Silver halides, dielectric constant

The use of ISEs in non-aqueous media(for a survey see [125,128]) is limited to electrodes with solid or glassy membranes. Even here there are further limitations connected with membrane material dissolution as a result of complexation by the solvent and damage to the membrane matrix or to the cement between the membrane and the electrode body. Silver halide electrodes have been used in methanol, ethanol, n-propanol, /so-propanol and other aliphatic alcohols, dimethylformamide, acetic acid and mixtures with water [40, 81, 121, 128]. The slope of the ISE potential dependence on the logarithm of the activity decreases with decreasing dielectric constant of the medium. With the fluoride ISE, the theoretical slope was found in ethanol-water mixtures [95] and in dimethylsulphoxide [23], and with PbS ISE in alcohols, their mixtures with water, dioxan and dimethylsulphoxide [134]. The standard Gibbs energies for the transfer of ions from water into these media were also determined [27, 30] using ISEs in non-aqueous media. 

In contrast to the silver halides for which the use of the static dielectric constant in Eq. (57) yields good agreement, for PbF2 with its high static dielectric constant a value more typical for this structure ( ficafj ) had to be used. Since at the small distances involved the full static permittivity is not operative, the neglect of the high static polarizability of the lead ion makes sense. (Similarly the interaction... 

In the experiments that follow, eight representative alkyl halides are treated with sodium iodide in acetone and with an ethanolic solution of silver nitrate. Acetone, with a dielectric constant of 21, is a relatively nonpolar solvent that will readily dissolve sodium iodide. The iodide ion is an excellent nucleophile, and the nonpolar solvent, acetone, favors the Sn2 reaction it does not favor ionization of the alkyl halide. The extent of reaction can be observed because sodium bromide and sodium chloride are not soluble in acetone and precipitate from solution if reaction occurs. 

The static dielectric constant (eo) of the alkali azides (cf. Table 11) is of the order of 6.5 while is 2.3. These values are of the same order as those of the alkali halides. Both the high and low frequency dielectric constants increase in the case of thallous. silver and cuprous azides. This is likely to be due to the increasing polarizibihty of the cations and a reflection of the decreasing ionicity of the lattices. The eo value for thallous azide and silver fulminate are however surprisingly high compared with the other azides. 

The silver halides, with the exception of one crystal modification of Agl, have cubic crystal structures. Crystal structure data, relevant lattice properties and low temperature dielectric constants, are collected in Table 3. AgF, AgCl, and AgBr all have the NaCl rocksalt structure in which there are four silver halide pairs per nonprimitive unit cell with cation-anion nearest neighbor distances equal to one-half a lattice constant. 

TABLE 3 The Space Groups, Lattice Constants (a) and Low and High Frequency Dielectric Constants (e) of the Silver Halides... 

Holes. Compared to the situation for photoelectrons in the silver halides, the properties of holes are less well understood. In the effective mass approximation, the binding energy is moderated by the background dielectric constant. When a mobile carrier is close to a trapping center, it does not experience the full dielectric constant of the perfect lattice. If the effective mass is sufficiently large or the dielectric constant sufficiently small, the... 

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