To find out whether teh w ater has been properly chlorinated, the test used is -

After chlorination,tge residual chlorine is tested at the end of one hour contact period.If free residaul chlorine level is less than 0.5 mg/litre, the chlorination procedure should be repeated before water is drawn.Reagent used in OT test is analytical grade ohotoluidone dissolved in 10% hydrochloric acid.On addition of reagent to water containing chlorine it turns yellow. Parks textbook of preventive and social medicine.K Park. Edition 23.Pg no: 718