COMPLEXOMETRIC REACTIONS AND TITRATIONS DR. A.K.M. SHAFIQUL ISLAM

Complexes Complexation reactions are widely applied through complexometric titration in order to determine the metal ions, present in the solution Metals ions, especially transition metals, act as Lewis acids, because they accept electrons from Lewis bases When metal cations combine with Lewis bases, the resulting species is called a complex ion This also called coordination complex The base is called a ligand 1 Harris, Daniel C. Quantitative Analysis. United States : W. H. Freeman and Company, 1999.

Complexes When the metals are covalently bonded with surrounding ions or molecules the resulting species are called metal complexes or coordinate complex The surrounding ions or molecules are called ligands

Coordination 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

For example, copper (II) has coordination number of four 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)42+, when complexed with glycine, a neutral complex, Cu(NH2CH2COO)2, when complexed with chloride, an anionic complex, CuCl42-.

Metal Cations Form Complex Ions Complex Ion = transition metal cation surrounded by LIGANDS Ligand = molecule or ions that have nonbonding electron pairs Bonding is called “coordination”

Terms Defined Complex formation – the process whereby a species with one or more unshared electron pairs forms coordinate bonds with metal ions. Ligand – an ion or molecule that forms a covalent bond with a cation or a neutral metal atom by donating a pair of electrons that are then shared by the two.

Terms Defined Chelating agent – substance with multiple sites available for coordination bonding with metal ions. Such bonding typically results in the formation of five or six member rings Dentate – (Latin) having toothlike projections

Some common inorganic ligands are ammonia, water, and halides. A ligand that has one donor group such as ammonia, is called unidentate. Glycine, which has two groups available for covalent bonding, (the carbonyl oxygen and the aminal nitrogen), is called bidentate. As titrants, multidentate ligands, particularly tetradentate and hexadentate chelating agents, those having four or six donor groups, have two advantages over their unidentate titrants.

When a metal cation is complexed to ligands forming a neutral compound, the complex is called coordinated compound.1 A chelate is produced when a metal ion coordinates with two or more donor groups of a single ligand to form a five or six membered heterocyclic ring. The copper complex of glycine, is an example of a simple chelate

Metal Chelate Complex

Chelon Effect Chelating is the ability of multidentate ligands to form more stable metal complexes than those formed by monodentate or bidentate ligands. These reactions happen over the monodentate because of favored thermodynamics. This results a larger Kf value for multidentate complexes. This is known as chelon effect or chelate effect.

Thermodynamic favorable The delta H’s for mono and multidentates are generally comparable. However, the delta S’ s (entropy) favors a reaction with the multidentate. ΔG° = ΔH° - TΔS° The chemical reaction is spontaneous when the free energy change, G is negative, and d G=H – TS. The enthalpy change for legands with similar groups is often similar. Fro example ammonia molecule complex to Cu2+ and four ammonia group from two ethylenediamine molecule complex to Cu2+ will result in about the same release of heat. However, more disorder or entropy is created by the dissociation of the Cu(NH3)42+ complex in which five species are formed than in the dissociation of the Cu(H2NCH2CH2NH2)22+ complex, in which three species are formed. Hence, S is greater for former dissociation, creating a more negative G and a greater tendency for dissociation. Thus, multidenate complexes are more stable (have large Kf values), largely because of the entropy effect.

The chemical reaction is spontaneous when the free energy change, G is negative, and d G=H – TS. The enthalpy change for legands with similar groups is often similar. For example, four ammonia molecules complexed to Cu2+ and four amino group from two ethylenediamine molecule complexed to Cu2+ will result in about the same release of heat. However, more disorder or entropy is created by the dissociation of the Cu(NH3)42+ complex in which five species are formed than in the dissociation of the Cu(H2NCH2CH2NH2)22+ complex, in which three species are formed. Hence, S is greater for former dissociation, creating a more negative G and a greater tendency for dissociation. Thus, multidenate complexes are more stable (have large Kf values), largely because of the entropy effect.

First, these multidentate titrants, generally react more completely with cations, thereby providing sharper more accurately end points. Second, they ordinarily react with metal ions in a single-step process, whereas with unidentate ligands usually involves two or more intermediate species.

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

EDTA Structure

Neutral EDTA is a tetrabasic acid EDTA is a polyprotic acid. Stepwise dissociation of EDTA as having four Ka Values. Overall ionization is sum of the individual steps and overall ionization constant is the product of individual ionization constant.

Disodium EDTA Since Y4- is the ligand species in complex formation, the complexation equilibria are affected markedly by the pH. H4Y has a very low solubility in water, and so that disodium salt Na2H2Y,2H2O is used. This salt dissociates in solution to give H2Y2-, pH of this solution is approximately 4 to 5.

EDTA Complex with Metal Ions (1) Forms strong 1:1 complexes regardless of the charge on the cation (2) Chelate with all cations

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

Table of Formation Constants for EDTA 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 equilibria From the overall equilibrium

Let us consider that CH4Y represent the total uncomplexed EDTA 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- .

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.

Effect of pH on EDTA Titration of Ca2+ Less distinct end point

EDTA Titration Curve Region 1 Region 2 Region 3 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 3 Excess EDTA. Virtually all metal in MYn-4 form.

EDTA Titration Curves for Ca2+ and Sr2+ (Buffered at pH 10) *Ca2+ end point more distinct. *Lower pH, Kf’ decreases, & End point less distinct. *We cannot raised pH arbitrarily: Metal hydroxides might precipitate.

Metal Ion Indicators Compounds changing colour when binding to metal ion. Kf for Metal-In < Kf for Metal-EDTA. 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) At the end point: 3. MgIn + EDTA  MgEDTA + In (red) (colourless) (colourless) (Blue)

Indicators for EDTA titration Erichrome Black T (EBT) The structure of Eriochrome Black T is as follows:-

Calmagite 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:-

EDTA 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 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 Titration: When to apply it *Analyte precipitates in the absence of EDTA. *Analyte reacts too slowly with EDTA. *Analyte blocks indicator 3. Displacement Titration *Metal ions with no satisfactory indicator. *Analyte treated with excess Mg(EDTA)2- Mn+ + MgYn-2  MYn-4 + Mg2+ * Kf’ for MYn-2 > Kf’ for MgYn-2

4. Indirect Titration *Anions analysed: CO32-, CrO42-, 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 analyte from 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+.

Releasing masking agent from analyte. *Demasking: Releasing masking agent from analyte. Metal-Cyanide Complex Formaldehyde *Oxidation with H2O2 releases Cu2+ from Cu+-Thiourea complex. pH control Masking Demasking *Thus, analyte selectivity: