Determination of hardness of water using EDTA

Question

Finding hardness of water by using EDTA. For determining permanent hardness we take the sample of about $\pu{50 mL}$ and then add Eriochrome Black T (EBT) indicator that results in wine red color due to calcium complex made by and also ammonia buffer. Then titrating it with ethylene diamine tetra acetic acid (EDTA) where change in color (i.e. sky blue) marks the end point of titration.

While carrying out experiment we add EBT as an indicator. Is there any criteria to add so in a fixed particular amount?

Answer 1

There really is no need to add EBT in a fixed amount. Calcium EDTA complex is far more stable at $\mathrm{pH} \approx 10$ (ammonia buffer) and will be the dominant Ca-compound in the solution even if there is a huge excess of EBT.

However, the amount of EBT is important to track the color change. If there is too much EBT, the color will be too dark, and one can easily miss the red-blue transition. If there are only trace amounts of EBT, the colors will be pale and again the transition is going to be barely visible.

The amounts of EBT given in the lab manuals are typically approximate and are estimated for the convenience of the personnel and to reduce the error in determining equivalence point due to unclear reading of the solution's color.