This acid belongs to a large class of compounds known as the heteropolyacids, of which the structures and exact formulæ have long been a subject for speculation. The heteropolyacids are compounds in which one atom of such elements as P, Si, As, B, Al, etc., is combined with a number of atoms of an element such as W or Mo, together with a relatively large number of atoms of oxygen. In addition to the elements mentioned, it has been shown that the oxides of a considerable number of other elements show a tendency to form heteropolyacids. All formulæ proposed indicate a relatively large number of atoms in the molecule, and a complex structure. In spite of the complexity of the molecule, many of these acids are quite stable, and form stable salts with practically all metals, The best known compounds of this group are the silicotungstic, silicomolybdic, phosphotungstic and phosphomolybdic acids, in which one atom of silicon or phosphorus is combined with a number of atoms of tungsten or molybdenum. Mixed acids are also known in which a number of tungsten atoms are replaced by molybdenum atoms, or
vice versa
. The heteropolyacids are classified according to the ratio of the numbers of the two types of cations present. Throughout the whole class of heteropolyacids, those acids which have the same cation-cation ratio tend to be isomorphous and have similar properties.
The structure and formula of 12-phosphotungstic acid
