The Chemical Analysis of Argonne Premium Coal Samples

Edited by Curtis A. Palmer
U.S. Geological Survey Bulletin 2144

Mercury in whole coal and bological tissue by continuous flow-cold vapor-atomic absorption spectrometry

By Richard M. O'Leary

PRINCIPLE

To determine mercury in whole coal and biological tissue, samples are digested with nitric acid, sulfuric acid, and vanadium pentoxide in a disposable glass test tube. After digestion, samples are diluted with water to a constant volume. All samples are then mixed with air and a solution of sodium chloride, hydroxylamine hydrochloride, and sulfuric acid and then Hg (II) is reduced to Hg° with stannous chloride solution in a continuous flow manifold. The elemental mercury vapor is separated and measured using cold vapor-atomic absorption spectrometry (CV-AAS).

INTERFERENCE

Sample solutions containing elevated concentrations of Ag, Au, Pt, Te, and Se may diminish the recovery of the Hg, as has been noted by previous investigators (Bartha and Ikrenyi, 1982 and Suddendorf, 1981). Of these, only Se poses a significant problem for nonmineralized geologic materials. Although a 1 ppm solution of the other elements causes greater than a 90 percent suppression of a 0.01 ppm Hg solution, these elements either will not be dissolved (Au, Pt) or are normally present at lower concentrations (Ag, Te). Silver does not become a problem until its concentration exceeds 12 ppm in the sample. Samples containing silver above 12 ppm need to be diluted. Selenium concentrations above 25 ppm suppresses recovery of mercury and the sample should be diluted.

SCOPE

The method offers a lower reporting limit of 0.02 ppm mercury in whole coal and biological samples. Samples exceeding the working range of 0.02–1.5 ppm mercury require dilution. Approximately 40 samples can be analyzed per person-day.

APPARATUS

• Perkin-Elmer 272 Spectrophotometer
• Perkin-Elmer 056 Strip Chart Recorder
• Technicon AAII Autosampler, modified by replacing the metal sampling probe with a glass tube
• Gilson Medical Electronic (Middleton, Wisconsin) Model Minipulse 2, eight channel, variable-speed peristaltic pump equipped with standard tygon pump tubing
• Standard laboratory hot plate with a 30x60-cm heating surface
• General Electric Chill Chaser Deluxe Infrared Heat Lamp. Position around the flow-through cell and the phase separator
• 7.5-cm-thick x 33-cm-wide x 43-cm-long aluminum heating block, with 18-mm holes drilled through in a 10 by 10 matrix (100 holes)
• Watch glass, 25-mm diameter

See figures 1 and 2 (pages 48 and 49) for the flow-through cell and phase separator used in this method. These have been described by Skougstad and others (1979). Mixing coils are available from Bran + Luebbe, Inc., Buffalo Grove, IL.

REAGENTS

Unless otherwise noted, chemicals are reagent grade and water is deionized (DI).

• Hydrochloric acid, 12 M conc ‘BAKER INSTRA-ANALYZED’
• Sulfuric acid, 18 M conc ‘BAKER INSTRA-ANALYZED’
• Vanadium Pentoxide, V₂O₅: Some brands of V₂O₅ contain detectable amounts of mercury. All V₂O₅ should be roasted prior to use in a chemical hood. Roast in a porcelain dish using a muffle furnace or a fisher burner, at a temperature below 690° C, the melting point of V₂O₅. Do not breath V₂O₅ dust as it is highly toxic, an irritant, and a possible mutagen.
• Nitric acid, 16 M conc ‘BAKER INSTRA-ANALYZED’

Nitric acid wash: Dilute 40 mL ‘BAKER INSTRA-ANALYZED’ grade HNO₃ to 4.0 L with DI water

30 percent potassium hydroxide solution: Dissolve 30 g KOH in DI water and dilute to
100 mL

Stannous chloride solution: Dissolve 100.0 g SnCl₂^.2H₂O (Baker, suitable for Hg determination grade) in 100 mL conc. ‘BAKER INSTRA-ANALYZED’ grade HCl. Let the solution stand for 20 to 30 minutes until the SnCl₂^.2H₂O totally dissolves. Dilute to 1.0 L with DI water. This solution is stable for about 1 week with refrigeration.

Complex-reducing solution: Dissolve 30 g hydroxylamine hydrochloride and 30 g NaCl in about 500 mL DI water. Add very slowly 100 mL conc H₂SO₄. When the solution is cool, dilute to 1 L with DI water.

Sodium dichromate, 25 percent (w/v) solution: Dissolve 25 g Na₂Cr₂O₇^.2H₂O in DI water and dilute to 100 mL.

Mercury standard solution: SRM 1641c (1.47 ppm mercury in DI water) available from the: National Institute of Standards and Technology

Mercury calibration standards, 0.00147, 0.00735, and 0.0147 ppm: Prepare by diluting in DI water, 0.5, 2.5, and 5.0 mL of 1.47 ppm mercury solution (SRM 1641c) in three 500 mL volumetric flasks containing 115 mL conc HNO₃, 50 mL conc H₂SO₄ and 10 mL of 25 percent (w/v) sodium dichromate. These standards correspond to 0.147, 0.735, and 1.47 ppm Hg in the whole coal sample. Make fresh every 3 months.

SAFETY PRECAUTIONS

Normal laboratory safety procedures should be observed, including the use of protective eyewear, laboratory coat, and gloves. All chemical digestion activities should be performed in a chemical hood. See the CHP and MSDS for further information concerning first-aid treatment and disposal procedures, etc. for chemical products used in this method. The atomic absorption spectrophotometer should be located under a vent exhaust hood to evacuate the acidic gases and mercury vapors that are generated by the continuous flow-cold vapor system.

PROCEDURE

1. Weigh 0.150 g of whole coal or dried biological tissue (0.75 to 1.5 g undried biological tissue) into 16x150-mm disposable test tube.
2. Add approximately (scooped) 0.1 g V2O5, 3.5 mL conc HNO₃, and 1.50 mL conc H₂SO₄ to the sample. Vortex to wet the entire sample solution.
3. Place test tube in the aluminum heating block, cover with watch glass, and ramp gradually to 150° C over a 2-hour period. Heat overnight at this temperature.
4. Remove the tube, allow to cool and dilute sample solution to 15 mL with DI water, cap and shake for 5 min.
5. Centrifuge at 1,000 rpm for 5 min and transfer approximately 12 mL sample solution to a 16x100 mm disposable tube.
6. Calibrate the instrument for each day’s analyses against the aqueous standards of 0.00147, 0.0075, and 0.0147 ppm Hg.
7. Using the manifold illustrated in figure 3 (page 50), modified from Koirtyohann and Khalil (1976) and Kennedy and Crock (1987), the digested geochemical materials are analyzed along with aqueous calibration standards. The modifications to the manifold include changes to reagent and sample flow rates and reagent composition. These were made to maximize the absorbance signal of a 0.00147 ppm mercury solution. Samples with mercury concentration greater than the highest standard (1.47 ppm in the sample) must be diluted and reanalyzed. Also, any sample following a sample that exceeds the concentration of the upper standard, should be reanalyzed due to the possibility of mercury carry over from the previous sample.
8. The calibration curve is checked at the beginning and after every 20 samples.

STANDARDIZATION OF INSTRUMENT

Instrument settings used for a Perkin-Elmer 272 AAS Spectrometer and a Perkin-Elmer 56 Recorder are outlined in table 11.

CALCULATION

Measure peak height to the nearest mm with a ruler and calculate the mercury concentration in the sample with the following formula:

ASSIGNMENT OF UNCERTAINTY

Low values for the aqueous standards and/or high values for reference materials suggest the apparatus needs to be disassembled and cleaned with 30 percent KOH. Upon heating this removes any residual mercury or organic carbon buildup. Each daily set of analyses is preceded by three aqueous calibration standards. Table 12 shows the analytical results of selected reference materials, duplicate samples, and method blanks obtained by this method.

BIBLIOGRAPHY

Bartha, A., and Ikrenyi, K., 1982, Interfering effects on the determination of low concentrations of mercury in geologic materials by cold-vapor atomic absorption spectrometry: Analytica Chimica Acta, v. 139, p. 329-332.

Govandaraju, K., ed., 1989, 1989 Compilation of working values and sample description of 272 geostandards: Geostandards Newsletter, v. 13, Special Issue, p. 67.

Kennedy, K.R., and Crock, J.G., 1987, Determination of mercury in geological materials by continuous-flow, cold-vapor, atomic absorption spectrophotometry: Analytical Letters, v. 20, p. 899-908.

Koirtyohann, S.R., and Khalil, M., 1976, Variables in the determination of mercury by cold vapor atomic absorption: Analytical Chemistry, v. 48, no. 1, p. 136-139.

Lengyel, J., DeVito, M.S., and Bilonick, R.A., 1994, Interlaboratory and intralaboratory variability in the analysis of mercury in coal: Consol Inc., Library, Penn.

National Bureau of Standards (now National Institute of Standards and Technology), 1974 and 1978, certificate of analysis: U.S. Department of Commerce, Washington, D.C.

National Institute of Standards and Technology, 1989, certificate of analysis: U.S. Department of Commerce, Gaithersburg, Md

National Research Council of Canada, 1983, certificate of analysis: Institute for Environmental Chemistry, Ottawa, Canada.

Potts, P.J., Tindle, A.G., and Webb, P.C., 1992, Geochemical reference material compositions: CRC Press Inc., Boca Raton, Fla., p. 254-255.

Skougstad, M.W., Fishman, M.J., Friedman, L.C., Erdman, D.E., and Duncan, S.S., eds., 1979, Methods for Determination of Inorganic Substances in Water and Fluvial Sediments: Techniques of Water-Resources Investigations of the United States Geological Survey Book 5, Chapter A1, p. 193-207.

Suddendorf, R.F., 1981, Interference by selenium or tellurium in the determination of mercury by cold vapor generation atomic absorption spectrometry: Analytical Chemistry v. 53, p. 2234-2236.

Wilson, S., May 1994, Branch of Geochemistry oral communication to the editor: U.S. Geological Survey, Denver, Colo.

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