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General information

The term actinometry refers to the measurement of the actinism, e.g. the photochemical effectivity of electromagnetic irradiation of different wavelength. According to that, an actinometer is a chemical system or a physical device (e.g. a bolometer or photo cell) to determine the number of photons in a light beam per time interval (photon flux) or integrally (amount of photons).[1] Chemical actinometers are of particular interest due to a number of additional capabilities physical measurement methods do not offer. Hence, there is a lot of benefit in their use, even if they are more challenging to handle and sometimes difficult to purchase or synthesize. The advantages are namely:

  • the physical condition of the probe solution can be adapted to the actual reaction components including the photon interactions,
  • the measurements are conducted right within the irradiated volume and thus include the effects of transmission, reflection and scattering,
  • for every experiment, the actinometer solution is freshly prepared and for that reason not subject to age effects.

Especially the last point is an important feature and guarantees reproducible results for the detected number of photons as the quantum yield for a given wavelength and concentration remains constant. In contrast to that, the sensitivity of a commonly used silicon photodiode to photons at a wavelength of 300nm decreases by about 18% per year, even without being in use.[2] Depending on the state of matter, chemical actinometers can be classified into three groups:

  • chemical actinometers in solid or a microheterogeneous phase,
  • chemical actinometers in liquid phase,
  • chemical actinometers in gaseous phase.

An overview of the most common systems is provided in a IUPAC Technical Report by KUHN et al.[3] Besides general information about preparation and measurement methods, notes for analytics and references for further reading are given.

Liquid chemical actinometers are suited best for an application in continuous setups because a solid might clog the reactor and a gas is way less accurate to control. Nevertheless, there are not many reports published in the literature. In fact, there are different approaches to create a model of the conducted experiment[4] or the radiation characteristics of the light source[5] to estimate the photon flux. In these modelings, the light source was assumed to be monochromatic and the photon flux that theoretically reached the reaction solution was calculated for the irradiated volume. Alternatively, the information about the power distribution of the lamp provided by the manufacturer can be used to calculate the received number of photons for each surface element. But even a combination of both methods can only result in an approximate value because reflection, transmission of the reactor material and the alignment of the light source and the reaction chamber are neglected.

In contrast to that, reliable conclusions on the efficiency and the optimization potential are possible when chemical actinometry is conducted as the results can be directly linked to the reactor geometry and the used construction materials.

The HATCHARD-PARKER/Ferrioxalate Actinometer

One of the most commonly used actinometers is the liquid HATCHARD-PARKER-actinometer firstly presented and well-investigated in 1956.[6] The reaction system relies on the reduction of Fe3+ in an oxalate complex to Fe2+, responsible for its other common name: ferrioxalate actinometer. The reaction is shown in Equation 1. Completing the redox reaction, one oxalate ligand is oxidized to carbon dioxide and a carbon dioxide anion radical in Equation 2. The latter may attack another ferrioxalate to a certain number, yielding in a second Fe2+, formed out of same absorbed photon as shown in Equation 3. This secondary reaction has a rather lower probability to happen, but this way reaching an overall quantum yield Φ significantly higher than 1 is possible.






In spite of the theoretical overall quantum yield of 2, a maximum of about 1.2 dependent on the irradiated wavelength and the concentration of the actinometer solution was found by HATCHARD and PARKER in systematic experiments[7] As references, the authors used the uranyl oxalate actinometer and a radiation thermopile. Another focus of former studies lied on the influence of the temperature coefficient on the quantum yield, but there was no detectable dependency of the quantum yield from varying the temperature.[8]

In a final step to analyze the actinometer after the reaction (this step is called “development”), an appropriate amount of 1,10-phenantroline in a buffered solution is added to quantitatively give a strongly red complex with the photochemically formed FeII. This way, the concentration can be easily determined via UV/VIS spectroscopy.

Over the past decades, some research groups adapted the experimental method and suggested newly developed analytics.[144147But it is still a problem that due to the subsequent developing process, no in-situ measurements (or at least measurements shortly after the irradiation process) are possible with the ferrioxalate actinometer. Additional problems occur implicated by contradictory recommended operating procedures ending up in different results. Hence, the results should be verified thoroughly using a valid model and the corresponding calculations.[3]

Other chemical actinometers

Combining some other actinometers with reliable accuracy and reproducibility of the IUPAC compilation, the spectral range between 130 nm to 750 nm can be investigated. The most convenient chemical actinometers are the uranyl oxalate photolysis (λ = 200 nm to 500 nm), the azobenzene system undergoing an (E)-(Z)-isomerization between λ = 230nm to 460nm, the photoisomerization of Aberchrome 540 under irradiation between λ = 310 nm to 375 nm and the 7,16-diphenyldibenzo[a,o]perylene at wavelengths between λ = 475 nm to 610 nm. Thus, complementary evaluations allow a defined characterization of a polychromatic light source in relatively small spectral ranges.


  1. WÖHRLE, D.; TAUSCH, M. W.; STOHRER, W.-D.: Photochemie, 3 edn., 2012, VILEYVCH.
  2. GARDNER, J. L.; WILKINSON, F. J.: Opt., 1985, 24, 1531–1534, DOI: 10.1364/ AO.24.001531.
  3. KUHN, H. J.; BRASLAVSKI, S. E.; SCHMIDT, R.: Chemical actinometry (IUPAC Technical Report), Pure and Applied Chemistry, 2004, 76 (12), 2105–2146.
  4. AILLET, T.; LOUBIERE, K.; DECHY-CABARET, O.; PRAT, L.: Photochemical synthesis of a "cage" compound in a microreactor: Rigorous comparison with a batch photoreactor, Eng. Process., 2013, 64, 38–47,                    DOI: 10.1016/j.cep.2012.10.017.
  5. SUGIMOTO, A.; FUKUYAMA, T.; SUMINO, Y.; TAKAGI, M.; RYU, I.: Microflow photoradical reaction using a compact light source: application to the Barton reaction leading to a key intermediate for myriceric acid A, Tetrahedron, 2009, 65 (8), 1593–1598, DOI:1016/j.tet.2008.12.063.
  6. HATCHARD, C. G.; PARKER, C. A.: A new sensitive chemical actinometer II. Potassium ferrioxalate as a standard chemical actinometer, Admiralty Materials Laboratory, 1956.
  7. BECKER, H.; BÖTTCHER, H.; DIETZ, F.; REHOREK, D.; ROEWER, G.; SCHILLER, K.; TIMPE, H.-J.: Einführung in die Photochemie, 3. bearb. aufl. edn., 1991, Deutscher Verlag der Wissenschaften GmbH, ISBN 978–3–3260–0604–8.
  8. PARKER, C. A.: Faraday Soc., 1954, 50, 1213.
  9. TAYLOR, H. A.; FITZGERALD, J. M.: Analytical methods and techniques for actinometry, Analytical Photochemistry and Photochemical Analysis, 1971, 91–115.
  10. MUROV, S. L.: Handbook of Photochemistry, vol. Sect. 13, 1993, Marcel Dekker, ISBN 978-0824761646.
  11. LEHÓCZKI, T.; JÓZSA, É.; ÖSZ, K.: Ferrioxalate actinometry with online spectrophotometric detection, Journal of Photochemistry and Photobiology A: Chemistry, 2013, 251, 63–68, DOI: 10.1016/j.jphotochem.2012.10.005.
  12. VINCZE, L.; KEMP, T. J.; UNWIN, R.: Flow actinometry in a thin film reactor: modeling and measurements, Journal of Photochemistry and Photobiology, A: Chemistry, 1999, 123.


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