ERT207 ERT207 Analytical ChemistryAnalytical Chemistry

Complexometric TitrationComplexometric Titration

Dr Akmal Hadi Bin Ma’ RadziDr Akmal Hadi Bin Ma’ Radzi

PPK BioprosesPPK Bioproses

Types of Titrimetric MethodsTypes of Titrimetric Methods

Classified into four groups based on type of Classified into four groups based on type of reaction involve;reaction involve;

1.1. Acid-base titrations Acid-base titrations

2.2. Complexometric titrationsComplexometric titrations

3.3. Redox titrationsRedox titrations

4.4. Precipitation titrationsPrecipitation titrations

ComplexesComplexes Most Most metal ionsmetal ions (cation) can react with (cation) can react with lone pair lone pair

electronelectron from a molecule or anion to form from a molecule or anion to form covalent covalent bondsbonds and produce and produce coordination compound or coordination compound or complexes.complexes.

Molecule or ion with at least 1 pair of unshared Molecule or ion with at least 1 pair of unshared electron can form covalent bond with metal ion = electron can form covalent bond with metal ion = ligands ligands

The bonding between metal and ligand generally The bonding between metal and ligand generally involves formal donation of one or more of the ligand's involves formal donation of one or more of the ligand's electron pairs pairs

Eg of ligands = ammonia, cyanide ions, halide ions, Eg of ligands = ammonia, cyanide ions, halide ions, water (neutral/-ve charge mols or ions) water (neutral/-ve charge mols or ions)

ComplexesComplexes

Complexation reactions are widely applied through complexometric titration in order to determine the metal ions, present in the solution

Metals ions, especially Metals ions, especially transition metals,transition metals, act as act as Lewis acidsLewis acids,, because they accept electrons from Lewis bases because they accept electrons from Lewis bases

When metal cations combine with Lewis bases, the resulting When metal cations combine with Lewis bases, the resulting species is called a species is called a complex ioncomplex ion

This also called This also called coordination complexcoordination complex

The base is called a The base is called a ligandligand

ComplexesComplexes

When the metals are covalently bonded with When the metals are covalently bonded with surrounding ions or molecules the resulting surrounding ions or molecules the resulting species are called species are called metal complexesmetal complexes or or coordinate complexcoordinate complex

The surrounding ions or molecules are called The surrounding ions or molecules are called ligandsligands

Coordination NumberCoordination Number

Coordination number = the number of ligands surrounding a central cation in a transition metal complex.

Common coordination numbers are 2, 4 and 6

The geometries of the ligands about the central atom are as shown

Compound form between ligand and metal ion = Compound form between ligand and metal ion = complexes or coordination compound (has charges or complexes or coordination compound (has charges or neutral)neutral)..

Examples of complex formation :Examples of complex formation :

AgAg++ + 2CN + 2CN-- Ag(CN) Ag(CN)22- -

AgAg++ + 2NH + 2NH33 Ag (NH Ag (NH33))22++

Metal ion ligandComplex/coordination compound

Metal ionMetal ion – lewis acid (electron pair acceptor) – lewis acid (electron pair acceptor)LigandLigand – lewis base (electron pair donor) – lewis base (electron pair donor)

Coordination number – number of covalent bond formed between metal ion and ligand

A ligand that has one pair of unshared electron A ligand that has one pair of unshared electron such as NHsuch as NH33, is called , is called unidentateunidentate. .

Glycine (NHGlycine (NH22CHCH22COOH) and ethylenediamine COOH) and ethylenediamine

(NH(NH22CHCH22CHCH22NHNH22) which has two pairs of ) which has two pairs of

unshared electron available for covalent unshared electron available for covalent bonding, is called bonding, is called bidentatebidentate. .

EDTA, has 6 pairs of unshared electrons = EDTA, has 6 pairs of unshared electrons = hexadentate hexadentate

EDTA EquilibriumEDTA Equilibrium

Chelating agentChelating agent - An organic agent that has - An organic agent that has two or more group capable of complexing with two or more group capable of complexing with a metal ion (also called ligand)a metal ion (also called ligand)

ChelateChelate – complex formed – complex formed Titration with chelating agent = Titration with chelating agent = chelometric chelometric

titrationtitration , a type of complexometric titration , a type of complexometric titration Most widely used chelating agent in titration – Most widely used chelating agent in titration –

ethylenediaminetetraacetic acid (EDTA).ethylenediaminetetraacetic acid (EDTA).

EDTA = Ethylenediaminetetraacetic acidHexadentate ligandHas six bonding sites (the four carboxyl groups and the two nitrogen providing six lone pairs electrons)Tetraprotic acid (H4Y), can exist in many forms H3Y-, H2Y2-, HY3- and Y4-

only unprotonated ligand (Y4-) can complex with metal ion

Since EDTA is a tetraprotic acid, the Since EDTA is a tetraprotic acid, the stepwise stepwise dissociationdissociation of EDTA as follows : of EDTA as follows :

H4Y H+ + H3Y- Ka1 = [H+][H3Y-] = 1.0 x10-2

[H4Y]

H3Y- H+ + H2Y2- Ka2 = [H+][H2Y2-] = 2.1 x10-3

[H3Y-]

H2Y2- H+ + HY3- Ka3 = [H+][HY3-] = 6.9 x10-7

[H2Y2-]

HY3- H+ + Y4- Ka4 = [H+][Y4-] = 5.5 x10-11

[HY3-]

Complex formation constant of Complex formation constant of EDTAEDTA

EDTA can form complex with CaEDTA can form complex with Ca2+2+ as the as the following equilibrium :following equilibrium :

CaCa2+2+ + Y + Y4-4- CaY CaY2- (1)2- (1)

The complex formation constant is :The complex formation constant is :

KKff = K = KCaYCaY2-2- = [CaY = [CaY2-2-] (2)] (2)

[Ca[Ca2+2+][Y][Y4-4-]]

Effect of pH on EDTA equilibriaEffect of pH on EDTA equilibria

CaY2 Ca2+ + Y4- HY3- H2Y2- H3Y- H4YH+

H+ H+H+

YH4C

][CaC 2YH4

][Y][HY]Y[H]Y[HY][HC 432234YH4

CaY2- + 4H+Ca2+ + H4Y

From the overall equilibrium

If H+ concentration increases, equilibrium in equation 1 will shift to the left. Chelating anion (Y4-) will react with H+. Dissociation of CaY2- in presence of acid

If we substitute the values of [HY3-], [H2Y2-], [H3Y-] and [H4Y] derived from the Ka values to this equation and divide each term with [Y4-], we will get the following equation:-

Where α4 is the fraction of the total EDTA exists as Y4- .

Let us consider that CH4Y represent the total uncomplexed EDTA

YHYHYHHYYC YH 432

234

4

4321

4

432

3

43

2

444

11

4

KKKK

H

KKK

H

KK

H

K

H

Y

C

a

YH

YHC

Y

4

4

4

YHCY44

4

Conditional formation constantConditional formation constant

The equation for the complex formation between The equation for the complex formation between Ca and EDTA is :Ca and EDTA is :

CaCa2+2+ + Y + Y4-4- CaY CaY2- 2- (3)(3)

KKff = K = KCaYCaY2-2- = [CaY = [CaY2-2-] ] (4) (4)

[Ca[Ca2+2+][Y][Y4-4-]]

αα44 = [Y = [Y4-4-] , hence [Y] , hence [Y4-4-] = ] = αα44 C CHH44Y Y (5)(5)

CCHH44YY

Replacing [YReplacing [Y4-4-] into equation (4) :] into equation (4) :

KKff = K = KCaYCaY2-2- = [CaY = [CaY2-2-] (6) ] (6)

[Ca[Ca2+2+] ] αα44CCHH44Y Y

KKffαα4 4 = [CaY = [CaY2-2-] = K’] = K’ff (7) (7)

[Ca[Ca2+2+]C]CHH44Y Y K’f is the conditional formation constant which depends on α4 therefore K’f depends on pH. We can use equation (7) to calculate the value of equilibirum concentration for EDTA species at specific pH to replace equation (4)

Metal-EDTA titration curvesMetal-EDTA titration curves

Titration is perform by adding the chelating Titration is perform by adding the chelating agent (EDTA) to the sample (metal).agent (EDTA) to the sample (metal).

Titration curve – plotting the changes in metal Titration curve – plotting the changes in metal ion concentration (pM) versus volume of ion concentration (pM) versus volume of titrant (EDTA)titrant (EDTA)

Example of complexometric titration is by Example of complexometric titration is by adding 0.100 M EDTA to 100 ml 0.100 M adding 0.100 M EDTA to 100 ml 0.100 M CaCa2+2+ solution buffered at pH 11 solution buffered at pH 11

CaCa2+2+ + Y + Y4-4- CaY CaY2-2-

Before titration started Before titration started – only have Ca– only have Ca2+ 2+ solution. solution.

pCa = - log [CapCa = - log [Ca2+2+]] Titration proceedTitration proceed – part of Ca – part of Ca2+2+ is reacted with EDTA is reacted with EDTA

to form chelate. [Cato form chelate. [Ca2+2+] gradually decrease.] gradually decrease.

pCa= -log [remaining CapCa= -log [remaining Ca2+2+] ] At equivalence pointAt equivalence point – have convert all – have convert all Ca2+ to CaY2-

So pCa can be determined from the dissociation of chelate at a given pH using Kf.

K’f = Kf α4 = [CaY2-]

[Ca2+] CH4Y

Excess titrant addedExcess titrant added – – pCa can be determined from the dissociation of chelate at a given pH using Kf.

ExerciseExercise

Calculate pCa in 100 ml of a solution of 0.100 Calculate pCa in 100 ml of a solution of 0.100 M CaM Ca2+2+ at pH10 after addition of 0, 50, 100, at pH10 after addition of 0, 50, 100, 150 ml of 0.100 M EDTA. K150 ml of 0.100 M EDTA. Kff for CaY for CaY2-2- is is

5.0x105.0x101010 and and α4 is 0.35.is 0.35.

K’K’ff = K = Kff x x αα44

= 5.0x10= 5.0x101010 x 0.35 x 0.35

= = 1.75x101.75x101010

a) Addition of 0.00 ml EDTAa) Addition of 0.00 ml EDTA

[Ca[Ca2+2+] = 0.100 M] = 0.100 M

pCa = - log 0.100pCa = - log 0.100

= = 1.001.00

b) Addition of 50.00 ml EDTAb) Addition of 50.00 ml EDTA

Initial mmol CaInitial mmol Ca2+2+ =100ml x 0.100 M =10 mmol =100ml x 0.100 M =10 mmol

mmol EDTA addedmmol EDTA added = 50ml x 0.100 M = 5 mmol = 50ml x 0.100 M = 5 mmol

mmol Cammol Ca2+2+ left left = 5 mmol = 5 mmol

[Ca[Ca2+2+] = 5 mmol = 0.0333 M] = 5 mmol = 0.0333 M

(100+50)ml(100+50)ml

pCa = - log 0.0333 = pCa = - log 0.0333 = 1.481.48

c) Addition of 100 ml EDTAc) Addition of 100 ml EDTAInitial mmol CaInitial mmol Ca2+2+ =100ml x 0.100 M =10 mmol =100ml x 0.100 M =10 mmolmmol EDTA addedmmol EDTA added =100ml x 0.100 M = 10 mmol =100ml x 0.100 M = 10 mmolEquivalence point is reached. We have convert all CaEquivalence point is reached. We have convert all Ca2+2+ to CaY to CaY2-2-. mmol CaY. mmol CaY2-2- = =

mmol initial Cammol initial Ca2+2+

[CaY[CaY2-2-] = 10 mmol = 0.05 M] = 10 mmol = 0.05 M (100+100)ml(100+100)ml

K’f = Kf α4 = [CaY2-]

[Ca2+] CH4Y

K’f = [CaY2-] = 1.75 x 1010

[Ca2+] CH4Y

0.05 = 1.75 x 1010

(x)(x)

x = 1.7 x 10-6 so pCa = - log 1.7x10-6 = 5.775.77

d) Addition of 150 ml EDTAd) Addition of 150 ml EDTAInitial mmol CaInitial mmol Ca2+2+ =100ml x 0.100 M =10 mmol =100ml x 0.100 M =10 mmolmmol EDTA addedmmol EDTA added =150ml x 0.100 M =15 mmol =150ml x 0.100 M =15 mmolmmol EDTA excessmmol EDTA excess = 5 mmol = 5 mmol

CCHH44YY = 5 mmol = = 5 mmol = 0.02M 0.02M [CaY [CaY2-2-] = 10 = ] = 10 = 0.04M0.04M

(100+150)ml (100+150)ml(100+150)ml (100+150)ml

K’f = [CaY2-] = 1.75 x 1010

[Ca2+] (0.02) 0.04 = 1.75 x 1010

(x)(0.02)

x = 1.14 x 10-10 so pCa = - log 1.14x10-10 = 9.949.94

EDTA Titration TechniquesEDTA Titration Techniques

1. Direct Titration

*Buffer analyte to pH where Kf’ for MYn-2 is large, and M-In colour distinct from

free In colour.*Auxiliary complexing agent may be used.

2. Back Titration2. Back Titration

*Known excess std EDTA added.

*Excess EDTA then titrated with a std sol’n of a second metal ion.

*Note: Std metal ion for back titration must not displace analyte from MYn-2 complex.

*To be used when the rate reaction is fast, and the stability of metal chelate is high

*To be used when the rate reaction is slow, and precipitation occurred.

2. Back Titration2. Back Titration: When to apply it: When to apply it

*Analyte precipitates in the absence of EDTA.

*Analyte reacts too slowly with EDTA.

*Analyte blocks indicator

3. Displacement Titration

*Analyte treated with excess Mg(EDTA)2-

Mn+ + MgYn-2 MYn-4 + Mg2+

* Kf’ for MYn-2 > Kf’ for MgYn-2

*Metal ions with no satisfactory indicator.

4. Indirect Titration

*Anions analysed: CO32-, CrO4

2-, S2-, and SO42-.

Precipitate SO42- with excess Ba2+ at pH 1.

*BaSO4(s) washed & boiled with excess EDTA at pH 10.

BaSO4(s) + EDTA(aq) BaY2-(aq) + SO42-(aq)

Excess EDTA back titrated:EDTA(aq) + Mg2+MgY2-(aq)

Alternatively: *Precipitate SO42- with excess

Ba2+ at pH 1.

*Filter & wash precipitate.

*Treat excess metal ion in filtrate with EDTA.

5. Masking

*Masking Agent: Protects some component of analytefrom reacting with EDTA.

*F- masks Hg2+, Fe3+, Ti4+, and Be2+.

*CN- masks Cd2+, Zn2+, Hg2+, Co2+, Cu+, Ag+, Ni2+, Pd2+, Pt2+, Hg2+, Fe2+, and Fe3+,

but not Mg2+, Ca2+, Mn2+, Pb2+.

*Triethanolamine: Al3+, Fe3+, and Mn2+.

*2,3-dimercapto-1-propanol: Bi3+, Cd2+, Cu2+, Hg2+, and Pb2+.

*Demasking: Releasing masking agent from analyte.

mHCOmHCNM mnm 2

mH2C

CN

OH

Mn+

Metal-CyanideComplex

Formaldehyde

*Oxidation with H2O2 releases Cu2+ from Cu+-Thiourea complex.

*Thus, analyte selectivity:1. pH control2. Masking3. Demasking