i
Table I. Results for Fluoride in Coals Determined with the Ion-Selective Electrode Fluoride, ppm Coal
Bomb combustion
Alkali fusion
NBS 1631 C-14796 C-15456
80 3: 4 (8 replicates)
86,88 120,122 100
113,111,115 103,100
Table 11. Results for Fluoride in Coals Determined with Test Solutions at Different pH Values Fluoride, ppm Coal
pH 7.2
pH 6.2
pH -5.0
C-14796 C-15448 C-15456 C-15038 C-16501 C-14774
123
113 98
... ... 51, 45, 49 ...
118
112 79 70 53
100
68 54 42
PH
Effect of p H on the potential of a lanthanum fluoride electrode in sodium fluoride solutions of various concentrations. Figure 1 .
The potential change with pH in the acidic region is caused by the formation of H F 2 - , [From Butler (7)-reproduced with the author's permission]
...
...
the fluoride that is free differs appreciably after the incremental change in total fluoride concentration. Crossley believed that fluorine in coal is present mainly as fluorapatite, CalOF2(P04)6. The occurrence of this phase in coal is also considered likely by Gluskoter et al. (9), who have described apatite petrifactions and nodules from Pennsylvanian shales in Illinois and have identified them as carbonate-fluorapatites. Some of these were in a black fissile shale immediately overlying a coal seam. One of our coal samples appeared unique in that it had a fluoride content appreciably outside the range of the other samples studied. Because the initial value of 265 ppm fluoride appeared anomalously high, replicates were run, and values of 153 and 240 ppm fluoride resulted. As mentioned a t the beginning of this report, the heterogeneity of coal frequently makes it very difficult in trace element analysis to achieve satisfactory analytical reproducibility. Some segregation of the inorganic fractions of higher density within the ground coal samples quite possibly can occur simply during sample handling. This was thought to be the likely explanation for the poor reproducibility with this particular sample. Unaltered mineral matter residue was obtained from this coal by means of a radiofrequency, low-temperature ashing technique de(9) H. J. Gluskoter, L H. Pierard, and H. W . Pfefferkorn. J . Sediment. Petrol.. 40, 1363 (1970).
scribed by Gluskoter (10). The heavy mineral concentrates, obtained by separating the low-temperature mineral matter residue in bromoform, were examined with a scanning electron microscope (Cambridge Stereoscan) that was equipped with an energy-dispersive X-ray spectrometer. A distinct calcium phosphate mineral phase, presumed to be apatite, was clearly defined. It is the only Illinois coal examined in this fashion to date in which this phase has been clearly discerned. Thus, for this particular sample, it would appear that the high fluoride values, together with the poor reproducibility, result from the presence of fluorine in relatively large discrete mineral fragments that have densities greater than twice that of coal, with consequent nonuniform dispersement in the ground sample.
ACKNOWLEDGMENT The authors acknowledge with thanks the use of the scanning electron microscope a t the Center for Electron Microscopy, University of Illinois. Received for review November 30, 1973. Accepted April 4, 1974. This study was supported, in part, by the U.S.Environmental Protection Agency under Grant R800059 and Contract 68-02-0246. (10) H. J. Gluskoter. Fuel. 44, 285 (1965)
Simultaneous Determination of Hafnium and Zirconium in Silicate Rocks by Isotope Dilution Lawrence B. Owen and Gunter Faure Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 432 10
The decay of naturally-occurring I7'jLu t o stable 176Hfis of great interest to geologists because of its potential use in determining the ages of Lu-bearing rocks and minerals.
Previous work has shown that the uncommon rare-earth minerals gadolinite (FeYpBep(Si0s)z) and priorite (Y, Er, Ca, Fe, T h ) (Ti, NbI2O6) can be dated by the Lu-Hf meth-
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 9, AUGUST 1974
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A. HAFNIUM
x
iI
!I1
Figure 1. ( A ) Mass spectrum of Hf modified by the addition of a spike Mass spectrum of Zr modified by the addition enriched in 179Hf;(6) of spike enriched in 91Zr
od (1-4). However, this method has not been used to date more common rocks and minerals because of the analytical difficulties in measuring the concentration and isotopic composition of Hf a t concentration levels of a few parts per million. A variety of analytical methods are available for the determination of Hf and Zr (5-7). However, in order to use the decay of 176Luto lTsHf for dating of rocks and minerals, a procedure is required which permits the determination of the concentration of Hf and its isotopic composition. The only method that is capable of providing this information is isotope dilution (8).This paper describes such a procedure and presents new determinations of the concentrations of Hf in a suite of silicate rock standards. The application of the method to the measurement of the isotopic composition of Hf in geological materials will be presented elsewhere. An advantage of the isotope dilution method presented here is the opportunity for the simultaneous determination of the concentration of Zr.
EXPERIMENTAL Known amounts of calibrated spike solutions enriched in li9Hf and 91Zr were evaporated to dryness in 30-ml Pt crucibles. Powdered samples (99.7%. Gold was obtained a s shot, >99.9% purity from K & K Laboratories. All inorganic chemicals used were reagent grade. Acetophenone and benzaldehyde (Aldrich Chemical Co.) were used as received. 1,3-Diphenyl-2-propen-l-one(chalcone) was synthesized by the procedure of Helmkamp and Johnson (2). 2.4,6-Triphenylpyrylium chloride was prepared a s previously described ( I ) . Elemental analyses for sulfur, platinum, and gold, were performed by the author (3).Elemental analyses for carbon and hydrogen were performed by Galbraith Laboratories, Inc.. Knoxville, Tenn. 2,4,6-Triphenylpvrylium Bisulfate. For a typical run, 42.76 g (0.205 mol) of chalcone, 12.35 g (0.103 mol) acetophenone, and 32.2 g (0.312 mol) of 95% sulfuric acid were mixed in an erlenmeyer flask and heated on a steam bath for 3 hours. After the flask was removed from the steam bath, the contents were mixed with 200 ml of distilled water, and the precipitate which had formed was dissolved by heating. During the process of heating, Helmkamp and H W Johnson, "Selected Experiments in Organic Chemistry.''W . H . Freeman, San Francisco, Calif., 1964, p 81 (3) J . 6. Niederl and V . Niederl, "Micromethods of Quantitative Organic Elementary Analysis," John Wiley and Sons, New York. N Y , 1938. p p 41-43.
(2) G . K .
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