Measured cell emf are the basis for standard electrode potentials. chemistry assignments
A method for the presentation of the data obtained from measurements of the equilibrium emf of electrochemical cells must now be developed. One expects (a thermodynamic proof of this is given in the description) that varying the concentration of any reagents involved in the electrode process will affect the emf of the cell. In view of this, the emfs of the cell with the reagents at standard states are reported. In keeping with standard states introduced with these standard state conditions are again chosen to consist of gases at unit activity, implying for most gases approximately 1 bar pressure, and solutes also at unit activity. For solutes, which for electrochemical cells are frequently ions, the activities can be appreciably different from molar concentrations. How activity coefficients can be determined to recognize that a standard state is chosen and that the emf's that cells would have for their variable reagents at unit activity are tabulated.
It is impractical, however, to list the emf's of all possible combinations of electrodes. It would be much more convenient to have some means of tabulating the relative electron accepting powers of the individual electrodes. This can be done by tabulating the emf of cells composed of a selected reference electrode and a variety of other electrodes. Quite arbitrarily, it is agreed that the hydrogen electrode, with a hydrogen pressure of 1 bar and a unit hydrogen ion activity in the solution, will be used as the reference electrode.
Tabulations can then be given of the emf measured for cells with the standard hydrogen electrode on the left and another electrode on the right. For example:
Pt| H2 (1 bar)| HCl (a = 1) | Cl2 (1 bar) | Pt
The emf of this particular cell is found to be + 1.3592 V and it is this value which is given for the Pt| Cl2 |Cl-electrode.
Such tabulated values also are interpreted as electrode emf's based on a zero emf being assigned to the reference hydrogen electrode. If the value is for all reagents at unit activity, such electrode potentials are known as standard electrodes potentials, indicated by the symbol €°. The results for the hydrogen chlorine cell can now be interpreted by recalling that the reported cell emf of +1.3592 V corresponds to the process in which electrons are gained from the external circuit by the right electrode and delivered to the external circuit by the left electrode.