At which pH does hypochlorous acid exist in 100% concentration?

Hypochlorous acid exists in 100% concentration at a pH of 4.0 due to its dissociation properties in water. As the pH decreases, the concentration of hydrogen ions increases, which drives the equilibrium of the dissociation reaction to favor the undissociated form of hypochlorous acid (HOCl) rather than its dissociated state (OCl- and H+).

At a pH around 4.0, hypochlorous acid is predominantly present as HOCl, making it more effective as a disinfectant. This characteristic is crucial for various applications, such as water treatment and swimming pool sanitation, where maintaining the proper pH is essential for ensuring the effectiveness of chlorine compounds.

In contrast, at lower pH levels (3.0), while HOCl is still present, the proportion of hypochlorous acid to hypochlorite becomes less favorable as the acidity increases. As the pH climbs above 4, hypochlorite (OCl-) becomes more prevalent, reducing the concentration of hypochlorous acid. Therefore, the optimal pH for the existence of hypochlorous acid in its fully undissociated form is at 4.0.