Determination of hardness of water using EDTA

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?

• Thank you for your edit, this clarified a lot. I retracted my close vote and fixed few typos. – andselisk Jan 2 '18 at 12:01

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.