Hydration number concentration dependence

Sluyters and coworkers [34] have studied the mechanism of Zn(II) reduction on DM E in NaCl04 solutions at different water activity (uw) using faradaic impedance method. Dqx and E p were determined from dc polarographic curves. Hydration numbers of Zn(Il) ion were estimated from the dependence of E[p on In Uw The obtained standard rate constant was changing with a NaCl04 concentration and the slope of the dependence of In k on potential was changing with potential (see Fig. 1). Therefore, the following mechanisms were proposed ... 

An X-ray diffraction study of aqueous Zn(N03)2 has been reported, and, as expected, a coordination number of six, almost independent of concentration, was found 613 however, it is interesting that although the [Zn(H20)6]2+ ion persists at most concentrations of aqueous solutions of zinc salts, the total hydration number is concentration dependent, and a study of aqueous zinc sulfate has revealed that the total number of water molecules associated with the Zn(S04) decreases linearly with log [ZnS04].614a More recent studies confirm octahedral coordination in [Zn(OH2)6]2+,614b but concentration dependence of hydration was not... 

If we calculated with the idealized co-operative model by the content of spectroscopic determined Op values the number Nei of H-bonded water molecules we would get — with different 1 molar salt solutions — the result of Fig. 11. The values Nei with salt additions depend strongly on the salt concentrations because of the disturbance of the big H-bonded system3At small concentrations the Nel-N0 numbers (7V0 association number in pure water) of structure-makers are in size of the order of Debye-Sack s or Azzam s calculations. They are of the same size of order as the secondary hydration numbers calculated by solubility measurements of organic substances in water (Chapter b) or as the hydration numbers of hydrophilic organic molecules (Chapter lld-e) or biopolymers (Chapter III). 

When the ion is hydrated by all the adjacent molecules (it = r — 1), there is a strong lattice-site exclusion effect at high ionic concentrations, since the central site can be available to an ion only when both its first and second neighbors are free of ions (a total of 1 + w 4- w(w — 1) = ia2 + 1 sites). On the other hand, when the hydration number (r — 1) is low, almost any free site can be occupied by an ion, since in this case there is a high chance to find at least (r — 1) free water molecules around a selected site. In the next section it will be shown that there is a strong dependence of the site-exclusion effect on the hydration number and, hence, that specific ion effects can be important in the double layer interactions. 

In order to characterize the hydration phenomena in more detail, it is worthwhile to obtain information on the dynamics of water molecules involved in the hydration shell. One of the useful techniques for such a purpose is 170-NMR spectroscopy. In the so-called two-state model, 170 nuclei in the aqueous solution are assumed to be distributed between the following two motional states the water in the hydration shell and the bulk water. Under this assumption, the analysis of concentration-dependent changes of the spin-lattice relaxation time of 170 nucleus gives the following important parameter known as the dynamic hydration number [17] ... 

Defined as the reciprocal of resistance (siemens, ft-1) conductance is a measure of ionic mobility in solution when the ions are subjected to a potential gradient. The equivalent conductance A of an ion is defined as the conductance of a solution of unspecified volume containing one gram-equivalent and measured between electrodes I cm apart. Due to interionic effects, A is concentration dependent, and the value, A0, at infinite dilution is used for comparison purposes. The magnitude of A0 is determined by the charge, size and degree of hydration of the ion values for a number of cations and anions at 298.15K are given in table 6.6. It should be noted that HjO and... 

With the exception of studies using NMR technique ) or Taube s isotopic dilution method or also X-ray diffraction techniqueof concentrated electrolyte solutions, the hydration numbers are not always integral numbers which quite often represent deviations of experimental data from results of a theoretical description of an equilibrium property. Therefore, these hydration numbers depend on the solution property studied. 

The mechanism of U02 " extraction by monoalkyl phosphoric acid reagents appears to be a more complex process than for their dialkyl counterparts. This results from the polymerization of the monoalkyl phosphate in the organic phase and the hydration of the extracted uranyl species so that variable stoichiometries arise for the extractant/water/UO complex. The extraction of from sulfuric acid by mono-2,6,8-trimethylnonyl phosphoric acid (H2DDP) and mono-n-butyl phosphoric acid (H2MBP) as 0.05 M solutions in benzene was shown to follow equations (61) and (62) when an excess of extractant was present. When an excess of uranium was present, equations (63) and (64) applied where n, x, y and z were variable numbers which depended upon the extent of extractant polymerization and hydration of the extracted species. Synergistic effects may also be found with the monoalkyl phosphoric acid extractants and in one recent example the use of tri-n-octylphosphine ocxide (TOPO) as a synergist with H2MEHP allowed the extraction of U02 from phosphoric acid solutions. The uranium may be returned to the aqueous phase by contact with concentrated acid, which reverses the extraction process by protonation of the phosphate. 

Another possible mechanism of trehalose molecules as a kinetic inhibitor is mentioned below. In the growth process of CO2 hydrate, trehalose may work as the kinetic inhibitor that prevents the rate-determining process of the crystal formation at the reaction site which might have small dependence on AT. Trehalose has been observed to prevent ice-crystal growth with the reduction of the free-water providing because trehalose strongly hydrated in the solution. This effect is found apparently when the trehalose concentration increases beyond the intrinsic hydration number of trehalose molecules. It is thus reasonable that the kinetic effect of trehalose on the inhibition of the hydrate formation would be resulted from the smaller supplement of free water from the solution of higher trehalose concentrations. 

The chemical shift observed for a given electrolyte depends on the nature of both the cation and the anion. Unlike the slow exchange where the chemical shift is the same whatever the concentration of the solution, the position of the resonance signal if the exchange of water is rapid depends on the concentration. This dependency is utilised in the determination of the overall hydration number for the electrolyte in question. 

Table 3 summarizes the results at low temperatures in this solvent mixture. These data show the hydration numbers n to be markedly sensitive to the type and the concentration of the anion, Low estimates of n were attributed to either replacement of water in the primary coordination sphere by the CIO4 or NO3 ions or to changes in water activity in solution as a result of hydrogen bonding between water and acetone (Brucher et al. 1985). Such hydrogen bonding has been detected by H-NMR spectroscopy in water-acetone mixtures at very low water content (Takahashi and Li 1966) and could involve either A. . . HOH or A. .. HOH. .. A species, depending on the relative concentrations of water and acetone. 

For substances that are not stable on heating to dryness, e.g. hydrates, their concentration in solution may be determined in some cases by chemical analysis and in others by measuring some concentration-dependent physical property. The latter is often convenient when a large number of determinations have to be made. Solution density and refractive index are probably the two properties most commonly measured for this purpose and the many well-established techniques available are fully described in standard textbooks of practical physical chemistry. 

The degrees of dissociation and hydration numbers calculated from vapor pressures correlate quantitatively with the properties of dilute as well as concentrated solutions of strong electrolytes. Simple mathematical relations have been provided for the concentration dependences of vapor pressure, e.m.f. of concentration cells, solution density, equivalent conductivity and diffusion coefficient. Non-ideality has thus been shown to be mainly due to solvation and incomplete dissociation. The activity coefficient corrections are, therefore, no longer necessary in physico-chemical thermodynamics and analytical chemistry. 

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