Potential chemical mechanisms causing false positive results in hexavalentchromium determination

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 des igned 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 V1 r esult. Hydrogen peroxide was found to oxidise Cr III to Cr VI in the phosph ate buffer. The peroxide was used as a surrogate for the transient-lived hy droperoxides formed juring oil oxidation. Hence fatliquors are implicated i ndirectly. The oxidation reaction did not occur in an acetate buffer, which is within the natural pH range of most leathers. Acetate buffer spiking ex periments demonstrated that Cr VI is stable at pH 4, validating the result. We attempted to identify specific leather chemicals able to cause the reac tion. Surprisingly, mimosa was shown to oxidise Cr III in the buffer soluti on in a time dependent reaction. Although previous work has proven a direct relationship between fatliquor unsaturation and Cr VI result, results in s olution were inconclusive but explainable. Magnesium ions added as an antio xidant reduced the Cr VI result by 60%, providing further evidence that an oxidation mechanism produces Cr VI during the phosphate buffer extraction p rocedure. By demonstrating that magnesium ions do not reduce Cr VI to Cr II I, we may conclude that the majority of the Cr VI originally detected had b een formed during the extraction, and was not present in the sample. Finall y, work utilizing artificial perspiration as a potential substitute extract ion solution provided further evidence that the official lest method buffer causes false positive results.