This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
[{'content': 'Orthotolidine test is used to determine – \nSelect and explain. \nA. Nitrates in water, B. Nitrites chlorine in water, C. Free and combined chlorine in water, D. Ammonia content in water', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Orthotolidine test (OT test) \n\nIt is used to determine both free and combined chlorine in water with speed and accuracy.\nIn this test, the reagent (O tolidine, dissolved in 10% solution of HCL) is dissolved in 1ml of water containing\nchlorine.\nThe water turns yellow and the intensity of the colour varies with the concentration of the gas.\nThe yellow colour is produced by both free and combined chlorine residuals.\nOrthotolidine instantaneously reacts with free chlorine, while it react more slowly with combined chlorine.\nTime taken to produce colour\n\n\nFree chlorine → within seconds (10s)\nCombined chlorine →15-20 minutes', 'role': 'assistant'}]