Page 223 - Modern physical chemistry

216 Electrochemistry

alternating current of 60 to 1000 hertz is suitable. But even then, some polarization would
take place if the electrodes did not present sufficient active surface. Platinized platinum
electrodes are suitable.
A person could calculate the specific conductance from the resistance of the cell and
its linear dimensions, using equation (9.37). However, it is difficult to determine these
dimensions accurately. Furthermore, some correction to the equation is generally needed
because of edge effects.
For routine determinations, the equation is employed in the form
k
1( =- [9.38J
R
where k is the ceU constant, to be determined by measuring the resistance of the cell
when it is filled with a solution of known specific conductance.
As a standard, one may use a solution of KCl, which is easily prepared. Careful absolute
measurements show that the specific conductance of 0.02000 M KCl at 25° C is

1( = 0.002768 O·! em-I. [9.39J

In studying poorly conducting solutions, one must remove impurity ions, ammonia,
and carbon dioxide from the distilled water and from the glassware that is used. Ordi-
nary distilled water can be improved if it is redistilled in clean apparatus after a little
KMn0 4 has been added. When in equilibrium with air at 25° C, such improved water may
have a specific conductance as low as 7 x 10- 7 n- 1 cm- 1 • Pure water, on the other hand,
has a 1( of 6.2 x 10-8 n- 1 cm- 1 at 25° C.

9.6 Equivalent Conductance

The specific conductance 1( of a strong electrolyte varies with concentration largely
because a unit volume of a concentrated solution contains more ions than a unit volume
of a dilute solution. But multiplying 1( by the volume containing one equivalent yields a
quantity that is free from this concentration effect. This new quantity can be broken down
into the separate contributions of each ion.
By definition, the equivalent conductance A of an electrolyte equals its specific con-
ductance times the volume containing one equivalent; thus
A = 1000K' [9.40J
c
where c equals the equivalents electrolyte in 1000 cm 3 • To interpret A, consider the fol-
lowing ideal experiment.
A rectangular cell with electrodes covering two opposite vertical walls 1 cm apart is
constructed. Into this is placed 1 equiv of the chosen electrolyte. Then water is added
and the temperature is kept constant. Mer all is dissolved, the number of ions conducting
the current does not vary unless the electrolyte is weak. Any change in the conductance
of the cell is due to a changing average interionic interaction, as long as the electrolyte
is completely ionized.
In the ideal cell, the cross sectional area conducting the current equals 10001e while
the distance between the electrodes is 1 cm. Consequently, the conductance of the cell
equals the equivalent conductance of the electrolyte. Any change in it is caused by chang-
ing average interionic interaction and a varying degree of dissociation. In going from 1(
to A, we have gotten rid of the concentration effect and have obtained a quantity that is
proportional to the conductivity of the ions from one molecule of electrolyte, on average.