a t 52 "C with trifluoroacetic anhydride in dry benzene to produce 111. Using these smaller amounts, the yield of the reaction averaged 85%. To determine if the derivatization reactions might be adapted to other urea herbicides, diuron, fenuron, and monuron were studied. All three were successfully reacted with methyl iodide to produce the trimethylated compounds. However, only fenuron and monuron reacted with trifluoroacetic anhydride. The reaction conditions were not optimized and as a result, yields were poor. Only the presence or absence of a product was noted on the chromatogram. In the five successful reactions on diuron, fenuron, and monuron, the expected product was obtained and was found to be stable under the conditions used during GC. The identity of these products was confirmed by GC-MS. Combined GC-MS was used as the primary identification tool for all derivatives prepared in these various experiments. This instrument proved to be ideal for such work; only a small amount of compound was required, extensive sample preparation was unnecessary, and spectra took only a few seconds each to record. In all cases, the molecular ion
in the spectrum corresponded to the molecular weight of the desired product. Further proof of identity came from studying the fragmentation patterns, which in general were similar to other substituted ureas (15). The studies reported here suggest that derivatization reactions can be performed on trisubstituted ureas to form products with enhanced thermal stability and increased sensitivity of detection. Such derivatives should be useful for developing sensitive analytical procedures for these compounds.
ACKNOWLEDGMENT The authors thank E. W. Day and J. W. Mosier for their assistance and suggestions. Received for review October 1, 1973. Accepted April 24, 1974.
(15) M. A . Baldwin, A. M. Kirkien-Konasiewicz, A. G. Loudon, A . Maccoll. and D. Smith, J. Chem. SOC.B, 1968, 34.
Determination of Fluoride in Coal with the Fluoride Ion-Selective Electrode Josephus Thomas, Jr., and Harold J. Gluskoter iiiinois State Geoiogicai Survey. Urbana. 111. 6180 1
Fluorine is one of several trace elements in coal currently receiving much attention owing to their possible harmful ecological effects upon being released from the large tonnages of coal being burned annually. As those engaged in coal analysis recognize, the determination of trace elements with satisfactory accuracy and precision is difficult, not only because of the heterogeneity of the material but also because of the lack of coal standards. Until the invention of the fluoride ion-selective electrode by Frant and Ross ( I ) a few years ago, fluorine analysis, particularly at the parts-per-million level, was a challenge for the analyst. Among the problems associated with fluorine analysis, mention may be made of the action of fluorides on glassware. the high solubility of nearly all fluorides thereby eliminating gravimetric methods, and the complexing of fluorides by iron, aluminum. and certain other cations in solution. Today, however, the use of the fluoride ion-selective electrode is commonplace for the reliable determination of fluoride at quite low (10-5 LO 10--6M) concentration (activity) levels. In many cases, separations are unnecessary and only simple solution chemistry is required for p H adjustment and/or freeing the fluoride ion from possible complexes. The occurrence of fluorine in British coals was the subject of an extensive investigation several years ago by Crossley (2-5) who developed a satisfactory method for fluoride determination. Finding that the existing methods (1) (2) (3) (4) (5)
M S Frant and J W Ross, Jr , S o e n c e . 1 5 4 , 1 5 5 3 ( 1 9 6 6 ) . H. E. Crossley, J SOC. Chem Ind.. London. 63,280 (1944) H. E Crossley J SOC.Chem. I n d . . London. 63, 284 (1944) H. E Crossley. J . SOC. Chem. l n d . London. 63, 289 (1944) H. E Crossley. J . SOC.Chem. I n d . . London. 63, 342 (1944)
a t the time were unsuitable for the determination of fluorine in coal, Crossley selected and modified a method that depends on the fading action fluorides have on zirconiumalizarin lake. In his method, the coal is decomposed. either in a combustion bomb or by alkali ignition, and the fluoride is separated by distillation as fluosilicic acid. The fluoride content of British coals. sampled from widespread locations, was found to range from 5 t o 200 ppm. The method we describe here requires no separations. Coal samples are decomposed in a combustion bomb, the pH of the bomb contents is adjusted, and the solution is buffered prior to the determination of the fluoride concentration with the fluoride ion-selective electrode. Analysis is by the known addition method (6) in which the change of potential (1.E) resulting from the addition of a known volume of standard fluoride solution to the initial test solution is used to determine the fluoride concentration of the initial solution. This technique is superior to direct potentiometry as problems resulting from interferences. complexation, and ionic strength variations are virtually eliminated. Reproducibility is good. To corroborate the quantitative recovery of fluoride b j the bomb combustion method. an alkali fusion method (-7) was used to decompose some of the coal samples. This method involves the ignition of coal mixed with an excess of sodium carbonate at approximately 473 "C, followed by fusion of the mixture at 1000 "C. In our study. the fused contents were then extracted with hot water and the resultant solution was treated like that from the bomb ('ombustion method. ( 6 ) Orion Research Inc
Cambridge Mass Newsleffer 2. 2 (1970)
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 9 , A U G U S T 1974
1321
EXPERIMENTAL Apparatus. A fluoride electrode and calomel reference electrode were used in conjunction with a Digital 112 Research pH meter (all from Corning Scientific Instruments, Medfield, Mass.). Plastic beakers and volumetric flasks were used in handling the fluoride-containing solutions. The contents were stirred with Teflon-coated magnetic stirring bars. A Parr oxygen combustion bomb containing the sample was fired in standard coal calorimetry apparatus. A platinum crucible was used in the fusion experiments. Reagents. All solutions were prepared with water that had passed through a mixed anion-cation resin (Model MB-575 IllcoWay Mixed-Bed Demineralizer, Illinois Water Treatment Co., Rockford, Ill.). The water contained less than 0.01 ppm fluoride. All reagents were reagent grade unless otherwise stated. Standard fluoride solutions were prepared as required from a 1-liter stock solution containing 4.199 grams (1900 ppm, or 0.1M F - ) ultrapure sodium fluoride (Alfa Inorganics, Inc., Ventron Corp., Beverly. Mass.). The NaF had been dried at 120 “C. A concentrated citrate ionic-strength-adjustment buffer was prepared by dissolving 294 grams sodium citrate dihydrate and 20.2 grams potassium nitrate in 1 1. of water and adjusting the pH to 6.0 with citric acid. Primary standard benzoic acid was used to ensure complete coal combustion, and primary standard anhydrous sodium carbonate was used in the fusion experiments. Five milliliters of 1M sodium hydroxide solution were used in the bomb for each combustion, and 0.25M sulfuric acid solution was used for pH adjustment. An ultrapure grade of oxygen (Linde Division, Union Carbide Corp.) was used for pressurizing the combustion bombs. Samples. Fluoride analyses were carried out on more than 100 coal samples, most of which were from the Illinois Basin. All of the coal samples were either face-channel or drill-core samples and were representative of the entire coal seam, although mineral bands over three-eights of an inch thick were excluded. The coal samples were first crushed to pass a one-eighth inch screen; a portion was then further comminuted to 20 mesh (‘740 pm) and then to 100 mesh (149 pm). At all stages of the sample preparation, portions were subdivided by riffle sample splitters or by quartering the sample. The final sample in each case, thus. was representative of the original coal sample. In addition, fluoride analysis was conducted on a sample blended at the National Bureau of Standards (Natl. Bur. Standards SRM 1631). This particular sample has been distributed to various laboratories for comparative trace element determinations and, hopefully, it will become a much needed “standard” coal sample. Procedure. Parr Bomb Combustion. One gram of coal ( < l o 0 mesh), weighed to the nearest 0.5 mg, is mixed with about 0.25 gram benzoic acid in a fused quartz sample holder and placed in a combustion bomb containing 5 ml 1M sodium hydroxide. If the coal is ground sufficiently fine, the benzoic acid addition. which assists combustion, is unnecessary. The bomb is pressurized with O2 to about 28 atmospheres and fired. At least 15 min are allowed to elapse before the bomb is depressurized to permit the bomb to cool and to condense the mists. The bomb is exhausted slowly over a period of about 1 min. Three approximately 5-ml aliquots of demineralized water are used to rinse the bomb contents into a 50-ml plastic beaker. The beaker contents are stirred while the pH is adjusted to 5.0-5.2 with 0.25M HzS04. (Before adjustment, the pH is generally about 7.0-7.5.) This reduces the initially high HCOs- content sufficiently to minimize its possible interference in the fluoride determination. The beaker and contents are heated in a hot water bath for about 10 min and then stirred again to drive off most of the dissolved COz. Five milliliters of the citrate ionic-strength-adjustment buffer are added to the beaker contents to buffer the solution a t a p H of about 6.0, and to release most of the fluoride from complexes with iron, aluminum, and hydrogen ions. The beaker contents are then brought to room temperature. The total volume is adjusted to 50 ml with demineralized water and the potential is determined with the fluoride ion-selective electrode. One milliliter of 0.01M F - (190 ppm) is added to the beaker contents, and the potential is again determined. During the determination, the contents are stirred moderately. The fluoride content in the coal is calculated from the AE resulting from the known fluoride addition. Alkali Fusion. In the alkali fusion method, 3 grams of coal (weighed to the nearest milligram) are mixed with about 5 grams of anhydrous sodium carbonate in a platinum crucible. Approximately 2 grams more of the XaZC03 are used to cover the mix1322
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO.
ture. The crucible and contents are heated in a furnace at about 475 “C for 24 hr. The crucible and contents are then heated over an air-boosted Meker burner (about 1000 “C) for 15 min to fuse the mixture. The crucible contents then are extracted with hot water. The pH is reduced to 5.0-5.2 (from -12) with 0.5M HzS04. The solution is warmed in hot water to assist the removal of COz. About 25 ml of the citrate ionic-strength-adjustment buffer are added to the solution. The total volume is built to a known value (generally to 250 m l ) . The potential is determined with the fluoride ion-selective electrode. Five milliliters of 0.01M standard fluoride solution are added and the potential is again determined. The fluoride concentration is calculated from the known addition and from AE.
RESULTS AND DISCUSSION
The fluoride content of the coal samples was found to range from 25 to 135 ppm, and to have a mean value of 60 ppm. This is within the range of values reported by Crossley for British coals. Three of the coals were decomposed by both the bomb decomposition method and by alkali fusion. Fluoride results from both methods are shown in Table I. The agreement between the two decomposition methods is satisfactory and serves to indicate that the major problems anticipated for the bomb combustion method-namely, the incomplete breakdown of the fluoride-containing component(s) in coal and the loss of fluoride during depressurization of the bomb-are not realized. As only about 1.0 to 1.5 grams of coal can be conveniently burned to completion in an oxygen combustion bomb, the concentration of fluoride in the 50-ml test solutions is W 4to 10-5M. At this concentration level, the major source of error in this method may arise from the pH a t which the initial potential is determined, as shown by Butler (7) (Figure 1).At a pH above 7.0, hydroxyl ion interferes and a less positive potential is obtained a t 10-5M fluoride concentration. With the known-addition method used here, the less positive initial potential reading leads to higher fluoride results because AE is less than it should be after the addition of the standard fluoride solution. In addition, according to the Corning Information Bulletin ElFL-2 concerning their fluoride electrode, the bicarbonate ion can also interfere if present at a concentration level 1000 times greater than that of the fluoride ion. The removal of the bicarbonate is therefore paramount. A t a pH less than about 5.0, hydrogen ions form well known complexes, such as HFz-, with available fluoride ions, thus producing low fluoride results. Table I1 shows comparative fluoride results for several coal samples. The test solutions were buffered at pH 6.2 and a t 7.2 with different citrate-citric acid ionic-strengthadjustment buffers. Results also are shown for one sample in unbuffered test solutions that had been adjusted merely to a pH near 5.0 with 0.25M H2S04. A t pH 7.2, the fluoride results are 10 to 22% higher than those determined at pH 6.2. In the unbuffered solutions with pH near 5.0, the fluoride results are much lower. In addition to fluoride complexing with hydrogen ions in unbuffered solutions, it is likely that considerable complexing also occurs with aluminum or silicon ions, which are present from the breakdown of clays normally present in coals. Edmond ( 8 ) has shown that the citrate buffer frees most of the fluoride from the latter complexes. As was pointed out earlier, the known addition method does not require complete freedom of the ion being determined. In the unbuffered solutions at pH 5 , however, it would appear that in such dilute fluoride solutions, the fraction of ( 7 ) J N Butler
Thermodynamic Studies Chap 5 in Ion-Selective Electrodes R A Durst Ed Nat Bur Stand Spec Pub/ 314, Washington D C 1969 ( 8 ) C R Edrnond Anal Chem 41, 1327 (1969)
9 , A U G U S T 1974
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 , V O L . 46, NO. 9, AUGUST 1974
1323
