doppelsalze

Doppelsalze, also known as double salts, are crystalline compounds that contain two different cations or two different anions (or both) in a fixed stoichiometric ratio, along with a common anion. They are formed by the crystallization of two different simple salts from a single solution. In the solid state, the ions are arranged in a regular lattice structure. Unlike double salts, double oxides are formed by combining two metal oxides.

A key characteristic of double salts is that their constituent ions can be detected individually in solution using standard qualitative tests. For instance, if a double salt containing potassium and magnesium cations is dissolved in water, both potassium ions (K+) and magnesium ions (Mg2+) will be present and can be identified. This distinguishes them from complex salts, where the central metal ion is bound to ligands and its individual properties are masked.

Common examples of double salts include alums, which have the general formula M⁺Al(SO₄)₂·12H₂O, where M⁺ is a univalent cation such as potassium (K⁺) or ammonium (NH₄⁺). Another well-known example is carnallite, a mineral with the formula KCl·MgCl₂·6H₂O. When dissolved, carnallite dissociates into K⁺, Mg²⁺, and Cl⁻ ions. The properties of double salts are typically intermediate between those of the individual salts that form them. They crystallize as a single phase, indicating a distinct chemical entity rather than a simple mixture.