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Measurement of recidual chlorine (Chlorotex reagent method)
Chlorine treatment as a means of purification is applied to public water supplies. Treatment with chlorine, whether in the form of gas or as sodium hypochlorite solution, will destroy all forms of bacteria and sufficient to ensure an excess of free or combined residual chlorine after the bacteria and organic matter have been destroyed. In addition to its use for sterilizing drinking water, chlorination also provides one of the most favoured methods for the treatment of swimming water.

The residual chlorine to extent of between 0.2 and 0.5 parts per million in water is considered to give adequate purification. For chlorination to be fully effective the water must be maintained between pH of 7.2 and 7.6. If pH value is less than 7.0 sodium carbonate should be added and if it is higher than 8.0, hydrochloric acid should be added. To find out efficient chlorination the examintion of water should be made for residual cholorine content. For this purpose, the BDH “Chlorotex” reagent provides a simply performed but accurate test which could be easily conducted by persons without chemical knowledge or experience.

Chlorinated water mixed with chlorotex reagent produces a colour which varies in intensity and shade according to the proportion of residual chlorine present in treated water.

The following are the indications of various colour shades developed with chlorotex reagent: