Chlorine in water can be present in two forms namely available chlorine which is naturally present in the water and residual chlorine which is extra and tends to be added externally. Externally additive chlorine comes in several forms like granules, tablets, powder, liquid and gas. It dissolves easily in water. When chlorine is added to water, it will attack organic matter and attempt to destroy it. If enough chlorine is added, some will remain in the water after all possible organisms have been destroyed and what is left is called residual chlorine. It does not remove chemical contaminants from the water.
Although chlorine does not destroy all micro-organisms but it is effective against a wide variety of disease causing microorganisms. It kills most of the microbial agents at the treatment point and its residual can further protect the water through long delivery pipeline to give the chemical time to perform its disinfecting action before it reaches the consumer. The presence of Residual Chlorine in drinking water indicates that most of the microbial organisms have been killed and the water will remain protected from recontamination during storage until the residual chlorine dissipates itself or is dissipated by heating of water. For domestic use the amount of residual chlorine at the point where the consumer collects water should be between 0.2 to 0.5 milligram per litre.
There are different methods of estimation of residual chlorine. The simplest and quickest method is detecting its concentration in water by adding Ortho-Toluidine indicator and comparing the colour change in water using a color comparator.
