Vanadates, isopoly

Reduced and Mixed-valence Isopoly anions 4.10.2.6.1 Vanadates... 

The condensed compounds of this type so far investigated are mainly phosphate-silicate, sulfate-phosphate, vanadate-phosphate, and arsenate-phosphate. All the component 0x0 acids or the anions of these condensed compounds can form isopoly acids or their anions. As can be seen in silicate minerals, there are several structural types of isopoly silicates which are polymers of interconnected Si04 tetrahedra. For example, pyroxene has a chain structure 1 (13). In the structure of... 

Thilo (25) classified oxo acids of various elements into three groups on the basis of the stability of the oxo acid in aqueous solutions, as shown in Table I. Although Thilo placed the elements in order of their ionic radii, covalent radii of the elements are used in Table I. The most stable phosphate in aqueous solutions is the monomer, orthophosphate. However, isopoly acid anions of phosphorus, i.e., condensed phosphates, are generally stable in an approximately neutral aqueous solution at room temperature. The rate of hydrolysis of the condensed phosphates is very low under these conditions. On the other hand, polysulfates and polyarsenates are very rapidly hydrolyzed into their monomers in aqueous solutions. In an alkaline solution, vanadate anions are present as monomer, i.e., orthovanadate. When the pH of the solution is de- creased, the orthovanadate anions are successively polymerized to form polymers with medium degrees of polymerization. Although silicate anions behave similarly, a highly polymerized form of silicate anions,... 

Derivatives of the weaker, oxygen-containing metallic acids, such as those of tungstic, molybdic or vanadic acids, exhibit a quite characteristic behavior, and may therefore be considered as the classical isopoly compounds. One property characteristic of these metallic acids is the more or less sharply pronounced hydrolysis of their salts in aqueous solution, particularly in the presence of H+ ions. The hydrolysis products then undergo, over a period of time, a secondary reaction, combining to more highly aggregated ions, that is, the isopolyanions. For example ... 

Heteropoly compounds are composed not only of the weak, oxygen-containing metallic acids (tungstic, molybdic and vanadic), but also of moderately strong to weak acids of nonmetals, e.g., boric, silicic, phosphoric, arsenic, telluric, etc., acids. Stable heteropoly compounds very frequently show nonmetallic to metallic acid ratios of 1 12, 1 6 or 1 9. Since the heteropoly compounds form under conditions similar to those in which isopoly compounds are obtained, that is, only in solutions containing H+ ions, it is assumed that the building blocks of the heteropolyanions are isopoly anions [1]. 

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