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86 CHAPTER 2

out that this is a nonevent for water because the rapid exchange between the two types,
bound and unbound, gives rise to only a broad peak.
In order to obtain information from nuclear resonance, the proceedings must be
a bit complicated. One adds a paramagnetic ion to a solution in which the solvation of
a diamagnetic entity is to be measured. Then, two types of water around, for example,
an ion, bound and unbound waters, can be distinguished by observing the resulting
nuclear magnetic resonance spectra of The nuclear spin in the interacts with
the electron spin vector of the paramagnetic ion added as a helpful auxiliary ion, and
this changes the field on the nucleus. This shift in the NMR spectra of between
water attached to the ion and bulk water has to be sufficiently large, and this in turn
may allow a separation to be made between water bound to the diamagnetic ion and
free water. In this rather complex and devious way, it is possible to obtain estimates
of the number of waters in the first layer next to an ion.
However, disappointingly, again the values obtained from this NMR spectro-
scopic approach (e.g., 6 for and are less than the values obtained for
these ions (e.g., 14 for ) from the relatively self-consistent values of mobility,
entropy, and compressibility. Is this simply because the nonspectroscopic measure-
ments are usually done at high dilutions (e.g., mol ) to diminish interionic
effects, and the spectroscopic ones have to be done at 0.5 mol or greater
concentrations, because the spectroscopic shifts are relatively insensitive, and hence
need the high concentration to score a detectable effect?
Swift and Sayne used concepts similar to those of Bockris and Saluja: if a
molecule stays associated with an ion for more than the time needed for a diffusional
jump, it “counts” as a primary hydration number. This approach yields approximately
4 solvation molecules for and and 5 for and whereas nonspec-
troscopic methods for these systems yield values that are two to three times larger.
Does NMR measure only water arranged in a first, octahedral layer in the first shell
near the ion and is it insensitive to the rest of the water structure near an ion?

Further Reading

Seminal
1. P. Debye, “The Vibrational Potential in Solution,” J. Chem. Phys. 1: 13 (1933).
2. D. D. Eley and M. G. Evans, “Statistical Mechanics of Ions in Solution,” Trans. Faraday
Soc. 34: 1093 (1938).
3. M. Passynski, “Compressibility and Solvation,” Acta Phys. Chim. USSR 8: 385 (1938).
4. K. Fajans and O. Johnson, “Heats of Hydration,” J. Am. Chem. Soc. 64: 668 (1942).
5. R. H. Stokes and R. A. Robinson, “Hydration Numbers from Activity Measurements,” J.
Am. Chem. Soc. 70: 1870 (1948).
6. J. B. Hasted, D. M. Ritson, and C. H. Collie, “Dielectric Constants of Solutions,” J. Chem.
Phys. 16: 1 (1948).
7. J. O’M. Bockris, “Primary and Secondary Solvation,” Quart. Rev. Chem. Soc. Lond. 3:
173 (1949).