mole

K(EDTA-Mg)= 4.9 x 108

K(EDTA-Ca)= 5.0 x 1010

EDTA

(y mole)Ca-EDTA+ (y mole)Mg-EDTA+ (x-ymole)Ca-EDTA

mole EDTA= C x V =(y mole)+(x-ymole)= x mole

Mg-EDTA


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1:1 Mg:EDTA

EDTA+Mg2 +Mg2EDTA

g

+Ca2 EDTA C 2 M EDTA EDTA C EDTA C EDTA M EDTA


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EDTA 0.01M

Ca-EDTA, Mg-EDTA

n(EDTA)=n(Ca2+)

Erio T

EDTA 0.01M

Ca2+, 20ml

~1ml Mg-EDTA

Buffer pH=10

Erio T-Ca

Ca2+ + Mg-EDTA+EDTA Ca-EDTA+ Ca-EDTA+ Mg-EDTA


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First

2nd


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3rd


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Documents

1 Complexation Titration