Decomposition of 2,6-dichlorophenolindophenol in strongly acidic solutions: a potential kinetic method for the determination of pH

Abstract

The problem of pH measurements at extreme pH values is well known. Higher pH values are obtained at pH values of below 1 because of the so-called ‘acid error’. The acid error depends on various parameters and it is not always reproducible. In order to determine accurate pH values at high acidities, a kinetic study of the decomposition of 2,6-dichlorophenolindophenol (DCPl) in strongly acidic solutions was investigated by applying the stopped-flow technique. The DCPl is unstable at lower pH values (below 2) and the reaction rate constant for its decomposition in the pH range 0–2 is dependent on the pH. A good correlation between pH and the observed reaction rate constant of the decomposition of DCPl (k_ob) was found for low pH values. The reaction rate constant, k₁, was calculated by three different approaches for the evaluation of the experimental data. The weighted mean value of k₁ was found to be (77 ± 1)× 10^–3 l mol^–1 S^–1, with confidence limits of 95%. A knowledge of k₁ is also important in analysis as DCPl is the main reagent for the determination of assorbic acid in acidic solutions.