Double salt

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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.

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

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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

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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

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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.

References

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  1. ^ 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
  2. ^ Vogel, A. I. (1951). Quantitative Inorganic Analysis (2nd ed.). Longmans Green and Co.
  3. ^ Schrödter, Klaus; Bettermann, Gerhard; Staffel, Thomas; Wahl, Friedrich; Klein, Thomas; Hofmann, Thomas (2008). "Phosphoric Acid and Phosphates". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_465.pub3. ISBN 978-3-527-30673-2.
  4. ^ 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.
  5. ^ Housecroft, C. E.; Sharpe, A. G. (2008). Inorganic Chemistry (3rd ed.). Prentice Hall. ISBN 978-0-13-175553-6.
  6. ^ 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.
  7. ^ 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.
  8. ^ 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.