Radii, hydrated anionic

While nearly all investigations of anion effects have focused on transport efficiency, a few recent works suggest a correlation between anion type and selectivity. At extraction of alkali metals into chloroform by dicyclohexano-18-crown-6 [101], selectivity for K+ over both Rb and Cs" " decreases dramatically depending upon anion type in the order NO3 > SCN > C1O/ I > Br. K+/Cs+ selectivity decreases from 16.0 for nitrate to 3.5 for bromide. The authors were unable to tie this trend to any particular parameter, although they discounted the possibihty that it is correlated to anion radius, hydration enthalpy, or anion softness. 

This is consistent with the values of Ahyd// becoming less negative as the anion radius increases, for any of the Group 1 halides. Note that for any particular Group 1 cation the sum of the enthalpies of hydration of the cation/fluoride combination is considerably more negative than the other three values. The cation and anion radii are separate as indicated by equations (3.32) and (3.33), and the relatively small fluoride ion has a more dominant influence than it does on lattice enthalpies, which are dependent on the sum of the cation and anion radii. 

According to Table 1, the free energy of hydration of anions decreases with increasing anionic radius. Despite this, the smallest, most highly solvated anions are bound most strongly by most host species. Explain this observation. 

It is obvious, that the cation and anion radius are mostly different, for example the hydrated radius of r(Na+) = 102 + 116, r(Cs+) = 170 + 49, r(Cl ) = 181 +43, r(I ) = 220 + 26 [pm]. As can be seen, the radii of the hydrated anions are usually larger than that of the hydrated cations. This relation is valid for the ion devoid of the hydration shield. From this reason the assumption of the separate IHP planes for adsorbing cations and anions is rational. 

There is considerable controversy over the value of the radius of the hydrated proton (51-57), and it has been reported from 0.945-4.5 A. However, overwhelming experimental as well as theoretical work seems to suggest that the hydrated proton species is H904+ with a radius of 3.0 A. Hydrated anionic radii (r.) were calculated using this value of the hydrated proton, and these are compiled in Table VIII. What do these radii mean The term (l/r+ + l/r ) in Equation 32 really implies the sum of reciprocals of the minimum distances of approach of the two ions or some function of it. In the Dennison and Ramsey (58) treatment of the Bom model this term is 1/r, where r is the minimum distance of approach of the two ions. Implicit in the expression (l/r+ + 1/r.) is the assumption that both ions are monatomic and spherical therefore, the minimum distance of approach of the two ions may be apportioned as the hydronium ion radius and the anion radius. Although it might be possible to assign a value to the hydronium ion radius, the anionic radius... 

The values of hj for different ions are between 0 and 15 (see Table 7.2). As a rule it is found that the solvation number will be larger the smaller the true (crystal) radius of the ion. Hence, the overall (effective) sizes of different hydrated ions tend to become similar. This is why different ions in solution have similar values of mobilities or diffusion coefficients. The solvation numbers of cations (which are relatively small) are usually higher than those of anions. Yet for large cations, of the type of N(C4H9)4, the hydration number is zero. 

Table 2.3 gives the self-diffusion coefficients of some important ions in submerged soils and Figure 2.2 shows the values for the elemental ions plotted against ionic potential ( z /r where z is the absolute ionic charge and r the crystal ionic radius). As the ionic potential increases the hydration layer of water molecules around the ion increases, and therefore the mobility tends to decrease. Also, at the same ionic potential, cations diffuse faster than anions. The mobilities... 

Concentrated solutions are strongly affected by ionic hydration. Its strength depends upon ionic radius and charge, therefore it is in general stronger for cations than anions. K2S04 and MgS04 both yield 3 ions,... 

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