Abstract
Abstract The acid dissociation constants of Calmagite [1-(2-hydroxy-5-methylphenylazo)-2-naphthol-4-sulfonic acid] and the formation constants of zinc(II) chelates have been determined spectrophotometrically at 25°C and at the ionic strengths of 0.1 and 1.0. In an ammoniacal medium zinc forms mixed-ligand complexes with Calmagite (I3−) and ammonia (X); ZnIXn−. The formation constants of the zinc(II)-Calmagite chelates are logKZnI=11.75, logKZnI2=7.82, logKZnIXx=2.09, and logKZnIX2x=1.70 (μ=1.0). Calmagite is recommended as an indicator for the titration of zinc(II) with EDTA. The color transition of Calmagite is clearer and sharper, and the aqueous solution of the indicator is more stable, than in the case of Eriochrome Black T.
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