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Sale double double double

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A closer look

Sale double double double: at a glance

Structure of sodium and potassium tartrate.

A double room is the product of the union of two simple salts having a common anion and the different cation, or more rarely, the opposite.

Very rarely you can meet double salts having both the anion, and the different cation.

Examples

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Examples of double salt are:

sodium and potassium tartrate
the series of allumi, characterized by the general formula M^THEMM^III(SO₄)₂•12H₂O
the so-called salts of Tutton, many of which are characterized by the general formula M^THE₂MM^II(SO₄)₂•6H₂O
the alstonite
the carnallite

Recognition

The double salts, contrary to the complex salts, are dissociated in a solution exactly as the simple salts that make it. More simply the...Explore this chapter

Double salts, contrary to complex salts, dissociate in solution in exactly the same way as the simple salts that compose it would. More simply, double salts are recognizable as they have two metals.

For example, sulphate magnesium potassium dissociates in this way:

${\displaystyle {\mathrm {K} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {Mg} (\mathrm {SO} {\vphantom {A}}_{\smash[{t}]{4}}{\vphantom {A}}_{\smash[{t}]{2}}{\mathrel {\longrightarrow } {}2\,\mathrm {K} {\vphantom {A}}^{+}{}\mathrm {Mg} {\vphantom {A}}^{2+}{\,\mathrm {SO} {\vphantom {A}}_{\smash[t]{4}}{\vphantom {A}}^{2-}}}{2-}}}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{"}{\"}{\"}{\"}{\"}{\"}{"}{"}{\"}{"}{\"}{\"}{\"}{"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{\"}{$

exactly as if the potassium sulphate and magnesium sulphate were soluble:

${\mathrm {K} {\vphantom {A}}_{\smash[{t}]{2}}\mathrm {SO} {\vphantom {A}}_{\smash[{t}]{4}♪ I don't know what you mean ♪2\,\mathrm {K} {\vphantom {A}}^{+}{}+{}\mathrm {SO} {\vphantom {A}}_{\smash[{t]}{4}}{\vphantom {A}}^{2-}}$

${\mathrm {MgSO} {\vphantom {A}}_{\smash[{t}]{4}}{\mathrel {\longrightarrow} {mathrm {Mg} {\vphantom {A}}^{2+}{}\mathrm {SO} {\vphantom {A}}_{\smash[{t}]{4}}{\vphantom {A}}^{2-}}$

as you can see, in this case the SO anion₄²⁻ is in common, while there are two cations, potassium and magnesium.

Bibliography

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Chr. Balarew, Mixed Crystals and Double Salts between Metal (II) Salt Hydrates. Z. Krist. 181, 35-82 (1987).

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