anode

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Anode

Vacuum tubes

Electrochemical cells

Anodes in practical use

The word anode is used in two different sets of circumstances: with respect to vacuum tubes and with respect to electrochemical cells.

Vacuum tubes

A vacuum tube is a tube (usually made of glass) with most of the air pumped out, which usually contains two electrodestwo pieces of metal with a potential difference (a voltage difference) applied between them. Electrons, being negatively charged, are repelled out of the negative electrode and fly through the vacuum toward the positive electrode, to which they are simultaneously attracted. The positive electrode is called the anode; the negative electrode is called the cathode.

Common examples of vacuum tubes in which electrons flow from a cathode to an anode are cathode ray tubes, such as those in television tubes, computer monitors, and x-ray tubes. In an x-ray tube, the kind of metal that the anode is made of determines the kind of x rays (i.e., the x-ray energy) that the tube emits.

Electrochemical cells

There are two kinds of electrochemical cells: those in which chemical reactions produce electricitycalled galvanic cells or voltaic cellsand those in which electricity produces chemical reactionscalled 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 an anode and a cathode in each cell.

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 reactionsthe oxidation and reduction reactions that take place inside the cell, whether it is generating electricity as a galvanic cell or consuming it as an electrolytic cell. The anode is 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.

Anodes in practical use

A sacrificial anode is a piece of metal that acts as an anode, and is therefore oxidized, to protect another piece of metal from being oxidized. For example, to keep iron or steel from oxidizing (rusting) when in contact with air and moisture, such as when it is being used as a fence post, the post can be connected to a piece of zinc that is buried in the ground next to it. The iron and zinc in the moist soil constitute the two electrodes of a galvanic cell. Because zinc oxidizes more easily than iron, the zinc acts as the anode and is preferentially oxidized. It is said to be sacrificed because it gradually gets eaten away by oxidation instead of the iron. For the same reason, a zinc cable was buried alongside the Alaskan oil pipeline, the huge steel pipe that transports petroleum from the Alaskan oil fields to the lower United States.

Galvanized iron has been coated with zinc so that it can be used outdoors or in the ground without rusting. The zinc oxidizes in preference to the iron. Galvanized iron is widely used in garbage cans, pails, and chain-link fencing.

Anodizing is a process in which a piece of metal is made the anode in an electrolytic cell to oxidize it deliberately. When aluminum is anodized in this way, an aluminum oxide coating builds up on its surface. This coating, unlike the metal itself, can take dyes. Many kinds of aluminum utensils and novelties in bright blue, green, red, and gold are made of anodized aluminum.

See also Cell, electrochemical; Oxidation-reduction reaction.

Robert L. Wolke

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Anode

The word anode is used in two different sets of circumstances: with respect to vacuum tubes and with respect to electrochemical cells.


Vacuum tubes

A vacuum tube is a tube (usually made of glass ) with most of the air pumped out, and usually containing two electrodes—two pieces of metal with a potential difference (a voltage difference) applied between them. Electrons, being negatively charged, are repelled out of the negative electrode and fly through the vacuum toward the positive electrode, to which they are simultaneously being attracted. The positive electrode is called the anode; the negative electrode is called the cathode .

Common examples of vacuum tubes in which electrons flow from a cathode to an anode are cathode ray tubes, such as television tubes and computer monitors, and x-ray tubes. In an x-ray tube, the kind of metal that the anode is made of determines the kind of x rays (i.e., the x-ray energy ) that the tube emits.


Electrochemical cells

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.


Anodes in practical use

A sacrificial anode is a piece of metal that is made to act as an anode and therefore be oxidized, in order to protect another piece of metal from being oxidized. For example, to keep iron or steel from oxidizing (rusting) when in contact with air and moisture, such as when it is being used as a fence post, the post can be connected to a piece of zinc that is buried in the ground next to it. The iron and zinc in the moist soil constitute the two electrodes of a galvanic cell. But because zinc oxidizes more easily than iron, the zinc acts as the anode and is preferentially oxidized. It is said to be "sacrificed" because it gradually gets eaten away by oxidation instead of the iron. For this reason, a cable made of zinc was buried alongside the Alaskan pipeline, the huge steel pipe that transports petroleum from the Alaskan oil fields to the lower states.

Galvanized iron is iron that has been coated with zinc so that it can be used outdoors or in the ground without rusting. The zinc oxidizes in preference to the iron. Galvanized iron is widely used in making garbage cans, pails, and chain-link fencing.

Anodizing is a process in which a piece of metal is made the anode in an electrolytic cell in order to oxidize it deliberately. When aluminum is anodized in this way, a coating of aluminum oxide is built up on its surface. This coating, unlike the metal itself, can take dyes. Many kinds of aluminum utensils and novelties in bright blue, green, red, and gold colors are made of anodized aluminum.

See also Cell, electrochemical; Oxidation-reduction reaction.

Robert L. Wolke

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an·ode / ˈanōd/ • n. the positively charged electrode by which the electrons leave a device. The opposite of cathode. ∎  the negatively charged electrode of a device supplying current such as a primary cell. DERIVATIVES: an·od·al / anˈōdl; āˈnōdl/ adj. an·od·ic / anˈädik/ adj.

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anodeabode, bestrode, bode, code, commode, corrode, download, encode, erode, explode, forebode, goad, implode, load, lode, middle-of-the-road, mode, node, ode, offload, outrode, road, rode, sarod, Spode, strode, toad, upload, woad •geode •diode, triode •barcode • zip code • unhallowed •carload • cartload • payload •trainload • caseload • freeload •peakload • shipload • coachload •boatload • truckload • wagonload •workload • anode • internode •epode • antipode • electrode •railroad •byroad, highroad •rhapsode • episode • cestode •nematode, trematode •cathode

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anode (electr.) positive electrode. XIX. — Gr. ánodos, f. ANA- + hodós way. Cf. CATHODE, ELECTRODE.

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anode Positive electrode of an electrolytic cell that attracts anions (negative ions) during electrolysis.

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anode A positive electrode, to which negative ions (anions) are attracted.

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anode A positive electrode. See ANION.

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anode A positive electrode. See anion.