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ION–SOLVENT INTERACTIONS 73

information obtained spectroscopically with that obtained using the partial molar
volumes and vibration potential methods (Section 2.7).
One point should be noted here: the importance of using a 10% mixture with
in IR spectroscopic measurements because of the properties of HOD, which
contributes a much more clearly resolved spectrum with respect to O-D. Thus, greater
clarity (hence information) results from a spectrum in the presence of HOD. However
the chemical properties (e.g., dipole moment) of HOD are very similar to those of
Raman spectra have a special advantage in analyzing species in solution. This is
because the integrated intensity of the spectral peaks for this type of spectroscopy is
15
proportional to the concentration of the species that gives rise to them. From
observations of the intensity of the Raman peaks, equilibrium constants K can be
calculated and hence from the thermodynamic equation can be
derived. Furthermore, if one carries out the Raman experiment at various temperatures,
one can determine both the heat and the entropy of solution. Since
a plot of ln K against 1/T gives the enthalpy of solvation from the slope and the entropy
from the intercept. This provides much information on the various relations of ions to
water molecules in the first one or two layers near the ion. In particular, the use of a
polarized light beam in the Raman experiments provides information on the shape of
complexes present in a solution.

2.11.2. IR Spectra

In obtaining information on solvation that can be deduced from IR spectra, the
first thing that must be understood is that the raw data, the peaks and their frequencies,
seldom speak directly but need to be decoded. Spectra in the IR region are mainly
messages fed back from the solvent, and it is from the interpretation of evidence for
changes in the solvent’s libration and rotation when ions are introduced (rather than
any new peaks) that information on solvation may sometimes be drawn. One has to
take the spectrum of the solvent, then that of the solution, and subtract them to obtain
the effect of the solute (Fig. 2.19). Vibration spectra have frequencies in the region of
but it is usual to refer to the inverse of the wavelength, that is, the wavenumber,
Since then It turns out then that the wavenumbers of
most covalent bonds are numerically in the thousands. 16
Intramolecular effects can be detected in the near infrared or high-frequency
region Intermolecular effects are seen in the far infrared or low-
frequency region down to 100 Early measurements showed that ions can
cause new peaks to arise that are at distinctly higher wavenumbers than those in pure
water. The explanation proposed is that some of the hydrogen bonds present in pure

15
This tends to be the case for all spectra. For other spectra it involves sensitivity factors or nonlinearity at
higher concentrations; that is, it is approximate.
16
A typical value for v is and Hence, the wave number, , is