Determining Total Hardness in Water by Complexometric Titration
- Calcium and magnesium salts dissolved in water cause water hardness.
- Water hardness can be measured using a titration with ethylenediaminetetraacetic acid (EDTA).
The ionised form of EDTA is shown on the right.
EDTA dissolved in water forms a colourless solution.
- At pH 10, calcium and magnesium ions form colourless, water soluble, complexes with EDTA:
calcium ion complexed by EDTA : CaEDTA2-
magnesium ion complexed by EDTA : MgEDTA2-
- An indicator, known as a metal ion indicator, is required for the titration.
Eriochromshwartz-T (Erio T) is a suitable indicator.
Eriochromshwartz-T is wine-red in hard water, but turns sky blue in the presence of free EDTA ions in the solution.
- Endpoint of the titration is when all the Ca2+ and Mg2+ ions have been complexed by the EDTA.
Before the endpoint, Ca2+ and Mg2+ are in excess, there is no free EDTA in solution.
Immediately after the endpoint, there is an excess of EDTA.
- It is essential that deionised water (water in which ions have been removed) is used in preparing all solutions, and in rinsing the conical flask.
Ca2+ and Mg2+ ions must NOT be present in the water used to prepare the solutions, or in rinsing.