ndicator used in the estimation of hard watera.all of the aboveb.EDTAc.Ammonia bufferd.EBT

The indicator used in the estimation of hard water is Eriochrome Black T (EBT). So, the correct answer is d. EBT.

Here's a step-by-step explanation:

1. Hard water contains multivalent cations such as calcium and magnesium.

2. To estimate the hardness of water, a complexometric titration is used. This is a form of volumetric analysis where the formation of a colored complex is used to indicate the end point of the titration.

3. Ethylenediaminetetraacetic acid (EDTA) is used as the titrant. It forms a complex with the calcium and magnesium ions in the water.

4. Eriochrome Black T (EBT) is used as the indicator. It is initially blue and forms a wine-red complex with the calcium and magnesium ions.

5. As EDTA is added, it reacts with the calcium and magnesium ions. When all the calcium and magnesium ions have reacted with the EDTA, there are none left to form a complex with the EBT.

6. At this point, the EBT changes from wine-red to blue, indicating the end point of the titration and hence the hardness of the water.