Principle of EDTA method-

The hardness of water can be determined by using EDTA method. EDTA is Ethylene diamine tetra acetic acid. It dissolves in water with great difficulty, but its disodium salt dissolve in water quickly & completely It is hexa dentate ligend. It binds the metal ions in water to give stable chelate complex. Hence it is called as complexometric titration method. This method is definitely preferable to the other method because of the greater accuracy, convenience & more rapid.

Ptinciple of EDTA method-

1. $Ca^{2+}$ & $Mg^{2+}$ + EDTA---- $\rightarrow$ Ca/Mg EDTA Stable complex

2. $Ca^{2+}$ & $Mg^{2+}$+EBT ------$\rightarrow$ Ca/Mg EBT $\rightarrow_{Unstable \ complex}$

This complex is unstable & quickly replaced by Ca EDTA complex pH=10

3) $Ca^{2+}/Mg^{2+}$ EBT +EDTA ------ $\rightarrow$ Ca/Mg EDTA + EBT

Unstable complex Titration Stable complex Blue

(Wine red color)