The hardness of a water sample is measured directly by titration with $\text{0.01 M}$ solution of ethylenediamine tetraacetic acid $\text{(EDTA)}$ using eriochrome black $\text{T (EBT)}$ as an indicator. The $\text{EBT}$ reacts and forms complexes with divalent metallic cations present in the water. During titration, the $\text{EDTA}$ replaces the $\text{EBT}$ in the complex. When the replacement of $\text{EBT}$ is complete at the end point of the titration, the colour of the solution changes from
- blue-green to reddish brown
- blue to colourless
- reddish brown to pinkish yellow
- Wine red to blue