Solvent Techniques for the Direct Colorimetric Determination of Copper and Iron in Oils

SUMMARY— The standard procedure for determining trace metals in biological materials requires the ashing of the sample to obtain an aqueous solution. Once this sample is prepared, numerous methods may be employed to determine the metal content. With oils the common ashing techniques lead to serious errors because of spattering, foaming, and volatilization of the sample. In the past, several methods have been developed to eliminate the ashing procedure but large samples (20–50 g) or special equipment was necessary. To allow use of very small oil samples (0.5–2 g) for determining iron and copper, solvent procedures which eliminated the ashing step were developed. The iron procedure is based on the standard aqueous 1,10‐o‐phenanthroline method modified to employ an initial solvent extraction procedure. Comparison of this method with the aqueous ash procedure on oils of known iron content showed the methods to be equivalent in precision. The solvent procedure is capable of determining 0.1 ppm of iron in the oil. The direct solvent copper procedure based on the sodium‐diethyldithiocarbamate aqueous method showed similar results. Because of the ease of performance and the small sample size required, these tests would be useful in food and biological research.