Residual chlorine in water is determined by

Residual chlorine in water can be determined using both the Starch iodide method and the Orthotolidine method.

The Starch iodide method involves adding a solution of starch and potassium iodide to the water sample containing residual chlorine. The residual chlorine oxidizes iodide ions to iodine, which then reacts with starch to form a blue complex, indicating the presence of residual chlorine.

The Orthotolidine method is another commonly used method for determining residual chlorine in water. In this method, orthotolidine (OT) reacts with residual chlorine to form a yellowish-brown color, which is then quantified spectrophotometrically.

Therefore, Option C: Both (A) and (B) is the correct choice, as both the Starch iodide method and the Orthotolidine method can be used to determine residual chlorine in water.