p-Nitroanisole/pyridine and p-nitroacetophenone/pyridine actinometers revisited: Quantum yield corrections based on ferrioxalate

Since Dulin & Mill introduced them in 1982,1 the p-nitroanisole/pyridine (PNA-pyr) and p-nitroacetophenone/pyridine (PNAP-pyr) actinometers have been widely adopted by environmental photochemists. They are convenient for solar and simulated solar experiments due to their insensitivity to visible radiation. Their basis is photonucleophilic substitution on either PNA or PNAP by pyridine, initiated by excitation of the nitroaromatic, which occurs in the UV. At low concentrations, nitroaromatic loss is pseudo-first order, and kinetic analysis yields the irradiance. Until now, the PNA-pyr and PNAP-pyr quantum yields have not been independently validated. We have conducted experiments using the well-charaterized actinometer ferrioxalate2 to examine the pyridine dependence of the PNA-pyr and PNAP-pyr quantum yields. Experiments at discrete UV wavelengths and with broadband radiation show that the photon flux (einsteins L-1 s-1) is systematically lower for PNA-pyr relative to ferrioxalate by 34%. Applying this correction to data from experiments with varying pyridine concentration in conjunction with Dulin and Mill’s original data leads to an updated equation for the PNA-pyr quantum yield: This work, PNA-pyr: Φ = 0.27[pyr] + 0.00027. Dulin and Mill,1 PNA-pyr: Φ = 0.44[pyr] + 0.00028. We recommend the new equation and present a standard molar absorption spectrum of PNA for future use. Based on the corrected PNA-pyr quantum yield, comparison between PNA-pyr and PNAP-pyr indicate that the previously reported PNAP-pyr quantum yield is approximately 46% too high. Preliminary results using broadband radiation suggest that a suitable equation for PNAP-pyr is: This work, PNAP-pyr: Φ = 7.4E-3[pyr] + 1.1E-5. Dulin and Mill,1 PNAP-pyr: Φ = 1.6E-2[pyr]. Work with PNAP-pyr is ongoing. This presentation will cover our methodology, results, and offer some cautionary notes about the use of the nitroaromatic based actinometers.