To determine chlorine residuals in water, two common tests are typically employed: the orthotolidine test and the starch-iodide test. The correct answer is option C, which states that both tests (i and ii) are employed.

1. Orthotolidine Test:
The orthotolidine test is a colorimetric method used to determine chlorine residuals in water. In this test, orthotolidine reagent is added to the water sample containing chlorine residuals. The chlorine reacts with orthotolidine, resulting in the formation of a reddish-brown color. The intensity of the color is directly proportional to the concentration of chlorine residuals present in the water. The color can be visually compared to a standard color chart to determine the chlorine residual concentration.

2. Starch-Iodide Test:
The starch-iodide test is another commonly used method to determine chlorine residuals in water. In this test, a starch solution and potassium iodide (KI) are added to the water sample. Chlorine reacts with iodide ions in the presence of starch, resulting in the formation of a blue color. The intensity of the blue color is directly proportional to the concentration of chlorine residuals in the water sample. The color can be visually compared to a standard color chart to determine the chlorine residual concentration.

Why is the correct answer option C?

The correct answer is option C, which states that both tests (i and ii) are employed to determine chlorine residuals in water. This is because both the orthotolidine test and the starch-iodide test are commonly used and have their own advantages and limitations. Using both tests allows for cross-validation and confirmation of results, enhancing the accuracy and reliability of the chlorine residual measurement.

It is important to note that other tests can also be used to determine chlorine residuals in water, such as the DPD (N,N-diethyl-p-phenylenediamine) test and the amperometric titration method. These tests may offer different sensitivities, detection limits, and ease of use compared to the orthotolidine and starch-iodide tests. The choice of test method depends on factors such as cost, availability of reagents, equipment, and the desired level of accuracy and precision.