This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Ohotoluidine Arsenite test determines - 
Select and explain. 
A. Free residual chlorine, B. Combined chlorine, C. Free and combined chlorine separately, D. Free and combined chlorine together
C: Free and combined chlorine separately
Answer: C: Free and combined chlorine separately 
Explanation: Oho-toluidine test is used to determine both free and residual chlorine with speed and accuracy. It is done using oho-toluidine dissolved in 10 per cent solution of hydrochloric acid. When added to water containing chlorine it turns yellow and its intensity varies with the concentration of the gas. Oho-toluidine Arsenite test is used to determine free and residual chlorine separately. (PARK TB (25th edition pg 775)