Abstract

Chemical analysis has indicated the presence of hexavalent chromium in some leather samples. However, Cr VI is not used in the leather tanning process, and leather normally exhibits a reductive capacity. This research was designed to explain the anomaly. Our work shows that once in solution, at the elevated pH of the phosphate buffer used, certain leather process chemicals may be able to oxidise leachable Cr III, giving a “false positive” Cr VI result. Hydrogen peroxide was found to oxidise Cr III to Cr VI in the phosphate buffer. The peroxide was used as a surrogate for the transient-lived hydroperoxides formed during oil oxidation. Hence fatliquors are implicated indirectly. The oxidation reaction did not occur in an acetate buffer, which is within the natural pH range of most leathers. Acetate buffer spiking experiments demonstrated that Cr VI is stable at pH 4, validating the result. We attempted to identify specific leather chemicals able to cause the reaction. Surprisingly, mimosa was shown to oxidise Cr III in the buffer solution in a time dependent reaction. Although previous work has proven a direct relationship between fatliquor unsaturation and Cr VI result, results in solution were inconclusive but explainable. Magnesium ions added as an antioxidant reduced the Cr VI result by 60%, providing further evidence that an oxidation mechanism produces Cr VI during the phosphate buffer extraction procedure. By demonstrating that magnesium ions do not reduce Cr VI to Cr III, we may conclude that the majority of the Cr VI originally detected had been formed during the extraction, and was not present in the sample. Finally, work utilizing artificial perspiration as a potential substitute extraction solution provided further evidence that the official test method buffer causes false positive results.