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This method, called a complexometric titration, is used to find the calcium content of milk, the ‘hardness' of water and the amount of calcium carbonate in various solid materials.
The method uses a very large molecule called EDTA which forms a complex with calcium ions. EDTA stands for ethylenediaminetetraacetic acid. A blue dye called Patton and Reeder's indicator (PR) is used as the indicator. This blue dye also forms a complex with the calcium ions changing colour from blue to pink/red in the process, but the dye–metal ion complex is less stable than the EDTA–metal ion complex. As a result, when the calcium ion–PR complex is titrated with EDTA the Ca2+ ions react to form a stronger complex with the EDTA.
For the titration, the indicator is added to the sample solution containing the calcium ions and forms the pink/red calcium ion-indicator complex (Ca-PR). This solution is then titrated with EDTA. The endpoint occurs when the solution turns blue, indicating that the Ca-PR complex has been completely replaced by the calcium ion-EDTA complex and the PR indicator reverts to its blue colour.
The reaction is:
Ca-PR + EDTA4- --> PR + [Ca-EDTA]2-
Note: Ca-PR is pink/red and PR is blue.
To download a printable version of this experiment (in pdf format) use the link below.
Calcium (PDF 84Kb)