The EDTA titration actually measures the combined concentration of calcium and magnesium ions, which are both commonly found in tap water.
Why does this procedure not distinguish between the two ions?
Outline a modification to the current procedure that could be used to analyze for these two ions separately?
Both Ca and Mg are in +2 oxidation state, belongs to the same group in the periodic table and have similar reactivity. Therefore, they both react in similar fashion with EDTA in solution.
Thus it is not possible to identify the two ions separately when present in solution with EDTA reagent. A total of Mg+Ca in solution is determined here.
We can use different pH values for the solution, which would result in precipitation of the two ions at different times from the solution. EDTA titration is greatly pH dependent. The more stable complex is formed first.
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