I have learned that sodium can be precipitated practically quantitatively as the mixed sodium-caesium hismuthinitrite (hexanitritobismuthate(III)) while potassium remains dissolved. hm...
@mona this is useful
@whitequark is it? *chuckles* I have a major fascination for these "wet" methods of analytical #chemistry but they are somewhat outmoded professionally. As an amateur however I take an interest in low-tech methods of analysis.
@mona not even necessarily analysis, I can imagine wanting to selectively precipitate sodium. Or at least I definitely wanted that for something in high school... so long ago I no longer remember what for
@whitequark I've had a very personal, strange fascination with chemical reagents that precipitate the alkali metals, all the weird little exceptions to the usual rules of solubility that everyone learns in high school or freshman #chemistry classes: all sodium and potassium salts are soluble. The classical reagents for precipitating sodium were uranyl acetate, usually in conjunction with magnesium or zinc acetates; and potassium "pyroantimonate", really KSb(OH)6 I believe.
@whitequark but then I kept poking at the scientific literature over the years and found more. A peculiar and unstable acid formed from careful oxidation of tartaric acid, "diketosuccinic acid" or tetrahydroxysuccinic acid rather, is another good reagent for sodium. So is the O-methyl ester of mandelic acid and a few related organic acids. None of these is particularly accessible without a bit of work, but the bismuthinitrite reagent is quite simple in comparison.