Nitrate and Nitrite Ultraviolet Actinometers

Abstract

Abstract We developed nitrate and nitrite actinometers to determine radiant fluxes from 290 to 410 nm. These actinometers are based on the reaction of the photochemically generated OH radical with benzoic acid to form salicylic acid (SA) and p‐hydroxybenzoic acid (pHBA). Actinom‐eter development included determination of the temperature and wavelength dependence of the quantum yield for formation of SA and pHBA from nitrate and nitrite photolysis in air‐saturated solutions. Quantum yields (at 25°C) for SA production from nitrate photolysis ranged from 0.00146 to 0.00418 between 290 and 350 nm, and from 0.00185 to 0.00633 for nitrite photolysis between 290 and 405 nm. The quantum yields for SA production were approximately 50–60% greater than quantum yields for pHBA production from nitrate and nitrite photolysis. For both actinometers, SA and pHBA formation was temperature dependent, increasing by approximately a factor of 2.2 from 0 to 35°C. Activation energies for SA formation varied with wavelength, ranging from 14.7 to 16.5 kj mol ‐1 between 290 and 330 nm for the nitrate actinometer and 12.3 to 17.8 kj mol‐1 between 310 and 390 nm for the nitrite actinometer. Activation energies for pHBA formation were 2–11% higher. Wavelength‐dependent changes in the quantum yield and activation energy for SA and pHBA formation from nitrate photolysis suggest multiple electronic transitions for nitrate from 290 to 350 nm. Quantum yields for OH radical formation from nitrate and nitrite photolyses were estimated from SA and pHBA quantum yields at 25°C. Wavelength‐dependent OH quantum yields ranged from 0.007 to 0.014 for nitrate photolysis between 290 and 330 nm and from 0.024 to 0.078 for nitrite photolysis between 298 and 390 nm. The nitrate and nitrite actinometers can maintain initial rate conditions for hours, are insensitive to laboratory lighting, easy to use and extremely sensitive; the minimum radiant energy that can be detected in our irradiation system is approximately 10‐9 einsteins.