Aims: Bactericidal activity of chlorine solution is enhanced by weak acidification. We compared the effects of various acids on the bactericidal activity of hypochlorite solution to establish a method for safe and effective use of an acidic hypochlorite solution. Methods and Results: The bactericidal activities of acidic hypochlorite solutions that had been adjusted to pH 5·0 with hydrochloric acid, acetic acid, citric acid, lactic acid, formic acid, phosphoric acid or sulphuric acid against Bacillus subtilis spores were compared. The acidic solutions prepared with hydrochloric acid and acetic acid showed the highest bactericidal activity, and all of the spores (5 × 106 cfu ml−1) were killed within 10 min. On the other hand, the solutions prepared with citric acid and lactic acid showed no bactericidal activity against any bacterial strains tested in this study despite the low pH. The amount of chlorine gas produced by the preparation using acetic acid was sixfold less than that produced from the preparation using hydrochloric acid. Conclusions: Acetic acid is the most suitable and safe acid for the preparation of an acidic hypochlorite solution. Significance and Impact of the Study: The results of this study provide useful information for establishing a method for safe and effective use of an acidic hypochlorite solution.

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