Chromic acid cell
Encyclopedia
The Chromic acid cell was a type of 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...

 which used chromic acid
Chromic acid
The 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...

 as a depolarizer
Depolarizer
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...

. The chromic acid was usually made by acidifying (with sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

) a solution of potassium dichromate. The old name for potassium dichromate was potassium bichromate and the cell was often called a Bichromate cell. This type of cell is now only of historical interest.

Construction

The main elements of the cell were:
  • Anode
    Anode
    An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

    , zinc
    Zinc
    Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

  • Electrolyte, dilute sulfuric acid
  • Depolarizer, chromic acid
  • Cathode
    Cathode
    A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

    , carbon
    Carbon
    Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...



The cell was made in two forms - the single-fluid type, attributed to Poggendorff
Johann Christian Poggendorff
Johann Christian Poggendorff , was a German physicist born in Hamburg.By far the greater and more important part of his work related to electricity and magnetism. Poggendorff is known for his electrostatic motor which is analogous to Wilhelm Holtz's electrostatic machine...

 and the two-fluid type, attributed to Fuller. In both cases, cell voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 was about 2 volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

s.

Poggendorff cell

The cell was set up in a long-necked glass bottle with a zinc plate located between two carbon plates. The electrolyte and depolarizer were mixed. The mixture would dissolve the zinc plate even when the cell was not in use, so there was a mechanism for lifting the zinc plate out of the liquid and storing it in the neck of the bottle.

Fuller cell

The cell was set up in a glass, or glazed earthenware
Ceramic glaze
Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...

, pot. This contained the chromic acid solution, the carbon plate and a porous pot. Inside the porous pot was dilute sulfuric acid, the zinc rod, and a small quantity of mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

. The mercury formed an amalgam
Amalgam (chemistry)
An amalgam is a substance formed by the reaction of mercury with another metal. Almost all metals can form amalgams with mercury, notable exceptions being iron and platinum. Silver-mercury amalgams are important in dentistry, and gold-mercury amalgam is used in the extraction of gold from ore.The...

with the zinc and this reduced "local action", i.e. unwanted dissolution of the zinc when the cell was not in use.

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