Successive chelatometric titration of calcium and magnesium using hydroxy naphthol blue (HNB) indicator

Abstract

Successive titration of calcium (pH 13) and magnesium (pH 10) using single indicator “Hydroxy Naphthol Blue” is proposed. Although HNB gives a very sharp end point in the titration of calcium at pH 13, an earlier end point color change is experienced at pH 10. On the other hand, magnesium can be titrated accurately at pH 10 with HNB indicator, if the photometric titration technique is employed. Thus, the sample containing calcium and magnesium is at first titrated for calcium with EDTA at pH 13 using HNB indicator to the visual end point. After Table III Successive determination of calcium and magnesium in the presence of various ions Table IV Successive determination of calcium and magnesium in natural waters neutralizing the resulting solution to dissolve magnesium hydroxide, pH of the solution is made to 10, and it is titrated for magnesium using the same indicator to the photometric end point. In spite of the drawback that the proposed method needs a photometric titration assembly, the successive determination of calcium and magnesium can be done with single indicator by simple procedure. The method is applied to the analysis of natural water such as seaor river water with satisfactory result. © 1970, The Japan Society for Analytical Chemistry. All rights reserved.