One cannot make a solid electrode out of hydrogen gas, of course, but the same effect can be achieved by bubbling a stream of hydrogen gas over an inert conductor such as a platinum electrode, as shown opposite.

The H₂ molecules can dissociate at the surface of the platinum, give up their electrons to the external circuit through the metal electrode, and go into solution as H + ions. Similar electrodes can be made with other gases.

In thinking about standard cell reactions, it is customary to forget that they are paired with the hydrogen half-reaction, and to speak of them as if they were isolated reactions of one half of the cell.

We can talk about half-reactions, and their free energies and half-cell potentials, as though the following had physical reality:

The potentials as written above are reduction potentials, positive if the ion is easier to reduce than H+ ions, and negative if harder to reduce than H+. A large positive reduction potential for a half-reaction is a sign that the reduced form of the substance is strongly favored, as with copper in the above examples.