Tests usually employed to determine chlorine residuals in water:

There are several tests that can be employed to determine chlorine residuals in water. Two commonly used tests are the orthotolidine test and the starch-iodide test.

(i) Orthotolidine Test:
- The orthotolidine test is a colorimetric test used to determine the presence and concentration of chlorine residuals in water.
- In this test, orthotolidine reagent is added to the water sample. If chlorine is present, it reacts with orthotolidine to form a yellow color.
- The intensity of the yellow color is directly proportional to the concentration of chlorine in the water.
- The test results can be determined by comparing the color of the water sample with a standard color chart.

(ii) Starch-Iodide Test:
- The starch-iodide test is another colorimetric test used to determine the presence and concentration of chlorine residuals in water.
- In this test, a solution containing potassium iodide and starch is added to the water sample. If chlorine is present, it reacts with the iodide ions to form iodine.
- The iodine then reacts with the starch, forming a blue-black color.
- The intensity of the blue-black color is directly proportional to the concentration of chlorine in the water.
- The test results can be determined by comparing the color of the water sample with a standard color chart.

Answer:
The correct answer is option 'C' - both (i) and (ii).
Both the orthotolidine test and the starch-iodide test are commonly employed to determine chlorine residuals in water. These tests are based on colorimetric reactions that produce distinct colors in the presence of chlorine. By comparing the color of the water sample with a standard color chart, the concentration of chlorine residuals can be determined. Therefore, both tests are useful in assessing chlorine levels in water.