What Does Combined Available Chlorine Mean?
Combined available chlorine is the amount of total chlorine which reacts with pollutants in water, forming chemical compounds. These compounds usually contain oxides of nitrogen, ammonia and organic amines. The word "available" shows that these chloro-derivatives are still capable of oxidizing the organic matter present in the water. Generally, combined available chlorine is more stable than other forms of chlorine.
Corrosionpedia Explains Combined Available Chlorine
Once a fixed concentration of chlorine is added to water, usually in the form of stabilizers or gas, some of it reacts with the compounds present in the water prior to the disinfection process. After that, a partial amount of the total chlorine forms compounds with organic and inorganic matter present in the water. Usually, these compounds are unable to participate in the disinfection process. However, the compounds which can disinfect water are referred to as combined available chlorine. This form of chlorine is very slow in disinfection action.
In pure water, the concentration of total chlorine does not include combined available chlorine. In that case, ammonia is added to water, which combines with chlorine and forms inorganic chloramine (combined available chlorine). This acts as a long-lasting disinfectant and allows the storage of water without any contamination.