Question

When Mg-EDTA is added to a sample to compensate for a lack of Mg2⁺, why is no blank correction necessary?

Answer 1

Usually, the concentration of EDTA is much lower than that of magnesium so that the significant part of magnesium ions remain unbound while EDTA exists as a Mg²⁺-EDTA complex. The undesirable metal ions have much higher affinity to EDTA than magnesium, often it is higher by many orders of magnitude. Therefore, even if trace amount of such a contaminant is present in your mixture, it will replace magnesium as EDTA ligand. In the titration the indicator (Eriochrome Black T (EBT),) itself also a chelating agent. As EDTA is added the free metal cation is complexed first because the Mg-EDTA complex is more stable than the Mg(ln)^-. Therefore a very small amount of Mg⁺² is added as Mg-EDTA for a sharper endpoint. This small amount has an insignificant effect on the final result. Hence we need not to have a blank correction.

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