How do I determine the amount of Calcium in a substance using EDTA titration?

Question

I need to determine the amount of calcium in a substance (that also contains Magnesium) using EDTA titration. From what I understand, this can be done by adding Eriochrome T indicator to the solution, then titrating it with EDTA. The solution will start out Red then turn blue as the Ca ions bond with the EDTA. Many of the labs I search up online seem to follow these procedures. However, doesn't Magnesium also bond with EDTA? Wouldn't that affect my calculations since I'd be measuring Ca AND Mg as they both bind with both Eri T and EDTA?

Thanks

Answer 1

Yes, EDTA will chelate to both calcium and magnesium. However, since the complex formation constant for $\ce{Ca^2+}$ is significantly greater than that for $\ce{Mg^2+}$ ($\log \beta_{f,\,\ce{Ca^2+}} = 10.65$; $\log \beta_{f,\,\ce{Mg^2+}} = 8.79$), we are justified in assuming that EDTA preferentially chelates to $\ce{Ca^2+}$ and only chelates to $\ce{Mg^2+}$ when no $\ce{Ca^2+}$ is available.

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Remark. How significant is "significant"? In principle we could avoid the approximation and solve for the system analytically, but then we would still have error from (i) imprecision in visual determination of endpoint, (ii) imprecision in determination of formation constants, and so on.