In a voltaic cell, there is always an oxidized species and a reduced species.
Oxidation is the loss of an electron (increase in the oxidation state).
Reduction is the gain of an electron (decrease in the oxidation state).
It is important to remember that if one of these processes is occurring then the other must occur, hence the term redox.
The likelihood that a metal will release an electron is called the potential. However, the potential of each metal cannot be measured directly since every reduction has to be accompanied by an oxidation (the species being oxidized will have its own potential). The international scientific community has resolved this problem by arbitrarily agreeing on:
a reference electrode (or redox couple). This reference electrode is the standard hydrogen electrode (SHE; E° = 0 V).
when tabulating potentials, they will all be represented as reductions (see a table of reduction potentials in a textbook or the "CRC Handbook of Chemistry and Physics."
reduction occurs at the cathode, and oxidation occurs at the anode. (Note: Reduction and cathode both start with a consonant; and oxidation and anode both start with a vowel).
in a cell, the more active metal will be the anode (it will have a more negative half-cell potential).
