Hydrolysis, solid electrolytes

Plausibly there is a correlation of the solubility of the stable oxide, hydroxide, or oxyhydroxide of a cation with the stability of the first hydrolysis product (Figure 6. 7). Many multivalent hydrous oxides are amphoteric because of the acid-base equilibria involved in the hydrolysis reactions of aquo metal ions. Alkalimetric or acidimetric titration curves for hydrous metal oxides provide a quantitative explanation for the manner in which the chaige of the hydrous oxide depends on the pH of the medium. The amphoteric behavior of solid metal hydroxides becomes evident from such titration curves. From an operational point of view, such hydrous oxides can be compared with amphoteric polyelectrolytes and can be considered hydrated solid electrolytes, fn -... 

Recent studies indicate that the adsorption of metal ions is controlled only in part by the concentration of the free (aquo) metal ion of considerable importance is the ability of hydroxo and other complex ions and molecules to adsorb. There have been two apparently divergent approaches to describe the role played by hydroxo metal complexes in adsorption at solid-aqueous electrolyte interfaces. Matijevic et al. (9) have proposed that specific hydrolysis products—e.g., Al8(OH)2o4+ in the A1(III)-H20 system, are responsible for extensive coagulation and charge reversal of hydrophobic colloids. It has also been demonstrated by Matijevic that the free (aquo) species of transition and other metal ions... 

Recent advances on the Ca-Br cycle were presented in an ANL paper. The original concept for this cycle involved solid phase reactions in a semi-continuous batch operation. The ANL paper reported on experiments that used a direct sparging reactor in the hydrolysis reaction to allow continuous production of HBr which is then electrolytically decomposed to produce hydrogen. The sparging steam was introduced into the molten bath of CaBr2 which yielded HBr in a stable and continuous operation. 

The equilibrium constant of dissolution of an electrolyte (describing the equilibrium between excess solid phase and solvated ions) is often called a solubility product, denoted Ksol or Ks (or KSoi or K as appropriate). In a similar way the equilibrium constant for an acid dissociation is often written Ka, for base hydrolysis Kb, and for water dissociation Kw. 

Hydrolysis equilibria can be interpreted in a meaningful way if the solutions are not oversaturated with respect to the solid hydroxide or oxide. Occasionally, it is desirable to extend equilibrium calculations into the region of oversaturation but quantitative interpretations for the species distribution must not be made unless metastable supersaturation can be demonstrated to exist. Most hydrolysis equilibrium constants have been determined in the presence of a swamping inert electrolyte of constant ionic strength (/ = 0.1, 1, or 3 M). As we have seen before, the formation of hydroxo species can be formulated in terms of acid-base equilibria. The formulation of equilibria of hydrolysis reactions is in agreement with that generally used for complex formation equilibria (see Table 6.2). 

Other alkali-metal chlorates are produced by analogous technology while sodium and potassium bromate are produced electrolytically starting both from bromide ion and bromine solutions. The production of bromate is, however, a very small-scale process and the cells have not been optimized to any extent for example while cells with lead dioxide and platinized titanium have been described, some plants still use solid platinum electrodes The mechanism of bromate formation is identical to that described for chlorate by reactions (5.10)—(5.13) the kinetics are, however, different. The hydrolysis of bromine is slower than chlorine but the disproportionation step is much faster (by a factor of 100) and it is therefore advisable to use a more alkaline electrolyte, about pH 11. 

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