| Abstract: | Six new results have been obtained in an investigation of the catalysed hydrolysis of atrazine. A procedure was developed for determining rate constants from the first 20 mol% of the reaction. The reduced length of the experiments increased the experimental productivity and minimised the risk of sample deterioration. The proton catalysis rate was measured at 25°C, over the H+ concentration range of 1 to 10 mmol litre?1. This shortened the extrapolation to the pH values of field conditions, and guaranteed the absence of doubly-protonated atrazine. Because spectrophotometry has been regarded as a controversial method for studying the hydrolysis kinetics of the 1,3,5-triazine herbicides, this method was re-examined. Excellent agreement was obtained from a comparison of the gas-liquid chromatographic and spectrophotometric methods of monitoring the kinetics experiments. Of the widely diverging relationships found in the literature for the extrapolation of laboratory half-lives to the pH range of field conditions, the one deduced from the reaction scheme of Plust et al. was found to be theoretically correct. Its parameters have been calibrated with several hundred experimental measurements. The proton is not the dominant catalytic agent in soils, but its calibration is essential for identifying those catalytic agents that are dominant. It has been deduced that the temperature dependence of the experimental rate constant is governed by the temperature responses of two parameters. |
