Page:NIOSH Manual of Analytical Methods - 7904.pdf/3

 CYANIDES, aerosol and gas: METHOD 7904, Issue 2, dated 15 August 1994 - Page 3 of 4 b.

Add a small amount (spatula tip) of powdered cadmium carbonate to the sample. Swirl to disperse the solid and recheck the liquid with lead acetate paper. If sulfide ion has not been removed completely, add more cadmium carbonate. Avoid a large excess of cadmium carbonate and long contact time with the solution. When a drop of liquid no longer discolors a strip of lead acetate paper, filter the sample through a small plug of glass wool in a Pasteur pipette and proceed with the analysis.

CALIBRATION AND QUALITY CONTROL: 8.

9. 10.

Prepare at least six working standards fresh daily to cover the range 50 to 2000 µg CN - per sample by diluting aliquots of 1000 µg/mL calibration stock solution with 0.1 N KOH (e.g., 0.05 to 2.0 mL calibration stock solution diluted to 25 mL). Analyze the working standards according to steps 11 and 12 together with the samples and blanks. Prepare a calibration graph on semilog paper by plotting cyanide ion concentration on the logarithmic axis and mV on the linear axis.

MEASUREMENT: 11. 12.

Transfer the solution to be measured to a 50-mL beaker. Immerse the cyanide ion electrode and reference electrode in the sample and start the magnetic stirrer. With the magnetic stirrer on, allow the potential reading to stabilize. Record the mV reading. NOTE 1: Potential readings are a function of temperature. Measure samples and working standards at the same temperature (± 2 °C). NOTE 2: The cyanide electrode will malfunction if chloride, iodide and bromide ions, which form insoluble silver salts, are present in sufficient quantity. Several metal ions are also known to complex with cyanide such as cadmium, zinc, silver, nickel, cuprous iron and mercury. Consult the electrode instruction manual for the procedure to use when such ions are present.

CALCULATIONS: 13. Read the mass, µg, of cyanide ion present in the sample filter (W f), sample bubbler (W b), average media blank filter (B f) and media blank bubblers (B b) from the calibration graph. 14. Calculate the concentration (mg/m 3) of particulate cyanide, C p, and hydrogen cyanide, C HCN, in the air volume sampled, V (L):

where 1.04 is the stoichiometric conversion factor from CN - to HCN. NOTE: Particulate cyanides will be collected on the filter. In humid atmospheres, however, it has been observed that during the collection of particulate cyanide, HCN is gradually liberated [2]; therefore, particulate cyanide interference is not completely removed.

EVALUATION OF METHOD: HCN: Method S288 was issued on September 2, 1977 [4]. Test atmospheres of HCN were generated by calibrated flow from a compressed mixture of HCN in nitrogen [3,8]. The range of HCN concentrations in air was 5 to 21 mg/m 3 for 12-L air samples. Eighteen HCN samples collected at 0.2 L/min for 60 min indicated overall precision of 6.2%, with a 96.7% recovery. An eight-day storage NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94