Depolarizer
Encyclopedia
A depolarizer or depolariser, in electrochemistry, according to an IUPAC definition, is a synonym of electroactive substance, i.e., a substance which changes its oxidation state
, or partakes in a formation or breaking of chemical bonds, in a charge-transfer step of an electrochemical reaction.
In battery industry, the term "depolarizer" has been used to denote a substance used in a primary cell
to prevent buildup of hydrogen gas bubbles. . Battery depolarizer takes up electrons during discharging the cell; therefore, it is always an oxidizing 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 The term "depolarizer" can be considered as outdated or misleading since it is based on the concept of "polarization" which is hardly realistic in many cases.
s can be converted into hydrogen atom
s and H2 molecules. In the extreme case, bubbles of hydrogen
gas might appear at one of the electrodes. If such a layer of hydrogen or even H2 gas bubbles appear on the positive plate of a battery, they interfere with the chemical action of the cell. An electrode covered with gases is called polarized. 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 However, this concept is outdated, since if enough depolarizer is present, it will react directly in most cases by getting electrons from the positive plate of the galvanic cell, i.e. there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.
the hydrogen to water. Examples include:
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.
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
, or partakes in a formation or breaking of chemical bonds, in a charge-transfer step of an electrochemical reaction.
In battery industry, the term "depolarizer" has been used to denote a substance used in a primary cell
Primary cell
A primary cell is any kind of battery in which the electrochemical reaction is not reversible, rendering the cell non-rechargeable. A common example of a primary cell is the disposable battery. Unlike a secondary cell, the reaction cannot be reversed by running a current into the cell; the chemical...
to prevent buildup of hydrogen gas bubbles. . Battery depolarizer takes up electrons during discharging the cell; therefore, it is always an oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
. 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.
Polarization
Under certain conditions for some electrochemical cells, especially if they use an aqueous electrolyte, hydrogen ionHydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
s can be converted into hydrogen atom
Hydrogen atom
A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively-charged proton and a single negatively-charged electron bound to the nucleus by the Coulomb force...
s and H2 molecules. In the extreme case, bubbles of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas might appear at one of the electrodes. If such a layer of hydrogen or even H2 gas bubbles appear on the positive plate of a battery, they interfere with the chemical action of the cell. An electrode covered with gases is called polarized. 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
These oxidizeRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
the hydrogen to water. Examples include:
- Nitric acidNitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
, used in the Grove cellGrove cellThe Grove cell was an early electric primary cell named after its inventor, British chemist William Robert Grove, and consisted of a zinc anode in dilute sulfuric acid and a platinum cathode in concentrated nitric acid, the two separated by a porous ceramic pot.-Cell details:The Grove cell voltage...
and Bunsen cellBunsen cellThe Bunsen cell is a zinc-carbon primary cell composed of a zinc anode in dilute sulfuric acid separated by a porous pot from a carbon cathode in nitric or chromic acid.- Cell details :... - Chromic acidChromic acidThe term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the...
, used in the Chromic acid cellChromic acid cellThe Chromic acid cell was a type of primary cell which used chromic acid as a depolarizer. The chromic acid was usually made by acidifying a solution of potassium dichromate. The old name for potassium dichromate was potassium bichromate and the cell was often called a Bichromate cell... - Manganese dioxide, used in the Leclanché cellLeclanché cellGeorges Leclanché invented and patented his battery, the Leclanché cell, in 1866. The battery contained a conducting solution of ammonium chloride, a cathode of carbon, a depolarizer of manganese dioxide, and an anode of zinc...
and Dry cellDry Cell-Dry Cell's formation:Part of the band formed in 1998 when guitarist Danny Hartwell and drummer Brandon Brown met at the Ratt Show on the Sunset Strip. They later met up with then-vocalist Judd Gruenbaum. The original name of the band was "Beyond Control"....
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
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 the Silver-oxide batterySilver-oxide batteryA silver oxide battery , not to be confused with a similar but different silver–zinc battery, which is a secondary cell, is a primary cell with relatively very high energy/weight ratio. They are costly due to the high price of silver...
- Copper sulphate, used in the Daniell cellDaniell cellThe Daniell cell was invented in 1836 by John Frederic Daniell, a British chemist and meteorologist, and consisted of a copper pot filled with a copper sulfate solution, in which was immersed an unglazed earthenware container filled with sulfuric acid and a zinc electrode...
- Mercurous sulphateMercury(I) sulfateMercury sulfate, commonly called mercurous sulfate or mercurous sulphate is the chemical compound Hg2SO4. It can be produced by a reaction of mercury nitrate with a source of sulfate ions:...
, used in the WestonWeston cellThe Weston cell, invented by Edward Weston in 1893, is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters...
and Clark standard cellsClark cellThe Clark cell, invented by English engineer Josiah Latimer Clark in 1873, is a wet-chemical cell that produces a highly stable voltage usable as a laboratory standard.-Chemistry:...