Which chlorine residual contains hypochlorous acid and hypochlorite ion?

The correct answer is free chlorine. Free chlorine refers to the concentration of chlorine in water that exists as hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). Both of these forms are effective in disinfecting water and are important for maintaining water quality.

When chlorine is added to water, it typically dissociates into these two species, with hypochlorous acid being the more potent disinfectant due to its ability to penetrate microbial cell walls more effectively. The presence of both hypochlorous acid and hypochlorite ion in free chlorine is a critical factor in ensuring that sufficient disinfectant levels are maintained.

Combined chlorine, in contrast, refers to chlorine that has reacted with contaminants in the water, such as ammonia, to form chloramines. While chloramines can still provide some disinfection, they are not as effective as free chlorine. Total chlorine encompasses both free chlorine and combined chlorine, so it does not exclusively represent the uninterrupted disinfection potential that free chlorine does. Lastly, 'residual chlorine' is a broader term that includes both free and combined chlorine but does not distinguish between the forms of chlorine that have disinfectant properties.