Page:NIOSH Manual of Analytical Methods - 3500.pdf/3

 FORMALDEHYDE: METHOD 3500, Issue 2, dated 15 August 1994 - Page 3 of 5 6.

7.

8.

9.

Pipette 4-mL aliquots from each sample solution into 25-mL glass-stoppered flasks. NOTE: Adjust aliquot size to contain between 2 and 20 µg formaldehyde for optimum absorbance. The calibration graph between becomes non-linear above an absorbance of ca. 1.0. A.U. Add 0.1 mL 1% chromotropic acid to the flask and mix. NOTE: This amount of chromotropic acid can react with ca. 40 µg of formaldehyde. Due to this fact, the range of the calibration curve should not exceed 36 µg (90% of theoretical). Add 6 mL conc. H 2SO 4 slowly to the flask. Replace the stopper gently. Gently swirl the solution to mix. CAUTION: Mixing of the sample solution with concentrated sulfuric acid is highly exothermic. Heat the solution to 95 °C for 15 min. Allow the solution to cool at room temperature for 2 to 3 hours. NOTE: Use caution due to the corrosive nature of hot sulfuric acid and the possible pressure buildup within the flask.

CALIBRATION AND QUALITY CONTROL: 10.

Calibrate daily with at least six working standards. a. Prepare a calibration stock solution by dilution of 1 mL of 1 mg/mL formaldehyde stock solution to 100 mL 1% sodium bisulfite solution. b. Pipet, e.g., 0.1, 0.3, 0.5, 0.7, 1.0, and 2.0 mL calibration stock solution into 25-mL glassstoppered flasks. c. Add 1% sodium bisulfite solution to bring the volume of each working standard to 4 mL. Also prepare a reagent blank. d. Analyze together with samples and blanks (steps 5 through 9, and steps 11 and 12). e. Prepare calibration graph (absorbance vs. µg formaldehyde/mL).

MEASUREMENT: 11. 12.

Set spectrophotometer according to manufacturer's recommendations and to conditions given on p. 3500-1. Fill 1-cm cuvette with sample. Read sample absorbance at 580 nm. NOTE: If absorbance is greater than the highest standard, take a smaller aliquot of the remaining unreacted sample solution, dilute to 4 mL with 1% sodium bisulfite solution, and analyze. (Steps 11 and 12). For optimum results, all samples containing over 20 µg formaldehyde should be diluted and reanalyzed.

CALCULATIONS: 13.

14.

Calculate the mass, µg, of formaldehyde in each front impinger (M f), back impinger (M b) and average reagent blank (M B). Use the appropriate aliquot factor (e.g., 4 mL aliquot from step 6) and the total sample volume noted in step 5. NOTE: Discard the sample if the mass found in the backup impinger exceeds 1/3 the mass found in the front impinger. Collection efficiency is ca. 95% for each impinger [15], but may be reduced due to chipped or mismatched impinger parts, or by carryover from the front impinger. Calculate the concentration, C, of formaldehyde in the air volume sampled, V (L):

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94