)Discussion:OT test enables both free and combined chlorine in water. Add 0.1ml of reagent to 1 ml of water. Yellow colour produced is matched against suitable standard colour.Modification is Orthotoludine Arsinite (OTA) test - to determine the free and combined chlorine separately. Also errors caused by the presence of interfering substances like nitrites, iron, manganese which also produce yellow colour to OT is overcome by OTA test.CHLORINATION# Chlorine kills pathogenic bacteria but it has no effect on spores and certain viruses (e.g., polio, viral hepatitis, protozoal cyst, helementhic).# The disinfecting action of chlorine is mainly due to hypochlorus acid.0# Chlorine acts best as a disinfectant when pH of water is around 7, because of the predominance of hypochlorus acid.# When pH value exceeds 8.5, it is unreliable as a disinfectant because about 90% of the hypochlorous acid gets ionized.# One tablet of chlorine, chlorinates 20 litres of water.0# Principles of chlorination- Water should be clear and free from turbidity because turbidity impedes efficient chlorination.- The chlorine demand of the water should be estimated. If further chlorine is added to the water beyond break point, free chlorine begins to appear in the water.- The pesence of free residual chlorine for a contact period of at least one hour is essential to kill bacteria and viruses; the chlorine has no effect on spores, protozoal cysts and helminthic ovas.- The minimum recommended concentration of free chlorine is 0.5 mg/L for one hour. The point at which the residual chlorine appears and all combined chlorines have been completely destroyed is the breakpoint and corresponding dosage is the break-point dosage. Presence of free residual chlorine for contact period of at least 1 hour.Orthotolidine Test# The yellow colour is produced by both free and combined chlorine residuals. The test is carried out by adding 0.1 mL of the reagent to 1 mL of water. The colour produced after 15-20 minutes is due to the action of both free and combined chlorine.# In this test, 10 ml of chlorinated sample of water is taken after the required contact period, in a glass tube. To this 0.1 mL of orthotolidine solution is added. The colour formed is observed after 5 minutes.# The formation of yellow colour normally indicates the presence of chlorine (either combined or free) in the water.# The more yellow the colour, the greater, is the chlorine residual.# The amount of residual chlorine can be ascertained by comparing the colour developed in the glass tube with the standard colours already kept in the laboratory.# This test, is therefore, very simple and does not require much technique or time. Under normal conditions, a lemon yellow colour is satisfactory for public water supply.# The orthotolidine test will normally give the total residual chlorine present in water. However, it may be adjusted so as to give separately the quantities of free residual as well as combined residual of chlorine.# The free residual chlorine forms the yellow colour during the first 5 seconds of the addition of orthotolidine, while the combined residual chlorine goes on forming the colour for about 5 minutes. Hence, the colour after 5 seconds will give the quantity of free residual chlorine, and the colour after 5 minutes will give the free and combined chlorine. The difference in value between the two values is the combined chlorine.# The orthotolidine test, however, is not accurate, because the impurities such as iron, manganese, nitrate etc., are likely to cause a false yellow colour, and indicating wrong and increased chlorine residual. To overcome this problem Orthotolidine - Arsenite (OT A) test is carried out.Now the different chlorine residuals can be easily determined as follows:# Free residual chlorine = (X2) - (XI)# Combine residual chlorine = (X3) - (X2)# In the orthotolidine test as well as in the OT A test, the temperature of water is to be controlled to room temperature.">