CHROMIUM, HEXAVALENT: METHOD 7600, Issue 2, dated 15 August 1994 - Page 3 of 4 additional 1 to 2 mL 0.5 N H2SO 4. Discard the filters. Start reagent blanks at this point. Filter the solution through a moistened PVC filter in a Buchner funnel to remove interferences from suspended dust. Collect the filtrate in a clean centrifuge tube. Rinse the bottle, which contained the filter, with 2 to 3 mL 0.5 N H2SO 4 and pour into the funnel. Rinse the funnel and filter with 5 to 8 mL 0.5 N H2SO 4. e. Add 0.5 mL diphenylcarbazide solution to each centrifuge tube. Bring the total volume in each centrifuge tube to 25 mL with 0.5 N H2SO 4. Shake to mix and allow color to develop (at least 2 min but no longer than 40 min. [6]). Transfer the solution to a clean 5-cm cuvette and analyze within 40 min of mixing (steps 9 through 11). Sample preparation for insoluble chromates and for Cr(VI) in the presence of iron or other reducing agents: NOTE: If significant amounts of Cr(III) are expected to be present, degas the sample solution by bubbling nitrogen through it for 5 min. before proceeding and purge the headspace above the solution during step 5.a. a. Remove the PVC filter from the bottle, place it in a 50-mL beaker, and add 5.0 mL filter extraction solution, 2% NaOH/3% Na 2CO 3. Start reagent blanks at this point. Purge the headspace above the solution with nitrogen throughout the extraction process to avoid oxidation of any Cr(III). Cover the beaker with a watchglass and heat it to near the boiling point on a hotplate with occasional swirling for 30 to 45 min. Do not boil the solution or heat longer than 45 min. Do not allow the solution to evaporate to dryness because hexavalent chromium may be lost owing to reaction with the PVC filter. An indication that hexavalent chromium has been lost in this manner is a brown-colored PVC filter. b. Cool the solution and transfer it quantitatively with distilled water rinses to a 25-mL volumetric flask, keeping the total volume about 20 mL. NOTE: If the solution is cloudy, filter it through a PVC filter in a vacuum filtration apparatus using distilled water rinses. c. Add 1.90 mL 6 N sulfuric acid to the volumetric flask and swirl to mix. CAUTION: CARBON DIOXIDE WILL BE EVOLVED CAUSING INCREASED PRESSURE IN THE FLASK. LET THE SOLUTION STAND FOR SEVERAL MINUTES UNTIL VIGOROUS GAS EVOLUTION CEASES. d. Add 0.5 mL diphenylcarbazide solution, dilute to the mark with distilled water and invert several times to mix thoroughly. Pour out about one-half of the contents of the flask, stopper the flask and shake it vigorously several times, removing the stopper each time to relieve pressure. NOTE: This step releases bubbles of carbon dioxide which otherwise would cause high and erratic readings. e. Transfer an aliquot of the solution remaining in the flask to a 5-cm cuvette and analyze (steps 9 through 11). d.
5.
CALIBRATION AND QUALITY CONTROL: 6.
7. 8.
Calibrate daily with at least six working standards. Transfer 6 to 7 mL 0.5 N H2SO 4 to each of a series of 25-mL volumetric flasks. Pipet 0 to 0.7 mL of 10 µg/mL calibration stock solution into the volumetric flasks. Add 0.5 mL diphenylcarbazide solution to each and sufficient 0.5 N H2SO 4 to bring the volume to 25 mL. These working standards contain 0 to 7 µg Cr(VI). Analyze the working standards together with blanks and samples (steps 9 through 11). Prepare a calibration graph [absorbance vs. µg Cr(VI)].
MEASUREMENT: 9. 10. 11.
Set wavelength on the spectrophotometer to 540 nm. Set to zero using a 0.5 N H2SO 4 reagent blank. Transfer sample solution to a cuvette and record the absorbance. NOTE 1: A sample containing 1.5 µg Cr(VI)/25 mL gives ca. 0.2 absorbance. NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
