This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and Health, Subject name: Social & Preventive Medicine
Which among the following is the most effective form of Chlorine for water disinfection? 
Select and explain. 
A. Hypochlorite ion, B. Hydrogen chloride, C. Hypochlorous acid, D. Chloride ion
C: Hypochlorous acid
Answer: C: Hypochlorous acid 
Explanation: (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.