Page 132 - MODERN ELECTROCHEMISTRY

72 CHAPTER 2

is a rough expression for the volume occupied by water molecules that move with one
ion. The expression is very approximate (Tables 2.8 and 2.9), because of the uncer-
tainties explained earlier.
On the other hand, the transport or mobility approach to determining the primary
hydration number does give a value for what is wanted, the number of water molecules
that have lost their own degrees of translational freedom and stay with the ion in its
motion through the solution. This approach has the advantage of immediately provid-
ing the individual values of the solvation number of a given ion, and not the sum of
the values of those of the electrolyte.
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.

2.11. SPECTROSCOPIC APPROACHES TO OBTAINING
INFORMATION ON STRUCTURES NEAR AN ION

2.11.1. General

There is nothing new about spectroscopic approaches to solvation, the first of
which was made more than half a century ago. However, improvements in instrumen-
tation during the 1980s and 1990s, and above all the ready availability of software
programs for deconvolving spectra from overlapping, mixed peaks into those of
individual entities, have helped spectra give information on structures near an ion. This
is not to imply that they supersede alternative techniques, for they do carry with them
an Achilles heel in that they are limited in sensitivity. Thus, by and large, only the
more concentrated solutions (>0.1 mol ) are open to fruitful examination. This is
not good, for in such concentrations, interionic attraction, including substantial ion
pairing and more, complicates the spectral response and makes it difficult to compare