A study of significant variables and their effects on the analysis of hexavalent chromium

Abstract

The recent reduction in the Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for hexavalent chromium Cr(VI) will most certainly cause employers to perform more exposure determinations. The standard applies to Cr(VI) in both the soluble and insoluble form. The National Institute for Occupational Safety and Health (NIOSH) Method 7600, an analytical technique utilizing visible spectrophotometry, was examined. The new OSHA PEL of 5 micrograms per cubic meter (μg/m 3 ) mandated that the lowest possible method detection limit (MDL) be obtained. Also, the solubilities of the test samples were in most cases unknown. Accordingly, this study focused on the following: (1) removal of filter debris or other turbidity, (2) filter type, storage stability and (3) the alkaline extraction of the samples. Preliminary analysis of blank polyvinyl chloride (PVC) filters resulted in noticeable debris and turbidity. To test turbidity a blank PVC filter was placed in a centrifuge tube and extracted with sulfuric acid. Eight replicates yielded a MDL of 4 micrograms per liter (μg/L). The aliquots were then filtered using a Buchner funnel and yielded 2 μg/L MDL. Finally, the filtrates were centrifuged for 2 minutes further reducing the MDL to 0.7 μg/L. The use of the centrifuge to eliminate turbidity was significant enough to necessitate further investigation. To determine the effect of an alkaline extraction on soluble Cr(VI), a standard addition of 0.5 μg of potassium chromate was performed on a filter, allowed to dry and then extracted with the alkaline solution. Three replicates had a 109.4% recovery. Centrifuging the filters was useful in eliminating the CO 2 interference for the alkaline extraction. Also, the use of an alkaline extraction did not negatively impact the soluble Cr(VI). The positive result of the preliminary extractions resulted in further investigation of an alkaline extraction to incorporate the inclusion of both soluble and insoluble Cr(VI).