Chemical Actinometry: Using o-Nitrobenzaldehyde to Measure Lamp Intensity in Photochemical Experiments

A simple actinometry experiment that could be used in graduate or undergraduate laboratory courses is described. The photochemical reaction of o-nitrobenzaldehyde (NBA) irradiated with a medium-pressure mercury vapor lamp is used to calculate the intensity of light over the wavelength range of 300-410 nm. The reactant disappearance rate can be quantitated by gas chromatography or by gas chromatographic mass spectrometry. The calculations necessary to convert the kinetic measurements to intensity are discussed. This experiment provides an easy way to determine the intensity of different light sources over a specific wavelength range, information that could be necessary in studying various environmentally relevant photochemical reactions.