Double salt

Mohrs zout

Mohrs zout

Ferric amm sulfate

Ferric amm oxalate

Ammonium alum crystals

Double salt refers to a type of chemical compound that forms through the combination of two distinct salts within the same crystal structure. These compounds are characterized by their unique ability to incorporate ions from both salts into a single crystalline entity. Double salts only maintain their integrity in the solid state; upon dissolution in water or other solvents, they dissociate into their constituent ions.

Formation and Characteristics

Double salts form when a solution containing two different salts is evaporated, leading to the crystallization of a compound that contains both types of ions. This process is influenced by the concentration of the ions, the temperature of the solution, and the ionic strength of the solution. The resulting crystal lattice of a double salt incorporates the ions of both salts in a fixed stoichiometric ratio, which is determined by the charges and sizes of the ions involved.

Examples

A well-known example of a double salt is Mohr's salt (ammonium iron(II) sulfate, \(\text{NH}_4\text{Fe}(\text{SO}_4\)_2·6\text{H}_2\text{O}\)), which is a combination of ammonium sulfate and iron(II) sulfate. Another example is Carnallite (\(\text{KCl}·\text{MgCl}_2·6\text{H}_2\text{O}\)), a naturally occurring mineral that is a double salt of potassium chloride and magnesium chloride.

Applications

Double salts have various applications, particularly in the field of analytical chemistry, where they are used in the preparation of standard solutions for titrations due to their stable and reproducible properties. They also find use in the agriculture industry as fertilizers, providing multiple nutrients to plants. In addition, double salts are utilized in the synthesis of other chemical compounds and in various industrial processes.

Distinction from Complex Salts

It is important to distinguish double salts from complex salts, as both can contain more than one type of ion. The key difference lies in their behavior in solution: double salts dissociate into all of their constituent ions, while complex salts dissociate into both simple ions and complex ions. Complex salts typically involve the formation of coordination complexes, which are not observed in the formation of double salts.

See Also

• Salt (chemistry)
• Crystal structure
• Ion
• Solubility