Question

explain how selectivity can be achieved in complexometric titrations.use examples to support your answer

Answer 1

Complexometric titration is a type of volumetric titration where a complexing agent forms a colored complex with one or more metal ions and the end point of the titration is detected by the formation of the colored complex(s). EDTA is possibly the most versatile complexing agent used in complexometric titrations. EDTA can bind almost all divalent, trivalent or tetravalent metal ions. Hence, some form of selectivity must be used in order to complex only a desired metal ion while the other ions in solution are left unreacted with EDTA.

Selectivity in EDTA titrations can be achieved by a number of ways, the principle of which are outlined below.

1) Control of pH: Most EDTA complexes are stable over a certain range of pH while these complexes break down beyond that pH range. For example, complexes of Bi are stable in acidic pH while most divalent cations form complexes which are stable at pH > 8. Therefore, a mixture of Bi and Pb can be made to selectively complex with EDTA at pH 2. The Pb comples, being unstable at this pH value, will decompose.

2) Masking Agents: Masking agents are used to selectively couple or complex off certain metal ions while keeping the desired ion(s) in solution. Fe(III) is known to form a stable complex with phosphoric acid. The complex is colorless and doesn’t interfere in the complexation of other metal ions with EDTA. Therefore, a mixture of Fe(III) and Ca²⁺ may be analyzed by first adding phosphoric acid (this removes Fe³⁺ from the solution) and then adding EDTA to selectively complex with Ca²⁺.

3) Demasking Agents: Demasking is the reverse process of masking where the masked metal is released from its complex and made available to complex with EDTA. Cyanide is a good masking agent for a number of metal ions like Cd²⁺, Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, Ag⁺, etc. Selective demasking is the process where a chemical is added to only react with a particular masked metal ion while the other metal ions do not participate in the reaction. Chloral hydrate is a good demasking agent for cyanide complexes of Zn and Cd. Therefore, a mixture of Zn, Ca and Cu can be titrated as below.

a) Standard EDTA is mixed with the solution and the excess EDTA is back-titrated with standard Zn solution. This gives the sum of the three metal ions taken, since all three metals can form stable complexes with EDTA.

b) A standard cyanide solution is added to the mixture to mask Zn and Cu as their cyanide complexes. EDTA is then added to the mixture. Ca²⁺ will selectively complex with EDTA and this step gives only the measure of Ca present.

c) Chloral hydrate is added to the mixture in step (b) to selectively damask or unmask Zn. Zn forms a stable complex with EDTA and therefore, the amount of Zn can be easily determined. The amount of Cu can be obtained by simply subtracting the sum of the amounts in steps (b) and (c) from the total sum in (a).