In case of qualitative analysis of anions, sodium carbonate extract is prepared for insoluble salts.

It is done by heating (for 10 minutes) an aqueous solution of a mixture of the solid salt and sodium carbonate in 1:4 ratio (by mass). As far as I understand, the principle behind preparing sodium carbonate extract is to prepare a clear solution containing the anion by precipitating the cation as insoluble oxide or carbonate.

But is this process applicable for salts that do not ionize at all in water, or covalent salts like $\ce{SnCl2}$, $\ce{SbCl3}$ etc.? I have never tried this myself.

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    $\begingroup$ You mean precipitate in reaction with sodium carbonate? "Extract" isn't correct, so edit it. $\endgroup$ – Mithoron Jan 7 '18 at 13:31
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    $\begingroup$ @Karl , All chemistry laboratory manuals in my state recommends this process. Otherwise I would not have written this. $\endgroup$ – Shoubhik R Maiti Jan 9 '18 at 4:54
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    $\begingroup$ Sorry, I remember now, after getting out old lecture notes. ;-) That's indeed the standard method ("Sodaauszug"=*soda extract*) for qualitative analysis of anions. de.wikipedia.org/wiki/Sodaauszug $\endgroup$ – Karl Jan 9 '18 at 18:13
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    $\begingroup$ Those chlorides are well soluble even in water, so that should work. $\endgroup$ – Karl Jan 9 '18 at 18:35
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    $\begingroup$ For BaSO4 and SrSO4, it doesn't work at all, their solubility is too low. You have to do what I proposed above, do it with soda in the melt, not in solution. $\endgroup$ – Karl Jan 9 '18 at 18:41

Soda extract is done to remove multivalent cations from your sample solution, by precipitating them as (mixed) carbonates and hydroxides.

Antimony or tin chlorides are well soluble even in water, and even if the solids have a strong covalent character, they are still ionic in solution.

The sulfates of barium, strontium and calcium are however very badly soluble, so it doesn't work for those. You can however do it in the melt, instead of solution. After cooling and powdering that melt, you add water. What dissolves are sodium sulfate and soda. Add $\ce{BaCl2}$ to detect the sulfate. The residue are the alkaline earth metal carbonates, which dissolve in $\ce{HCl}$.

(Please check your textbook/lab manual for the actual procedure. I have Jander, Blasius, Einführung in das anorganisch-chemische Praktikum. Perhaps someone else can recommend a good English book.)

  • $\begingroup$ By melt do you mean molten sodium carbonate? $\endgroup$ – Shoubhik R Maiti Jan 29 '18 at 6:36
  • $\begingroup$ Yes. In a sodium carbonate melt, the sulfates dissolve. $\endgroup$ – Karl Jan 29 '18 at 14:01
  • $\begingroup$ But I am confused by that. The wikipedia page shows that sodium carbonate does not melt but decomposes at about 800 degree celsius. Is it possible to heat the mixture to that high a temperature? Because glass should start to melt at that temperature. $\endgroup$ – Shoubhik R Maiti Jan 30 '18 at 3:59
  • $\begingroup$ The melting point of mixtures is depressed, works perfectly in a borosilicate test tube on a bunsen burner. $\endgroup$ – Karl Jan 30 '18 at 7:22
  • $\begingroup$ Ok, I understand now. $\endgroup$ – Shoubhik R Maiti Feb 5 '18 at 9:08

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