The potential developed by a single electrode in a solution is caused by the tendency of the solution either to donate or accept electrons and can be
> The Nernst equation expresses the potential developed by an electrode in terms of the activity and number of ions Involved in the reaction.
>• A half-cell describes a single electrode and its associated chemical reaction which forms part of a voltaic cell.
>■ A half-reaction is the reaction that occurs at a single electrode but that requires another reaction to accept or donate the electrons involved in the overall process.
>■ An active electrode is composed of an element that is in an equilibrium with its ions in the surrounding solution.
>- The activity of an ion reflects the proportion of the molecules that are actually ionized at the time.
> Inert electrodes are used to make electrical contact with solution and are not part of the chemical reaction.
> Standard electrode potential Is the potential developed by an active electrode when in equilibrium with a molar solution of its ions.
calculated using the Nernst equation: 23026RT
X log a where E is the electrode potential at the specified concentration,
E° is the standard electrode potential,
R is the gas constant,
T is the absolute temperature, n is the number of electrons involved,
F is the Faraday constant, a is the activity of the ion. For measurements made at 25 °C the equation simplifies to:
There can be no chemical reaction in such a system without a complementary electron donor or acceptor to complete the process. Each of these electrode systems is known as a half-cell and the potential developed by a half-cell cannot be measured in absolute terms but only compared with that of another half-cell. The chemical reaction occurring at each half-cell is known as a half-reaction.
An active electrode consists of an element (M) in its uncombined state which is capable of establishing an equilibrium with a solution that contains its ions:
The ionization of atoms or molecules results in a potential being developed by such an electrode, the intensity of the potential being related to the concentration of the ions. The effective concentration of the ions (known as the activity of the ions) is more significant than the molar concentration. The values for activity and concentration are only the same in very dilute solutions.
Inert electrodes, such as silver, platinum, and carbon, are used solely to make electrical contact with the solution and only reflect the potential of the solution. They are used to measure the potential of solutions containing mixtures of ions which have a tendency to transfer electrons between them, e.g. ferric and ferrous ions:
Such reactions are know as redox reactions and, in this case, the potential developed by the electrode system depends on the tendency of ferrous ions to donate an electron compared with the tendency of the ferric ions to accept one.
The standard electrode potential of an element is defined as its electrical potential when it is in contact with a molar solution of its ions. For redox systems, the standard redox potential is that developed by a solution containing molar concentrations of both ionic forms. Any half-cell will be able to oxidize (i.e. accept electrons from) any other half-cell which has a lower electrode potential (Table 4.1).
Table 4.1 Standard electrode potentials at pH 7.0
Oxygen/hydrogen peroxide 0.30
Cytochrome c ferric/ferrous 0.22
Dehydroascorbic acid/ascorbic acid 0.08
Ferrous/iron - 0.44
It is impossible to measure the potential of a half-cell directly and a reference half-cell must be used to complete the circuit. The hydrogen electrode (Figure 4.3) is the standard reference electrode against which all other half-cells are measured and is arbitrarily attributed a standard electrode potential of zero at pH 0. Because it is difficult to prepare and inconvenient to use, the
Standard HCl solution
Figure 4.3 The hydrogen electrode. An electrode of platinum foil, covered with platinum black, dips into a solution of hydrochloric acid (1.0 mol I '). Hydrogen gas at a pressure of 1 atm (101 kPa) is bubbled over the electrode and is absorbed by the platinum black. The half-reaction for the electrode can be represented as: 2H+ + 2e~ = H2
> A saturated calomel electrode is conveniently used as a reference electrode in potentiometnc measurements.
saturated calomel electrode is frequently used instead. This has a standard electrode potential of +0.242 V relative to the hydrogen electrode. It consists of a mercury electrode in equilibrium with mercuric ions in the slightly soluble salt, mercuric chloride (Figure 4.4). A high concentration of chloride ions is maintained by using a saturated solution of potassium chloride, which also provides a means of ensuring electrical contact with any other half-cell used.
Mercury • Paste
■ Saturated potassium chloride
Figure 4.4 The saturated calomel electrode. A platinum wire makes electrical contact with an electrode which is composed of a paste of metallic mercury, mercuric chloride (calomel) and potassium chloride. A saturated solution of potassium chloride completes the half-cell and provides electrical contact through a porous plug.
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