Explain the principle of EDTA method .
1) EDTA or its sodium salt forms stable complex ion with Ca2+ or Mg2+( hardness producing ions ) in water .The titration is carried out in presence of indicator such as Eriochrome Black T ( EBT ) .
2) In a hard water sample ,the total hardness can be determined by titrating Ca2+ and Mg2+ present in an aliquot of the sample with Na EDTA solution ,using NH4Cl , NH4OH buffer solution of pH =10 and the metal indicator EBT .
3)At pH 10 ,EBT indicator forms wine red coloured unstable complex with Ca2+ or Mg2+ ions in hard water .
4)When EDTA is added from burette to this wine red complex ,there is formation ofmore stable Ca-EDTA but colourless complex . Near the end point , when all Ca2+ ions get complexed with EDTA ,indicator EBT ions set free giving blue colour to the solution .