Hydration number exchange

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 controhing effect of various ions can be expressed in terms of thermodynamic equhibria [Karger and DeVivo, Sep. Sci., 3, 393 1968)]. Similarities with ion exchange have been noted. The selectivity of counterionic adsorption increases with ionic charge and decreases with hydration number [Jorne and Rubin, Sep. Sci., 4, 313 (1969) and Kato and Nakamori, y. Chem. Eng. Japan, 9, 378 (1976)]. 

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. 

It is clear from the above equations that numerous parameters (proton exchange rate, kcx = l/rm rotational correlation time, tr electronic relaxation times, 1 /rlj2e Gd proton distance, rGdH hydration number, q) all influence the inner-sphere proton relaxivity. Simulated proton relaxivity curves, like that in Figure 3, are often used to visualize better the effect of the... 

JHNMR spectra of zirconyl perchlorate in water-acetone mixtures at —70°C indicate that ZrIV has an average hydration number of —four.151 This is in accord with [Zr4(0H)8(H20)i6]8+ if it is assumed that only the bound water molecules are observed the lack of an NMR signal for the hydroxy protons could be due to rapid proton exchange. 

In recent years, X-ray diffraction studies of aqueous solutions have established primary hydration numbers for several fast-exchange cations 45,187-190 the timescale of X-ray diffraction is very much shorter than that of NMR spectroscopy. Octahedral hydration shells have been indicated for Tl3+,191 Cd2+, Ca2+, Na and K+, for example. For the lanthanides, [Ln(OH2)9]3+ is indicated for La, Pr and Nd, but [Ln(OH2)8]3 for the smaller Tb to Lu.192,193 Sometimes there are difficulties and uncertainties in extracting primary hydration numbers from X-ray data. Thus hydration numbers of eight and of six have been suggested for Na+ and for K+,194 and for Ca2+,195 and 8 and 9 for La3+, 196 In some cases rates of water exchange between primary and secondary hydration shells are so fast as to raise philosophical questions in relation to specific definitions of hydration numbers.197... 

Partially Dehydrated Ba6 A. Full-matrix least-squares refinement was initiated using the atomic parameters of the framework atoms and of the barium ions (Ba(l), Ba(2), Ba(3), and Ba(4)) in fully hydrated Ba2+-exchanged zeolite A. Full anisotropic refinement converged to the error indices = 0.113 and R2 = 0.088. Simultaneously, the occupancy numbers at Ba(l),... 

The NMR spectrum of an aqueous A1(C104)3 solution in [Dgjacetone shows nicely the two different signals of bulk water and hydration water in the AP inner shell, even at room temperature [245]. The addition of acetone slows down the proton exchange rate. A primary hydration number of six for Al has been obtained in this way [245]. 

As we have seen above, a large number of parameters (proton exchange rate, kex = 1/Tm5 rotational correlation time, r, electronic relaxation times, 1/Ti 2e> Gd - proton distance, hydration number, q) influence the inner sphere proton relaxivity. If the proton exchange is very slow (Ti , t ), it will be the only limiting factor (Eq. (5)). If it is fast (t Ti ,), proton relaxivity will be determined by the relaxation rate of the coordinated protons,, which also depends on the rate of proton exchange, as well as on rotation and electronic relaxation. The optimal relationship is ... 

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