The effects of pH on the degradation of isothiazolone biocides

In order to measure the effective lifetime and understand the mechanism of degradation, the stability of the active component, 5-chloro-2-methyl-4-isothiazolin-3-one in Kathon 886 MW and Kathon MWC biocides was investigated in both acidic and alkaline solutions. For kinetic data at different pH levels, the concentration of the chlorinated component was measured at regular intervals using reversed-phase liquid chromatography. It was found that the biocides are stable in acidic media. However, the active component undergoes degradation in alkaline solutions, and the rate of degradation is faster with an increase in pH. The first-order rate constant and half-life values for the degradation of the active component in the pH range of 8.5 to 10 were determined. The typical half-life values were found to be 47, 23, 3.3 and 2 days in media of pH 8.5, 9.0, 9.6 and 10, respectively. A high stability in acidic medium, and a linear relationship between hydroxyl ion concentration and first-order rate constant in the alkaline range, suggest that chemical degradation of Kathon biocides results from hydrolysis of the chlorinated isothiazolone.