Erichrome Black T is an indicator commonly used in the determination of water hardness. The indicator changes its color in the presence of certain metal ions, such as calcium and magnesium, which are the main contributors to water hardness.

Water hardness is a measure of the concentration of these metal ions in water. It is usually expressed in terms of calcium carbonate (CaCO3) equivalents. Hard water can cause various problems, such as scale formation in pipes and appliances, reduced effectiveness of soaps and detergents, and decreased efficiency of water heaters.

The determination of water hardness is important in various industries, including water treatment, agriculture, and manufacturing. It helps in assessing the quality of water and determining the appropriate treatment methods to reduce hardness if necessary.

The test for water hardness using Erichrome Black T as an indicator involves the following steps:

1. Sample collection: A representative sample of water is collected for analysis. The sample should be stored in a clean and sterile container to avoid contamination.

2. Preparation of reagents: The reagents required for the test are prepared. This usually includes a buffer solution to maintain a constant pH, a complexing agent to bind with metal ions other than calcium and magnesium, and Erichrome Black T as the indicator.

3. Titration: The prepared sample is titrated with a standardized solution of ethylenediaminetetraacetic acid (EDTA). EDTA is a chelating agent that forms stable complexes with metal ions, including calcium and magnesium.

4. Color change: As the EDTA solution is added to the sample, it complexes with the calcium and magnesium ions present in the water. When all the metal ions have reacted, the excess EDTA reacts with the Erichrome Black T indicator, causing a color change from red to blue.

5. Endpoint determination: The titration is continued until the color change is permanent, indicating that all the metal ions have been complexed. The volume of EDTA solution used is then measured.

6. Calculation of hardness: The hardness of the water is calculated based on the volume of EDTA solution used and the concentration of calcium and magnesium ions in the sample.

In conclusion, the test for water hardness employs Erichrome Black T as an indicator. This indicator undergoes a color change in the presence of calcium and magnesium ions, allowing for the determination of water hardness.