Evaluation of 2-hydroxy-1-(2-hydroxy-4-sulpho-1-naphthylazo)-3-naphthoic acid and hydroxynaphthol blue as metallochromic indicators in the EDTA titration of calcium

Abstract

The acid dissociation constants and the chelate stability constants for calcium have been determined spectrophotometrically on purified samples of 2-hydroxy-1-(2-hydroxy-4-sulpho-1-naphthylazo)-3-naphthoic acid (Patton and Reeder's dye)(HSN) and hydroxynaphthol blue, both of which have been used as indicators in EDTA titrations of calcium. By comparison of experimental and theoretical curves of the indicator colour change at the end-point, it was found that these two dyes are suitable indicators for the titration of calcium at pH 12 to 13, with a slightly more satisfactory end-point indication given by hydroxynaphthol blue.