Trace determination of gaseous chlorine: a comparison between an electrometric method and the methyl orange photometric method

Two methods for the determination of molecular chlorine in air are critically compared. An electrometric method based on measurement of the reduction current of chlorine to chloride is superior to the photometric determination based on bleaching of an acidic solution of methyl orange. The limit of detection for the electrometric method is 50 μg m^-3; the standard error of the mean is better than 2%, and the sensitivity is 5.9 μg m^-3 μ A^-1 at +50 mV applied potential versus SCE with a mercury cathode. The limit of detection for the photometric method is 300 μg m^-3; the precision is of the order of 5%, and the sensitivity is 80 μg m^-3 for an error of 1% in the absorption measurements. The accuracy of the electrometric method is good whereas the photometric method yields consistently low results. The electrometric method is less subject to interferences than the photometric method.