anode

Diagram of a zinc anode in a galvanic cell.

An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID (Anode Current Into Device). Electrons flow in the opposite direction to the electric current (flow of hypothetical positive charge)

A widespread misconception is that anode polarity is always positive. This is often incorrectly inferred from the correct fact that in all electrochemical devices negatively charged anions move towards the anode (hence their name) and/or positively charged cations move away from it. In fact anode polarity depends on the device type, and sometimes even in which mode it operates, as per the above electric current direction-based universal definition. Consequently, as can be seen from the following examples, in a device which consumes power the anode is positive, and in a device which provides power the anode is negative:

• In a discharging battery or galvanic cell (diagram at right) the anode is the negative terminal, where the hypothetic charges constituting a conventional current flow in, and electrons out. Since this inwards current is carried externally by electrons moving outwards, the negative charge moving one way amounts to positive charge flowing into the electrolyte from the anode, i.e., away (surprisingly) from the more negative electrode and towards the more positive one (chemical energy is responsible for this "uphill" motion). If the anode is composed of a metal, electrons which it gives up to the external circuit must be accompanied by metal atoms missing those electrons (cations) moving away from the electrode and into the electrolyte.
• In a recharging battery, or an electrolytic cell, the anode is the positive terminal, which receives current from an external generator. The current through a recharging battery is opposite to the direction of current during discharge; In other words, the electrode which was the cathode during battery discharge becomes the anode while the battery is recharging.
• In a diode, it is the positive terminal at the tail of the arrow symbol, where current flows into the device. Note electrode naming for diodes is always based on the direction of the forward current (that of the arrow, in which the current flows "most easily"), even for types such as zener diodes or solar cells where the current of interest is the reverse current.
• In a cathode ray tube, it is the positive terminal where electrons flow out, i.e., where positive electric current flows in.

An electrode through which current flows the other way (out of the device) is termed a cathode.

Etymology

The word was coined in 1834 from the Greek ἄνοδος (anodos), 'way up', by William Whewell, who had been consulted^[1] by Michael Faraday over some new names needed to complete a paper on the recently discovered process of electrolysis. In that paper Faraday explained that when an electrolytic cell is oriented so that electric current traverses the "decomposing body" (electrolyte) in a direction "from East to West, or, which will strengthen this help to the memory, that in which the sun appears to move", the anode is where the current enters the electrolyte, on the East side: "ano upwards, odos a way ; the way which the sun rises" (^[2], reprinted in ^[3]).

The use of 'East' to mean the 'in' direction (actually 'in' → 'East' → 'sunrise' → 'up') may appear unnecessarily contrived. Previously, as related in the first reference cited above, Faraday had used the more straightforward term "eisode" (the doorway where the current enters). His motivation for changing it to something meaning 'the East electrode' (other candidates had been "eastode", "oriode" and "anatolode") was to make it immune to a possible later change in the direction convention for current, whose exact nature was not known at the time. The reference he used to this effect was the Earth's magnetic field direction, which at that time was believed to be invariant. He fundamentally defined his arbitrary orientation for the cell as being that in which the internal current would run parallel to and in the same direction as a hypothetical magnetizing current loop around the local line of latitude which would induce a magnetic dipole field oriented like the Earth's. This made the internal current East to West as previously mentioned, but in the event of a later convention change it would have become West to East, so that the East electrode would not have been the 'way in' any more. Therefore "eisode" would have become inappropriate, whereas "anode" meaning 'East electrode' would have remained correct with respect to the unchanged direction of the actual phenomenon underlying the current, then unknown but, he thought, unambiguously defined by the magnetic reference. In retrospect the name change was unfortunate, not only because the Greek roots alone do not reveal the anode's function any more, but more importantly because, as we now know, the Earth's magnetic field direction on which the "anode" term is based is subject to reversals whereas the current direction convention on which the "eisode" term was based has no reason to change in the future.

Since the later discovery of the electron, an easier to remember, and more durably correct technically although historically false, etymology has been suggested: anode, from the Greek anodos, 'way up', 'the way (up) out of the cell (or other device) for electrons'.

Flow of electrons

The flow of electrons is always from anode to cathode outside of the cell or device, regardless of the cell or device type and operating mode, with the exception of diodes, where electrode naming always assumes current flows in the forward direction (that of the arrow symbol), i.e., electrons flow in the opposite direction, even when the diode reverse-conducts either by accident (breakdown of a normal diode) or by design (breakdown of a Zener diode, photo-current of a photodiode or solar cell).

Electrolytic anode

In electrochemistry, the anode is where oxidation occurs, and is the positive polarity contact in an electrolytic cell. At the anode, anions (negative ions) are forced by the electrical potential to react chemically and give off electrons (oxidation) which then flow up and into the driving circuit.

This process is widely used in metals refining. For example, in copper refining, copper anodes, an intermediate product from the furnaces, are electrolysed in an appropriate solution (such as sulfuric acid) to yield high purity (99.99%) cathodes. Copper cathodes produced using this method are also described as electrolytic copper.

Battery or galvanic cell anode

In a battery or galvanic cell, the anode is the negative electrode from which electrons flow out towards the external part of the circuit. Internally the positively charged cations are flowing away from the anode (even though it is negative and therefore would be expected to attract them, this is due to electrode potential relative to the electrolyte solution being different for the anode and cathode metal/electrolyte systems); but, external to the cell in the circuit, electrons are being pushed out through the negative contact and thus through the circuit by the voltage potential as would be expected. Note: in a galvanic cell, contrary to what occurs in an electrolytic cell, no anions flow to the anode, the internal current being entirely accounted for by the cations flowing away from it (cf drawing).

In the United States, many battery manufacturers regard the positive electrode as the anode, particularly in their technical literature. Though technically incorrect, it does resolve the problem of which electrode is the anode in a secondary (or rechargeable) cell. Using the traditional definition, the anode switches ends between charge and discharge cycles.

Vacuum tube anode

In electronic vacuum devices such as a cathode ray tube, the anode is the positively charged electron collector. In a tube, the anode is a charged positive plate that collects the electrons emitted by the cathode through electric attraction. It also accelerates the flow of these electrons.

Diode anode

In a semiconductor diode, the anode is the P-doped layer which initially supplies holes to the junction. In the junction region, the holes supplied by the anode combine with electrons supplied from the N-doped region, creating a depleted zone. As the P-doped layer supplies holes to the depleted region, negative dope ions are left behind in the P-doped layer ('P' for positive charge-carrier ions). This creates a base negative charge on the anode. When a positive voltage is applied to anode of the diode from the circuit, more holes are able to be transferred to the depleted region, and this causes the diode to become conductive, allowing current to flow through the circuit. The terms anode and cathode should not be applied to a zener diode, since it allows flow in either direction, depending on the polarity of the applied potential (i.e. voltage).

Sacrificial anode

In cathodic protection, a metal anode that is more reactive to the corrosive environment of the system to be protected is electrically linked to the protected system, and partially corrodes or dissolves, which protects the metal of the system it is connected to. As an example, an iron or steel ship's hull may be protected by a zinc sacrificial anode, which will dissolve into the seawater and prevent the hull from being corroded. Sacrificial anodes are particularly needed for systems where a static charge is generated by the action of flowing liquids, such as pipelines and watercraft.

A less obvious example of this type of protection is the process of galvanising iron (though the name of the process provides the essential clue). This process coats iron structures (such as fencing) with a coating of zinc metal. As long as the zinc remains intact, the iron is protected from the effects of corrosion. Inevitably, the zinc coating becomes breached, either by cracking or physical damage. Once this occurs, corrosive elements act as an electrolyte and the zinc/iron combination as electrodes. The resultant current flow ensures that the zinc coating is sacrificed but that the base iron does not corrode. Such a coating can potentially protect an iron structure for a few decades, but once the protecting coating is consumed, the iron rapidly corrodes.

At least one anode is found in tank-type water heaters. The anode should be removed and checked after 5 years (sooner if there is a sodium based water softner inline), and replaced if 15 cm (6 inches) or more of bare wire is showing. This will greatly extend the life of the tank.

Water heater anode information

Related antonym

The opposite of an anode is a cathode. When the current through the device is reversed, the electrodes switch functions, so anode becomes cathode, while cathode becomes anode, as long as the reversed current is applied, with the exception of diodes where electrode naming is always based on the forward current direction.

See also

References

1. ^ Ross, S, Faraday Consults the Scholars: The Origins of the Terms of Electrochemistry in Notes and Records of the Royal Society of London (1938-1996), Volume 16, Number 2 / 1961, Pages: 187 - 220, [1] consulted 2006-12-22
2. ^ Faraday, Michael, Experimental Researches in Electricity. Seventh Series, Philosophical Transactions of the Royal Society of London (1776-1886), Volume 124, 01 Jan 1834, Page 77, [2] consulted 2006-12-27 (in which Faraday introduces the words electrode, anode, cathode, anion, cation, electrolyte, electrolyze)
3. ^ Faraday, Michael, Experimental Researches in Electricity, Volume 1, 1849, reprint of series 1 to 14, freely accessible Gutenberg.org transcript [3] consulted 2007-01-11

External links

• The Cathode Ray Tube site
• How to define anode and cathode
• Valence Technologies Inc. battery education page
• Cathodic Protection Technical Library

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