Double salt
Type of salt
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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.
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.