# How to measure the amount of calcium in a supplement?

In chemistry lab, my group was assigned to determine the actual amount of calcium in a supplement. We did this successfully using HCl to dissolve the tablet, vacuum filtration, and then the addition of sodium carbonate. Next week, it has been suggested that we attempt it again using a different method. Thus, we are seeking the most interesting way, albeit using elementary tools and chemicals, that one might do this.

For the determination of calcium (at least) two methods are conceivable:

• gravimetric analysis
Sulfate is a rather bad choice here. As compared to the carbonate, the sulfate has a much higher solubility. A better choice would be the oxalate with a solubility around $6\,\mathrm{mg\,l^{-1}}$.

• photometry
Calcium forms a purple complex with the Arsenazo III dye that can be detected at wavelengths of 660 and 700 nm.
Alternatively, you can use o-cresolphthalein and detect the complex at 545 nm.

• The photometric methods are typically used in automated high-throughput clinical analysis of calcium in serum, but given that a photometer isn't available in the students' lab, Karl's comment on EDTA is exactly what you might be looking for. Complexometric titration is the keyword you might want to look for. Here, an indicator that forms a coloured complex with $\ce{Ca}$ is replaced by EDTA. The liberated indicator then shows a different colour.

• Titration with EDTA? – Karl Oct 29 '16 at 5:39
• @Karl Yes, that's definitely the classic. But there was a twist, wasn't it? $\ce{Mg}$ could be determined directly with Eriochrome Black T as the indicator, while $\ce{Ca}$ was run in a repacement titration because the binding to the indicator was somewhat sluggish. – Klaus-Dieter Warzecha Oct 29 '16 at 5:49
• I also remember something like that. But second semester is too long ago. I'd have to dig up Jander-Blasius in my basement. ;-) – Karl Oct 29 '16 at 5:56
• @Karl Same to me ;) You probably had the Merck broschure on complexometric titrations too :) – Klaus-Dieter Warzecha Oct 29 '16 at 6:02
• If I remember correctly, the real problem about the experiment of complexometric titration was that you had a mixture of Mg/Ca (at least at my school). If you only have Ca present there should not be any twist to it. – Martin - マーチン Nov 30 '16 at 7:34