Anode
The term anode, from the Greek words hodos, meaning way, and ana, meaning up, was used by experimenters in the late ninteenth and early twentieth century to designate the conducting to which negatively charged particles, electrons, were drawn when they subjected partially evacuated tubes to high voltage. The tubes were made of glass and typically had a voltage source connected to two pieces of conducting materials sealed into the glass and separated by a few inches. The piece of conducting material from which the electrons emanated was called the cathode and the one to which they were attracted was called the anode.
In chemistry the most common use of the term anode occurs in electrochemistry. Electrochemical cells consist of one half-cell in which oxidation occurs and a second half-cell in which reduction occurs. Electrons flow though a conductor connecting the two half-cells. At the two ends of the conductor are the electrodes, which gather and disperse the electrons. The anode is the electrode at which oxidation takes place and the cathode is the electrode at which reduction takes place.
A voltaic cell, or a sources of energy, is an electrochemical cell is one in which an oxidation-reduction reaction takes place simultaneously. The everyday batteries we commonly use are voltaic cells. Because electrons originate at the anode in a voltaic cell, the anode has a (-) charge; electrons enter the cathode, which has a (+) charge. This polarity of the terminals and the role of the electrodes are accurate for any cell that is operating spontaneously and producing electricity.
In an electrolytic cell, energy must be supplied in the form of electric current from an external source. Electrolytic cells are used to electroplate tableware and jewelry and to produce useful chemicals from naturally occurring materials. In an electrolytic cell, the power source pumps electrons into the electrode to which it is attached; that electrode is the cathode. Therefore, in electrolytic cells, the cathode is the negative terminal and the anode is the positive terminal.
The polarities of the anode and cathode are reversed in comparison to their assignments in voltaic cells, but the function of the anode, being the site of oxidation, and of the cathode, being the site of reduction, is the same.
The materials that are useful for anodes must be good conductors and must not corrode too easily under oxidizing conditions. If the anode itself is the substance that one wishes to oxide, it should be the most easily oxidized material in the half-cell. If another substance is to be oxidized, the anode material should not be readily oxidized. Some materials commonly used as unreactive anodes are platinum and graphite.
One of the most common materials using an anode is zinc metal. The zinc containers of carbon-zinc and alkaline dry cell batteries and mercury batteries serve as the anode and are oxidized as the battery is used. In NiCad batteries, the anode is composed of cadmium metal and cadmium hydroxide. As the battery is used, cadmium metal is oxidized to cadmium hydroxide and as it is recharged the process is reversed.
Electrochemical techniques are also widely used in chemical analysis. In electrochemical analysis by such methods as anodic stripping voltammetry, cyclic voltammetry, differential pulse polarography, and potentiometry, the anode always functions as the site of oxidation. In general, the anode is a highly conductive and relatively inert material such as platinum, graphite or mercury.
This is the complete article, containing 559 words
(approx. 2 pages at 300 words per page).
