Hach Disinfection Series - Step 2 

2. Chlorine Chemistry

Chlorine is the predominant method of disinfecting water and wastewater and has been used for almost 100 years in the United States. Chlorine can be added to water as chlorine gas, an aqueous sodium hypochlorite solution, or solid calcium hypochlorite. When chlorine is added to water in any of these forms, it creates hypochlorous acid. Hypochlorous acid (HOCl) is a weak acid that dissociates into hypochlorite ion (OCl) according to the following equation:

HOCl ↔ H+ + OCl

Together, HOCl and OCl are known as free chlorine. The two species exist in an equilibrium that is pH dependent. The equilibrium is also slightly affected by temperature. As the pH increases, the ratio of hypochlorous acid to hypochlorite ion decreases. Below a pH of 7.5, hypochlorous acid is the dominant species. Above a pH of 7.5, hypochlorite ion is the dominant species. The disassociation curve below illustrates the relationship between the chemical forms of chlorine and pH at 20° C. The graph indicates a significant change in the ratio of hypochlorous acid to hypochlorite between pH 6 and 9, within the typical pH range for drinking water treatment. The steepest portion of the curve is between pH 7 and 8. Even a 0.1 unit change in pH can cause a significant change in the ratio between HOCl and OCl. This is significant because HOCl is a stronger disinfectant than OCl. Therefore, the chlorination process is pH dependent. The germicidal effects of HOCl will be realized by chlorination at a lower pH.

germicidal effects of HOCl chart

When free chlorine is combined with ammonia, the compounds react to form chloramines, such as monochloramine, dichloramine, and nitrogen trichloride. The sum of the free chlorine and chloramines in a sample is known as total chlorine.

The various forms of chlorine-based disinfectants have varying degrees of germicidal strength and persistence in the water and are therefore used in different applications. Free chlorine is a much stronger disinfectant than chloramine, but its residual is not as persistent as a chloramine residual. Because of its germicidal properties, free chlorine is commonly used whenever possible; however free chlorine forms trihalomethanes and is often replaced with chloramines when trihalomethanes or other disinfection by-products are a problem. Since chloramines are a weaker disinfectant their concentration levels are kept at much higher than free chlorine. Since the chloramine residual lasts much longer than free chlorine residuals, chloramines are often used as a secondary residual disinfectant to ensure the presence of a disinfectant residual throughout the distribution system.