Orthotolidine Test for Chlorine Residue in Water

The correct answer for testing chlorine residue in water is option 'A', the orthotolidine test. The orthotolidine test is a commonly used method to detect the presence of free residual chlorine in water. This test is based on the reaction between orthotolidine (OT) and free chlorine, which results in the formation of a yellow-brown color.

Here is a detailed explanation of the orthotolidine test for chlorine residue in water:

1. Principle:
The orthotolidine test is based on the oxidation of orthotolidine by free chlorine present in water. The reaction between orthotolidine and free chlorine produces a yellow-brown color, which is directly proportional to the chlorine concentration.

2. Procedure:
The procedure for the orthotolidine test involves the following steps:

a) Preparation of Reagents:
- Prepare a 0.1% orthotolidine solution by dissolving orthotolidine in a suitable solvent, such as glacial acetic acid.
- Prepare a 0.1 N sodium thiosulfate solution as a titrant.

b) Sample Collection:
- Collect a representative sample of water to be tested in a clean container.

c) Test:
- Take a measured volume of the water sample in a test tube.
- Add a few drops of the orthotolidine solution to the test tube.
- Mix the contents of the test tube thoroughly.
- Observe the color change in the solution.
- Compare the color with a standard color chart or use a colorimeter to measure the absorbance of the solution.
- Titrate the solution with sodium thiosulfate solution to determine the chlorine concentration accurately.

3. Interpretation:
The color developed in the test tube indicates the presence of free chlorine in the water sample. The intensity of the color is directly proportional to the chlorine concentration. A darker yellow-brown color indicates a higher concentration of chlorine.

4. Advantages of Orthotolidine Test:
- The orthotolidine test is simple and easy to perform.
- It provides a quick and qualitative estimation of free chlorine in water.
- The test does not require sophisticated equipment and can be performed in the field.

In conclusion, the orthotolidine test is a widely used method for testing chlorine residue in water. It is based on the reaction between orthotolidine and free chlorine, resulting in the formation of a yellow-brown color. This test is simple, quick, and provides a qualitative estimation of free chlorine in water.