(C) Hypochlorous acid[?]Chlorination:oChlorination is one of the greatest advances in water purification. It is supplement, not a substitute to sand filtration.oAction of chlorine: When chlorine is added to water, there is formation of Hydrochloric & Hypochlorous acids.oHydrochloric acid is neutralized by the alkalinity of the water.oDisinfecting action of chlorine is mainly due to the hypochlorous acid, & to a small extent due to the hypochlorite ions.oIt is more effective (70-80 times) than the hypochlorite ion.oChlorine acts best as a disinfectant when the pH of water is around 7 because of the predominance of hypochlorous acid.oWhen the pH value exceeds 8.5 it is unreliable as a disinfectant because about 90 per cent of the hypochlorous acid gets ionized to hypochlorite ions.oPrinciples of chlorination: Mere addition of chlorine to water is not chlorination.oRules to ensure proper chlorination:-First of all, the water to be chlorinated should be clear and free from turbidity. Turbidity impedes efficient chlorination-Secondly, "chlorine demand" of the water should be estimated.-Thirdly the contact period.-Presence of free residual chlorine for a contact period of at least one hour is essential to kill bacteria & viruses-Chlorine has no effect on spores, protozoal cysts and helminthic ova, except in higher doses.-The minimum recommended concentration of free chlorine is 0.5mg/L for one hour.-Free residual chlorine provides a margin of safety against subsequent microbial contamination such as may occur during storage and distribution.-Sum of the chlorine demand of the specific water plus the free residual chlorine of 0.5mg/L constitutes the correct dose of chlorine to be applied.[?]Break Point Chlorination-Addition of chlorine to ammonia In water produces chloramines which do not have the same efficiency as free chlorine.-If chlorine dose in the water is increased, a reduction in the residual chlorine occurs, due to the destruction of chloramine by the added chlorine.-End products do not represent any residual chlorine.-Fall in residual chlorine will continue with further increase in chlorine dose and after a stage, the residual chlorine begins to increase in proportion to the added dose of chlorine.-This point at which the residual chlorine appears and when all combined chlorines have been completely destroyed is the breakpoint and corresponding dosage is the breakpoint dosage.-Breakpoint chlorination achieves the same results as superchlorination in a rational manner and can therefore be construed as controlled superchlorination.[?]Super ChlorineoSuperchlorination followed by dechlorination comprises the addition of large doses of chlorine to the water, & removal of excess of chlorine after disinfection, this method is applicable to heavily polluted waters whose quality fluctuates greatly.[?]Ortho Tolidine (OT) Test-Enables both free & combined chlorine in water to be determined with speed and accuracy.-Reagent consists Orthotolidine, dissolved in 10% solution of hydrochloric acid.-Reagent is added to water containing chlorine, it turns yellow & intensity of the colour varies with the concentration of the gas.-Yellow colour is produced by both free & combined chlorine residuals.-OT reacts with free chlorine instantaneously but reacts more slowly with combined chlorine.-Test is carried out by adding 0.1 ml of the reagent to 1 ml of water.-Yellow colour produced is matched against suitable standards or colour discs.-It is essential to take the reading within 10 seconds after addition of reagent to estimate free chlorine in water-Colour that is produced after a lapse, say 15-20 minutes, is due to the action of both free and combined chlorine.[?]Ortho Tolidine-Arsenite (OTA) Test-Modification of the OT test to determine free & combined chlorine residuals separately.-Errors caused by the presence of interfering substances such as nitrites, iron & manganese all of which produce a yellow colour with Orthotolidine, are overcome by the OTA test.