Actinometers are instruments used to measure the heating

power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

. They are used in

meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...

to measure

solar radiation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...

pyranometer A pyranometer is a type of actinometer used for measuring solar irradiance on a planar surface and it is designed to measure the solar radiation flux density (W/m2) from the hemisphere above within a wavelength range 0.3 μm to 3 μm. The name pyra ...

pyrheliometer A pyrheliometer is an instrument for measurement of direct beam solar irradiance. Sunlight enters the instrument through a window and is directed onto a thermopile which converts heat to an electrical signal that can be recorded. The signal vo ...

s and net radiometers. An actinometer is a chemical system or physical device which determines the number of photons in a beam integrally or per unit time. This name is commonly applied to devices used in the

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

and visible wavelength ranges. For example, solutions of iron(III) oxalate can be used as a chemical actinometer, while

bolometer A bolometer is a device for measuring radiant heat by means of a material having a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley. Principle of operation A bolometer ...

thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoele ...

s, and

photodiode A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons. The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...

s are physical devices giving a reading that can be correlated to the number of photons detected.

History

The actinometer was invented by

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wo ...

in 1825; he introduced the term ''actinometer'', the first of many uses of the prefix ''actin'' for scientific instruments, effects, and processes. The

actinograph An actinograph is an instrument for measuring or estimating the amount of light available, in terms of its ability to expose photographic film. That is, it measures the ''actinic'' or ''chemical'' intensity of light, as opposed to radiometric o ...

is a related device for estimating the actinic power of lighting for photography.

Chemical actinometry

Chemical actinometry involves measuring

radiant flux In radiometry, radiant flux or radiant power is the radiant energy emitted, reflected, transmitted, or received per unit time, and spectral flux or spectral power is the radiant flux per unit frequency or wavelength, depending on whether the spe ...

via the yield from a chemical reaction. This process requires a chemical with a known

quantum yield The quantum yield (Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system. Applications Fluorescence spectroscopy The fluorescence quantum yield is defined as the ratio of the numb ...

and easily analyzed reaction products.

Choosing an actinometer

Potassium ferrioxalate is commonly used, as it is simple to use and sensitive over a wide range of relevant wavelengths (254 nm to 500 nm). Other actinometers include

malachite green Malachite green is an organic compound that is used as a dyestuff and controversially as an antimicrobial in aquaculture. Malachite green is traditionally used as a dye for materials such as silk, leather, and paper. Despite its name the dye is ...

leucocyanides, vanadium(V)–iron(III) oxalate and

monochloroacetic acid Chloroacetic acid, industrially known as monochloroacetic acid (MCA), is the organochlorine compound with the formula ClCH2CO2H. This carboxylic acid is a useful building block in organic synthesis. It is a colorless solid. Related compounds a ...

, however all of these actinometers undergo dark reactions, that is, they react in the absence of light. This is undesirable since it will have to be corrected for. Organic actinometers like

butyrophenone Butyrophenone is an organic compound with the formula C6H5C(O)C3H7. It is a colorless liquid. The butyrophenone structure—a ketone flanked by a phenyl ring and a butyl chain—forms the basis for many other chemicals containing various sub ...

piperylene Piperylene or 1,3-pentadiene is an organic compound with the formula . It is a volatile, flammable hydrocarbon. It is one of the five positional isomers of pentadiene. Reactions and occurrence Piperylene is a typical diene. It forms a sul ...

are analysed by gas chromatography. Other actinometers are more specific in terms of the range of wavelengths at which quantum yields have been determined.

Reinecke's salt Reinecke's salt is a chemical compound with the formula NH4 r(NCS)4(NH3)2� H2O. The dark-red crystalline compound is soluble in boiling water, acetone, and ethanol. The chromium atom is surrounded by six nitrogen atoms in an octahedral geometry. ...

K r(NH₃)₂(NCS)₄reacts in the near-UV region although it is thermally unstable.

Uranyl oxalate Uranyl oxalate (UO2C2O4) is a pale yellow powdered uranyl compound. It is often encountered in industrial nuclear processes at both the front and back-end of the nuclear fuel cycle. Due to its hygroscopicity, uranyl oxalate rarely exists in the d ...

has been used historically but is very toxic and cumbersome to analyze. Recent investigations into nitrate

photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

have used 2-nitrobenzaldehyde and

benzoic acid Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin ...

as a

radical scavenger A scavenger in chemistry is a chemical substance added to a mixture in order to remove or de-activate impurities and unwanted reaction products, for example oxygen, to make sure that they will not cause any unfavorable reactions. Their use is wide- ...

for

hydroxyl radical The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...

s produced in the photolysis of

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3 ...

and

sodium nitrate Sodium nitrate is the chemical compound with the formula . This alkali metal nitrate salt is also known as Chile saltpeter (large deposits of which were historically mined in Chile) to distinguish it from ordinary saltpeter, potassium nitrate. ...

. However, they originally used

ferrioxalate Ferrioxalate or trisoxalatoferrate(III) is a trivalent anion with formula . It is a transition metal complex consisting of an iron atom in the +3 oxidation state and three bidentate oxalate ions anions acting as ligands. The ferrioxalate anio ...

actinometry to calibrate the quantum yields for the hydrogen peroxide photolysis. Radical scavengers proved a viable method of measuring production of hydroxyl radical.

Chemical actinometry in the visible range

Meso-diphenylhelianthrene can be used for chemical actinometry in the visible range (400–700 nm). This chemical measures in the 475–610 nm range, but measurements in wider spectral ranges can be done with this chemical if the emission spectrum of the light source is known.

References

{{reflist Radiometry Measuring instruments English inventions

History

Chemical actinometry

Choosing an actinometer

Chemical actinometry in the visible range

See also

References