Acids isopoly

Iso-polysaure,/. isopoly acid, -purpurinskure, /. isopurpuric acid. -saccharinsaure, /. isosaccharinic acid, isosmotisch, a. isosmotic. iso-therm, -thermisch, a. isothermal. Iso-therme, /. isotherm, -thlocyanat, -thio-cyanid, n. isothiocyanate, -thiocyansaure, f. isothiocyanic acid, -tonie, /. isotonicity. 

Much synthetic and descriptive chemistry of heteropolyanions dates from 1860-1920 when the terms isopoly acid and heteropoly add were introduced. Reviews of the early work are available7 and they provide much valuable, if descriptive, information. In the following sections we shall discuss the chemistry of isopolyanions (general formula [MmOJ,]p ) as well as that of heteropolyanions ([XIMmOy] x m). Excluded from consideration here are substances with similar formula that are the result of high temperature solid state or melt reactions and which are polymeric mixed oxides with no defined solution chemistry. 

The classic poly adds of molybdenum and tungsten are of two types (a) the isopoly acids and their related anions, which contain only molybdenum or tungsten along with oxygen and hydrogen, and (b) the heteropoly acids and anions, which... 

A. Actinyl V s and Vi s give monomeric oxo cations in acid, in contrast to the d element proclivity towards polymerization (isopoly acid formation). Protactinium is similar to d elements in this regard. 

Finite polynuclear anions of far greater complexity are found in the extensive series of salts of the iso- and hetero-polyacids. The salts of the isopoly acids contain ions [NyOz], where N is W, Mo or V, and y and z are large an example of such an ion is Mo70246. The salts of the heteropolyacids contain ions [MxNyOJ, where M can be one of many elements (B, Si, P, As, S, Te, I and others), N is again W, Mo... 

In contrast to the isopoly acids, many of the heteropoly acids are stable without depolymerization in quite strongly acid solutions they are often themselves strong acids. In general, heteropolymolybdates and tungstates of small cations, including those of some heavy metals, are water-soluble, but with larger cations insolubility is frequently found. Thus Cs+, Pb2+ and Ba2+ salts are usually insoluble, and NH, K + and Rb+ salts are sometimes insoluble salts of [(/ri-C5H5)2Fe]+,R4N+, R4P+ and alkaloids, are invariably insoluble. Table 26-C-2 lists the principal types of heteropolymolybdates,... 

As i. he isopoly acids, there have been extensive studies of heteropol anions in s hons. X-ray studies of solutions of 12WO3-SiO2-2H20 hav demonstrated t. on [SiW1204u]4 with the structure shown in Fig. 26-C-"/ 26-C-7, which is consistent with the crystal structure (Figure 26-C-3). 

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... 

As for isopoly acids or their salts of sulfur(VI)—i.e., polysulfuric acids or their salts—disulfuric acid and its salts (structure 3) and the nitro-nium salt of trisulfuric acid (structure 4) (14) are the known crystalline... 

From the facts mentioned above, it appears that the 0x0 acid that can form condensed compounds wdth phosphate would be an 0x0 acid capable of forming isopoly acids. 

There are, in general, chemical equilibria between monomers and various polymers of a given isopoly acid in an aqueous solution. If, for the sake of convenience, one divides the polymers of the isopoly acid into two groups of polymers which have high and medium degrees of polymerization, the following equilibria can be considered in the aqueous solution. 

Equilibria of Isopoly Acids of Various Elements in Aqueous Solution Stable species and Covalent Radius (in A.) of the Elements... 

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,... 

The steps involved in the formation of the ions of isopoly acids (for example, isopolywolframates)... 

Specific isopolyanions of definite degrees of condensation and specific chemical properties predominate in the solution their existence is a function of the H+ concentration and their crystalline salts may be isolated if certain conditions are observed. When solid, almost all salts of these isopoly acids contain water of crystallization. In keeping with their structure, these compounds... 

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