Amperometric methods in water analysis are used to measure chlorine residual.

• A. Chlorine residual
• B. Turbidity
• C. Temperature
• D. pH

Answer: (A)

Amperometric sensing translates the amount of an oxidizing species in a sample into an electrical current by driving a redox reaction at an electrode. For chlorine residual, the chlorine species present (Cl2, HOCl, OCl−) undergo a redox reaction at the sensing electrode when a fixed potential is applied, producing a current that is proportional to how much chlorine is present. This direct current–to–concentration relationship makes amperometric methods ideal for online monitoring of residual chlorine in water treatment and distribution. Turbidity is read by how light is scattered or transmitted, not by electrochemical current; temperature is measured with thermistors/RTDs; pH is measured with a glass electrode that detects hydrogen-ion activity, not a redox current.