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.