Disodium ethylenediaminetetraactetic acid (Na2EDTA)
                     Na2EDTA |
Na2EDTA reacts with several cations to form stable chelate compounds. The reaction has a definite stiochiometry and so it is possible to use it as a technique to titrate for the amount of the cation present in the solution. All that is needed is a suitable indicator. This is not difficult as the pH or the conducitivity of the solution can in most cases be monitored easily. Of course there are also indicators that can be used, like Murexide.
Cations that can form chelate compounds with Na2EDTA are: Magnesium, calcium, bismuth, thorium, lead, zinc, cadmium, mercury, etc.
CALCIUM
Take a solution of calcium salt and adjust the ph to about 10. Add a few drops of Murexide. The complexes formed by the calcium ions with the indicator is salmon-pink in colour. Titrate the solution with a solution of disodium ethylenediaminetetraacetic acid of appropriate concentration. When all the calcium cation is chelated with the EDTA added the solution will return to colourless.
The mole of calcium cations present in the solution will be equivalent to the moles of EDTA used, since the stiochiometry of the reaction is 1:1.
Note: To standardise the Na2EDTA, use anhydrous calcium carbonate of high purity. The calcium carbonate should be heated for an hour at about 150°C to remove any traces of water before use.