Biology Reference
In-Depth Information
6.2. POTENTIOMETRY
Potentiometry measures, through a high input impedance equipment
(potentiometer), the oxidation-reduction potentials difference of a solution
between a working or indicator electrode and a counter electrode under
static conditions. Electrodes used for providing a reference potential usually
include hydrogen, saturated calomel, or silver chloride (AgCl). However, for
routine analytical work, a hydrogen electrode is rarely chosen due to difficulty
in preparation and use of the setup. The silver chloride electrode is preferred
when the temperature of the system is not controlled. The working electrode
measures the current generated by the system without any external excita-
tion. In modern arrangement, each electrode is placed in a separate solution
and connected to the potentiometer instrument; while a salt bridge, contain-
ing an inert electrolyte, such as potassium chloride, is exposed to each sample,
completing an electrical circuit (
Fig. 6.2
). Usually, the counter electrode is
called the anode and the working electrode is referred to as the cathode.
Measurements are always made when no or very little current is present,
so the composition of the substance measured is not altered, making quantita-
tive analysis possible. In such conditions, the measured potential can be related
to the concentration of one or more electroactive analytes present in the sam-
ple. Indeed, this potential is the difference between the reduction potentials
Figure 6.2
Typical arrangement for a potentiometric sensor. Source:
Adapted from
Ref.
8
.
It consists of an indicator and a reference electrode that are immersed into the
sample solution and connected to the two terminals of a voltmeter. The working elec-
trode is made of an ion-selective membrane. This membrane separates the sample solu-
tion and the inner filling solution of the working electrode. (For color version of this
igure, the reader is referred to the online version of this topic.)
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