Electrode potentials, standard calculating electromotive force from

It is impossible to measure directly the electrode potentials. Only the electromotive force (emf) of a voltaic cell arising from a combination of two electrodes can be directly measured, which is given as the arithmetical sum or difference of the two electrode potential depending upon their signs. If one of the electrode potential be accurately measured, that of the other may be calculated. The reference electrode arbitrarily chosen for this purpose is the standard hydrogen electrode. Hydrogen gas at 1 atm. pressure and at a temperature of 25°C is slowly bubbled over a platinised platinum electrode which is immersed in a solution of hydrogen ions of unit activity. By convention potential of the half cell reaction... 

Before we discuss standard electrode potential, we will talk about electromotive force (emf). The electromotive force of a cell is the potential difference between the two electrodes. This can be measured using a voltmeter. The maximum voltage of a cell can be calculated using experimentally determined values called standard electrode potentials. By convention, the standard electrode potentials are usually represented in terms of reduction half-reactions for 1 molar solute concentration. The standard electrode potential values are set under ideal and standard-state conditions (latm pressure and 25°C temperature). From the MCAT point of view, you can assume that the conditions are standard, unless stated otherwise. Table 12-1 shows a list of standard electrode potentials (in aqueous solution) at 25°C. 

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