Which of the following methods are employed for determination of free and combined chlorine residuals in water?
1. Starch-iodide method
2. Orthotolidine method
3. Amperometric titration method
4. SNORT method
5. DPD method
Chlorine residual determination relies on specific biochemical and colorimetric indicators to measure how much
active disinfectant remains in water.
The Starch-iodide, Orthotolidine, SNORT (Stabilized Neutral Orthotolidine), and DPD (N,N-diethyl-p
phenylenediamine) methods are all standard laboratory and field procedures specifically designed to differentiate or
measure free and combined forms of residual chlorine. For example, the modern DPD method is highly favored
because it quickly turns a distinctive pink colour to distinguish free available chlorine from combined chloramines.
In contrast, the Amperometric titration method (statement 3) is a highly precise electrochemical technique used to
determine the total concentration of chlorine or oxidants in a sample rather than dynamically isolating the individual free
and combined residuals in general field testing setups. Therefore, methods 1, 2, 4, and 5 are the correct answers
grouped together.
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