Simple or “single” salts consist of one “cationic,” or positively charged, component and one “anionic,” or negatively charged component — each salt resulting from the chemical combining of an acid with a base. A double salt is similar, except it requires two specific conditions be met. There must be either two cationic components or two anionic components, and the two salts must be capable of crystallizing together in regular fashion. The latter property imparts to the salt an appearance of being one pure substance, rather than two separate substances. In reality, a crystallized double salt is a solid solution.
There are a number of well-known double salts found in the kitchen. One of these double salts is alum, chemically potassium aluminum sulfate dodecahydrate — KAl(SO4)2·12H2O. It is used to keep pickles crisp and is a mildly acidic component of some baking powders. The salt is also used in water treatment plants to hasten sedimentation and improve water clarity through the process of flocculation, in which small, undesirable particles — through the use of electric charge imparted by the alum — are increased in size, enabling easy filtration and removal. Alum is used as an astringent in styptic pencils, and for its antibacterial properties and cathartic properties in other medical preparations.
Used in commercial food preparation, the double salt potassium sodium tartrate, tetrahydrate, is better known as Rochelle salt — KNa(C4H4O6)·4H2O. It is most commonly used in the manufacture of cheeses, jellies and fruit butters, although it is sometimes used for buffering in the cosmetics industry. Buffering is the property of maintaining a near-constant pH in the presence of incremental additions of acids or bases. This salt was one of the first substances known to produce electricity when its crystals are squeezed; this phenomenon is called the “piezoelectric effect.” Another historic use of Rochelle salt was in the process of silvering mirrors.
Of spectacular ongoing interest are double salts that exhibit metallophilic interactions. These interactions are closed-shell electronic interactions in which the metals possess d10 and d8 electron configurations. In crystalline form, the salt molecules are aligned by design to produce, in effect, single-atom-thick electrical wire or conduit. Such wires are “insulated” by the organic portions — called ligands — of their double salt structures. Devices employing wires such as these are of special interest to the field of communications, but though pursued for decades, practical application has not yet been achieved.