HETEROPOLY ACIDS AND THEIR SALTS. 151HETEROPOLY ACIDS AND THEIR SALTS.
Among the compounds with complex anions belong a great number of
complicated acids, many of them known only in the form of their salts, which
are formed by condensation of oxy-acids. Such acids are often called poly
acids; heteropoly acids if the condensed acids are unlike and isopoly acids if
the condensation takes place from like constituents. The first classic repre-
sentative of a heteropoly acid is phosphomolybdic acid (No. 116) which was
discovered by Berzelius; almost equally well known is silicotungstic acid,
discovered by Marignac. The components of condensed acids which come
chiefly into consideration are, aside from those just mentioned, boric, stannic,
arsenic and vanadic acids. To the group of isopoly acids belong the poly-
chromic, polysilicic and polyboric acids. The study of such acids is com-
plicated by the fact that the acids condense not only in one but often in several
proportions. An acid of a particular composition, therefore, is produced only
under certain definite conditions. Thus, a compound such as ammonium
phosphomolybdate is well fitted for use in analytical chemistry for the pre-
cipitation of phosphoric acid, but it is only when the precipitate is produced
under definite conditions that it is suitable as a form for weighing. There
are, however, often clearly defined limits to the composition of the polyacids,
since with a number of pairs no product can be obtained containing more than
12 molecules of the one acid anhydride to one molecule of the other.
The heteropoly acids crystallize exceptionally well. The crystals are often
very soluble, sometimes even in organic solvents such as alcohols and ether.
Drechsel has made use of their affinity for ether as a means for their purifica-
tion. In the field of organic chemistry, certain of the heteropoly acids have
been used as precipitants for high-molecular bases. Thus phosphomolybdic
and phosphotungstic acids have been used for the detection of alkaloids and
the latter can serve in the separation of the cleavage products of albumin.
Further information concerning the preparation and analysis of polyacids may
be found, for example, in a compilation by Rosenheim and Jaenicke, Z. anorg.
Chem. 101, 215 (1917).
The constitution of the polyacids is still a subject for speculation. Miolati
and Rosenheim have assumed that one atom can be regarded as the central
atom, around which six substituents are grouped. As the central constituent,
phosphorus, silicon or boron can serve. Sometimes water, H 2 O, serves as one
of the six substituents and we have the following hypothetical acids:
or H 7 [PO 6 ]; Si gj^^1 or H,[SiOJ; B (°g» or H,[BOJ.If to each of the 6 atoms of oxygen foreign acid anhydrides are added the follow-
ing types of acids result:
H 7 P(W 2 O,) 6 H 8 Si(W 2 O 7 ) 3 H 9 B(W 2 O 7 )e
phosphotungstic acid silicotungstic acid borotungstic acid
Pfeiffer has assumed an inner complex with a coordination number of 6
and an outer one with a coordination number of 12 and deduced a similar
formula for phosphotungstic acid: — H 7 [(PO6)(WO3)i 2 J.