Chemistry – Experimental techniques and chemical analysis - Acid-base titrations | e-Consult

Ferroin is a suitable indicator for the titration of potassium thiocyanate with iodine because it exhibits a distinct colour change over the pH range of the reaction. The reaction between iodine and thiocyanate results in the formation of a complex ion, which is a relatively weak acid. The pH of the solution changes as the reaction proceeds.

Ferroin is a diprotic indicator, meaning it has two distinct colour forms depending on the pH. It typically appears colourless in acidic solutions (pH 6.2). The reaction between iodine and thiocyanate produces HI, which is a strong acid. As the reaction proceeds, the pH of the solution decreases. The end-point is reached when the pH falls below 2.3, causing the ferroin to change from colourless to a deep blue colour. This sharp colour change signals the complete consumption of iodine by the thiocyanate.