A Hofmann voltameter is an apparatus for electrolysing water, invented by

August Wilhelm von Hofmann August Wilhelm von Hofmann (8 April 18185 May 1892) was a German chemist who made considerable contributions to organic chemistry. His research on aniline helped lay the basis of the aniline-dye industry, and his research on coal tar laid the g ...

(1818–1892) in 1866. It consists of three joined upright cylinders, usually glass. The inner cylinder is open at the top to allow addition of

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

which contains a low concentration of a compound such as sulfuric acid to improve conductivity and complete the circuit . A

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

electrode is placed inside the bottom of each of the two side cylinders, connected to the positive and negative terminals of a source of

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as describ ...

. When

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

is run through Hofmann's voltameter, gaseous

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

forms at the

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...

and gaseous

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

at the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...

. Each gas displaces water and collects at the top of the two outer tubes. The tubes which collect the product gases are usually graduated, so that the volume of gas can be measured.

Name

The name ' voltameter' was coined by Daniell, who shortened

Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, ...

's original name of "volta-electrometer". Hofmann voltameters are no longer used as electrical measuring devices. However, before the invention of the

ammeter An ammeter (abbreviation of ''Ampere meter'') is an instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. For direct measurement, the ammeter is connected in series with the circuit ...

, voltameters were often used to measure

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...

, since current through a voltameter with

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

electrodes electroplates the cathode with an amount of metal from the anode directly proportional to the total coulombs of charge transferred (

Faraday's law of electrolysis Faraday's laws of electrolysis are quantitative relationships based on the electrochemical research published by Michael Faraday in 1833. First law Michael Faraday reported that the mass (m) of elements deposited at an electrode is directly prop ...

). The modern name is "electrochemical

coulometer A voltameter or coulometer is a scientific instrument used for measuring electric charge (quantity of electricity) through electrolytic action. The SI unit of electric charge is the coulomb. The voltameter should not be confused with a voltmeter ...

". Although the correct spelling of Hofmann contains only one 'f', it is often incorrectly depicted as Hoffmann.

Uses

The amount of electricity that has passed through the system can then be determined from the volume of gas.

Thomas Edison Thomas Alva Edison (February 11, 1847October 18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation, mass communication, sound recording, and motion pictures. These inventi ...

used voltameters as electricity meters. A Hofmann voltameter is often used as a demonstration of

stoichiometric Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...

principles, as the two-to-one ratio of the volumes of

and

gas produced by the apparatus illustrates the chemical formula of water, H₂O. However, this is only true if oxygen and hydrogen gases are assumed to be diatomic. If hydrogen gas were monatomic and oxygen diatomic, the gas volume ratio would be 4:1. The volumetric composition of water is the ratio by volume of hydrogen to oxygen present. This value is 2:1 experimentally; this value is determined using Hofmann's water voltameter.

References

{{DEFAULTSORT:Hofmann Voltameter Electrolytic cells Hydrogen technologies Hydrogen production

Name

Uses

See also

References