Overall hydration numbers

Similarly, concepts of solvation must be employed in the measurement of equilibrium quantities to explain some anomalies, primarily the salting-out effect. Addition of an electrolyte to an aqueous solution of a non-electrolyte results in transfer of part of the water to the hydration sheath of the ion, decreasing the amount of free solvent, and the solubility of the nonelectrolyte decreases. This effect depends, however, on the electrolyte selected. In addition, the activity coefficient values (obtained, for example, by measuring the freezing point) can indicate the magnitude of hydration numbers. Exchange of the open structure of pure water for the more compact structure of the hydration sheath is the cause of lower compressibility of the electrolyte solution compared to pure water and of lower apparent volumes of the ions in solution in comparison with their effective volumes in the crystals. Again, this method yields the overall hydration number. 

The fact that the water molecules forming the hydration sheath have limited mobility, i.e. that the solution is to certain degree ordered, results in lower values of the ionic entropies. In special cases, the ionic entropy can be measured (e.g. from the dependence of the standard potential on the temperature for electrodes of the second kind). Otherwise, the heat of solution is the measurable quantity. Knowledge of the lattice energy then permits calculation of the heat of hydration. For a saturated solution, the heat of solution is equal to the product of the temperature and the entropy of solution, from which the entropy of the salt in the solution can be found. However, the absolute value of the entropy of the crystal must be obtained from the dependence of its thermal capacity on the temperature down to very low temperatures. The value of the entropy of the salt can then yield the overall hydration number. It is, however, difficult to separate the contributions of the cation and of the anion. 

This is probably the most powerfiil spectroscopic technique, and with X-ray and neutron diffraction is now the technique of choice. A shift in the proton resonance frequency and the intensity of the signal teUs how many water molecules are responsible. Proton relaxation shifts have proved to be a major advance, and are progressively being applied to solutions containing complex ions. For simple ions they suggest six water molecules around a cation are fairly typical. In favourable cases individual hydration numbers are obtained using this technique. In this respect they are superior to the more traditional methods which on the whole only measure overall hydration numbers and require some arbitrary way of splitting these into cation and anion contributions. Diffraction studies also furnish individual hydration numbers. 

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. 

When the exchange is fast only overall hydration numbers can be found. But these, in conjunction with diffraction techniques which do give individual hydration nmnbers, can extend the range of cations for which individual hydration numbers can be found. 

The experimental measurements that provide the hydration number and hydrated radius information are made on lanthanide solutions of moderate concentration with dilFerent counter ions. The data in Rizkalla and Choppin (1991) indicate that hydration numbers and Ln-O distances change slightly with both the nature of the counter ion and the concentration of the salt. It appears likely that composition of the primary coordination sphere of the lanthanide ion does not vary appreciably with the concentration (or identity of the counterion) of the lanthanide salts. However, the reduced water activity that occurs in concentrated salt solutions would suggest that overall hydration numbers will be higher in dilute solutions. Thus the values reported for overall hydration and hydrated radii determined in concentrated aqueous salt solutions probably underestimate the hydration of lanthanide cations in the dilute solutions that are typical of analytical applications. It has been suggested that as many as 40 water molecules may feel the presence of a trivalent lanthanide ion in solution (Choppin 1997). Using Lundqvist s (1981) estimate of 30 for the volume of a water molecule, the radial distance of the lanthanide iQrdration sphere... 

Diffusion measurements were also used to estimate the overall hydration numbers of the trivalent Am, Cm, Cf and Es ions (Fourest et al. 1982,1983,1984,1988, Latrous et al. 1982). The open-end capillary method was claimed to have an accuracy better than 1% for y-emitters and 1-2% for a-emitters. The results are included in table 3. Comparison of the data obtained by the electrophoretic and the diffusion methods reflect the uncertainty of the measurements e.g., the individual h values for the Ln(III) cations are roughly the same for both methods while those for the An(III) cations from the diffusion measurements are lower than those from electrophoresis. 

Fig. 4. S-shaped variation of the overall hydration number, h, derived from diflii-sion measurements, as a function of crystallographic radius, (C = 8) for some trivalent 4f and 5f ions.

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. 

The sequential reactions 4.1 and 4.2 represent the self-dissociation of water as the exchange of a proton between water molecules, where hydration of the proton according to reaction 4.2 is the driving force for its separation (reaction 4.1) although the proton hydration is not limited to one H20 (hydration number 1), nor is the occurrence of unhydrated OH ion realistic, the overall reaction 4.3 is generally written as the simplest form to show the principle of proton acidity. 

The three Ndm, Er111, and Ybm chelates display sizeable metal-centered NIR luminescence in HBS-buffered (pH 7.4) aqueous solutions. Their photophysical characteristics are summarized in table 18. The hydration numbers calculated from eqs. (10a) and (9a) are very small, 0.31 and 0.16 for Ndm and Ybm, respectively, and compare well with the results obtained for the tetrapodal ligand H890a. The overall luminescence quantum yields in aqueous solution are comparable to those obtained for H890a, but smaller than those determined for chelates withH890b (compare tables 17 and 18). Upon deuteration of the solvent, from 3- to 10-fold increases are observed in the luminescence quantum yields. Moreover cytotoxicity studies on several cell lines have shown the Ybm chelate to be non-toxic, opening the way for applications in cell imaging (Comby et al., 2007). 

If the ion-dipole center distance is 2.75 A and the overall packing corresponds to the closest approach distance for water (taken as 2.76 A), then the second set of dipoles are 15% further away than the first, whereas if it is 2.5 A, the second group are 31% further away. The concept of a tightly bound inner shell and a more loosely bound second shell offers a good explanation for the coordination number (c) and hydration number (n). 154,155... 

Why bother about these hydration numbers What is the overall purpose of chemical investigation It is to obtain knowledge of invisible structures, to see how things work. Hydration numbers help to build up knowledge of the environment near ions and aid our interpretation of how ions move. 

A number of metals salts can be used as the source of electrophiles in reactions with alkenes. One of the most interesting of these involves the attack of mercury(II) acetate in acetic acid. Reductive cleavage of the organomercury compound with sodium borohydride leads to the overall hydration of the alkene in a Markownikoff sense. There are a number of preparative advantages, such as a reduced tendency to rearrange, associated with this and similar relatively mild procedures when compared to the direct protonation of a double bond (Scheme 3.14)... 

Hydration numbers (the number of water molecules in the primary hydration layer) can be determined by various physical techniques (for example, compressibility) and the values obtained tend to differ depending on the method used. The overall total action of the ion on water may be replaced conceptually by a strong binding between the ion and some effective number (solvation number) of solvent molecules this effective number may well be almost zero in the case of large ions... 

A factor that can influence C02 hydration/dehydration reactivity is the overall coordination number of the zinc center and the coordination mode of a bicarbonate ligand (Fig. 7). It is reasonable to suggest that a unidentate coordinated HCO will be easier to displace, which could influence the rate of the overall hydration reaction. Data discussed below in terms of single turnover experiments supports the notion that bidentate bicarbonate coordination inhibits catalytic C02 hydration. Similarly, bidentate coordination of HCO could be expected to slow the dehydration reaction. Notably, X-ray crystallographic studies of bicarbonate-bound forms of a mutant CA-II, and a Co(II)-substituted form of the enzyme, have revealed both monodentate and bidentate coordination modes for the bicarbonate anion.28,32,45... 

In conclusion, many questions remain about the mechanism of water exchange. While activation volume data support an association mechanism (I ), the ultrasound and kinetic data are interpreted in terms of an overall dissociative mechanism. Kinetic results show a discontinuity in plots of log k values versus lanthanide radius in the region of Sm -Tb which would seem to be good evidence for a change in hydration number in this region. 

The coordination number (or primary hydration number) of an R aqua complex has been the subject of continuous debate. Many of the techniques used for its determination provide only indirect evidence, being open for different interpretations. Recent reviews give a good overall picture of the situation, see for instance Marcus (1981), Choppin (1984), and Lincoln (1987). 

As is suggested frequently , this term might well result from the restriction of the hydrogen bonding possibilities experienced by the water molecules in the first hydration shell. For each individual water molecule this is probably a relatively small effect, but due to the small size of the water molecules, a large number of them are entangled in the first hydration shell, so that the overall effect is appreciable. This theory is in perfect agreement with the observation that the entropy of hydration of a nonpolar molecule depends linearly on the number of water molecules in the first hydration shell ". ... 

Recently Blake et al.153) made such studies in the case of human (HL) and tortoise egg-white (TEWL) lysozyme based on crystallographic refinements at 1,5 and 1,6 A resolution, respectively. By these investigations they attempted to obtain information on the perturbations of water structure in the hydration shell by neighboured protein molecules and by high salt concentrations as well as on the degree of order of the bound water. The authors came to the conclusion that the number of ordered water molecules are 128 in TEWL and 140 in HL, whereas the overall content is made up of 650 and 350 water molecules per lysozyme molecule. 

The hydrated ion [Cu(H20)6]2+ is an example of a complex, a species consisting of a central metal atom or ion to which a number of molecules or ions are attached by coordinate covalent bonds. A coordination compound is an electrically neutral compound in which at least one of the ions present is a complex. However, the terms coordination compound (the overall neutral compound) and complex (one or more of the ions or neutral species present in the compound) are often used interchangeably. Coordination compounds include complexes in which the central metal atom is electrically neutral, such as Ni(CO)4, and ionic compounds, such as K4[Fe(CN)6]. 

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