The method of Seeber, White, and Ferber (2) is used to determine traces of iron in adipic acid by the measurement of the baseline absorbance of the ferric chloride complex at 360 millimicrons in a 15% solution of HCl in 95% ethanol (v/v) to which has been added a small amount of hydrogen peroxide to ensure that all the iron is in the ferric state. We have successfully applied this method to other high purity organic compounds (e.g., succinic acid) and to a few inorganic compounds (e.g., concentrated sulfuric acid). However, the method is limited to substances having little or no intrinsic absorption in the region from 330 to 400 millimicrons. Many organic compounds either display appreciable absorption themselves or contain traces of strongly absorbing impurities.
Robert G. White and Richard E. Seeber, "Correction for Organic Background in the Determination of Traces of Iron by Ultraviolet Absorption," Appl. Spectrosc. 18, 158-159 (1964)