adsorption indicator (absorption indicator)

A type of indicator used in reactions that involve precipita­tion. The yellow dye fluorescein is a common example, used for the reac­tion

NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)

As silver nitrate solution is added to the sodium chloride, silver chlo­ride precipitates. As long as Cl" ions are in excess, they adsorb on the precipitate particles. At the end point, no Cl ions are left in solution and negative fluorescein ions are then adsorbed, giving a pink colour to the precipitate. The technique is sometimes known as Fajan's method.