Sodium potassium sulphite, NaKSO₃

When sodium hydrogen sulphite is neutralized by potassium hydroxide or carbonate, a double sulphite of sodium and potassium is formed. It is known as anhydrous salt, and also as monohydrate and dihydrate. For the heat of solution of the anhydrous form Hartog gives -1.19 Cal.

Rohrig and Schwicker claim to have prepared two isomeric forms of this salt, and attribute to them the constitutional formulae

Na-SO₂-OK and NaO-SO₂-K

Schwicker's procedure for making the compounds was to neutralize potassium hydrogen sulphite with sodium carbonate, and sodium hydrogen sulphite with potassium carbonate. The first isomeride is represented as potassium sodiosulphonate, and crystallizes with two molecules of water; the second is assumed to be sodium potassiosulphonate, and crystallizes with one molecule of water.

Barth failed to distinguish the supposed isomerides in aqueous solution, an illustration of his difficulty being the identity in their electric conductivities. Since both isomerides might yield the same ions, Barth's evidence is inconclusive; but his work on the mercury salts appears to confirm the assumption of the sulphonic formula.

Schwicker claimed to have prepared distinct derivatives by the action of ethyl iodide, but a repetition of his work by Fraps led to negative results. Arbusoff investigated the interaction of methyl iodide and the double sulphites obtained by Schwicker's method, the sodium atom in each instance being replaced by a methyl-group, with formation of the same compound,

CH₃•SO₃K.

If two isomerides do exist, that with potassium attached directly to sulphur must be unstable, and change readily into the isomeric form.