Actinometer
Encyclopedia
Actinometers are instruments used to measure the heating power
of radiation
. They are used in meteorology
to measure solar radiation as pyrheliometer
s.
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
and visible wavelength ranges.
For example, solutions of iron(III) oxalate
can be used as a chemical
actinometer, while bolometer
s, thermopile
s, and photodiode
s are physical
devices giving a reading that can be correlated to the number of photons
detected. The actinometer was invented by Sir John Frederick William Herschel(March 7, 1792 – May 11, 1871) to measure the direct heating power of the sun's rays, and his work with the instrument is of great importance in the early history of photo-chemistry.
in 1825; he introduced the term actinometer, the first of many uses of the prefix actin for scientific instruments, effects, and processes.
The actinograph
is a related device for estimating the actinic power of lighting for photography.
via the yield from a chemical reaction. It requires a chemical with a known quantum yield
and easily analyzed reaction products.
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 leucocyanides, vanadium(V)-iron(III) oxalate and monochloroacetic acid, however all of these 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
or piperylene 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 K[Cr(NH3)2(NCS)4] reacts in the near-UV region although it is thermally unstable
. Uranyl oxalate has been used historically but is very toxic and cumbersome to analyze.
Recent investigations into nitrate photolysis
have used 2-nitrobenzaldehyde and benzoic acid as a radical scavenger for hydroxyl radicals produced in the photolysis of hydrogen peroxide and sodium nitrate. However, they originally used ferrioxalate actinometry to calibrate the quantum yields for the hydrogen peroxide photolysis. Radical scavengers proved a viable method of measuring production of hydroxyl radical.
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
of radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
. They are used in meteorology
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
to measure solar radiation as pyrheliometer
Pyrheliometer
A pyrheliometer is an instrument for direct measurement of solar irradiance. Sunlight enters the instrument through awindow and is directed onto a thermopile which converts heat to an electrical signal that can be recorded. The signal voltage is converted via a formula to measure watts per square...
s.
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
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
and visible wavelength ranges.
For example, solutions of iron(III) oxalate
Potassium ferrioxalate
Potassium ferrioxalate, also known as potassium oxalatoferrate, is a chemical compound with the formula K3[Fe3], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions are bound to an iron center. Potassium acts as a counterion,...
can be used as a chemical
actinometer, while bolometer
Bolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...
s, thermopile
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....
s, and photodiode
Photodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
s are physical
devices giving a reading that can be correlated to the number of photons
detected. The actinometer was invented by Sir John Frederick William Herschel(March 7, 1792 – May 11, 1871) to measure the direct heating power of the sun's rays, and his work with the instrument is of great importance in the early history of photo-chemistry.
History
The actinometer was invented by John HerschelJohn Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...
in 1825; he introduced the term actinometer, the first of many uses of the prefix actin for scientific instruments, effects, and processes.
The actinograph
Actinograph
An actinograph is an instrument for measuring or estimating the amount of light available, in terms of its ability to expose photographic film...
is a related device for estimating the actinic power of lighting for photography.
Chemical actinometry
Chemical actinometry involves measuring radiant fluxRadiant flux
In radiometry, radiant flux or radiant power is the measure of the total power of electromagnetic radiation...
via the yield from a chemical reaction. It requires a chemical with a known quantum yield
Quantum yield
The quantum yield of a radiation-induced process is the number of times that a defined event occurs per photon absorbed by the system. The "event" may represent a chemical reaction, for example the decomposition of a reactant molecule:...
and easily analyzed reaction products.
Choosing an actinometer
Potassium ferrioxalatePotassium ferrioxalate
Potassium ferrioxalate, also known as potassium oxalatoferrate, is a chemical compound with the formula K3[Fe3], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions are bound to an iron center. Potassium acts as a counterion,...
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 leucocyanides, vanadium(V)-iron(III) oxalate and monochloroacetic acid, however all of these 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
Butyrophenone is a chemical compound ; some of its derivatives are used to treat various psychiatric disorders such as schizophrenia, as well as acting as antiemetics....
or piperylene 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 K[Cr(NH3)2(NCS)4] reacts in the near-UV region although it is thermally unstable
. Uranyl oxalate has been used historically but is very toxic and cumbersome to analyze.
Recent investigations into nitrate photolysis
have used 2-nitrobenzaldehyde and benzoic acid as a radical scavenger for hydroxyl radicals produced in the photolysis of hydrogen peroxide and sodium nitrate. However, they originally used ferrioxalate actinometry to calibrate the quantum yields for the hydrogen peroxide photolysis. Radical scavengers proved a viable method of measuring production of hydroxyl radical.