Which method is used to determine free available chlorine in water?

The DPD-colormetric method is a widely accepted technique for determining free available chlorine in water. This method involves the use of a reagent called N,N-diethyl-p-phenylenediamine (DPD), which reacts with chlorine to form a pink-colored complex. The intensity of the pink color, which is proportional to the concentration of free chlorine, can then be measured using a spectrophotometer or by visual comparison against a color chart.

This method is favored because of its sensitivity, accuracy, and the ability to deliver rapid results in various water types, including drinking water and swimming pools. It's especially effective in detecting low concentrations of chlorine, ensuring that water treatment processes can be monitored effectively.

Other methods listed, such as membrane filtration, ion chromatography, and turbidity measurement, are not specifically designed for measuring free available chlorine. Membrane filtration is primarily used for microbial detection, ion chromatography focuses on analyzing ions and charged species, and turbidity measurement assesses the clarity of water, not specific chemical concentrations. Thus, the DPD-colormetric method is uniquely suited for chlorine analysis, making it the correct choice.