Ionic properties individual

Ionic movements, and the random walk, 372 Ionic properties individual. 98... 

Conway, B.E., VerraU, R.E., and Desnoyers, J.E. Specificity in ionic hydration and the evaluation of individual ionic properties. Z. Phys. Chem. (Leipzig) 1965, 230, 157-178. 

One of the challenges of solvation studies consists in separating effects among the ions of a salt (e.g., those due to the anion and those due to the cation) and this difficulty, that of determining the individual solvation heats (see Section 2.15), invades most methods devoted to the determination of individual ionic properties (Fig. 2.46). When it comes to the solvation number of an ion, an unambiguous determination is even more difficult because not all workers in the field understand the importance of distinguishing the coordination number (the nearest-neighbor first-layer number) from... 

G. A. Krestov, Individual Ionic Properties, in Thermodynamic Structure of Solvation, Ellis Harwood, New York (1990). 

The great convenience of being able to think in terms of individual ionic properties means that we always look at ionic solutions this way, rather than as a simple changing of solute component. 

These quantities, however, are objectionable on account of the difficulty of estimating individual ionic properties. 

Impedance measurement can be considered a third way to evaluate electrochemical sensors besides potentiometry and amperometry. Electrochemical impedance studies in a narrower sense deal with phenomena at the electrode surface. The overall impedance of a chemosensor also includes effects of charge carrier properties far from the electrode. This was visualized by equivalence circuits presented in Chaps. 2 and 5. By individual experimental design, the study can be focused more on processes at the electrode surface or otherwise on ion properties in homogeneous solution. Even the variation of the dielectric constant in a layer will affect the overall impedance. If impedimetry is designed only to acquire data corresponding to ionic properties or value of the dielectric constant, it is not really an electrochemical method, in a strict sense. 

Under such conditions, it is indispensabte to make at Uasi one assumption, if evaluations of these individual ionic properties are aimed at. For example, Eq. (5) ows that estimation of the value requires an assumption on the a, if the and are known (remember that the a or the is measurable.), and the validity of the assumption depends in turn on the correctness of the value. After all, it is impossible to estimate the single-ion activity experimentalty. This is also the case with the ringle-ion activity coefficients of cations or anions. 

Specificity of ionic hydration and the evaluation of individual ionic properties 233... 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

However they are formed, and from whatever source, aqueous ions are individual species with properties not possessed by the materials from which they came. Furthermore, the properties of a particular kind of ion are independent of the source. Chloride ions from sodium chloride, NaClfsJ, have the same properties as chloride ions in an aqueous solution of hydrochloric acid, HQ. In a mixture of the two, all of the chloride ions act alike none remembers whether it entered the solution from an ionic NaQ lattice or from a gaseous HC1 molecule. 

Consider the fluorides of the second-row elements. There is a continuous change in ionic character of the bonds fluorine forms with the elements F, O, N, C, B, Be, and Li. The ionic character increases as the difference in ionization energies increases (see Table 16-11). This ionic character results in an electric dipole in each bond. The molecular dipole will be determined by the sum of the dipoles of all of the bonds, taking into account the geometry of the molecule. Since the properties of the molecule are strongly influenced by the molecular dipole, we shall investigate how it is determined by the molecular architecture and the ionic character of the individual bonds. For this study we shall begin at the left side of the periodic table. 

---