Disinfection Using Chlorine Bleach pH Effect (contradiction to the above !! with respect to the effective pH for chlorine based disinfection)... When sodium hypochlorite is dissolved in water, the resulting solution is quite basic due to the production of OH– ions by the following reaction: NaOCl (s) + H20(l) → HOCl (aq) + NaOH (aq) The various free chlorine components exist in equilibrium in water; the predominant form depends on the pH. When the pH is between 2 – 7, the equilibrium favors HOCL. As the pH falls below 2, the main form is Cl2. At a pH of 7.4, HOCl and OCl– are about equal, and as the pH goes above 7.4, increasing proportions of OCl– are present. Maximum disinfecting efficacy is achieved at pH 4-5, because essentially all the chlorine is present as HOCl which is two orders of magnitude more effective than OCl– . Below pH 4, increasing amounts of Cl2 are present which will evolve from solution and be lost to the system at atmospheric pressure. Chlorine gas is also quite toxic, so pH below 4 should be avoided. On balance, for safety and efficacy a pH of 5-7 works best; addition of a small amount of HCl to achieve this pH should be considered when disinfecting materials that may be contaminated with bacterial endospores or other resistant forms of microorganisms. For most routine disinfection, no adjustment of pH is necessary. Extreme caution should be exercised when acidifying bleach solutions; too much acid will cause toxic Cl2 to off-gas. - oregonstate.edu/dept/larc/sites/default/files/pdf/chlorine-fact-sheet.pdf
Posted on: Sun, 23 Nov 2014 03:39:31 +0000
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