Question

In your own words, explain what chemically happens in Equation 1 and Equation 2.

Equation 1: M^2+ (aq) +In^2- (aq) --> MIn(aq)

Where M is colorless, In is blue, and MIn is pink

Equation 2: MIn (aq) + EDTA^4- (aq) --> M(EDTA)^2- (aq) + In^2- (aq)

Where MIn is pink and In is blue

Answer 1

In equation 1: metal cation M^2+ forms a complex with indicator anion In^2-, complex formation change the colour of Indicator ion from blue to pink.

In equation 2: metal-indicator 'MIn' complex react with EDTA^4-, EDTA stands for ethylene diamine tetraacetate, which is a hexadentate ligand and forms a stable complex with metal cation such as M^2+, so EDTA^4- replace the indicator anion from metal-indicator complex and form a stable complex M(EDTA)^2- , this leaves indicator ion In^2- free, so colour change form pink to blue happens again.

Above reactions shows the basic principle behind the complexometric titration in which the color change is a indication of end point.