Chlorine (Cl) is one of the most common chemical elements used to eliminate microorganisms that can infect our water. All chlorinated products produce hypochlorous acid (HCLO-) when reacted with water. As we will see, the amount of hypochlorous acid is strongly conditioned by the pH value. These two forms of chlorine are found in water in equilibrium, although this depends on the pH values.

The purpose of disinfection is to eliminate pathogenic microorganisms and to ensure the absence of any infectious germ (bacteria or virus) in the water. Chlorinated products are the most frequently used substances in the chemical treatment of water due to their safety and ease of control of their levels.

All chlorinated products used in treatment produce hypochlorous acid (HCLO) when reacted with water. Hypochlorous acid is a weak acid that dissociates to hypochlorite (ClO-) in water according to an equilibrium conditioned by the pH value. The sum of these 2 forms constitutes what is called free chlorine. In high pH waters, most of the hypochlorous acid (active chlorine) is converted to hypochlorite ion (potential chlorine), a form of chlorine with very low disinfectant power.

Combined chlorine is the result of the combination of chlorine with ammonia and nitrogenous organic matter contained in the water. The sum of free chlorine and combined chlorine constitutes total chlorine.

Free Chlorine: This form of chlorine has the highest disinfectant and oxidizing power, mainly due to the presence of hypochlorous acid and hypochlorite anion.

Combined chlorine: It has a very low disinfectant power and its presence causes irritations and bad odors. The combination of free chlorine with ammonia and nitrogenous organic matter in the water gives rise to combined chlorine (chloramines).

Total chlorine: The sum of free chlorine and combined chlorine results in total chlorine. Total chlorine should not exceed more than 0.6 mg/l of the free residual chlorine level.