Hydration number peak area

Both type I and type II water forms were detected, and the I/II mole ratio was rather constant at 1 2.2 for the Na+ form having the low H2O/SO3 mole ratios of 0.06, 0.5, and 1.2. These numbers were derived from the areas under the two deconvoluted peaks. The ratio of type I to II water molecules decreases in the order for the series Na+ > K+ > Rb+ > Cs+, which is reasonable considering that the cation hydration number decreases in this order and shows the structure-breaking action of cations with... 

There are a number of alternative approaches to lability measurements, and three which involve different timescales are listed in Table 2.8. The level of labile electro-active metal species present in aqueous samples can be determined using ASV (see Section 2.7.4). The electrochemical response (i.e. peak area values) reflects the hydrated metal ion content together with contributions from any metal complexes present which rapidly dissociate in the diffusion layer (around the mercury cathode). The labile metal is measured either at the natural sample pH, or after pH adjustment (e.g. after addition of pH 5 acetate buffer). 

These are vibrational spectra and lead to fundamental frequencies of vibration and the strength of ion-solvent interactions. When a salt is dissolved in water it will affect the frequencies of the water absorption and in favourable cases it will give rise to new peaks due to ion-solvent interactions. Alteration in the vibrational spectrum of the water due to the presence of the ion gives information regarding the effect of the ion on the water structure. The really useful information, however, comes from a study of the new lines due to the actual bonding of the ion to the solvent molecules. The frequencies and intensities of the vibrational lines give a measure of the strength of the bond between ion and solvent, and the peak areas can in favourable cases lead to hydration numbers. 

For slow exchange the positions of the chemical shifts do not alter with concentration, though of course the areas under the peaks do. The areas under the peak for coordinated water and the peak for bulk water can be easily measured and the ratio of these areas along with a knowledge of the number of mol of ion and water will lead directly to the hydration number. 

Paramagnetic ions such as Ni and give only one proton resonance, and so cannot be studied in H2O. However, if enriched water is used, then two peaks are found and the areas under the two peaks will give the hydration number. 

Table 13.2 Typical values for hydration numbers found from areas of peaks in NMR...

Figure 13.12 Radial distribution function showing how to estimate the average hydration number for an ion from the peak area.

The information relating to solvation numbers of tetravalent actinide ions is rather sparse. From NMR peak area, an estimate of the hydration number of Th(IV) in an aqueous-acetone solution of Th(C10 J4 at — 100°C indieated a value of nine (Butler and Symons 1969, Fratiello et al. 1970b) whereas an indirect NMR linewidth method gave a hydration number of ten (Swift and Sayre 1966). An entirely different method for the estimation of hydration numbers from conductivity measurements has been proposed and developed by Gusev (1971,1972,1973). The dependence of conductance on coneentration in acidic solutions of metal salts shows the pattern given in fig. 5. 

The results for a 1.5 molal solution of NiCl in D O are shown in Fig. 6. The first peak corresponds to the Ni-0 distance and the second peak to the Ni-D distance the ratio of the areas under these peaks is of course 1 2, and integration of the peaks yields a hydration number of 5.8 0.2 which is invariant of concentration. This fact, taken together with the sharpness of the p aks reflects the stability of the first hydration shell of the Ni ion. From the position of the two peaks and a knowledge of the geometry of the D O molecule it becomes apparent that the D O molecules are tilted at an angle to the Ni-0 axis this angle increases with increasing concen-... 

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