Effects of isocyanuric acid on the monochlorodimedone chlorinating rates with free chlorine and ammonia chloramine in water

Changes in monochlorodimedone (MCD) chlorinating rates with free chlorine (mixture of HOCl and OCl−) and ammonia monochloramine (NH2Cl) in water at pH 7 by the addition of isocyanuric acid (H3Cy) were determined at room temperature. Decreases in MCD absorbance at 290 nm in equimolar (0.04 mM) reactions of MCD and free available chlorine solutions containing H3Cy (0.01–1.60 mM) were recorded in a stopped-flow spectrophotometer. The rates indicate second-order reactions. Since the rate with free chlorine was high (>7.6×106 M−1 s−1), the amounts of free chlorine in the solutions could be distinguished from that of chlorinated cyanurates. The chlorinating rates with chlorinated cyanurates decreased with an increase in H3Cy concentrations. Plotting the rates against the molar ratio of chlorine to H3Cy showed a linear correlation and the rates with chlorinated cyanurates (H2ClCy) was estimated at 0.5×105 M−1 s−1. In contrast, the rates with the NH2Cl solution containing H3Cy increased with an increase in H3Cy concentrations, increasing from 1.2×10 to 2.7×10 M−1 s−1 by the addition of 1.55 mM H3Cy. The DPD color development rates (OD512/t1/2/M) with free available chlorine (0.015 mM) declined from 1.3×105 to 0.9×105 M−1 by the addition of 0.61 mM H3Cy.