Solution states, variable valency

In Section 24 it was shown that, under favorable conditions, two oxides of the same metal, in different states of valency, may form solid solutions which have been described as compounds with variable composition. The stabilizing factor in this case is the increase in entropy, due to the random distribution of the two positive ions these systems, strictly speaking, are stable only at elevated temperature. The conditions may be such that two oxides form a real compound, because this process is connected with a decrease in energy. The compounds formed in this way have a stoichiometric composition, with two kinds of positive ions in fixed positions, so arranged that the energy of the system is minimal. A good example of a compound of this type is Fe304. 

Oxidation of variable valence metal ions to higher valence state is desirable, for example, in hydrometallurgy, water conditioning, etc. The kinetics of Fe(II) and Ti(III) oxidation by oxygen in acidified water solutions was investigated. An investigation of oxidation process kinetics was carried out in the same equipment as mentioned above. It was established that... 

Thus, elements in the same oxidation state exhibit similar properties while the same element in different oxidation states exhibits quite different properties. The problem of variable valencies is particularly important for Pu and Np because these two elements display multiple oxidation states in aqueous solutions. 

For other elements of variable valence, su( h as technetium, the amount of the element in solution is determined by the stable valence state under reactor conditions. In general, the higher valence states lietter resist hydrolysis and remain in solution. Thus at 275°C in 0.02 m UO2SO4 Tc(VII) is reduced to Tc(IV) if hydrogen is present, and only 12 mg/kg H2O remains in solution. However, a slurry of TCO2 in the same solution but with oxygen present dissolves to give a solution at 275 C with a technetium concentration of more than 9 g/kg H2O. The same qualitative behavior is observed with ruthenium. Selenium and tellurium in the hexapositive state are much more soluble than when in the tetrapositive state [4]. 

In the metals, the same type of interatomic force acts between atom of different metals that acts between atoms of a single element. We have already stated that for this reason liquid solutions of many metals with each other exist in wide ranges of composition. There, are many other cases in which two substances ordinarily solid at room temperature are soluble in each other when liquefied. Thus, a great variety of molten ionic crystals are soluble in each other. And among the silicates and other substances held by valence bonds, the liquid phase permits a wide range of compositions. This is familiar from the glasses, which can have a continuous variability of composition and which can then supercool to essentially solid form, still with quite arbitrary compositions, and yet perfectly homogeneous structure. 

Although this treatment will he restricted to the dissolution of metals in aqueouvS solutions, particularly metallic fcorrosion, the ideas should oe pa rtially applicable to the behavior of semiconductors. As discussed in part HI of this volume, semiconductors have a low and variable electron density and a rather wide energy gap between the valence and conduction electrons. These and other factors allow the existence of "surface states" and the presence of electric fields within the surface layers which may affect their electrochemical behavior. 

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