[nateireann]

Hello all,

let me introduce myself, I am a Chemical Engineering and I am working for an Abrasives Company. My boss asked me what is the best and simplest test to do to Calcium Carbonate and Calcium Sulphate to tell them appart.

I have been looking through the internet (google is not my friend anymore!) and in this forum and, sorry if I am wrong, but I didn't find the answer to my question.

The only thing I found is that CaCO₃ reacts with HCl generating bubbles (CO₂) and my question is,

what happens if we add HCl to Calcium Sulphate? because if there are no bubbles (and no explosion, of course!) I have my answer, the test that produces bubbles demonstrate that the sample is Calcium Carbonate, doesn't it?

I am really sorry for my English (I am Spanish and working in Ireland)

If I broke any rule of the forum, please, don't doubt in telling me.

Thank you very much for your time,
Nat

[Borek]

what happens if we add HCl to Calcium Sulphate?

Nothing.

[nateireann]

Thanks Borek, my boss will be glad

[AWK]

On the test scale you can selectively solubilize CaSO4 using concentrated ammonium sulfate solution.

Borek is not precisely right. Medium concentrated HCl also solubilize quite a lot of CaSO4 because of protolysis of SO₄^2- to HSO₄^- and ionic strength effect. Of course, diluted HCl increases solubility of CaSO4 insignificantly (which in pure water is of order 2 g/dm³, solubility of CaCO₃ in water is about 300 times lower).

[resc]

Hi Nateireann

I think if you add dilute HCl and then a drop of indicator (eg phenolhthalein), this will give off CO2 in the first instance and then turn pink if it is carbonate,

and add BaCl2, this will precipitate the sulfate, and the BaSO4 precipitate will not dissolve in dilute HCl.

Greetings from Ireland also!

[Borek]

I think if you add dilute HCl and then a drop of indicator (eg phenolhthalein), this will give off CO2 in the first instance and then turn pink if it is carbonate

It will not turn pink. pH will not go that far up.

[resc]

I think if you add dilute HCl and then a drop of indicator (eg phenolhthalein), this will give off CO2 in the first instance and then turn pink if it is carbonate

It will not turn pink. pH will not go that far up.

Hi

I have done this myself and it works. Get formation of HCO3-

Thanks

[Borek]

I think if you add dilute HCl and then a drop of indicator (eg phenolhthalein), this will give off CO2 in the first instance and then turn pink if it is carbonate

It will not turn pink. pH will not go that far up.

I have done this myself and it works. Get formation of HCO3-

Funny. Carbonates solutions have high pH, that's nothing new, but I was sure CaCO₃ solubility is too low to move pH into the phenolphthalein range. Seems my intuition was wrong and it is high enough.

But it looks like just saturated solution of CaCO₃ has pH around 9.9 (so it is pink), no need for HCl.

[AWK]

But it looks like just saturated solution of CaCO3 has pH around 9.9 (so it is pink), no need for HCl.

THis statement is true only for pure CaCO₃, not in the presence od CaSO₄

[nateireann]

Hi all!

first, thanks to all for your answers and help.

So, if I understood well, I have to add HCl to my samples, if there are bubbles, it means that the sample contains CaCO₃, what we can check adding phenolthalein and seeing if it turns pink. If there are no bubbles, sample is only CaSO₄, which we can check adding BaSO₄, generating a precipitate that will not dissolve in dilute HCl. Am I correct?

And my new question is, What concentration is medium concentrate? and dilute? I finished my Chemical Engineering studies 2 years and a half ago and I have been working as a programmer for Accenture (in Barclays Bank!!) and as Technical Support in natural gas operations and as Power Market analyst for Shell, so I have almost forgotten all my knowledge about chemical staff, what it is a shame because I love it!

thanks again and pleased to meet you!

[Rabn]

If you need to run tests on solids you can just add equal masses of each solid to an equal volume of distilled water and check the pH.  The carbonate will make the solution much more basic since it has a much smaller monoprotic Ka(weaker acid has a stronger conjugate base). If you're going to do this you should determine the mass you'll use and then use each substances respective Ksp and Ka to find what range you expect the pH to be. If you have solutions to test you can use copper II sulfate.  As long as the solutions have nearly equivalent molarity Copper II carbonate is not very soluble and will form a precipitate upon addition of the copperII sulfate while the sulfate solution shouldn't form one at all.

[resc]

which we can check adding BaSO₄, generating a precipitate that will not dissolve in dilute HCl.

Add BaCl2, which will form BaSO4 precipitate if it is CaSO4.