A double salt is a

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

that contains two or more different

cations An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by con ...

anions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

. Examples of double salts include alums (with the general formula ) and Tutton's salts (with the general formula ). Other examples include potassium sodium tartrate,

ammonium iron(II) sulfate Ammonium iron(II) sulfate, or Mohr's salt, is the inorganic compound with the formula (NH4)2Fe(SO4)2(H2O)6. Containing two different cations, Fe2+ and NH4+, it is classified as a double salt of ferrous sulfate and ammonium sulfate. It is a co ...

(Mohr's salt), potassium uranyl sulfate (used to discover radioactivity) and bromlite . The fluorocarbonates contain fluoride and carbonate anions. Many

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. ...

es form double salts. Double salts should not be confused with complexes. Double salts only exist in the solid. 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 A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

remains unchanged. Similarly, potassium hexaiodoytterbate(II) is a complex salt and contains the discrete hexaiodoytterbate(II) ion , which remains intact in aqueous solutions. In many cases, the

is indicated by square brackets " . Double salts are distinct from mixed-crystal systems where two salts

cocrystal Cocrystals are "solids that are crystalline single phase materials composed of two or more different molecular or ionic compounds generally in a stoichiometric ratio which are neither solvates nor simple salts." A broader definition is that cocryst ...

lise; 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.

Gallery

References

* {{Chem-stub