The method uses a very large molecule called EDTA
which forms a complex with calcium and magnesium
ions. EDTA is short for ethylenediaminetetraacetic acid.
A blue dye called Eriochrome Black T (ErioT) is used
as the indicator. This blue dye also forms a complex
with the calcium and magnesium ions, changing
colour from blue to pink in the process. The dye–metal
ion complex is less stable than the EDTA–metal
ion complex. For the titration, the sample solution
containing the calcium and magnesium ions is reacted
with an excess of EDTA. The indicator is added and
remains blue as all the Ca2+ and Mg2+ ions present are
complexed with the EDTA.
A back titration is carried out