The pH of the solution has a significant effect on the level of chlorination activity. When
chlorine gas or one of the hypochlorite salts is added to water, each will generate chlorine
gas (Cl2), hypochlorous acid (HOCl), or hypochlorite ions (OCl) in various proportions,
depending on the pH of the solution. The form desired for chlorination is hypochlorous
acid (HOCl). Hypochlorite ions are relatively inactive, and chlorine gas quickly bubbles
out of the solution, causing worker discomfort and serving no useful purpose.
At a pH slightly above neutral, half of the chlorine will be in the form of hypochlorous
acid and the other half in the form of hypochlorite ions. Very little will be in the gaseous
form. Solutions that are more acid have a higher percentage of hypochlorous acid but are
very unstable, allowing more of the chlorine to escape from the solution as a gas. To
maximize the proportion of hypochlorous acid and hence the effectiveness of the solution,
the pH should be kept in the practical range between 6.5 and 7.5. Because well water
varies from moderately acid to moderately alkaline, the pH should be checked with a pH
meter or test papers before and after the chemicals are added and frequently during
operation. Furthermore, even if the water initially has a near-neutral pH, the addition of
hypochlorites will change the pH.