[curiouscat]

Given a mixture of Na2CO3 + Na2SO4 + CaCl2 as solids what would be a good way to differentially estimate the three?

Doesn't need to be too accurate. 5% accuracy would be fine.

[Corribus]

There's probably some fancy reagent based method, but the most direct analytical way would be ICP-MS or ICP-AES. Calcium and sodium are simple. Sulfur is tricky but doable. I think EPA has ICP-based methods validated for sulfate quantification in drinking water.

You might be able to get at carbonate by TGA because carbonates are susceptible to thermal decomposition (so are nitrates for that matter), but I think the temperature for sodium carbonate is extremely high, maybe over 1000 C. If you could do some kind of ion exchange and make everything calcium, TGA would be a piece of cake to determine the carbonate content.

You could also probably get at the carbonate content by treatment with acid. So, maybe you could get the easy CaCl₂ content by ICP-MS and then measure the volume of CO₂ liberated when treated with sulfuric acid or something. 

[curiouscat]

Thanks very much @corribus.

PS. An ICP-MS is fancy enough for me. I'll probably look for an outside lab to do it using ICP-MS. I was wishfully thinking there was an old-school titration based method.

[Borek]

Calcium with EDTA, carbonate with HCl, whatever is left is sodium sulfate?

[curiouscat]

Calcium with EDTA, carbonate with HCl, whatever is left is sodium sulfate?

Elegant. I will try this. It ought to suffice for our purposes.

Thanks @Borek.

[Hunter2]

I am not sure it is so easy. If this mixture gets wet Calcium carbonate and Calcium sulfate will precipitate depending of there solubility. The solution mainly containing sodium chloride, but also small amount of sulfate and carbonate. Most Calcium is precipitated.

[Borek]

Good point. However, whether it will interfere or not depends on the initial composition. To be sure you can go for backtitration for carbonates (using excess HCl and boiling the solution to make sure you got rid of CO₂, then titrating with NaOH).

While calcium sulfate has low solubility, its saturated solution is around 0.007 M, and can be easily titrated with EDTA. Unless you have a huge excess of sulfate it should be not a problem.