A Hofmann voltameter is an apparatus for electrolyzing water

Electrolysis of water

Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through the water.-Principle:...

, invented by August Wilhelm von Hofmann

August Wilhelm von Hofmann

August Wilhelm von Hofmann was a German chemist.-Biography:Hofmann was born at Gießen, Grand Duchy of Hesse. Not intending originally to devote himself to physical science, he first took up the study of law and philology at Göttingen. But he then turned to chemistry, and studied under Justus von...

 (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

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and an ionic compound to improve conductivity, such as a small amount of sulphuric acid. A platinum

Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 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

Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

. When current

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 is run through Hofmann's Voltameter, gaseous oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 forms at the anode

Anode

An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 and gaseous 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...

 at the 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...

. Each gas displaces water and collects at the top of the two outer tubes.

Name

The name 'voltameter

Voltameter

A Voltameter is a scientific instrument used for measuring quantity of electricity. It should not be confused with a voltmeter which measures electric potential...

' was coined by Daniell

John Frederic Daniell

John Frederic Daniell was an English chemist and physicist.Daniell was born in London, and in 1831 became the first professor of chemistry at the newly founded King's College London. His name is best known for his invention of the Daniell cell , an electric battery much better than voltaic cells...

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

Ammeter

An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes , hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters...

, voltameters were often used to measure direct current

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

, since current through a voltameter with iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 or copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 electrodes electroplates

Electroplating

Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...

 the cathode with an amount of metal from the anode directly proportional to the total coulombs of charge transferred (Faraday's law of electrolysis). The modern name is "electrochemical coulometer". 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 by weighing the cathode. Thomas Edison

Thomas Edison

Thomas Alva Edison was an American inventor and businessman. He developed many devices that greatly influenced life around the world, including the phonograph, the motion picture camera, and a long-lasting, practical electric light bulb. In addition, he created the world’s first industrial...

 used voltameters as electricity meters. (A Hofmann voltameter cannot be used to weigh electric current in this fashion, as the platinum electrodes are too inert for plating.)

A Hofmann voltameter is often used as a demonstration of stoichiometric

Stoichiometry

Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...

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

 and oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

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 monoatomic 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 Hoffman's water voltameter.

PRINCIPLE USED:
Water is split into hydrogen and oxygen by the action of electric current. Pure water doesn't conduct electricity but sulphuric acid (H₂SO₄) when added to water conducts electricity.