An indicator is added that forms a colored complex with some of the metal ion being determined. When the endpoint is reached, all of the free metal ion has been complexed by the EDTA. The last addition of EDTA then removes the metal ion from its complex with the indicator, producing a color change. The relative strength of the EDTA-metal ion complex compared to that of the indicator-metal ion complex determines whether a direct titration or a back-titration is applicable.

EDTA is most commonly used to determine hardness (i.e., Calcium and Magnesium ions) in water, where the pH must be approximately 10 and is usually controlled by adding a buffer such as Water Hardness Buffer. Water Hardness Indicator (Eriochrome Black T), Eriochrome Blue Black R, Calmagite, and Murexide are used for hardness or Calcium determinations using EDTA.