• The potential difference in the double layer of a given electrode is considered its electrode potential. This absolute difference cannot be measured. It can only be measured relative to another electrode.
• An electrode with a known or a defined potential used to measure the potential of a given electrode is called a reference electrode. Standard electrode potentials are measured with reference to the standard hydrogen electrode. The potential of the standard hydrogen electrode is defined as 0.00 V {[H+ (aq)] is 1.0 mol dm-3 and pressure of H2(g) is 1 atm and temperature is 298 K}. Calomel electrode and AgCl(s)|Ag(s) electrode are used as practical reference cells. Standard electrode potentials of Calomel electrode and AgCl(s)|Ag(s) electrode are +0.22 V and +0.2415V respectively.
• The value of the potential of a given electrode under standard conditions measured relative to the standard hydrogen electrode (SHE) is its standard electrode potential.
• The following diagram illustrates a set up that can be used for measuring the standard electrode potential of the Zn2+ (aq)|Zn(s) electrode.
• When the standard electrode potentials (reduction potential) of different electrodes are arranged in the ascending order, the electrochemical series is obtained.
Factors affecting the electrode potential
• Temperature
• Concentration of the electrolyte
• Nature of electrolyte
• Pressure (of a gas)
• Type of electrode
• Electromotive force (e.m.f.) is defined as the potential difference between the two electrodes when no current flows through the cell.
• Temperature, concentration of the electrolyte, electrode type and the nature of the electrolyte are the factors that affect the electromotive force. It is independent of the distance betweenthe electrodes and the surface area of the electrodes.
• Electrolytes in the two electrodes of an electrochemical cell are connected to complete the internal circuit through a salt bridge or a permeable membrane/diaphragm/porous partition.A tube filled with a solution of a salt such as KCl or NH4NO3 which is gelified with agar is used as a salt bridge. Such an electrochemical cell is known as a cell with a liquid junction.
• In a cell, when both electrodes are not separated by a salt bridge or a permeable membrane/diaphragm/porous partition is referred as a cell without a liquid junction.
• According to the international convention, electrode potentials are measured relative to the standard hydrogen electrode. On this standard, the electrode potential of the standard silver-silver chloride electrode is 0.22 V. Similarly, the electrode potentials of other standard electrodes can also be calculated. Show that in arranging these in the ascending order, electrochemical series is obtained.
Ecell = Ecathode – Eanode
