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Are there any sodium salts (e.g. NaCl) that are insoluble in water? If so, what are their approximate $K_\mathrm{sp}$ values?

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From here:

Sodium tends to form water-soluble compounds, such as halides, sulfate, nitrate, carboxylates and carbonates. But examples of many insoluble sodium compounds such as sodium bismuthate ($\ce{NaBiO3}$), sodium octamolybdate ($\ce{Na2Mo8O25·4H2O}$), sodium thioplatinate ($\ce{Na4Pt3S6}$), and sodium uranate ($\ce{Na2UO4}$) is found in nature.

Other examples include:

  1. Feldspars(aluminosilicates of sodium, potassium, calcium)
  2. Sodium metaantimonate, $\ce{NaSbO3.7H2O}$ in the pyro form $\ce{Na2H2Sb2O7.H2O}$ has a solubility 0.7g/L.
  3. Sodium metaphosphate, $\ce{NaPO3}$ has both soluble and insoluble form.
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If your definition of a "sodium salt" is any solid compound that contains monovalent cationic sodium, then all you need to do is to look below your feet. The earth has abundant sodium bearing minerals (aka "salts").

Scroll down in The Mineralogy of Sodium to where it says Most widespread minerals containing Sodium. Out of all of these, only halite is soluble. The rest are mostly insoluble. I'm not sure their ksp values will mean anything here, because the dissolution of these minerals is up to kinetics and not thermodynamics at STP.

You can also have a look at Mineral Species containing Sodium (Na). Other than a few that are soluble (particularly some of the first on the list), the rest are not that soluble. Two reasons for that - they are silicates or phosphates which are in general insoluble, and they have other things in them.

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Cryolite, $\ce{Na3AlF6}$, is one such compound. https://en.wikipedia.org/wiki/Cryolite

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  • 1
    $\begingroup$ Of course,h ow did i not think of that,gj $\endgroup$ – Omkaar.K May 16 '18 at 16:57
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The salt $\ce{NaK2AlF6}$ ("elpasolite") is insoluble. Its exothermic precipitation is used as the basis for the thermometric titrimetric quantitative determination of sodium in a wide range of sample matrices. The determination is fast, accurate and highly precise.

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