Question

The calmagite indicator changes from a pink to a blue color during the titration. Describe why the color change occurs.

This question is part of the experiment "determination of water hardness", and you should answer the question based on the following background information(abstract information and you can get the answer).

In this titration Calmagite is used an indicator for both calcium and magnesium. The equation below shows the competition reaction between the calmagite indicator (In) and EDTA for Mg and the colors associated with each state.

MgIn2⁺+ EDTA^4-⇆MgEDTA^2-+ In

Ca²⁺+ EDTA^2-⇆CaEDTA^2-+ 2H⁺

Ca²⁺+ MgEDTA^2-⇆CaEDTA^2-+ Mg²⁺

Left side is Red and right side is Blue

Explanation: before the titration begins, a small amount of the magnesium is complexed to the calmagite and the rest of the calcium and magnesium ions are free in solution. As the EDTA titrant is added, the calcium complexes with the EDTA and produces protons (see Equation). The presence of a pH 10 buffer will keep the solution within the proper pH range.

The calmagite complexes to any free Mg2+ or Ca2+ ions in solution. However, during the titration, an equilibrium exists between the Ca-EDTA and Mg-EDTA complexes. The calcium-EDTA complex is favored over both the Ca-calmagite and the Mg-EDTA complexes(refer to Equation). Therefore, as the titrant is added to the analyte, both the free calcium in the analyte and the calcium bound to the indicator will form Ca-EDTA complexes. Once all of the free and indicator-bound Ca2+ ions have been complexed with EDTA, the free Mg2+ions and then those complexed with the indicator will bind to the EDTA. The red solution will begin to turn purple when some of the calmagite is metal-free. Once all of the magnesium is bound to the EDTA rather than the calmagite, the end point is reached and the indicator changes color to blue as shown in Equation

Answer 1