Need Testing method

How can I test residual chlorine in water

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I am using potassium iodide,starch and acetic acid for residual chlorine and titrate with 0.01sodium thiosulphate…but acc to procedure color change in a night but my sample doesn’t change colour in one day…I don’t know what is reason plz elaborate this

Thanks for this but I have to follow this method in which KI ,acetic acids and starch are used