A voltameter or coulometer is a

scientific instrument A scientific instrument is a device or tool used for scientific purposes, including the study of both natural phenomena and theoretical research. History Historically, the definition of a scientific instrument has varied, based on usage, laws, an ...

used for

measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. In other words, measurement is a process of determining how large or small a physical quantity is as compared to ...

electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

(quantity of electricity) through electrolytic action. The

SI unit The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...

of electric charge is the

coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...

. The voltameter should not be confused with a

voltmeter A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit. A ...

, which measures electric potential. The SI unit for electric potential is the

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 ...

Etymology

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

used an apparatus that he termed a "volta-electrometer"; subsequently

John Frederic Daniell John Frederic Daniell (12 March 1790 – 13 March 1845) was an England, English chemist and physicist. Biography Daniell was born in London. In 1831 he became the first professor of chemistry at the newly founded King's College London; and in ...

called this a "voltameter".Frank A. J. L. James, (1991), ''The correspondence of Michael Faraday'', IET, , letter 872, 9/1/1836

Types

The voltameter is an

electrolytic cell An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would otherwise not occur. The external energy source is a voltage applied between the cell's two electrodes; ...

and the measurement is made by weighing the

element Element or elements may refer to: Science * Chemical element, a pure substance of one type of atom * Heating element, a device that generates heat by electrical resistance * Orbital elements, parameters required to identify a specific orbit of o ...

deposited or released at the

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 ...

in a specified time.

Silver voltameter

This is the most accurate type. It consists of two

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

plates in a solution of

silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...

. When current is flowing, silver dissolves at the

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 ...

and is deposited at the

. The cathode is initially massed, current is passed for a measured time and the cathode is massed again.

Copper coulometer

This is similar to the silver voltameter but the anode and cathode are

copper Copper is a chemical element; it has symbol Cu (from Latin ) 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 pinkish-orang ...

and the solution is

copper sulfate Copper sulfate may refer to: * Copper(II) sulfate, CuSO4, a common, greenish blue compound used as a fungicide and herbicide * Copper(I) sulfate, Cu2SO4, an unstable white solid which is uncommonly used {{chemistry index Copper compounds ...

, acidified 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, ...

. It is cheaper than the silver voltameter, but slightly less accurate.

Mercury voltameter

In this device, mercury is used to determine the amount of charges transformed during the following reaction: : + 2e- <=> Hg^\circ These oxidation/reduction processes have 100% efficiency with the wide range of the current densities. Measuring of the

quantity of electricity The word ''electricity'' derives from Neo-Latin and ultimately Greek. It first appears in English in Francis Bacon's writings. Depending on context, the word may refer to "electric charge", "electric power" or "electric energy". Historical drift ...

(

s) is based on the changes of the mass of the mercury electrode. Mass of the

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

can be increased during

cathodic 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 current ...

deposition of the mercury ions or decreased during the

anodic 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 devi ...

dissolution of the metal.

Sulfuric acid voltameter

The anode and cathode are

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

and the solution is dilute sulfuric acid.

Hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

is released at the cathode and collected in a graduated tube so that its volume can be measured. The volume is adjusted to

standard temperature and pressure Standard temperature and pressure (STP) or standard conditions for temperature and pressure are various standard sets of conditions for experimental measurements used to allow comparisons to be made between different sets of data. The most used ...

and the

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

of hydrogen is calculated from the volume. This kind of voltameter is sometimes called Hofmann voltameter.

Coulometer

A coulometer is a device to determine

s. The term comes from the unit of charge, the

. There can be two goals in measuring charge: Coulometers can be devices that are used to determine an

amount of substance In chemistry, the amount of substance (symbol ) in a given sample of matter is defined as a ratio () between the particle number, number of elementary entities () and the Avogadro constant (). The unit of amount of substance in the International ...

by measuring the charges. The devices do a quantitative analysis. This method is called

coulometry In analytical electrochemistry, coulometry is the measure of charge (Coulomb, coulombs) transfer during an Electrochemistry, electrochemical Redox, redox reaction. It can be used for precision measurements of charge, but coulometry is mainly used ...

, and related coulometers are either devices used for a coulometry or instruments that perform a coulometry in an automatic way. Coulometers can be used to determine electric quantities in the direct current circuit, namely the total charge or a constant current. These devices invented by

were used frequently in the 19th century and in the first half of the 20th century. In the past, the coulometers of that type were named voltameters.

Sources

* ''Practical Electricity'' by W. E. Ayrton and T. Mather, published by Cassell and Company, London, 1911, pp 12–26

References

{{Electroanalytical Measuring instruments Electroanalytical chemistry devices