ION-CHROMATOGRAPHIC METHOD FOR DISSOLVED HEXAVALENT CHROMIUM IN DRINKING-WATER, GROUNDWATER, AND INDUSTRIAL WASTE-WATER EFFLUENTS - COLLABORATIVE STUDY

A collaborative study was conducted by the U.S. Environmental Protection Agency (U.S. EPA) and the American Society for Testing Materials (ASTM) on an ion chromatographic method for the determination of hexavalent chromium (U.S. EPA method 218.6, and the ASTM equivalent method). This study was designed to determine the mean recovery and precision of analyses for hexavalent chromium in reagent water, drinking water, groundwater, and industrial wastewaters. The study design was based on Youden's nonreplicate plan for collaborative studies of analytical methods. The test waters were spiked with hexavalent chromium at 8 concentration levels, prepared as 4 Youden pairs. A fifth Youden concentration pair was also included to determine method performance close to the method detection limit. Twenty-one laboratories were instructed to filter their test waters through a 0.45 mum filter and to adjust the pH of the filtrate to 9-9.5 with an ammonium sulfate/ammonium hydroxide buffer solution before spiking with the hexavalent chromium concentrates. A known volume, 50-250 muL, was injected into an ion chromatograph which separated the Cr(VI), as CrO4(2-), on an anion exchange column. After separation, the Cr(VI) was derivatized with diphenylcarbazide and the colored complex was detected at 530 nm. The submitted data were corrected for background concentrations and analyzed by applying ASTM D-2777-86 statistical procedures and a U.S. EPA computer program. U.S. EPA method 218.6 and the equivalent ASTM method were judged acceptable for the measurement of hexavalent chromium concentrations at 1-1000 mug Cr(VI)/L. The study found that the use of a single linear calibration extending over 3 orders of magnitude yielded biased results at the very lowest concentration levels. Shorter range calibration curves yielded more accurate results. Analysis of variance (ANOVA) tests indicated that method performance was significantly different between the reagent water matrix and the other matrixes used. The recovery of Cr(VI) from the other matrixes was lower at all concentration levels with slightly less precision when compared with the reagent water data set. For reagent water, the mean recovery and the overall and single-analyst relative standard deviations were 105%, 7.8% and 3.9%, respectively. For the other matrixes, the same values were 96.7%, 11.9% and 6.3%, respectively. The method was adopted first action by AOAC INTERNATIONAL.