COMPLEXOMETRIC COMPLEXOMETRIC REACTIONS AND TITRATIONSREACTIONS AND TITRATIONS

DR. A.K.M. SHAFIQUL ISLAMDR. A.K.M. SHAFIQUL ISLAM

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 Lewis acidsacids,, because they accept electrons from Lewis bases because they accept electrons from Lewis bases

• When metal cations combine with Lewis bases, the When metal cations combine with Lewis bases, the resulting species is called a resulting 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 When the metals are covalently bonded with surrounding ions or bonded with surrounding ions or molecules the resulting species are molecules the resulting species are called called metal complexesmetal complexes or or coordinate coordinate complexcomplex

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

Coordination Coordination NumberNumber

• 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

• For example, copper (II) has coordination number of four. The species formed from such coordination or complexing, can be electrically positive, neutral or negative.

• Copper when complexed with ammonia results in a cationic complex, Cu(NH3)4

2+,

• when complexed with glycine, a neutral complex, Cu(NH2CH2COO)2,

• when complexed with chloride, an anionic complex, CuCl4

2-.

Metal Cations Form Complex IonsMetal Cations Form Complex Ions

• Complex Ion = transition metal cation surrounded by LIGANDS Complex Ion = transition metal cation surrounded by LIGANDS

• Ligand = molecule or ions that have nonbonding electron pairsLigand = molecule or ions that have nonbonding electron pairs

• Bonding is called “coordination”Bonding is called “coordination”

Terms DefinedTerms Defined• Complex formation – the process Complex formation – the process

whereby a species with one or more whereby a species with one or more unshared electron pairs forms unshared electron pairs forms coordinate bonds with metal ions.coordinate bonds with metal ions.

• Ligand – an ion or molecule that forms Ligand – an ion or molecule that forms a covalent bond with a cation or a a covalent bond with a cation or a neutral metal atom by donating a pair neutral metal atom by donating a pair of electrons that are then shared by of electrons that are then shared by the two.the two.

Terms DefinedTerms Defined

• Chelating agent – substance with Chelating agent – substance with multiple sites available for multiple sites available for coordination bonding with metal ions. coordination bonding with metal ions. Such bonding typically results in the Such bonding typically results in the formation of five or six member ringsformation of five or six member rings

• Dentate – (Latin) having toothlike Dentate – (Latin) having toothlike projectionsprojections

• Some common inorganic ligands are ammonia, Some common inorganic ligands are ammonia, water, and halides.water, and halides.

• A ligand that has one donor group such as A ligand that has one donor group such as ammonia, is called unidentate. ammonia, is called unidentate.

• Glycine, which has two groups available for Glycine, which has two groups available for covalent bonding, (the carbonyl oxygen and the covalent bonding, (the carbonyl oxygen and the aminal nitrogen), is called bidentate. aminal nitrogen), is called bidentate.

• As titrants, multidentate ligands, particularly As titrants, multidentate ligands, particularly tetradentate and hexadentate chelating agents, tetradentate and hexadentate chelating agents, those having four or six donor groups, have two those having four or six donor groups, have two advantages over their unidentate titrants. advantages over their unidentate titrants.

• When a metal cation is complexed to When a metal cation is complexed to ligands forming a neutral compound, the ligands forming a neutral compound, the complex is called complex is called coordinated compoundcoordinated compound..11

• A A chelatechelate is produced when a metal ion is produced when a metal ion coordinates with two or more donor coordinates with two or more donor groups of a single ligand to form a five or groups of a single ligand to form a five or six membered heterocyclic ring. The six membered heterocyclic ring. The copper complex of glycine, is an example copper complex of glycine, is an example of a simple of a simple chelatechelate

2 H2N CHC

H

OH

O

CuNH2

NH2

OOOO

Cu2+ +

Metal Chelate ComplexMetal Chelate Complex

Chelon EffectChelon Effect

• Chelating is the ability of multidentate ligands Chelating is the ability of multidentate ligands to form more stable metal complexes than to form more stable metal complexes than those formed by monodentate or bidentate those formed by monodentate or bidentate ligands.ligands.

• These reactions happen over the monodentate These reactions happen over the monodentate because of favored thermodynamics. because of favored thermodynamics.

• This results a larger Kf value for multidentate complexes. This is known as chelon effect or chelate effect.

Thermodynamic favorableThermodynamic favorable

• The delta H’s for mono and The delta H’s for mono and multidentates are generally comparable.multidentates are generally comparable.

• However, the delta S’ s (entropy) favors However, the delta S’ s (entropy) favors a reaction with the multidentate.a reaction with the multidentate.

• ΔG° = ΔH° - TΔS° ΔG° = ΔH° - TΔS°

• The chemical reaction is spontaneous when the free The chemical reaction is spontaneous when the free energy change, energy change, G is negative, and d G is negative, and d G=G=H – TH – TS.S.

• The enthalpy change for legands with similar groups is The enthalpy change for legands with similar groups is often similar. For example, four ammonia molecules often similar. For example, four ammonia molecules complexed to Cucomplexed to Cu2+2+ and four amino group from two and four amino group from two ethylenediamine molecule complexed to Cuethylenediamine molecule complexed to Cu2+2+ will result will result in about the same release of heat.in about the same release of heat.

• However, more disorder or entropy is created by the However, more disorder or entropy is created by the dissociation of the Cu(NHdissociation of the Cu(NH33))44

2+2+ complex in which five complex in which five species are formed than in the dissociation of the species are formed than in the dissociation of the Cu(HCu(H22NCHNCH22CHCH22NHNH22))22

2+2+ complex, in which three species complex, in which three species are formed. are formed.

• Hence, Hence, S is greater for former dissociation, creating a S is greater for former dissociation, creating a more negative more negative G and a greater tendency for G and a greater tendency for dissociation.dissociation.

• Thus, multidenate complexes are more stable (have Thus, multidenate complexes are more stable (have large Kf values), largely because of the entropy effect.large Kf values), largely because of the entropy effect.

• First, these multidentate titrants, generally First, these multidentate titrants, generally react more completely with cations, thereby react more completely with cations, thereby providing sharper more accurately end providing sharper more accurately end points. points.

• Second, they ordinarily react with metal ions Second, they ordinarily react with metal ions in a single-step process, in a single-step process,

• whereas with unidentate ligands usually whereas with unidentate ligands usually involves two or more intermediate species.involves two or more intermediate species.

LigandLigand

• An example of a hexadendate ligand An example of a hexadendate ligand is EDTA (Ethylenediaminetetraacetic is EDTA (Ethylenediaminetetraacetic Acid). Acid). It has six potential sites for It has six potential sites for complex formation – the electron complex formation – the electron pairs on the two nitrogen atoms and pairs on the two nitrogen atoms and the four electron-rich carboxyl the four electron-rich carboxyl groups. groups.

EDTA Structure EDTA Structure

Y][H][Y ][H

105.5K YHHY

Y][H][HY ][H

106.9K HYHYH

Y][H]Y[H ][H

102.2K YHHYH

Y][H]Y[H ][H

101.0K YHHYH

4

411-

a443

4

37-

a332

2

4

223-

a22

23

4

32-a134

Neutral EDTA is a tetrabasic Neutral EDTA is a tetrabasic acidacid

Disodium EDTADisodium EDTA

• Since YSince Y4-4- is the ligand species in complex is the ligand species in complex formation, the complexation equilibria are formation, the complexation equilibria are affected markedly by the pH.affected markedly by the pH.

• HH44Y has a very low solubility in water, and Y has a very low solubility in water, and so that disodium salt Naso that disodium salt Na22HH22Y,2HY,2H22O is used. O is used.

• This salt dissociates in solution to give HThis salt dissociates in solution to give H22YY2-2-, , pH of this solution is approximately 4 to 5.pH of this solution is approximately 4 to 5.

EDTA Complex with Metal EDTA Complex with Metal IonsIons(1) Forms strong 1:1 complexes regardless

of the charge on the cation(2) Chelate with all cations

(3) Since the anion Y(3) Since the anion Y4-4- is the ligand species in is the ligand species in complex formation, the complexation equilibria complex formation, the complexation equilibria are affected markedly by the pH.are affected markedly by the pH.

(4) The formation constant are in Table (next slide)(4) The formation constant are in Table (next slide)

[Y4-] ][Ca][CaY

K

CaYYCa

2

2

f

242

Table of Formation Constants for EDTA Table of Formation Constants for EDTA Complexes Complexes

Cation Kf Log Kf

Ag+  2.1 x 107  7.32 

Mg2+  4.9 x 108  8.69 

Ca2+  5.0 x 1010  10.70 

Sr2+  4.3 x 108  8.63 

Ba2+  5.8 x 107  7.76 

Mn2+  6.2 x 1013  13.79 

Fe2+  2.1 x 1014  14.33 

Co2+  2.0 x 1016  16.31 

Ni2+  4.2 x 1018  18.62 

Cu2+  6.3 x 1018  18.80 

Zn2+  3.2 x 1016  16.50 

Cd2+  2.9 x 1016  16.46 

Hg2+  6.3 x 1021  21.80 

Pb2+  1.1 x 1018  18.04 

Al3+  1.3 x 1016  16.13 

Fe3+  1.3 x 1025  25.1 

V3+  7.9 x 1025  25.9 

Th4+  1.6 x 1025  23.2 

Effect of pH on EDTA Effect of pH on EDTA equilibriaequilibria

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 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

Effect of pH on EDTA equilibria Effect of pH on EDTA equilibria

From the distribution of EDTA species as function of pH we can see that above pH 10, most of the EDTA exist as Y4- form.

At lower pH values, the protonated species are dominating, hydronium ions compete with EDTA for binding the metal ions. Thus, at those pH using Kabs to calculate the formation of EDTA metal complex will be misleading.

Obviously as the pH goes down, there will be more dissociation than formation. In this situation, the Kabs and all the Ka values of EDTA will be involved for calculation.

If we consider the following chemical reaction of EDTA with any metal, Mn+

Mn+ + Y4-= MY-(4-n)

Then, the formation constant or Kf will be

Kf = [MY-(4-n)] / [Mn+] [Y4-]

Now, substituting the [Y4-], we can rewrite the above equation as follows:-

Kf = [MY-(4-n)] / [Mn+] α4 CH4Y

Kf’ = Kf α4 = [MY-(4-n)] / [Mn+] CH4Y

Kf’ is called conditional solubility constant or effective solubility constant.

Less distinctend point

Effect of pH on EDTA Titration of Effect of pH on EDTA Titration of CaCa2+2+

Region 1

Excess Mn+ left after each additionof 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 3

Excess EDTA. Virtually all metalin MYn-4 form.

EDTA Titration CurveEDTA Titration Curve

*Ca*Ca2+2+ end point more distinct. end point more distinct.

*Lower pH, K*Lower pH, Kff’ decreases, &’ decreases, &

End point less distinct.End point less distinct.

*We cannot raised pH *We cannot raised pH arbitrarily:arbitrarily:

Metal hydroxides might precipitate.

EDTA Titration Curves for CaEDTA Titration Curves for Ca2+2+ and and SrSr2+2+ (Buffered at pH 10) (Buffered at pH 10)

At the end point:3. 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.Kf for Metal-In < Kf for Metal-EDTA.

Metal Ion IndicatorsMetal Ion Indicators

Erichrome Black T (EBT)

The structure of Eriochrome Black T is as follows:-

Indicators for EDTA titration

Eriochrome Black T is, unfortunately, unstable in solution and solutions must be freshly prepared in order to obtain the proper color change. It is still widely used, but another indicator of similar structure, called calmagite, has been developed. Its structure is as follows:-

CalmagiteCalmagite

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.

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