PHOSPHORUS TRICHLORIDE: METHOD 6402, Issue 2, dated 15 August 1994 - Page 3 of 4 8.
9.
10.
Add saturated hydrazine sulfate dropwise until the orange-yellow color of excess bromine is gone; add one drop in excess. Transfer the solution to a second 50-mL volumetric flask. Wash the beaker several times with a few mL of distilled water, adding the wash to the second volumetric flask. Pipet 5 mL sodium molybdate solution and 2 mL of 1.5 g/L hydrazine sulfate solution into the second volumetric flask (containing the oxidized sample). Add identical amounts of reagents to the unoxidized 25-mL sample in the volumetric flask from step 5. Dilute both solutions to the mark with distilled water and shake well. Immerse the volumetric flasks in a boiling water bath for 10 min. Remove and cool rapidly to room temperature in cold water bath. Proceed immediately with the measurement (steps 12 through 14).
CALIBRATION AND QUALITY CONTROL: 11.
Calibrate daily with at least six working standards over the range 3 to 500 µg PCl 3 (2 to 360 µg H3PO 4) per sample. a. Pipet aliquots of calibration stock solution into 50-mL volumetric flasks. b. Prepare (steps 5 through 10) and measure (steps 12 through 14) together with the samples and blanks. c. Construct calibration graph (absorbance vs. µg phosphoric acid).
MEASUREMENT: 12. 13. 14.
Set the spectrophotometer according to manufacturer's directions to read at 830 nm. Transfer several mL of the sample or standard solution to a cuvette and place in the spectrophotometer. Record the absorbance reading vs. reagent blank.
CALCULATIONS: 15. 16.
Determine from the calibration graph the mass (µg) of phosphoric acid present in each oxidized sample (M o) and unoxidized sample (M u) and the average media blank (M b). Calculate the concentration (C) of PCl 3 in the air volume sampled, V (L):
where: 1.4 = M.W. PCl 3/M.W. H 3PO 4 = 137.3/98.0. EVALUATION OF METHOD: This method is based on P&CAM 305 [1,2]. In that work, recoveries were determined for samples spiked at 0.5, 1, and 2 times the OSHA standard to be 0.992, 1.04, and 1.05. The overall precision and accuracy have not been completely determined. In one experiment, the efficiency of bubbler collection of PCl 3 was found to be 0.99 ±0.06 based on six pairs of bubblers sampling from a 3.1 mg/m 3 atmosphere generated using a diffusion cell. At the concentrations studied, PCl 3 tended to react with water vapor in the air to produce fogs of H 3PO 3. Attempts to separate the H 3PO 3 aerosol from PCl 3 vapor by means of polyvinyl chloride membrane filters were unsuccessful because of reaction between the filters and PCl 3. The reactions for the molybdenum blue color development are: (1) PCl 3 + 3H 20 → H3PO 3 + 3HCl (2) H 3PO 3 + Br 2 + H 2O → H3PO 4 + 2HBr (3) H 3PO 4 + Na 2MoO 4 + N 2H6SO 4 → molybednum blue NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
