Question

Pyrocatechol violet(Table 12-3) is to be used as a metal ion indicator in an EDTA titration. The procedure is as follows:

1. Add a known excess of EDTA to the unknown metal ion.

2. Adjust the pH with a suitable buffer.

3. Back-titrate the excess chelate with standard Al³⁺.

From the following available buffers, select the best buffer, and then state what color change will be observed at the end point. Explain your answer.

1. pH 6–7 (ii) pH 7–8 (iii) pH 8–9 (iv) pH 9–10

Short answer

From table 12-3 it can be identified that buffer solution of pH 6-7 is the most suitable one as in this range the color change occurs from yellow to blue while for the other pH ranges the color changes from violet to blue. Identifying change from yellow to blue is much easier than that of violet to blue. So (i) pH6-7 will be the best buffer to operate with.

Step-by-step solution

EDTA

EDTA, also known as ethylenediaminetetraacetic acid forms strong complexes with metal ions and plays a major role in quantitative analysis. In industries it is used as a metal binding agent and used in production of detergents, cleaning agents, food additives etc.

Reaction procedure

A titration reaction between pyrocatechol violet (a metal ion indicator) and EDTA follows the steps below;

For an unknown concentration of metal ion, a known excess of EDTA must be added. Then the pH must be adjusted using a suitable buffer. After that we need to back titrate the excess amount of chelate with standard solution of Al³⁺

Selection of best buffer

From table 12-3 it can be identified that buffer solution of pH 6-7 is the most suitable one as in this range the color change occurs from yellow to blue while for the other pH ranges the color changes from violet to blue. Identifying change from yellow to blue is much easier than that of violet to blue. So (i) pH6-7 will be the best buffer to operate with.