METHODS FOR SAMPLING AND
INORGANIC ANALYSIS OF COAL
U.S. Geological Survey Bulletin 1823
Edited by D.W. Golightly and F.O. Simon
THE DETERMINATION OF FLUORIDE IN COAL BY ION-SELECTIVE ELECTRODE
By Herbert Kirschenbaum
A method is described for determining the concentration of fluoride in coal. Coal ash is fused with sodium hydroxide, and the resulting melt is leached with distilled water. This solution is diluted with an ammonium citrate buffer to produce a final solution having a pH in the range of 5.5 to 7.0. The fluoride concentration then is determined by measurement of the potential produced by a fluoride ion-selective electrode. The lower limit for determination of fluorine in a 250-mg ash sample is 20 m /g. The precision of the method is approximately 6-percent relative standard deviation.
The method described herein for determining fluoride in coal (Ingram, 1970; Swanson and Huffman, 1976) requires that coal ash be fused with sodium hydroxide and that the resulting melt be leached with distilled water, filtered, and then diluted to a fixed volume with an ammonium citrate buffer. The pH of the resulting citrate-buffered solution must range from 5.5 to 7.0 for accurate measurement of the fluoride concentration with an ion-selective electrode (ISE). At a pH below 5.5, the measurement gives concentrations that are biased low because of the complexation of F- by H+. A high bias in the measured concentration occurs at pH values above 7.0 because of the presence of high concentrations of OH- or HCO3-.
EXPERIMENTAL METHODS AND PROCEDURES
Measurements are made with an Orion Research IonalyzerTM (model 801), equipped with a fluoride-ion electrode (model 94-09) and a single-junction calomel reference electrode (model 90-01).
The quality of each chemical reagent is reagent grade or better.
1. MgO-Mg(NO3)2 solution: Dissolve 38 g of MgO and 53 g of Mg(NO3)2 in 500 mL of distilled water.
2. 1 M ammonium citrate solution: Dissolve contents of a 454-g bottle of ammonium citrate in 2 L of distilled water. The pH of this solution is 4.7 to 4.9.
3. Sodium hydroxide pellets: Baker AnalyzedTM sodium hydroxide pellets, low in carbonates. These pellets are used in fusions described later.
4. Sodium fluoride standard: Purify the sodium fluoride by first adding from 10 to 15 mL of concentrated hydrofluoric acid to 3 g of sodium fluoride in a platinum dish. Evaporate the resulting solution to dryness on a steam bath. Transfer the purified NaF to a polyethylene vial.
5. Standard A: Prepare a stock solution of 1,000 m g/mL of fluoride in distilled water by first dissolving 1.105 g of the purified NaF and then by diluting the resulting solution to 500 mL.
6. Standard B: Prepare a 100 m g/mL fluoride standard by diluting 10.0 mL of standard A to 100 mL.
7. Standard C: Prepare a 10 m g/mL fluoride standard by diluting 1.0 mL of standard A to 100 mL.
8. Calibration standards: Add 2.7 to 3.0 g (16 pellets) of NaOH to each of nine 100-mL volumetric flasks. Then, add approximately 20 mL of distilled water to each flask to dissolve the NaOH pellets.
The dilutions used to prepare the individual calibration solutions are detailed in table 26. The calibration standard having a F- concentration of 8 m g/mL is used to equilibrate between all measurements with the ISE on standards and samples. When these calibration standards are mixed 1 to 1 with the ammonium citrate buffer, the F- concentration is diluted to half the value given in table 26.
1. Thoroughly clean each 35-mL zirconium crucible by first fusing NaOH within the crucible and then washing the crucible in dilute (1 to 1) hydrochloric acid. This step is necessary to avoid contamination.
2. Weigh 0.250 g of pulverized (100 mesh) coal sample, and transfer the sample to a 35-mL zirconium crucible.
3. Add 1 mL of isopropanol into the crucible to wet the sample.
4. Add 1 mL of the MgO-Mg(NO3)2 solution to the wet sample and mix with a glass stirring rod. Rinse the stirring rod with a little distilled water while collecting the rinse solution in the crucible.
5. Place the crucible into an oven operated at 110° C and dry the sample for 30 min.
6. Place the zirconium crucible that contains the sample into a muffle furnace and ash the sample in the following uninterrupted sequence: 200° C for 30 min, 300° C for 30 min, 400° C for 90 min, and 525° C for 135 min. If the coal is of high rank, such as an anthracite, then ash the sample overnight.
7. Allow the sample to cool to room temperature. Add 3 g of NaOH (approximately 16 pellets). Fuse in the uncovered crucible over an open Meeker burner at a "dull-red" temperature for 2 min.
8. Allow the sample to cool, add 25 mL of distilled water to the crucible, and place the crucible on a steam bath to leach the fused mass. After heating the crucible contents for approximately 30 min, transfer the contents of the crucible into a 100-mL polyethylene beaker. Do not put the crucible into the polyethylene beaker because contamination from the outside surface of the crucible is likely to occur.
9. Filter the contents of the beaker through a 9-cm-diameter WhatmanR number 40 paper into a 100-mL volumetric flask. Wash the residue on the paper three times with a 1 percent (w/v) NaOH solution. Dilute to the 100-mL mark with distilled water and mix well.
10. Pipet 10 mL of the sample solution into a 100-mL polyethylene beaker; pipet 10 mL of 1 M ammonium citrate solution into the beaker, and stir the resulting solution. Measure the fluoride concentration in this final solution by ISE. As previously explained, it is extremely important that the pH of this solution be in the range of 5.5 to 7.0.
Measurements of electrode potentials are accomplished in the following sequence. Prepare each solution for measurements in a 100-mL polyethylene beaker a few minutes before the potential is measured. Before each measurement on a sample solution, equilibrate the electrodes for 5 min in a 4 m g/mL fluoride solution. Remove excess solution from the electrodes by blotting with laboratory tissue paper, such as KimwipeTM, and rinse the electrodes with a portion of the prepared solution in a 5-mL polyethylene beaker. Place the electrodes in the sample solution, and after exactly 10 min of equilibration, measure the potential produced by the fluoride-ion electrode. Typically, the potential ranges from 100 to 170 mV. The actual electrode potential has been observed to change as the electrodes age. The variability of single potential measurements commonly is ±0.5 mV. Upon completion of all measurements of electrode potentials for samples and calibration standards, graphically establish the functional relation between fluoride concentration and electrode potential by plotting concentration (in micrograms per milliliter) versus measured potential (in millivolts). This plot generally is accomplished on one-cycle semilogarithmic paper, as shown in figure 15. For an ash sample weighing 250 mg, the concentration of fluorine in the sample (in micrograms per gram) equals 800 times the concentration of fluoride in solution (in micrograms per milliliter), as extrapolated from the calibration curve (fig. 15).
ACCURACY, PRECISION, AND DETECTABILITY
Sets of coal-ash samples are always taken through the procedure with three blanks and National Bureau of Standards (NBS) coal standard reference material 1632. A fluoride concentration of approximately 10 m g/mL is typical for the blank solutions. The mean value for 51 determinations of fluorine on ash from NBS standard reference material 1632 over a 1-year period is 83 m g/g, with a standard deviation of 5 m g/g. This value agrees quite well with 80 m g/g reported by Gladney and others (1984) as the median of seven determinations by several different methods of analysis. The American Society for Testing and Materials (ASTM) method (ASTM, 1984) gives a fluorine concentration of 76 ± 10 ,ug/g (Gladney and others, 1984). Swaine (1985) noted that the ASTM method generally gives lower results than those from pyrohydrolysis methods. The lower limit of determination by the ISE method for fluorine in a 250-mg sample is 20 m g/g.
American Society for Testing and Materials (ASTM), 1984, D3761-79(1984) Standard test method for total fluorine by the oxygen bomb combustion--ion selective electrode method, in 1984 annual book of ASTM standards, petroleum products, lubricants, and fossil fuels, sect. 5, v. 05.05: Gaseous fuels, coal, and coke: Philadelphia, ASTM, p. 474-477.
Gladney, E.S., Burns, C.E., Perrin, D.R., Roelandts, Iwan, and Gills, T.E., 1984, 1982 compilation of elemental concentration data for NBS biological, geological, and environmental standard reference materials: NBS Special Publication 260-88: Washington, D.C., U.S. Government Printing Office.
Ingram, B.L., 1970, Determination of fluoride in silicate rocks without separation of aluminum using a specific-ion electrode: Analytical Chemistry, v. 42, no. 14, p. 1825-1827 .
Swaine, D.J., 1985, Modern methods in bituminous coal analysis: trace elements: CRC Critical Reviews in Analytical Chemistry, v. 15, no. 4, p. 336-337.
Swanson, V.E., and Huffman, Claude, Jr., 1976, Guidelines for sample collecting and analytical methods used in the U.S. Geological Survey for determining chemical composition of coal: U.S. Geological Survey Circular 735, p.7-8.
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