Salt effects on rates of alkaline fading of sulphonphthalein dye ions

Abstract

Salt effects on the rates of alkaline fading of bromophenol blue and bromophenol red have been investigated. The rate depends on the character and concentration only of the cation of the added salt and not on the ionic strength, anions being without effect. The relative effects of univalent cations in enhancing the rate lie in the order K⁺ > (CH₃)₄N⁺ > Na⁺≃ Li⁺. The tervalent cation Co(NH₃)₆³⁺ is much more enhancing. The results are discussed in terms of the distribution of the charge on the bivalent dye ion and consequent increased charge density at or near the site of reaction.