DECOMPOSITION OF 2,6-DICHLOROPHENOLINDOPHENOL IN STRONGLY ACIDIC SOLUTIONS - A POTENTIAL KINETIC METHOD FOR THE DETERMINATION OF PH

The problem of pH measurements at extreme pH values is well known. Hig her pH values are obtained at pH values of below 1 because of the so-c alled 'acid error'. The acid error depends on various parameters and i t is not always reproducible. In order to determine accurate pH values at high acidities, a kinetic study of the decomposition of 2,6-dichlo rophenolindophenol (DCPI) in strongly acidic solutions was investigate d by applying the stopped-flow technique. The DCPI is unstable at lowe r pH values (below 2) and the reaction rate constant for its decomposi tion in the pH range 0-2 is dependent on the pH. A good correlation be tween pH and the observed reaction rate constant of the decomposition of DCPI (k(ob)) was found for low pH values. The reaction rate constan t, 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) x 10(-3) l Mol-1 S-1, with confidence limits of 95%. A k nowledge of k1 is also important in analysis as DCPI is the main reage nt for the determination of ascorbic acid in acidic solutions.