National Institute for Occupational Safety and Health

“ Calibrate daily with at least six working standards over the range 1 to 300 μg CN - per sample. a. Prepare a working standard solution, 1.00 μg /mL.CN -, by diluting 100 μL of calibration stock solution to 100 mL with 0.1 N NaOH. b. Pipet 0.5-, 1.00-, 1.50-, 2.00- and 2.50-mL of the working standard solution into 25-mL volumetric flasks to make 0.50-, 1.00-, 1.50-, 2.00- and 2.50- μg CN - standards. c. Analyze together with field samples and blanks (steps 12 through 19) . d. Prepare calibration graph (absorbance vs. μg CN -) . ”

“ After at least 5 min. standing (but not longer than 15 min) , starting with the reagent blank, add 1.0-mL barbituric acid-pyridine coupling reagent. Mix. Adjust sample volume to 25 mL with deionized-distilled water and allow to stand at least 12 min (but not longer than 30 min) for color development. Read absorbance at 580 nm in a 1-cm light path cuvette on a spectrophotometer. If sample absorbance is outside the range of the calibration standards, take an aliquot, re-analyze (steps 12 through 19) , and apply the appropriate aliquot factor in calculations. ”

“ Pipet a sample aliquot estimated to contain 0.5 to 2.5 μg CN - into a 25-mL volumetric flask. Alternately, to cover an unknown sample concentration range, pipet 0.5-, 1.00-, and 3.00-mL aliquots into separate 25-mL vol. flasks for each field sample. Larger or smaller aliquots may be taken, based on prior knowledge of expected analyte level. Pipet 0.5 mL 0.1 N NaOH into a 25-mL volumetric flask for reagent blank. Add one drop phenolphthalein solution to each standard or sample. NOTE: Add a little deionized-distilled water to increase volume for easier mixing. ”