Double salts are formed when two cationic or anionic components can crystallize together. They appear as one pure substance but are a solid solution. Alum and Rochelle salt are examples used in cooking and other industries. Some double salts exhibit metallophilic interactions, which could be useful in the communications field.
Simple or “single” salts consist of a “cationic” or positively charged component and an “anionic” or negatively charged component, each salt resulting from the chemical combination of an acid with a base. A double salt is similar, except that it requires two specific conditions to be met. There must be either two cationic components or two anionic components and the two salts must be able to crystallize together in a regular fashion. This last property gives salt the appearance of one pure substance, rather than two separate substances. In reality, a double crystallized salt is a solid solution.
There are a number of known double salts found in cooking. One such double salt is alum, potassium aluminum sulfate dodecahydrate, KAl(SO4)2 12H2O. It is used to keep pickles crisp and is a slightly acidic component of some yeasts. Salt is also used in water treatment plants to accelerate sedimentation and improve water clarity through the process of flocculation, where small and unwanted particles – through the use of the electrical charge imparted by alum – are increased in size, allowing for easy filtration and removal. Alum is used as an astringent in emotional pencils and for its antibacterial and cathartic properties in other medical preparations.
Used in commercial food preparation, double salt sodium potassium tartrate, tetrahydrate, is better known as Rochelle salt – KNa(C4H4O6) 4H2O. It is most commonly used in the production of cheeses, jellies and fruit butters, although it is sometimes used as a dab in the cosmetics industry. Buffering is the property of maintaining a nearly 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 crushed; this phenomenon is called “piezoelectric effect”. Another historical use of Rochelle salt was in the silvering process of mirrors.
Of spectacular ongoing interest are double salts that exhibit metallophilic interactions. These interactions are closed-shell electron interactions in which the metals possess d10 and d8 electron configurations. In crystalline form, salt molecules are aligned by design to produce, in effect, a single-atom electrical wire or conduit. These strands are “insulated” by the organic portions – called ligands – of their double salt structures. Devices employing wires such as these are of particular interest in the communications field, but although they have been pursued for decades, practical application has not yet been achieved.
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