The ferrioxalate and iodide–iodate actinometers in the UV region

The ferrioxalate and iodide–iodate actinometers have been re-studied in view of apparent inconsistencies and disagreements of results obtained using different methods and laboratories. The quantum yields have been determined with the aid of highly accurate and sensitive calibrated photo-diodes in the range 205–365 nm. In the case of ferrioxalate, a pronounced change between 240 and 270 nm was observed with a plateau below 240 nm, Φ(FeII) = 1.48 ± 0.02, and above 270 nm, Φ(FeII) = 1.25 ± 0.02. The latter value agrees with other literature reports and is attributed to the known ligand to metal charge transfer band around 300 nm. A shoulder at 215–230 nm is apparently associated with the higher quantum yield below 240 nm. The quantum yield of I3− in the iodide–iodate system is essentially constant between 205 and 245 nm, Φ(I3−) = 0.92 ± 0.02. The results agree with part of the literature values and provide reliable Φ(I3−) for the range 205–290 nm for the purpose of actinometry. The steep change above 245 nm introduces a high uncertainty unless well-defined monochromatic light is used. The integrated results of both actinometers are consistent, and apparent discrepancies in literature are resolved.