Double salt

A double salt is a salt that contains two distinct cations or two distinct anions. More specifically, the two cations (or anions) are not statistically distributed.^[1] 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.

Unit cell of the double salt ferrous ammonium sulfate (Mohr's salt or Mohrite). Color code: N is violet, O is red, S is orange, Fe is large red.

Examples

Examples of double salts with two cations

formula	cation A	cation B	anion	Comment
MM'(SO4)2·12H2O (alums)	[M(H2O)6]+	[M'(H2O)6]3+	SO2−4	many examples
BaCa(CO3)2 (mineral bromlite)	Ba2+	Ca2+	CO2−3	Although called a salt, it is insoluble in water
potassium sodium tartrate	Na+	K+	(CH(OH))2(CO−2)2	rare mixed Na-K salt
[NH4]2[Fe(H2O)6](SO4)2 (Mohr's salt)	[NH4]+	[Fe(H2O)6]2+	SO2−4	one of the Tutton's salts
K2Na[Co(NO2)6]·H2O	K+	Na+	[Co(NO2)6]3−	Sodium cobaltinitrite (Na3[Co(NO2)6) is highly soluble but NaK2[Co(NO2)6 is not.[2]

Examples of double salts with two anions

formula	cation	anion A	anion B	Comment
Pb2(CO3)Cl2 (mineral Phosgenite)	Pb2+	Cl−	CO2−3	Although called a salt, it is insoluble in water
(NH4)3PO4.(NH4)2HPO4	NH4+	PO3−4	HPO2−4	one of several ammonium phosphates[3]
K5(HSO3)3(S2O5)	K+	HSO−3	S2O2−5	A rare crystalline bisulfite (HSO3-).[4]

Counter examples

Many coordination complexes could be viewed as double salts, but they usually are not. Species like sodium ferrocyanide Na₄[Fe(CN)₆] are not classified as double salts. It contain the discrete hexacyanoferrate(II) ion [Fe(CN)₆]⁴⁻.^[5] Thus, it is simply a 4:1 salt. It is nontoxic by virtue of the fact that [Fe(CN)₆]⁴⁻ 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;^[6] the former involves a chemical combination with fixed composition, whereas the latter is a mixture.^[7]

Triple salts

An example of a triple salt is Oxone, which is a widely used disinfectant. With the formula 2KHSO₅·KHSO₄·K₂SO₄, Oxone features three distinct anions: HSO−5, HSO−4, and SO2−4. X-ray crystallography confirms the triple salt formulation, revealing hydrogen-bonding network that entraps the persulfate anion.^[8] The active ingredient in Oxone, KHSO₅ or potassium peroxysulfate, is less stable than the triple salt.