Depolarizer
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Adepolarizerordepolariser,in electrochemistry, according to anIUPACdefinition,[1][2]is a synonym ofelectroactive substance,i.e., a substance which changes itsoxidation state,or partakes in a formation or breaking of chemical bonds, in acharge-transferstep of anelectrochemical reaction.
In thebattery industry,the term "depolarizer" has been used to denote a substance used in aprimary cellto prevent buildup of hydrogen gas bubbles.[3]A battery depolarizer takes up electrons during discharge of the cell; therefore, it is always anoxidizing agent.The term "depolarizer" can be considered as outdated or misleading, since it is based on the concept of "polarization"which is hardly realistic in many cases[citation needed].
Polarization
[edit]Under certain conditions for some electrochemical cells, especially if they use an aqueous electrolyte,hydrogen ionscan be converted intohydrogen atomsand H2molecules. In the extreme case, bubbles ofhydrogengas might appear at one of the electrodes. If such a layer of hydrogen or even H2gas bubbles appear on the positive plate of a battery, they interfere with the chemical action of the cell. An electrode covered with gases is said to bepolarized.Polarization in galvanic cells causes the voltage and thus current to be reduced, especially if the bubbles cover a large fraction of a plate. Depolarizers are substances which are intended to remove the hydrogen, and therefore, they help to keep the voltage at a high level. However, this concept is outdated, since if enough depolarizer is present, it will react directly in most cases by getting electrons from the positive plate of the galvanic cell, i.e. there will be no relevant amount of hydrogen gas present. Therefore, the original concept of polarization does not apply to most batteries, and the depolarizer does not react with hydrogen as H2.Still, the term is used today, however, in most cases, it might be replaced with oxidizing agent.
Many different substances have been used as depolarizers; the most notable are listed below.
Oxidizing agents
[edit]Theseoxidizethe hydrogen to water. Examples include:
- Nitric acid,used in theGrove cellandBunsen cell
- Chromic acid,used in theChromic acid cell
- Manganese dioxide,used in theLeclanché cellandDry cell
Nitric and chromic acids are powerful oxidizing agents, and effective depolarizers, but their hazardous nature makes them unsuitable for general use. Manganese dioxide is, therefore, the most widely used depolarizer.
Salts of metals
[edit]The hydrogen ions displace metal from the salt so that metal, instead of hydrogen, is deposited on the positive plate. Examples:
- Silver oxide,used in theSilver-oxide battery
- Copper sulphate,used in theDaniell cell
- Mercurous sulphate,used in theWestonandClark standard cells
References
[edit]- ^IUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "depolarizer".doi:10.1351/goldbook.D01599
- ^IUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "electroactive substance".doi:10.1351/goldbook.E01940
- ^"McGraw-Hill Dictionary of Scientific & Technical Terms", McGraw-Hill, Inc., 2003.
- Everyman's Encyclopaedia,1931, volume 3, pp. 520–522