Many complex ions are coloured because the energies required for the electronic transitions within their partially-filled d-subshells lie in the visible region of the electromagnetic spectrum.
The wavelength of light absorbed by the crystal and hence its colour will vary both with co-ordination number and ligand type, resulting in a change in colour of the solution when a new ligand is added.
Part I: Colour of complex ion
Water dissolves many ions as it flows through minerals. Although water hardness is defined as the quantity of cations with a +2 or +3 charge, calcium ion and magnesium ion are the most common of such ions in natural water.
The quantity of “hardness ions” will be determined by titration. Dihydrogen-ethylenediaminetetraacetate (EDTA). The indicator added to the sample is Eriochrome Black T. The indicator will form a complex with the cations and give a red-coloured solution. As the EDTA is added drop-wise to the sample, it replaces the Eriochrome Black T and forms more stable complexes with calcium and magnesium. When the indicator is released by the metal ions, it has a distinct blue colour. Therefore, the endpoint of the titration is marked by the colour change form red to blue.
Part II: Determination of water hardness