Which statement correctly describes HOCl versus hypochlorite at low pH and low temperature?

Which statement correctly describes HOCl versus hypochlorite at low pH and low temperature?

• A. Low pH and high temperature
• B. High pH and high temperature
• C. Low pH and low temperature
• D. High pH and low temperature

Answer: (C)

The balance between HOCl and hypochlorite (OCl−) is set by pH. At pH below about 7.5 (the pKa of HOCl), the acidic form HOCl dominates; above that pH, the hypochlorite ion becomes more prevalent. Temperature also matters: the dissociation of HOCl into H+ and OCl− is endothermic, so increasing temperature shifts the balance toward more OCl−, while lowering temperature shifts it toward HOCl. Therefore, at a low pH and low temperature, HOCl is the predominant species. Since HOCl is a stronger disinfectant than OCl−, this condition yields the most effective disinfection.

The balance between HOCl and hypochlorite (OCl−) is set by pH. At pH below about 7.5 (the pKa of HOCl), the acidic form HOCl dominates; above that pH, the hypochlorite ion becomes more prevalent. Temperature also matters: the dissociation of HOCl into H+ and OCl− is endothermic, so increasing temperature shifts the balance toward more OCl−, while lowering temperature shifts it toward HOCl. Therefore, at a low pH and low temperature, HOCl is the predominant species. Since HOCl is a stronger disinfectant than OCl−, this condition yields the most effective disinfection.