p-Nitroanisole/Pyridine and p-Nitroacetophenone/Pyridine Actinometers Revisited: Quantum Yield in Comparison to Ferrioxalate

Chemical actinometry is often used to measure irradiance in environmental photochemistry. Two widely adopted actinometers are p-nitroanisole/pyridine (PNA-pyr) and the related but less popular p-nitroacetophenone/pyridine (PNAP-pyr). We report that PNA-pyr predicts systematically lower (−29%) photon irradiance than the well-characterized ferrioxalate actinometer. Thus, quantum yields previously determined using PNA-pyr should be correspondingly lower. Experiments at various pyridine concentrations produced an updated equation for the pyridine (pyr) dependence of the PNA-pyr quantum yield: Φ = 0.29[pyr] + 0.00029. Additionally, we present a standard molar absorption spectrum of PNA for future use. A comparison between PNA-pyr and PNAP-pyr suggests the previously reported PNAP-pyr quantum yield is also too high. Preliminary results suggest a suitable equation for the PNAP-pyr system: Φ = 7.4 × 10–3[pyr] + 1.1 × 10–5.