Double salts are salts containing more than one cation or anion, 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•6H2O) or Tutton salts (with the general formula [MI]2MII[SO4]2•6H2O). Other examples are potassium sodium tartrate and bromlite.
Note that double salts should not be confused with a complex. When dissolved in water, a double salt completely dissociates into simple ions while complexes do 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 a square bracket "[ ]" around it.
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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