<prompt> Ohotolidine test is used to determine ?Nitrates in waterNitrites chlorine in waterFree and combined chlorine in waterAmmonia content in water  <response>: Ans. is c i.e., Free and combined chlorine in water Ohotolidine test OT test It is used to determine both free and combined chlorine in water with speed and accuracy. In this test. the reagent 0 tolidine, dissolved in 10 solution of HCL is dissolved in tml of water containing o chlorine. The water turns yellow and the intensity of the colour varies with the concentration of the gas. The yellow colour is produced by both free and combined chlorine residuals. o Ohotolidine instantaneously reacts with free chlorine. n h i le it react more slowly with combined chlorine. o Time taken to produce colour Free chlorine within seconds 10s Combined chlorine 15-20 minutes 