<prompt> Orthotolidine test is used to determineNitrates in waterNitrites chlorine in waterFree and combined chlorine in waterAmmonia content in water  <response>: Orthotolidine test OT test It is used to determine both free and combined chlorine in water with speed and accuracy.In this test, the reagent O tolidine, dissolved in 10 solution of HCL is dissolved in 1ml of water containingchlorine.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.Orthotolidine instantaneously reacts with free chlorine, while it react more slowly with combined chlorine.Time taken to produce colourFree chlorine within seconds 10sCombined chlorine 15-20 minutes 