Question

which buffer solution is added during determination of hardness of water by edta titration and why

Answer 1

When determining the hardness of water an indicator dye is used which gives either a pink color to the solution when Mg2+ and Ca2+ are still present, or a blue color when all of the metal ions have reacted with the EDTA4-. 

As far as I know the pH needs to be at least 10 for the EDTA to let go of its H+ ions so we get the EDTA4- solution we need for the reaction with Mg2+ and Ca2+.  

The reaction [EDTA --> EDTA4- + 4H+] by itself will change the pH of the total solution (it's releasing H+ ions, after all) and we're constantly adding EDTA-solution until the solution is blue and all hints of pink have gone. For this reason we need a buffer which will keep the total solution at pH10 even if we have to add considerable amounts of EDTA.