Water Hardness by EDTA Titration

Determination of Calcium

chem 128 September 20, 2002

Web document edited: September 17, 2001


To determine the Calcium content of a sample such as tap water, milk.


Titrate the sample with the chelating agent known as EDTA.

  • Remember how ammonia, NH3, acts as a ligand because it can offer a lone pair of electrons for bond formation.

  • Ethylenediamine can be though of as two ammonia molecules bound together with a two atom carbon change. Each nitrogen offers a pair of electrons so each is capable of forming a coordinate covalent bond with metal ions.

  • Now remove each of the hydrogens and replace with a bound acetate ion, CH3CO2-. One of the oxygens on the acetate is capable of forming another bond to the metal ion. (Actually there are two oxygen atoms with four electron pairs, but only one can be oriented towards the metal ion. Note that the acetate ion loses one H to form the C-N bond.))

  • Since there are four such acetate ions, we now have six bonds to the metal ion. The EDTA literally wraps around the metal ion and binds it quite strongly.

    Color Indicator:

    Experimental Directions: