Agriculture Reference
In-Depth Information
The classification of electrodes is based upon the chemical nature of the
substances participating in the electrochemical process. Electrodes of the
first type are systems in which the reduced forms are metals of electrodes
and oxidized forms are ions of the same metal. Electrodes of the second
type are systems in which the metal is covered by a layer of low-solubility
salts (or oxide), and the solution contains anions of these salts (for oxide -
OH
−
ions).TheNernstequationforelectrodesofthesecondtypecanbe
written as
E
0
−
RT
=
E
nF
ln
a
A
n
−
,
(24.2)
where
E
0
is the standard potential of an electrode of the second kind. As
follows from Eq. 24.2 that electrodes of the second kind are anions re-
versible. The potential of an electrode of the second kind depends on the
anionic activity of the sparingly soluble compound of the electrode mate-
rial. The values of the potentials of electrodes of the second kind are readily
reproducible and stable. These electrodes are often employed as standard
half-cell or reference electrodes with respect to which the potentials of
other electrodes are measured (Volkov et al. 1998). Of greatest interest in
practice are reversible Ag/AgCl electrodes. Ag/AgCl electrodes consist of
a piece of silver wire covered with a layer of silver chloride and immersed
in an electrolyte solution containing chloride ions. Ag/AgCl electrodes are
initially unstable; however, stabilization can be accomplished by placing
two electrodes for 24 h in 0.05 M KCl solution and connecting a short cir-
cuit between them. Ag/AgCl electrodes should be stored in the dark and
protected from light while in use. Ag/AgCl electrodes are sensitive to tem-
perature and should be maintained at constant temperature (Ksenzhek and
Volkov 1998).
24.4
Measuring of Action, Graded,
and Variation Potentials in Plants
Two distinct classes of cell bioelectrochemical potential measurements
exist. The intracellular action potential is measured with one electrode
placed inside a cell while the reference electrode is situated in the con-
ducting medium surrounding the cell. The extracellular action potential is
measured with both electrodes in contact with the conducting tissue em-
bodying larger groups of cells. In the latter case the signal observed is due
to the depolarization-repolarization process in a group of cells. Measure-
ments of these two types have been made in animal electrophysiology. The
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