Comparative Toxicity of Hypochlorous Acid and Hypochlorite Ions to Mosquitofish

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Abstract
We examined the relative toxicity of hypochlorous acid (HOCl) and the hypochlorite ion (OCl−) by exposing mosquitofish Gambusia affinis for 1 hour to predominantly free residual chlorine (FRC) at six levels of pH following a 7‐day acclimation to the test pH. Median lethal concentrations (LC50), in terms of total residual chlorine (TRC), increased with increasing pH, ranging from 0.41 mg/liter at pH 6.05 to 1.28 mg/liter at pH 8.42. Because of the influence of hydrogen ion concentration on dissociation of HOCI, the percent of FRC present as HOCl is about 97% and 13%, respectively, at the low and high pH. Cursory examination of toxicity of monochloramine (NH2Cl) and mixtures of NH2Cl and dichloramine (NHCl2) suggested that their contributions to toxicity were negligible at the concentrations existing at any test pH. Free residual chlorine concentrations at the LC50 for each pH were fitted to a theoretical model derived from an assumption that toxicities of HOCl and OCl− were additive. More than 99% of the total variability was explained by the model, providing strong support for the assumption. Hypochlorous acid is about four times as toxic as OCl−.