Page 361 - Physical chemistry understanding our chemical world

328 ELECTROCHEMISTRY

Why ‘earth’ a plug?

Reference electrodes

An electrical plug has three connections (or ‘pins’): ‘live’, ‘neutral’ and ‘earth’. The
earth pin is necessary for safety considerations. The potential of the earth pin is the
same as that of the ground, so there is no potential difference if we stand on the
ground and accidentally touch the earth pin in a plug or electrical appliance. We will
not be electrocuted. Conversely, the potentials of the other pins are different from
that of the earth – in fact, we sometimes cite their potentials with respect to the earth
pin, effectively defining the potential of the ground as being zero.
The incorporation of an earth pin is not only desirable for safety, it also enables
us to know the potential of the other pins, because we cite them with respect to the
earth pin.
A reaction in an electrochemical cell comprises two half-cell reactions. Even when
we want to focus on a single half-cell, we must construct a whole cell and determine
its cell emf, which is defined as ‘E (positive electrode) − E (negative electrode) ’. Only when we
know both the emf and the value of one of the two electrode potentials can we
calculate the unknown electrode potential.
A device or instrument having a known, predetermined electrode
A reference electrode potential is called a reference electrode. A reference electrode is
is a constant-potential always necessary when working with a redox couple of unknown
device. We need such E O,R . A reference electrode acts in a similar manner to the earth
a reference to deter- pin in a plug, allowing us to know the potential of any electrode
mine an unknown with respect to it. And having defined the potential of the refer-
electrode potential. ence electrode – like saying the potential of earth is zero – we then
know the potential of our second electrode.

Aside

At the heart of any reference electrode lies a redox couple of known composition: any
passage of current through the reference electrode will change its composition (we argue
this in terms of Faraday’s laws in Table 7.1). This explains why we must never allow a
current to flow through a reference electrode, because a current will alter its potential.

The standard hydrogen electrode – the primary reference

The internationally accepted primary reference is the standard
We define the value hydrogen electrode (SHE). The potential of the SHE half-cell is
of E (SHE) as zero at all defined as 0.000 V at all temperatures. We say the schematic for
temperatures. the half-cell is

+
Pt|H 2 (a = 1)|H (a = 1, aq)|