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Interference of Sb(III) in the Determination of Hexavalent Chromium in Thermoplastic Matrices
Keywords: Cr(VI) , RoHS, Interference
Hexavalent chromium, Cr(VI), has traditionally been used as a pigment for paints and plastics. Both Chromium Yellow (PbCrO4) and Chromium Red (PbCrO4•Pb(OH)2) were extensively used in the US and Europe. However, due to the adverse health effects associated with Cr(VI) and lead, these pigments and other hexavalent chromium compounds were largely banned for use in electronic products under the EU RoHS Directive. With product manufacturing continuing to shift into regions with less stringent environmental regulatory requirements, the possibility of finding these pigments in plastic electronic assemblies is of concern. In some cases, random batch to batch testing may be necessary to ensure product compliance. X-ray Fluorescence spectrometry is an effective screening tool for detection of total chromium but it is unable to speciate Cr(VI) from Cr(III). International test standards organizations are working to develop test standards that can discern the two forms of but fully validated methods are not currently available. A challenge in developing such a test method is the low recovery of hexavalent chromium compounds in certain plastic matrices. While the base resins can influence the extraction efficiency, trivalent antimony was discovered as another key factor that affects hexavalent chromium recovery in plastics. Hhexavalent chromium recovery in various plastic matrixes were evaluated via colorimetric reaction of Cr(VI) with diphenylcarbazide (DPC). Low Cr(VI) recovery was noted even in cases where the majority of the plastic matrices were dissolved. This pointed to potential matrix interference rather than extraction efficiency. Halogenated flame retardant plastics are often used in electronic products to prevent the spread of fire. The authors have demonstrated that antimony trioxide, a synergist to enhance the activity of halogenated flame retardants, reacts with hexavalent chromium by converting Cr(VI) to Cr(III), leading to an artificially low detected hexavalent chromium levels.
Sophia Lau, Senior Engineer
Rochester, MN

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