Dynamic analysis of chemical relaxation effects: A study of salicylate-indicator systems by a T-jump apparatus with laser monitoring

The kinetics and equilibria of proton-transfer reactions involving 3,5-dinitrosalycilate ion (DNSA) and the indicators Bromothymol Blue (BTB), Thymol Blue (TB), and Phenol Red (PR) have been investigated at ionic strength 0.1 M by a T-jump technique where a system of lasers was employed for monitoring the relaxation effects. The reacting systems display biexponential curves that have been analyzed in terms of normal reactions. A method is presented that leads to a quick evaluation of the stoichiometric coefficients of the species taking part in the normal modes of reaction. The kinetic results show that the rates of proton exchange are lower than those of proton recombination by about 3 orders of magnitude, thus indicating that a strong intramolecular hydrogen bond is present in DNSA. The shapes of the plots of log k vs Delta pK suggest that a two-step mechanism involving a fast preequilibrium between a closed (unreactive) and an open (reactive) form of DNSA should be preferred to the one-step mechanism where the acceptor is able to react also with the proton of the closed DNSA. The analysis of the relaxation amplitudes allowed Delta G(0) and Delta H-0 of the individual steps to be simultaneously evaluated. A comparison between the performance of the T-jump with conventional lamp and that of the T-jump with laser monitoring is in favor of the latter technique, which, in turn, has been demonstrated to be as accurate and precise as classical spectrophotometry.