In an electrochemical cell, each electrode is placed in contact with the electrolytic solution. Thus at the surface of separation of the electrode and the electrolyte solution, there exists an electrostatic potential which is known as electrode potential. Let us see in detail how the electrostatic potential arises?
All metallic element and hydrogen have a tendency to pass into solution in the form of positive ions. This property of the metal is known as the ‘solution pressure’, ‘electrolytic solution pressure’ or ‘solution tension’ of the metal and is constant at a given temperature. Due to the migration of the positive ions, the metallic electrode is left negatively charged and thus electrical double layer is set up at the electrode.
If a metallic electrode is dipped in a solution of one of its salts, the position becomes slightly different. In such a case, the tendency of the ions is to be deposited on the electrode. This backward reaction is attributed to be osmotic pressure of ions in solution. Thus, the standard potential of a metal is equal to the difference between its solution pressure and the osmotic pressure of its ions. Hence there arises three possibilities:
a) If the solution pressure is greater than the osmotic pressure, the tendency of the metal to lose ions predominates. A potential difference is therefore set up with the metal left with negative charge with respect to the solution. The net result will be that the positive ions will enter the liquid and leave the metal negatively charged with respect to solution. The formation of double layer prevents the further expulsion of ions from the metal, and thus there is rapidly established a state of equilibrium with a definite potential difference, termed as the electrode potential.
b) If the solution pressure is less than the osmotic pressure of the metal in solution, it means that the ions have greater tendency to leave the solution and get deposited on the metal. So potential difference is established due to the charge separations as shown in the following diagram.
c) When the solution pressure becomes equal to that of osmotic pressure, no relative charge is developed and hence no potential difference exists. Such a system is sometimes termed as null electrode as shown below:
Thus, the tendency of an electrode to lose or gain electrons when it is in contact with its own ions in solution is called electrode potential. Since the tendency to lose electrons means also the tendency to get oxidised, this tendency is called oxidation potential. Similarly, the tendency to gain electrons means the tendency to get reduced. Hence this tendency is called reduction potential. Needless to mention that reduction potential is the reverse of oxidation potential. So far, we have seen how the potential was developed. Now we are going to see how to determine the electrode potential. It is not possible to determine experimentally the potential of a single electrode. It is only the difference of potentials between two electrodes that we can measure by combining them to give a complete cell.
By arbitrarily fixing potential of one electrode as zero (just as the boiling point of water at atmospheric pressure has been arbitrarily fixed as 100 on temperature scale), it is possible to assign numerical values to potentials of the various other electrodes. Accordingly, the potential of a reversible hydrogen electrode in which the gas at one atmospheric pressure is bubbled through a solution of hydrogen ion of unit activity (or to be approximate, unit concentration has been fixed as zero. This electrode is known as Standard Hydrogen Electrode (SHE) and is represented as, Pt, H2(1atm), H+(C = 1)
All other single electrode potentials are referred to as potentials on the hydrogen scale. If it is required to find the electrode potential of, say zinc electrode dipping in a solution of zinc sulphate ((i.e.,) Zn, Zn2+ electrode), all that is needed is to combine it with the standard hydrogen electrode so as to have a complete cell represented as,
The emf of the cell is determined potentiometrically, is then equal to the potential of the electrode (on the hydrogen scale) since the potential of the standard hydrogen electrode is taken as zero.
Since , Ecell = ERP(cathode) − ERP(Anode)
In this case S.H.E. is undergoing oxidation. So, whatever reading we are getting in the potentiometer directly gives the potential of Zn/Zn2+ electrode.
In case, the electron accepting tendency of the metal electrode is more than that of a S.H.E., its standard reduction potential gets a positive sign. On the other hand, if the electron accepting tendency of the metal electrode is less than that of S.H.E, its standard electrode potential gets a negative sign. According to latest convention, all standard electrode potentials are taken as reduction potentials.
Thus, the electrode at which reduction occurs with respect to S.H.E has positive electrode potential and while the electrode at which oxidation occurs with respect to S.H.E has negative electrode potential