A double salt is a salt that contains more than one cation or more than one anion, and is obtained by combination of two different salts which were crystallized in the same regular ionic lattice. Examples of double salts include alums (with the general formula MIMIII[SO4]2·12H2O) and Tutton's salts (with the general formula [MI]2MII[SO4]2·6H2O). Other examples include potassium sodium tartrate, ammonium iron(II) sulfate (Mohr's salt), and bromlite. Aluminium sulfacetate, Al
4, is an example of a double salt with two distinct anions; another is the salt Tl
3 where hydroxyl-centred triangles of thallium, [Tl
, are a recurring structural motif.
Double salts should not be confused with complexes. When dissolved in water, a double salt completely dissociates into simple ions while a complex does not; the complex ion remains unchanged. For example, KCeF4 is a double salt and gives K+, Ce3+ and F− ions when dissolved in water, whereas K4[YbI6] is a complex salt and contains the discrete [YbI6]4− ion which remains intact in aqueous solutions. It is therefore important to indicate the complex ion by adding square brackets "[ ]" around it. Double salts are distinct from mixed-crystal systems where two salts cocrystallise; according to an Ida Freund publication from 1904 reprinted in 2014, the former involves a chemical combination with fixed composition, whereas the latter is a mixture.
In general, the properties of the double salt formed will not be the same as the properties of its component single salts.
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