Although it is not difficult to measure relative half-cell potentials, it is impossible to determine absolute half-cell potentials because all voltage measuring devices measure only differences in potential. To measure the potential of an electrode, one contact of a voltmeter is connected to the electrode in question. The other contact from the meter must then be brought into electrical contact with the solution in the electrode compartmenet via another conductor. This second contact, however, inevitably involves a solid/solution interface that acts a a second half-cell at which chemical change must take place if charge is to flow and a potential difference is to be measured. A potential is associated with this second reaction. Thus an absolute half-cell potential is not obtained but rather the difference between the half-cell potential of interest and a half-cell made up of the second contact with the solution.
Our inability to measure absolute half-cell potentials presents no real obstacle because relative half-cell potentials are just as useful provided they are all measured against the same half-cell. Relative half-cell potentials can be combined to generate cell potentials. The standard reference half-cell is the standard hydrogen electrode.