Page 215 - Modern physical chemistry

208 Electrochemistry

When this ratio varies with the time t, it is replaced with the derivative

1= dQ. [9.4]
dt
The ratio of the voltage l between two cross sections to the amperage I flowing
between them is the resistance R between the cross sections in ohms (abbreviated n).
We have
R=&. [9.5]
I
When this ratio varies with the current, it is replaced with the derivative

R=d&. [9.6]
dI
The reciprocal of resistance is known as the conductance L:

L=~. [9.7]
R

The unit for conductance is the siemens (8), which is one reciprocal ohm (n -1) or mho.
To measure conductance, a person may attach or insert two equivalent electrodes
onto or into the phase to be studied, apply a potential, and determine the resulting
current.

9.2 Electrode Reactions
A change in carrier of current generally occurs at the interface between an electrode
and a solution. This is effected by a chemical reaction in which the electron is a product
or a reactant.
In a conductance experiment, electrons are drawn away from the anode by an exter-
nal source of potential. The necessary electrons may be produced in several different
ways. These include the following possibilities:
1. An anion discharges. As an example

[9.8]

2. A cation is produced; thus
[9.9]

Cu ~ Cu++ +2e. [9.10]
3. A cation is oxidized; for example

[9.11]

[9.12]
4. The hydroxide ion from water is discharged when no other oxidation readily occurs:

[9.13]