Orthotoludine test can detect?

ANSWER: (C) Free Sl combined chlorineREF: Park 20^ edition page 626"Orthotolidine (OT) test enables both free and combined chlorine in water to be determined with speed and accuracy"Orthotolidine reacts with free chlorine instantaneously but reacts more slowly with combined chlorine.Orthotolidine test = 0.1 ml of reagent + 1 ml of water (read after 10 sec for free chlorine and after 15-20 min for both free and combined chlorine)Orthotolidine reagent = Orthotolidine + 10% solution of HCLWhen this reagent is added to water containing chlorine, it turns yellow and the intensity of colour varies with the concentration of the gasIf free residual chlorine level is less than O.Smg/litre. the chlorination procedure should be repeated before any water is drawnDose of chlorine = chlorine demand + residual chlorine (0.5mg/lit)Orthotolidine arsenite (OTA) test is a modification of OT test to determine the free and combined chlorine residuals separatelyFurther the errors caused by interfering substances like nitrates, iron and manganese all of which causes yellow colour with OT, are overcome by OTA test.