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.
Examples
Examples of double salts with two cations
| formula | cation A | cation B | anion | Comment |
|---|---|---|---|---|
| Page Module:Chem2/styles.css has no content.MM'(SO4)2·12H2O (alums) | Page Module:Chem2/styles.css has no content.[M(H2O)6]+ | Page Module:Chem2/styles.css has no content.[M'(H2O)6]3+ | Page Module:Chem2/styles.css has no content.SO2−4 | many examples |
| Page Module:Chem2/styles.css has no content.BaCa(CO3)2 (mineral bromlite) | Page Module:Chem2/styles.css has no content.Ba2+ | Page Module:Chem2/styles.css has no content.Ca2+ | Page Module:Chem2/styles.css has no content.CO2−3 | Although called a salt, it is insoluble in water |
| potassium sodium tartrate | Na+ | K+ | Page Module:Chem2/styles.css has no content.(CH(OH))2(CO−2)2 | rare mixed Na-K salt |
| Page Module:Chem2/styles.css has no content.[NH4]2[Fe(H2O)6](SO4)2 (Mohr's salt) | Page Module:Chem2/styles.css has no content.[NH4]+ | Page Module:Chem2/styles.css has no content.[Fe(H2O)6]2+ | Page Module:Chem2/styles.css has no content.SO2−4 | one of the Tutton's salts |
| Page Module:Chem2/styles.css has no content.K2Na[Co(NO2)6]·H2O | K+ | Na+ | Page Module:Chem2/styles.css has no content.[Co(NO2)6]3− | Sodium cobaltinitrite (Page Module:Chem2/styles.css has no content.Na3[Co(NO2)6]) is highly soluble but Page Module:Chem2/styles.css has no content.NaK2[Co(NO2)6] is not.[2] |
Examples of double salts with two anions
| formula | cation | anion A | anion B | Comment |
|---|---|---|---|---|
| Page Module:Chem2/styles.css has no content.Pb2(CO3)Cl2 (mineral Phosgenite) | Page Module:Chem2/styles.css has no content.Pb2+ | Page Module:Chem2/styles.css has no content.Cl− | Page Module:Chem2/styles.css has no content.CO2−3 | Although called a salt, it is insoluble in water |
| (NH4)3PO4.(NH4)2HPO4 | NH4+ | Page Module:Chem2/styles.css has no content.PO3−4 | Page Module:Chem2/styles.css has no content.HPO2−4 | one of several ammonium phosphates[3] |
| Page Module:Chem2/styles.css has no content.K5(HSO3)3(S2O5) | Page Module:Chem2/styles.css has no content.K+ | Page Module:Chem2/styles.css has no content.HSO−3 | Page Module:Chem2/styles.css has no content.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 Page Module:Chem2/styles.css has no content.Na4[Fe(CN)6] are not classified as double salts. It contain the discrete hexacyanoferrate(II) ion Page Module:Chem2/styles.css has no content.[Fe(CN)6]4−.[5] Thus, it is simply a 4:1 salt. It is nontoxic by virtue of the fact that Page Module:Chem2/styles.css has no content.[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 Page Module:Chem2/styles.css has no content.2KHSO5·KHSO4·K2SO4, Oxone features three distinct anions: Page Module:Chem2/styles.css has no content.HSO−5, Page Module:Chem2/styles.css has no content.HSO−4, and Page Module:Chem2/styles.css has no content.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, Page Module:Chem2/styles.css has no content.KHSO5 or potassium peroxysulfate, is less stable than the triple salt.
References
Page Template:Reflist/styles.css has no content.
- ^ Page Module:Citation/CS1/styles.css has no content.Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, p. 120, ISBN 0-12-352651-5
- ^ Page Module:Citation/CS1/styles.css has no content.Vogel, A. I. (1951). Quantitative Inorganic Analysis (2nd ed.). Longmans Green and Co.
- ^ Script error: No such module "Template wrapper".
- ^ Page Module:Citation/CS1/styles.css has no content.Chen, I C.; Wang, Y. (1984). "Reinvestigation of potassium pyrosulfite, K2S2O5". Acta Crystallographica Section C Crystal Structure Communications. 40 (11): 1780–1781. Bibcode:1984AcCrC..40.1780C. doi:10.1107/S0108270184009525.
- ^ Page Module:Citation/CS1/styles.css has no content.Housecroft, C. E.; Sharpe, A. G. (2008). Inorganic Chemistry (3rd ed.). Prentice Hall. ISBN 978-0-13-175553-6.
- ^ Page Module:Citation/CS1/styles.css has no content.Balarew, Christo (1987). "Mixed crystals and double salts between metal(II) salt hydrates". Zeitschrift für Kristallographie. 181 (1–4): 35–82. Bibcode:1987ZK....181...35B. doi:10.1524/zkri.1987.181.1-4.35.
- ^ Page Module:Citation/CS1/styles.css has no content.Freund, Ida (2014) [1904]. "Mitscherlich and the Connection between Crystalline Form and Chemical Composition". The Study of Chemical Composition: An Account of its Method and Historical Development with Illustrative Quotations. Cambridge University Press. pp. 385–453. ISBN 9781107690301.
- ^ Page Module:Citation/CS1/styles.css has no content.Ermer, Otto; Röbke, Christof (2003). "Crystal Structure and Chemical Stabilization of the Triple Salt (KHSO5)2⋅KHSO4⋅K2SO4". Helvetica Chimica Acta. 86 (8): 2908–2913. doi:10.1002/hlca.200390238.
