Question

The orthotolidine test is used for assessing the concentration of which of the following in water?

• A. Chlorine
• B. Nitrites
• C. Nitrates
• D. Fluorine

Hint:

Think of the yellow colour test done to check residual disinfectant in chlorinated water.

Answer 1

The Correct Option is A

The question asks which water parameter the orthotolidine (OT) test measures. In public health practice, the OT test is the standard bedside method for checking the adequacy of water chlorination. The reagent orthotolidine combines with chlorine in the sample and develops a yellow tint, and the depth of that colour reflects how much chlorine is present. A free residual chlorine level of $0.5$ mg/L maintained for a one-hour contact time is the accepted marker of effective disinfection, and the OT test is what confirms this in the field. Nitrites and nitrates are indicators of organic or fertiliser contamination and are measured by separate analytical procedures, while fluorine levels are checked to prevent fluorosis using fluoride-specific methods. None of these three are detected by orthotolidine. Therefore the only parameter linked to this test is chlorine. \[\boxed{\text{Chlorine}}\]