This is arbitrarily assigned an electromotive force of zero volts. A very cumbersome and temperamental electrode for this reaction can be fabricated from a catalyst called platinized platinum over which hydrogen in bubbled. This inconvenient electrode is very seldom used except for laboratory calibrations or with non-aqueous systems where it is necessary to return to the definition of the hydrogen electrode. Hydrogen ion concentration is conveniently expresses as pH. Glass electrodes that are sensitive to hydroxyl ions are used for measuring pH, and the EMF is compared to a reference cell. As anyone who has used a pH meter knows, there are convenient, inexpensive, reliable reference electrodes.
Metals can react with solutions to reduce ions.
All solids are assigned an activity of one. Metals tend to displace from solution other metals that have a lower standard potential. However, standard potential is not the whole story. Potential for a given system depends on activity. As with other discussions for environmental engineering, assume that activity and concentration are roughly equal for dilute solutions.