Double salt

{{Short description|Type of salt}}

[[File:MohriteD.jpg|thumb|right|244px|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.<ref>{{Holleman&Wiberg|page=120}}</ref> 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 {| class="wikitable" |+ Some double cation salts| ! formula !! cation A !! cation B !! anion!! Comment |- |{{chem2|MM'(SO4)2*12H2O}} (alums) || {{chem2|[M(H2O)6]+}}|| {{chem2|[M'(H2O)6](3+)}} || {{chem2|SO4(2-)}}||many examples |- | {{chem2|BaCa(CO3)2}} (mineral bromlite) || {{chem2|Ba(2+)}} || {{chem2|Ca(2+)}} || {{chem2|CO3(2-)}}||Although called a salt, it is insoluble in water |- | potassium sodium tartrate || Na⁺ || K⁺ || {{chem2|(CH(OH))2(CO2-)2}}||rare mixed Na-K salt |- || {{chem2|[NH4]2[Fe(H2O)6](SO4)2}} (Mohr's salt)|| {{chem2|[NH4]+}}|| {{chem2|[Fe(H2O)6](2+)}}||{{chem2|SO4(2-) }}||one of the Tutton's salts |- |{{chem2|K2Na[Co(NO2)6]*H2O}}||K⁺||Na⁺||{{chem2|[Co(NO2)6](3-)}}||Sodium cobaltinitrite ({{chem2|Na3[Co(NO2)6]}}) is highly soluble but {{chem2|NaK2[Co(NO2)6]}} is not.<ref name = "Vogel">{{cite book|first=A. I. |last=Vogel|date=1951 |title=Quantitative Inorganic Analysis |edition=2nd|publisher=Longmans Green and Co.}}</ref> |} Examples of double salts with two anions {| class="wikitable" |+ Some double anion salts| ! formula !! cation!! anion A !! anion B!! Comment |- |{{chem2|Pb2(CO3)Cl2}} (mineral Phosgenite)|| {{chem2|Pb(2+)}}|| {{chem2|Cl-}} || {{chem2|CO3(2-)}}|| Although called a salt, it is insoluble in water |- |(NH₄)₃PO₄^.(NH₄)₂HPO₄||NH₄⁺||{{chem2|PO4(3-)}}||{{chem2|HPO4(2-)}}||one of several ammonium phosphates<ref name=Ullmann>{{Ullmann|first1=Klaus|last1=Schrödter|first2=Gerhard|last2=Bettermann|first3=Thomas |last3=Staffel|first4=Friedrich|last4=Wahl|first5=Thomas|last5=Klein||first6=Thomas|last6=Hofmann|title=Phosphoric Acid and Phosphates|year=2008|doi=10.1002/14356007.a19_465.pub3}}</ref> |- |{{chem2|K5(HSO3)3(S2O5)}}||{{chem2|K+}}||{{chem2|HSO3-}}||{{chem2|S2O5(2-)}}||A rare crystalline bisulfite (HSO₃^-).<ref name=chen>{{cite journal |last1=Chen |first1=I C. |last2=Wang |first2=Y. |title=Reinvestigation of potassium pyrosulfite, K2S2O5 |journal=Acta Crystallographica Section C Crystal Structure Communications |date=1984 |volume=40 |issue=11 |pages=1780–1781 |doi=10.1107/S0108270184009525 |bibcode=1984AcCrC..40.1780C }}</ref> |}

==Counter examples== Many coordination complexes could be viewed as double salts, but they usually are not. Species like sodium ferrocyanide {{chem2|Na4[Fe(CN)6]}} are not classified as double salts. It contain the discrete hexacyanoferrate(II) ion {{chem2|[Fe(CN)6](4−)}}.<ref name=InorgChem>{{Housecroft3rd}}</ref> Thus, it is simply a 4:1 salt. It is nontoxic by virtue of the fact that {{chem2|[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;<ref>{{cite journal|title= Mixed crystals and double salts between metal(II) salt hydrates|last= Balarew|first= Christo|journal= Zeitschrift für Kristallographie|volume= 181|issue= 1–4|pages= 35–82|doi= 10.1524/zkri.1987.181.1-4.35|year= 1987|bibcode= 1987ZK....181...35B}}</ref> the former involves a chemical combination with fixed composition, whereas the latter is a mixture.<ref>{{cite book|chapter-url= https://books.google.com/books?id=9ttkAwAAQBAJ&pg=PA448|title= The Study of Chemical Composition: An Account of its Method and Historical Development with Illustrative Quotations|first= Ida|last= Freund|authorlink= Ida Freund|publisher= Cambridge University Press|orig-year=1904|year=2014|isbn= 9781107690301|chapter= Mitscherlich and the Connection between Crystalline Form and Chemical Composition|pages= 385–453}}</ref>

==Triple salts== An example of a '''triple salt''' is Oxone, which is a widely used disinfectant. With the formula {{chem2|2KHSO5*KHSO4*K2SO4}}, Oxone features three distinct anions: {{chem2|HSO5-}}, {{chem2|HSO4-}}, and {{chem2|SO4(2-)}}. X-ray crystallography confirms the triple salt formulation, revealing hydrogen-bonding network that entraps the persulfate anion.<ref>{{cite journal |last1=Ermer |first1=Otto |last2=Röbke |first2=Christof |title=Crystal Structure and Chemical Stabilization of the Triple Salt (KHSO₅)₂⋅KHSO₄⋅K₂SO₄ |journal=Helvetica Chimica Acta |date=2003 |volume=86 |issue=8 |pages=2908–2913 |doi=10.1002/hlca.200390238}}</ref> The active ingredient in Oxone, {{chem2|KHSO5}} or potassium peroxysulfate, is less stable than the triple salt.

==References== {{Reflist}}

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