Double salt

A double salt is a salt that contains two distinct cations or two distinct anions. More specifically, the two cations are not statistically distributed. Many examples are known. Double salts only exist in the solid state. For those that can be dissolved in water, they completely dissociate into simple ions. They have no characteristic appearance.

Examples

Examples of double salts with two cations

formula	cation A	cation B	anion	Comment
				many examples
				Although called a salt, it is insoluble in water
potassium sodium tartrate	Na⁺	K⁺		rare mixed Na-K salt
				one of the Tutton's salts
	K⁺	Na⁺		Sodium cobaltinitrite is highly soluble but is not.

Examples of double salts with two anions

formula	cation	anion A	anion B	Comment
				Although called a salt, it is insoluble in water
₃PO₄^.₂HPO₄	NH₄⁺			one of several ammonium phosphates
				A rare crystalline bisulfite.

Counter examples

Many coordination complexes could be viewed as double salts, but they usually are not. Species like sodium ferrocyanide are not classified as double salts. It contain the discrete hexacyanoferrate ion. Thus, it is simply a 4:1 salt. It is nontoxic by virtue of the fact that remains intact in solution, vs. releasing free and highly toxic cyanide. In many cases, the complex ion is indicated by square brackets "".
Double salts are distinct from mixed-crystal systems where two salts cocrystallise; the former involves a chemical combination with fixed composition, whereas the latter is a mixture.

Triple salts

An example of a triple salt is Oxone, which is a widely used disinfectant. With the formula, Oxone features three distinct anions:,, and. X-ray crystallography confirms the triple salt formulation, revealing hydrogen-bonding network that entraps the persulfate anion. The active ingredient in Oxone, or potassium peroxysulfate, is less stable than the triple salt.