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). Actinometer
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
258C) 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 358C. Activation energies for SA
formation varied with wavelength, ranging from 14.7 to
16.5 kJ mol21 between 290 and 330 nm for the nitrate
actinometer and 12.3 to 17.8 kJ mol21 between 310 and
390 nm for the nitrite actinometer. Activation energies
for pHBA formation were 2–11% higher. Wavelengthdependent
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 258C. 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 1029 einsteins.
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