<prompt> Oho-toluidine test measures the levels of:-Combined chlorineBoth free and combined chlorineNitrates in waterHardness of water  <response>: 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 nitratesironmanganese Thus, superior to OT test. 