Halide ions electrolyte solutions

The character of the passivation phenomena that can be observed at a given metal strongly depends on the composition of the electrolyte solution, particularly on solution pH and the anions present. The passivating tendency as a rule increases with increasing pH, but sometimes it decreases again in concentrated alkali solutions. A number of anions, particularly the halide ions CU and Br, are strong activators. 

Divided cells — Electrochemical cells divided by sintered glass, ceramics, or ion-exchange membrane (e.g., - Nafion) into two or three compartments. The semipermeable separators should avoid mixing of anolyte and - catholyte and/or to isolate the reference electrode from the studied solution, but simultaneously maintain the cell resistance as low as possible. The two- or three-compartment cells are typically used a) for preparative electrolytic experiments to prevent mixing of products and intermediates of anodic and cathodic reactions, respectively b) for experiments where different composition of the solution should be used for anodic and cathodic compartment c) when a component of the reference electrode (e.g., water, halide ions etc.) may interfere with the studied compounds or with the electrode. For very sensitive systems additional bridge compartments can be added. 

Solutions of non-electrolytes contain neutral molecules or atoms and are nonconductors. Solutions of electrolytes are good conductors due to the presence of anions and cations. The study of electrolytic solutions has shown that electrolytes may be divided into two classes ionophores and ionogens [134]. lonophores (like alkali halides) are ionic in the crystalline state and they exist only as ions in the fused state as well as in dilute solutions. Ionogens (like hydrogen halides) are substances with molecular crystal lattices which form ions in solution only if a suitable reaction occurs with the solvent. Therefore, according to Eq. (2-13), a clear distinction must be made between the ionization step, which produces ion pairs by heterolysis of a covalent bond in ionogens, and the dissociation process, which produces free ions from associated ions [137, 397, 398]. 

The use of a silver anode in the presence of chloride or another halide ion in the electrolyte solution is the most commonly used consumable anode for delivery of positively charged drugs. 

Studies of the nuclear resonances of Cl, Br, and have also been carried out in various alkali metal halide solutions [17]. The magnitude of the chemical shift increases with electrolyte concentration and also with atomic number of the anion. In the case of the alkali metal ions the chemical shift becomes more positive in the series Na+ < K+ < Li" " < Rb+ < Cs+. The results were attributed to direct interaction between the cation and anion in solutions containing K, Rb, and Cs. With the smaller cations, interactions between the halide ion and the water molecules solvating the cation are more important. 

In many electrolytes, one or more of the constituent ions are specifically adsorbed at the interface. Specific adsorption implies that the local ionic concentration is determined not just by electrostatic forces but also by specific chemical forces. For example, the larger halide ions are chemisorbed on mercury due to the covalent nature of the interaction between a mercury atom and the anion. Specific adsorption can also result from the hydrophobic nature of an ion. Thus, tetra-alkylammonium ions, which are soluble in water, are specifically adsorbed at the mercury water interface because of the hydrophobic nature of the alkyl groups. Specific adsorption of molecular solutes, such as the alcohols, occurs for the same reason. 

Values of the PZC at the Hg solution interface are shown as a function of electrolyte concentration in fig. 10.6. In the case of NaF, the PZC with respect to a constant reference electrode is independent of electrolyte concentration. However, in the cases of the other halides, the PZC moves to more negative potentials as the electrolyte concentration increases. The latter observation is considered to be evidence that the anion in the electrolyte is specifically adsorbed at the interface. Specific adsorption occurs when the local ionic concentration is greater than one would anticipate on the basis of simple electrostatic arguments. Anions such as Cl , Br , and 1 can form covalent bonds with mercury so that their interfacial concentration is higher than the bulk concentration at the PZC. Furthermore, the extent of specific adsorption increases with the atomic number of the halide ion, as can be seen from the increase in the negative potential shift. A more complete description of specific adsorption will be given later in this chapter. 

In some electrolytic solutions, especially those strong in halides, many metal eations are complexed by anions to the extent that they exist primarily as neutral ion pairs or anionic species. The formation of anionic complexes may retard metal extraction by solvation or cation exchange, but it can be exploited by use of anion-exchanging extractants. 

According to Fig. 7.3.1 the isotherm slopes are approximately equal in formamide and methanol whereas the slope for the aqueous system is considerably larger. The mutual repulsion of adsorbed anions is therefore evidently stronger in methanol and formamide than in water. The interaction parameter is also found to depend strongly on the anion for a given solvent. For example in the formamide system the second virial coefficient (which is directly related to the interaction parameter) for adsorption of 1 ions is 310 A /ion compared with 2000 A /ion for Cl ions. Thus the simple adsorption model of point charges undergoing lateral coulombic repulsion represents a considerable oversimplification in non-aqueous solutions as in aqueous solutions. Studies of adsorption of halide anions from mixed electrolyte solutions in formamide and methanoF reveal complex behaviour which cannot be explained in terms of a simple model. 

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