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What is Orthotolidine test is used to determine – and explain in detail?
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 containing
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.
Orthotolidine instantaneously reacts with free chlorine, while it react more slowly with combined chlorine.
Time taken to produce colour


Free chlorine → within seconds (10s)
Combined chlorine →15-20 minutes