This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Water, Subject name: Social & Preventive Medicine
Oho-toluidine test measures the levels of:- 
Select and explain. 
A. Combined chlorine, B. Both free and combined chlorine, C. Nitrates in water, D. Hardness of water
B: Both free and combined chlorine
Answer: B: Both free and combined chlorine 
Explanation: 1. OHOTOLUIDINE (OT) TEST Gives value of free and total (combined + free) chlorine levels. Combined chlorine can be easily calculated by subtracting free chlorine from total chlorine. Reagent: Ohotoluidine dissolved in 10% solution of hydrochloric acid. Yellow color produced on reacting with chlorine. Reading within 10 seconds of addition of chlorine --> Free chlorine. Reading after a lapse of 15-20 mins --> Total chlorine. 2. OHOTOLUIDINE-ARSENITE (OTA) TEST Modification of OT test. Gives value of free and combined chlorine separately. Not affected by presence of nitrates/iron/manganese (Thus, superior to OT test).