There are two kinds of electrochemical cells: those in which chemical reactions produce electricity—called galvanic cells or voltaic cells—and those in which electricity produces chemical reactions—called electrolytic cells. An example of a galvanic cell is a flashlight battery, and an example of an electrolytic cell is a cell used for electroplating silver or gold. In either case, there are two electrodes called the anode and the cathode.
Unfortunately, there has been much confusion about which electrode is to be called the anode in each type of cell. Chemists and physicists correctly consider electricity to be a flow of negative electrons, but for historical reasons, engineers have considered electricity to be a flow of positive charge in the opposite direction. Furthermore, even chemists have been confused because negative charge flows away from one of the electrodes inside the cell, but in the external circuit negative charge flows toward that same electrode. This has led to a variety of conflicting definitions of anodes in various textbooks and reference works.
The confusion can be cleared up by defining the anode and cathode in terms of the actual chemical reactions—the oxidation and reduction reactions—that are taking place inside the cell, whether the cell is generating electricity as a galvanic cell or consuming it as an electrolytic cell. The anode is now defined as the electrode at which an oxidation reaction is taking place in the cell. The cathode, then, is the electrode at which the corresponding reduction reaction is taking place.
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