FerrioxalatActinometer

Ferrioxalate actinometer, commonly referred to as the ferrioxalate actinometer, is a chemical actinometer used to quantify photon flux in photochemical experiments. It is based on the photoreduction of the ferrioxalate complex to Fe2+ upon irradiation, with the oxalate ligand oxidized to carbon dioxide. The ferrioxalate used is typically potassium ferrioxalate, K3[Fe(C2O4)3], prepared in an acidic solution to stabilize the complex.

Mechanism and measurement: When exposed to light, Fe3+ within the ferrioxalate complex is reduced to Fe2+. The amount of Fe2+ produced is proportional to the number of photons absorbed, with a quantum yield that is close to unity over a broad range of wavelengths, though the yield is wavelength dependent. Fe2+ is quantified by forming a stable complex with 1,10-phenanthroline to yield [Fe(phen)3]2+, whose absorbance at 510 nm allows determination of Fe2+ concentration by Beer-Lambert law. Calibrations and literature provide wavelength-specific quantum yields, which are used to convert Fe2+ amounts into photon flux or absorbed photons.

Procedure: Prepare an acidic ferrioxalate solution, irradiate for a known time under the study’s light source, and then quench the reaction. Add phenanthroline and measure the 510 nm absorption to determine Fe2+. Using the known ε value for the Fe(phen)3 complex and the solution’s geometry, calculate the moles of Fe2+ formed. With Φ(λ) for the irradiation wavelength, compute the photon flux or radiant energy corresponding to the absorbed photons.

Applications and history: The ferrioxalate actinometer is a widely used primary chemical actinometer, introduced by Hatchard and Parker in 1956, and remains a standard for calibrating photochemical experiments and light sources.