Crystalline salts electrolyte effects

The ionic conductivities of most solid crystalline salts and oxides are extremely low (an exception are the solid electrolytes, which are discussed in Section 8.4). The ions are rigidly held in the crystal lattices of these compounds and cannot move under the effect of applied electric fields. When melting, the ionic crystals break down, forming free ions the conductivities rise drastically and discontinuously, in some cases up to values of over 100 S/m (i.e., values higher than those of the most highly conducting electrolyte solutions). 

Broadly considered, solubilities depend in part on nonspecific electrolyte effects and in part on specific effects. The nonspecific effects can be considered in terms of activity coefficients (Chapter 2). But activity-coefficient effects often are negligible compared with the uncertainties arising from disregarded or unknown side reactions and also with uncertainties arising from the crystalline state, the state of hydration, the extent of aging of the precipitate, and intrinsic solubility, all of which may contribute to the solubility of the precipitate. To the extent that each can be identified and measured, each can be accounted for. Nevertheless, the magnitude of unsuspected effects makes it expedient to assume activity coefficients of unity unless otherwise specifically indicated for relatively soluble salts or solutions containing moderate amounts of electrolytes. 

The above-discussed acid retardation and base retardation in the immobiUzed Uquid phase could be compared with the so-called salting-out effects. However, this term is hardly applicable to the case of salt retardation, the first example of which was demonstrated by a successful removal of small amounts of NH4CI from a concentrated brine of (NH4)2S04. This practically important problem arises in the manufacture of caprolactam, where large amounts of sulfuric acid are converted into ammonium sulfate used for the preparation of the crystalline fertilizer. The new process of ISE on nanoporous NN-381 resin allowed an effective purification of very large volumes of concentrated sulfate brine, due to the fact that small ions of NH and Cl are efficiently squeezed into and retained in the finest pores of the sorbent [172]. We consider this salt retardation process as a convincing proof of our interpretation of the mechanisms of the new electrolyte separation process. 

The Mechanism of Corrosion.—An attractive theory of the mechanism of corrosion has been outlined by Aitchison.2 Compact iron, when examined under the microscope (see Part III.), is seen to consist of crystals of ferrite separated from each other by an amorphous cement. It is reasonable to suppose that the solution pressure of this cement differs from that of the ferrite, for differences of this kind invariably occur between amorphous and crystalline varieties of substances. Upon immersion in an electrolyte, therefore, such as ordinary tap water or aqueous solutions of inorganic salts, a difference of potential exists leading to corrosion. If the cement is positive to the ferrite, it is the cement that will oxidise away and vice versa. In a perfectly annealed specimen, in which there is but little mechanical strain, the action will, in the main, be confined to that between the cement and ferrite. If, however, there is any appreciable potential difference between the crystals of ferrite themselves, this will increase the effect, the total observed corrosion being the sum of the two actions. 

The electrostatic effects on the phase behavior of the CTAB and SOS mixture with added salt have been studied [26]. The phase behavior of this surfactant system changes markedly when an electrolyte is added (Fig. 5). At certain compositions, there is a vesicle-to-micelle transition with increasing salt concentration, and surface charge density measurements show that aggregate composition changes with added electrolyte. A thermodynamic cell model for micellization of mixtures of anionic and cationic surfactants which provides an accurate account of surfactant inventory, micelle composition, and counterion binding (as probed by electrical conductivity) has been developed. Model predictions for the phase equilibria between spherical micelles and a crystalline precipitate phase are in agreement with experimental data [24,26]. 

The behaviour of PEO-based electrolytes with multivalent cation salts is strongly related to the nature and concentration of ionic species involved, preparation conditions, thermal history and state of hydration of the sample. Martins and Sequeira analysed the effect of these factors on the observed performance of PEO-based systems with salts of divalent metals. One of the properties reviewed was the composition of maximum conductivity, and a huge variation was observed. In a number of studies of the PEO -Znl2 system, different preparation conditions for the polymeric films were shown to affect the crystallinity of the electrolyte and displace the conductivity maximum (as a function of salt concentration) for different compositions. Examples include the compositions n = 30, for the range... 

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