Page 156 - Instant notes

E6
MACROSCOPIC ASPECTS OF IONIC
MOTION

Key Notes
The conductance of an ionic solution is measured by applying an
oscillating voltage between two parallel plate electrodes, which
avoids concentration polarization and ensures the current is
proportional to the applied voltage. The conductance is the ratio
of the current to the voltage. The conductance, L, is used to
calculate the conductivity, κ, which is a measure of the charge
carrying ability of the ionic solution and is independent of the cell
in which the measurement is performed.
The conductivity is proportional to the number of ions in
solution. The molar conductivity is κ/c, where c is the salt
concentration, and gives a measure of the charge carrying
capabilities for the same amount of dissolved salt. This allows
comparison of the charge carrying capabilities for the same
amount of different dissolved electrolytes. The molar
conductivity is an addition of the molar conductivities of the
cations and anions.
A strong electrolyte is an electrolyte that completely dissociates
into its constituent ions. Its molar conductivity slowly decreases
with increasing concentration, due to the increasing importance
of ionic interactions.
The limiting molar conductivity of an electrolyte is the molar
conductivity as c→0, where there are no ionic interactions. The
limiting molar conductivity is a combination of the limiting
molar conductivities of the cations and of the anions. The limiting
molar conductivity of a particular ion is constant.

Weak electrolytes do not completely separate into their
constituent ions except at high dilution. As c increases, the molar
conductivity falls relatively rapidly, as the proportion of
undissociated electrolyte increases. Molar conductivity
measurements allow the degree of electrolyte dissociation to be
calculated at any c.
The transport number of an ion is a measure of the fraction of the
total current carried by the ion. The sum of the transport numbers
for the ions in solution add up to 1. The transport number for an
ion varies with the nature of the counterion(s) and with c.
Related topics Solubility (C5) Thermodynamics of ions in solution