Question

Calcium ion was titrated with EDTA at pH 11, using Calmagite as indicator (Table 12-3). Which is the principal species of Calmagite at pH 11? What color was observed before the equivalence point? After the equivalence point?

Short answer

The principal species of Calmagite at pH 11 will be ${\mathrm{HIn}}^2$ Wine color was observed before the equivalence point and blue color was obtained after the equivalence point

Step-by-step solution

Information given

A titration reaction between calcium ion and EDTA at pH 11 in presence of calmagite indicator. is as follows

$$\begin{gathered} \mathrm{Ca{\rm I}n}+\mathrm{EDTA}\to \mathrm{CaEDTA}+\mathrm{{\rm I}n} \\ \mathrm{Wine}\mathrm{Red}\mathrm{ColorlessColorlessBlue} \end{gathered}$$

How the reaction proceeds

Little amount of indicator is induced at the beginning of the experiment. It is added to the colorless Ca²⁺complex and forms a red complex. After that EDTA is added. This reacts with the Ca²⁺ solution first.After completion of reaction between Ca²⁺ and EDTA, the remained indicator sample reacts with EDTA and turns into blue unbound .

Reason

The principal species of Calmagite at pH 11 will be ${\mathrm{HIn}}^{2-}$Wine color was observed before the equivalence point and blue color was obtained after the equivalence point.