Explain why the change from red to blue in Reaction 12-19 occurs suddenly at the equivalence point instead of gradually throughout the entire titration.

Equation 12-19 is a typical illustration of a titration reaction between Mg²⁺ and EDTA at pH 10 in presence of calmagite indicator. The reaction is as follows

$$\begin{gathered} \mathrm{Mg{\rm I}n}+\mathrm{EDTA}\to \mathrm{MgEDTA}+\mathrm{{\rm I}n} \\ \mathrm{Re}\mathrm{d}\mathrm{ColorlessColorlessBlue} \end{gathered}$$

Little amount of indicator is induced at the beginning of the experiment. It is added to the colorless Mg²⁺complex and forms a red complex. After that EDTA is added. This reacts with the Mg²⁺ solution first. After completion of reaction between Mg²⁺ and EDTA, the remained indicator sample reacts with EDTA and turns into blue unboundIn.

As very small of indicator is introduced into the sample, many of the Mg²⁺ ions is not bounded with the indicator. Free ions of Mg²⁺ are preferable for reaction with EDTA than MgIn . Therefore, concentration of MgIn remains constant until all the free Mg²⁺ ions have been consumed. Only after that MgIn gets chance to react with EDTA. As soon as the reaction starts with the indicator the color change from red to blue. This is the reason that the change from red to blue in Reaction 12-19 occurs suddenly at the equivalence point instead of gradually throughout the entire titration