The chromic acid cell is a type of primary cell which uses

chromic acid Chromic acid is a chemical compound with the chemical formula . It is also a jargon for a solution formed by the addition of sulfuric acid to aqueous solutions of dichromate. It consists at least in part of chromium trioxide. The term "chromic ...

as a depolarizer. The chromic acid is usually made by acidifying (with

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

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

History

Construction

The main elements of the cell are: *

Anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

* Electrolyte, dilute sulfuric acid * Depolarizer, chromic acid *

Cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

The cell is made in two forms - the single-fluid type, attributed to Poggendorff and the two-fluid type, attributed to Fuller. In both cases, cell

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

is about 2

volt The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...

Poggendorff cell

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

Fuller cell

The cell is set up in a glass, or glazed earthenware, pot. This contained the chromic acid solution, the carbon plate and a porous pot. Inside the porous pot is dilute sulfuric acid, the zinc rod, and a small quantity of mercury. The mercury formed an amalgam with the zinc and this reduced "local action", i.e. unwanted dissolution of the zinc when the cell is not in use.

References

External links