Double salt

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).

[1] Other examples include potassium sodium tartrate, ammonium iron(II) sulfate (Mohr's salt), potassium uranyl sulfate (used to discover radioactivity) and bromlite BaCa(CO3)2.

When dissolved in water, a double salt acts as a mixture of the two separate salts: it completely dissociates into simple ions while a hexaaquo complex does not; the complex ion remains unchanged.

Similarly, potassium hexaiodoytterbate(II) K4[YbI6] is a complex salt and contains the discrete hexaiodoytterbate(II) ion [YbI6]4−, which remains intact in aqueous solutions.

Double salts are distinct from mixed-crystal systems where two salts cocrystallise;[2] the former involves a chemical combination with fixed composition, whereas the latter is a mixture.