Hypochlorous acid is favored over hypochlorite ion when the pH is _____ and the temperature is ________.

Hypochlorous acid is favored over hypochlorite ion when the pH is _____ and the temperature is ________.

• A. Low, High
• B. High, High
• C. Low, Low
• D. High, Low

Answer: (C)

The key idea is that hypochlorous acid (HOCl) is the more effective disinfectant, and its presence relative to hypochlorite (OCl−) depends on both pH and temperature. The HOCl/OCl− balance is governed by the acid-base equilibrium HOCl ⇌ H+ + OCl−. At lower pH (more acidic conditions), the equilibrium shifts toward HOCl, increasing the amount of the active, uncharged disinfectant that can penetrate microbial cells. Temperature also matters: as temperature increases, the balance shifts toward OCl−, reducing the HOCl fraction. So the combination that best favors HOCl is low pH and low temperature, making disinfection more effective.

The key idea is that hypochlorous acid (HOCl) is the more effective disinfectant, and its presence relative to hypochlorite (OCl−) depends on both pH and temperature. The HOCl/OCl− balance is governed by the acid-base equilibrium HOCl ⇌ H+ + OCl−. At lower pH (more acidic conditions), the equilibrium shifts toward HOCl, increasing the amount of the active, uncharged disinfectant that can penetrate microbial cells. Temperature also matters: as temperature increases, the balance shifts toward OCl−, reducing the HOCl fraction. So the combination that best favors HOCl is low pH and low temperature, making disinfection more effective.