Anal. Chem. 1986, 58.3266-3269
3266
-T
Table I. Comparison of Asphaltene Data
filter paper method
--20 rnL Glass syringe
,
-2
1
-
t
-
asphaltene, wt Yo
crude oil residue Arab Light 340+ "C
I : i- -
1 - 1
av
Arab Medium 340+ "C
4% O,
av 5 p Millex filter
Arab Heavy 340+
O C
av
North Slope 340+ "C
Flgure 1. Apparatus for rapid filtration of asphaltene sample mixture.
This prevents loss of vacuum when a filter goes dry. This apparatus provides a rapid filtration technique and also permits collection of the filtrate for further analysis (such as the determination of hydrocarbon types) if so required. The sample and asphaltene suspension were poured into the 20-mL syringe, and vacuum was applied to the filter flask. A small amount of hexane (from a wash bottle) was used to rinse down the walls of the syringe before it went completely dry. Air was pulled through the filter for 1-2 min, and then the syringe and filter were rinsed with three 10-mLportions of n-hexane without allowing the filter to go dry until after washing was complete. Air was pulled through the filter until all the residual solvent was removed. The filter was heated at 100 "C for 1 h and then cooled and weighed.
RESULTS AND DISCUSSION This work describes an evaluation of Millex filters as a rapid and precise way to determine asphaltenes in crude oil residues. The data obtained were compared with data from a conventional filter paper method. The operating parameters applied in both cases were those recommended by Speight et al. (6): (1)the use of at least 30 mL of paraffin/g of heavy oil; (2) the contact time between the paraffin and the oil to be in the range of 8-20 h; and (3) all operations, precipitation, and washing to be carried out a t room temperature. A comparison of the values obtained for each of the six crude oil residues is given in Table I. The agreement between the values obtained for the filter paper and Millex filtration procedures is very good. The greatest difference is for Hondo crude, which has the highest asphaltene values. The pooled standard deviation based on triplicate determinations for each of the six samples was 0.1% in the range of 4.3-21.4 wt % asphaltenes. The precision of both techniques is very good. The Millex filter procedure proved to be a t least 4 times as fast as the fiiter paper technique. In addition, the Millex filters did not require the lengthy equilibration to room atmosphere needed by the filter papers. We have found from experience
av
Hondo 340+ "C av refinery residue 750+
O C
av
std dev
miilex filter method asphaltene, w t Yo
4.4 4.3 4.4 4.4
4.3 4.3 4.3 4.3
6.4 6.7 6.9 6.7
6.7 6.8 6.8 6.8
12.4 12.5 12.5 12.5
12.3 12.1 12.1 12.2
4.1 4.0 4.1 4.1
4.0 4.2 4.0 4.1
22.2 22.0 22.3 22.2
21.4 21.5
5.5 5.5 5.6 5.5
5.4 5.4 5.3 5.4
0.1%
0.1%
21.4
that the use of less than 2 g of sample for the filter paper technique causes unacceptable loss of precision, especially for samples with low asphaltene content. This is because of variations in the weight of the filter paper due to changes in its moisture content. The Millex filters, however, are hydrophobic and their weights are very reproducible as they are not as affected by moisture in the air. This makes it possible to use sample sizes as low as 0.2 g. The filtrate can be used for further analysis, such as the determination of hydrocarbon types. The purity of a blank filtrate was established by UV and GC analyses. Maximum absorbance was a t 200 nm with a typical absorbance of 0.02. Gas chromatography indicated the c6 content of the filtrate as greater than 99.9%.
LITERATURE CITED (1) Pearson. C. D.; Gharfeh, S . G. Anal. Chem. 1986, 58, 307. (2) Altgelt, K. H.; Gouw, T. H. Chromatography in Petroleum Analysis; Marcel Dekker: New York, 1979; Chapter 9. (3) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel
Dekker: New York,
(4) Hoberg,
A. J.
1980.
Bituminous Materials: Asphalts, Tars and Pitches ; Wiley-Interscience: New York, 1965. (5) Standards for Petroleum and Its Products; Institute of Petroleum: London, Standard No. IP 143157. (6) Speight, J. G.; Long, R. B.; Trowbridge, T. D. Fuel 1984, 6 3 , 616.
RECEIVED for review May 28,1986. Accepted August 1,1986.
Determination of Oxygen in Molten Alkali Halide Salts by Proton Activation Analysis C. M. Wai* and M. E. Dysart Department of Chemistry, University of Idaho, Moscow, Idaho 8 3 8 4 3 Charged-particle-induced reactions such as 180(p,n)18F, '60(3He,p)18F,and 160(4He,pn)18Fhave been used to analyze 0003-2700/86/0358-3266$0 1.50/0
oxide films and to determine oxygen in metals (1-4). The radioisotope 18F(a positron emitter with t l p = 110 m) pro@ 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table I Potential Interfering Positron Emitters Produced during Proton Activation of Oxygen element 0 F Ti
Ni
cu Zn
nuclear reaction threshold E , MeV 180(p,n)18F 18F(p,d)18F "Ti(p,n)"V -I'i(p,n)@V 'gTi(~,n)~~V @"i(p,n)@Cu 61Ni(p,n)61Cu 62Ni(p,n)s2Cu B4Ni(p,n)@Cu 'W~(p,n)~~Zn 65Cu(p,n)GSZn mZn(p,n)mGa 67Zn(p,n)67Ga BBZn(p,n)ssGa
2.4 8.2 3.7 4.8 1.4 6.9 3.0 4.7 2.5 4.2 2.1 6.0 1.8 3.7
t l l z product 109.8 min 109.8 min 32.6 rnin 16 days 330 days 23.4 rnin 3.4 h 9.7 min 12.7 h 38 min 250 days 9.2 h 3.3 days 68 rnin
duced from these reactions can be identified through the 0.511-MeV y radiation emitted during the positron annihilation process. A problem associated with the determination of small quantities of oxygen in a complex system using the charged-particle activation technique is that of spectral interferences resulting from other positron emitters produced by concomitant elements during irradiation. For instance, in the case of the proton activation process, the cross section for the reaction 180(p,n)1sFreaches a maximum of about 0.5 b around 5 MeV proton energy (5). At this proton energy, (p,n) reactions with other elementa, including Ti, Ni, Cu, and Zn can also take place, forming product nuclides which are positron emitters with half-lives comparable to that of 18F (Table I) (6, 7). Radiochemical separation of 18Fbecomes necessary in order to apply particle activation techniques for determining low levels of oxygen in complex systems. We have recently used a lanthanum fluoride precipitation method to separate 18Fproduced from proton activation of l80in alkali chloride and fluoride salts. The procedure was developed for studying dissolved oxide species in alkali halide melts. Determination of oxygen in these systems is important for research in batteries and in extractive metallurgy utilizing molten salts as solvents. The details of the experimental procedures, and a discussion on the applications of this method to oxygen determination in alkali chloride and fluoride systems, are given in the following sections.
EXPERIMENTAL SECTION Polarographic grade LiC1-KC1 (58:42) and LiF-KF-NaF (295912) eutectic mixtures were obtained from Anderson Physics Laboratory, Inc. (Urbana IL). A LiF crystal was obtained from Harshaw Chemical Co. Labeled Alz03(enriched to 89.3% l80) was purchased from Stohler Isotope Chemicals, Inc. The labeled A1203was heated in a vacuum oven at 450 "C for several hours before the experiment. Because of the hygroscopic nature of the salts involved, samples were prepared in a helium-filled glovebox with a furnace well attached to the bottom of the box. The glovebox atmosphere was continuously circulated through a purifier, and the oxygen and water contents were kept below 2 ppm. Two types of experiments were performed. One involved mixing of anhydrous alkali halide salts with various amounts of 180-labeled alumina with a mortar and pestle. The homogenized salt mixtures were made into pellets for proton-activation analysis. The other type of experiments involved melting of the anhydrous alkali halide eutectic mixtures in a quartz tube of 1 in. i.d. and 18 in. length heated to 500 OC. Varying amounts of 180-labeled A1203in f i e powder form were added to the melts, and the molten salt solutions were stirred vigorously with quartz spoons. Bath samples were then taken with the quartz spoons for chemical analysis. Soluble compounds of Cu, Ni, Ti, and Zn in chloride or other forms were also added to the melts to test the LaF3 separation procedures. For proton irradiation, salt samples were ground and homogenized with a mortar and pestle. The ground materials were
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made into pellets of 1.27 cm (0.5 in.) diameter and approximately 2-3 mm in thickness, using a Parr pellet maker. Each sample pellet was placed in an aluminum disk of 0.51 in. i.d. and 0.12 in. depth and sealed with 10-mil-thickaluminum foil with a small amount of superglue. The sealed disks were then transported from the drybox to a nearby cyclotron in a helium-filled container for proton irradiation. A 60-in. cyclotron at the Argonne National Laboratory was used for proton irradiation. The actual energy of the protons bombarded on sample surface was reduced to 7.5 MeV by using aluminum absorbers. At this energy, protons were totally absorbed by the sample pellet. This proton energy also prevents the formation of 18Ffrom the 19F(p,d)18F reaction. Each sample was irradiated with a total charge of 300 FC from a proton beam (about 0.75 in. diameter) of 0.5 MA. After irradiation, samples were removed from the sealed disks and dissolved in 5 mL of 4 M HC1. The radioactive 18F produced in the chloride salt samples was precipitated in the form of LaF3by the addition of 2 mL of 2 M LaC1, and about 40 mg of NH,F.HF to the acid solution. The fluoride precipitate was separated from the solution by centrifugation. After removing the solution, the precipitate was counted with a Ge(Li) detector connected to a 4096-channel analyzer. For fluoride salt samples, an excess amount of La3+was added to the solution in order to ensure complete precipitation of F-.
RESULTS AND DISCUSSION A lead fluoride precipitation method was reported by Lee et al. for the separation of 18Fproduced by 3He activation of l60in copper (8). The method involves dissolving the metal in acid, followed by removal of Cu2+,Zn2+,and Ga3+ as hydroxides a t pH 9. The solution was then acidified to precipitate lead fluoride a t p H 5 in an acetate buffer. After counting, the chemical yield of F- was determined gravimetrically by reprecipitation of the F- as PbClF. The solubility product of PbF2 is 1.07 X lo-' and that of PbC12 is 1.7 X (9). This PbF2 precipitation method is tedious and causes precipitation of PbC12 in alkali chloride systems. The radiochemical separation used in our procedure is based on the fact that the solubility of LaF3in water is extremely small (Ksp = 3 X 10-19) (10). Furthermore, in acid solutions, the LaF3 precipitate is essentially insoluble, whereas LaC1, is very soluble (IO). Therefore, small amounts of fluoride in alkali chloride salts can be effectively precipitated as LaF3 in an HC1 solution. Other fluorides, including those of Cu, Ga, and Zn, which have significant solubilities in HC1, are not expected to precipitate with LaF, (9). The precipitation procedure therefore serves two purposes. First, it separates 18Ffrom other positron emitters present in the system. Second, it concentrates 18Fin a precipitate form with fixed geometry for counting. The latter one is important because, after irradiation, the salt sample pellet may break into fragments, making it difficult to handle for y counting. By use of the precipitation procedure, the original salt samples can be in a pellet form or in a uniformly packed powder form, as long as they are thick enough to absorb all bombarding protons. It is important that the whole irradiated sample be dissolved in HC1, in order to ensure the precipitation of all 18Fproduced in the system. The alkali halide pellets became colored during the proton bombardment. The depth of penetration of the proton beam in salt samples was recognizable by the visual inspection of the irradiated samples. The LaF, precipitate is difficult to filter because it is gelatinous. Centrifugation is necessary in order to separate the precipitate from the supernate. The possibility of coprecipitation of interfering metal ions with LaF3 in HC1 solution has been tested by two different experiments. In the first experiment, the radioisotopes 72Ga(tllz = 14.1 h), 69mZn( t l , z = 14.0 h), and 64Cu(tl12= 12.7 h) were spiked in KC1 and KF salt solutions (in 4 M HC1) with the metal concentrations fixed at parts-per-million levels. The radioisotopes were produced from thermal neutron irradiation of their chloride solutions
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
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8.5 8.0
1 -
0
0
6.0
0
50
100
150
200
250
Time ( m i n i
Flgure 1. Decay of the 51 1-keV y peak for a proton-activated LiF sample spiked with 1000 ppm Ti and 50 ppm "0 and processed by
the LaF, precipitation procedure: (closed circles) 51 1-keV y in LaF, precipitate, (open circles) 51 1-keV y in supernate. in a 1-MW TRIGA reactor. Vanadium was not included in this experiment because the half-life of 52V (tllS = 3.8 m) produced from the thermal neutron capture process is too short for the radiochemical study. The LaF, precipitation and separation procedures were then carried out according to the descriptions given in the Experimental Section. After phase separation, the distributions of the isotopes in the precipitate and in the solution were measured by the characteristic y rays (72Ga,834 keV; 69mZn,438.6 keV; 64Cu, 511 keV) emitted during their decay. The procedures for thermal neutron irradiation and y counting are similar to those reported in the literature (11). In all cases, less than 2% of the activities were found in the precipitate, indicating that trace amounts of Ga, Zn, and Cu would remain in the acid solution during the LaF3 precipitation process. The radioactivities in the LaF, precipitate could be reduced by washing with 4 M HC1, suggesting that small amounts of the radioactive isotopes were probably trapped in the precipitate in solution form or adsorbed to the container walls. In the second experiment, Cu, Ga, Zn, and V were added separately to the alkali halide salts spiked with 180-labeled alumina, and the mixtures were processed according to the procedures described for proton activation and radiochemical separation. The radioactivities in the precipitate and in the solution were both measured under the 0.511-MeV y peak at different time intervals after the separation. In each case, the precipitate clearly showed a half-life of 110 min, consistent with that of 18F,and the radioactivities remaining in solution showed varied half-lives, characteristics of a mixture of isotopes. To illustrate this, Figure 1 shows the rates of decay of the 511-keV peak for the precipitate and the supernate obtained from a LiC1-KCl sample spiked with about lo00 ppm Ti and 50 ppm 180-labeledalumina. Titanium produces three isotopes from the (p,n) process, 47V,48V,and 49V,with halflives of 32.6 min, 16 days, and 330 days, respectively. The isotopes with mass number 47 and 49 are primarily positron emitters, and the one with mass number 48 decays by both electron capture and positron emission associated with 983.5and 1312.1-keV 7's. As shown in Figure 1,the 511-keV peak of the precipitate definitely shows a half-life of 110 min, and the decay curve of the supernate shows the presence of a short-lived and a long-lived component. It was also noticed that about 1% or less of the y radiations from 48Vwere found in the precipitate. The recovery of 18Ffrom proton activation of LiCl-KCl containing different concentrations of 180-labeled A1203has been tested using this LaF3 separation procedure. The Samples were prepared by mixing l80-labeled alumina in LiC1-KCl eutectic melt in the quartz tube at 500 "C. The 18Factivities in the LaF, precipitates were found to increase linearly with the amount of l80present in the salt samples. The blank in
this experiment showed approximately 1ppm l80.With this level of blank, the proton activation technique is capable of detecting the dissolved l80in molten alkali chloride salts at above 3 X lo4 mol fraction. Lower l80blank values were found in other experiments using freshly ground KCl or NaCl crystals without melting. These results indicate that quantitative proton activation analysis measurement of partsper-million levels of l80in alkali chloride melts can be achieved by use of this radiochemical separation method, by comparing 18Factivities in the samples with a spiked standard irradiated and counted under identical conditions. In the case of proton activation of l80in alkali fluoride salts, the 19F(p,d)18Freaction, which has a Q value of -8.2 MeV, can cause interference for oxygen determination. However, this interference can be easily eliminated by irradiating samples at proton energies less than 8.2 MeV. We chose to use 7.5MeV protons for sample irradiation to avoid the interference of fluorine. According to our experiments, pure LiF crystals irradiated with this proton energy showed 18F activities equivalent to less than 1 ppm l80,clearly indicating the absence of F interference. If the proton energy was above 8.5 MeV, large 18Factivities were found in the irradiated LiF samples. Proton activation of LiF or a LiF-KF-NaF eutectic mixture spiked with varying amounts of l80-labeled A1,03 also showed linear calibration curves similar to that obtained from the alkyl chloride systems. It should be pointed out that when LiF-KF-NaF eutectic mixture was melted in the quartz tube at 500 "C, much higher l80blank values (around 10 ppm) were observed. This high blank value was attributed to the interactions of molten alkali fluorides with the quartz container. A neutron-activation technique has been used by Haupin to determine oxygen in alkali chloride salts via the l60(n,p)l6N reaction induced by 14-MeV neutrons produced from a neutron generator available at the Oak Ridge National Laboratory (22). The product nuclide 16N has a half-life of 7.2 s and can be identified by the 6.1-MeV y emitted during its decay. However, this activation technique cannot be applied to oxygen determinations in fluoride systems because of the interfering 19F(n,cy)16N reaction, which is about one-third as great as that of the l60reaction with the fast neutrons (13). Other charged-particle activation techniques, such as the cy activation process, also cannot be applied to the fluoride systems because of the 19F(a,cyn)18Freaction, which has a Q value of -10.4 MeV, about 5.9 MeV lower than the Q value of the 160(cy,d)'8F reaction. Therefore, the interfering reaction from fluorine cannot be eliminated by simply adjusting the energy of the bombarding cy particles. In the case of 3He activation, the Q values for the 160(3He,p)18F and the 19F(3He,cy)18F reactions are 2.0 and 10.1 MeV, respectively. This means that both reactions will proceed spontaneously with 3He of almost zero kinetic energy if the particles can penetrate the potential barriers of the target nuclides. The maximum cross section for the (3He,p)reaction occurs around 7.6 MeV. At 6.5 MeV, the cross sections of the 160(3He,p)18Fand the 19F(3He,cy)18F reactions are approximately 350 mb and 15 mb, respectively (8). Although the former has a much greater cross section, in a fluoride matrix with a small amount of oxygen, the latter reaction can cause serious interference for the l60determination. Correction for the interference from the fluoride matrix in the cy or the ,He activation process is by no means simple. The proton activation procedure, which is free of fluorine interference a t energies less than 8.2 MeV, appears to be a unique technique suitable for determining low levels of oxygen in molten fluoride salts. This technique is especially useful for measuring solubilities of dissolved oxide species in molten fluoride salts when l80-labeled compounds are used. The proposed LaF, precipitation method for the isolation of 18F
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Anal. Chem. 1986, 58, 3269-3270
in HCl solution should be a general radiochemical separation procedure for oxygen determination by other charged-particle-activation processes in samples with relatively low fluorine contents. ACKNOWLEDGMENT We thank Milan C. Oselka for performing sample irradiations at the Argonne cyclotron facilities, Robert R. Heinrich for the use of counting equipment, and Milton Blander for his support of this project and many helpful discussions. Marilou Dysart wishes to thank the Chemical Technology Division, ANL, for a summer student fellowship which allowed her to perform the proton activation experiments. Registry No. 02, 7782-44-7;LiC1,7447-41-8;KC1,7447-40-7; LiF, 7789-24-4; KF, 7789-23-3; NaF, 7681-49-4; ISF, 13981-56-1; LiCl,, 10099-58-8. LITERATURE C I T E D (1) Thompson, B. A. Anal. Chem. 1981, 33, 583. (2) Markowitz, S. S.; Mahony, J. D.'Anal. Chem. 1982, 3 4 , 329.
Rook, H. L.; Schweikert, E. A. Anal. Chem. 1989, 4 1 , 958. Ricci, E.; Hahn, R. L. A n d . Chem. 1988, 4 0 , 54. Blaser, J. P.; Boehm, F.; Marmier, P.; Scherrer, P. Heiv. Phys. Acta 1951, 2 4 , 465. Blosser, H. G.; Handley, T. H. Phys. Rev. 1955, 100, 1340. Tanaka, S.; Furukawa, M. J . Phys. SOC.Jpn. 1959, 14, 1269. Lee, D. M.; Stauffacher, C. V.; Markowitz, S. S. Anal. Chem. 1970, 4 2 , 994. Handbook of Chernisfry and Physics. 64th ed.; CRC: Boca Raton, FL, 1983. Burgess, J.; Kljowske, J. Adv. Inorg. Radiochem. 1981, 2 4 , 5 7 . Mok, W. M.; Shah, N. K.; Wai, C. M. Anal. Chem. 1988, 5 8 , 110. Haupin, W. E. Light Met.; New York, 1979; p 475. Lauff, J. J.; Champlin, E. R.: Przybylowicz, E. P. Anal. Chem. 1973, 4 5 , 52.
RECEIVED for review May 12,1986. Accepted September 4, 1986. This work was performed a t Argonne National Laboratory (ANL) with the support of ANL and a grant from the Aluminum Company of America (Alcoa).
Syringe Pycnometer for Use with Electronic Microbalance Hugh A. Wyllie
Australian Atomic Energy Commission, Research Establishment, Lucas Heights Research Laboratories, Private Bag, Sutherland, New South Wales 2232, Australia A syringe pycnometer that is light enough to be weighed on an electronic microbalance has been developed from the apparatus first described by Lowenthal and Page (1). A silicone-treated glass needle of simpler design is fused directly to a disposable syringe to form the pycnometer. The new pycnometer holder, which is employed during the dispensing of solutions, allows weighing to be carried out on a Mettler M3 electronic microbalance, whose pan can bear loads of up to 3.05 g. The apparatus is used for dispensing radioactive solutions when preparing counting sources for the standardization of radioactivity.
A
EXPERIMENTAL SECTION Pycnometer Construction. The pycnometer is made from a 2-mL polypropylene syringe (supplied by Pharma-Plast (Australia) Pty Ltd.). Nonsterile syringes are used because sterilization with ethylene oxide reduces the tightness of fit between plunger and barrel. Glass needles are prepared from 2 mm i.d. by 7 mm 0.d. tubing. A 30-cm length is heated in the middle by an oxy-gas flame, drawn out, cooled, and bisected. Half (see left-hand side of Figure 1) is inserted into the Luer fitting of a syringe from which the plunger has been removed, withdrawn with thumb nail marking point A, and cut into two at that point. A needle of the required length is made from the piece shown above point A in Figure 1,by cutting off the excess at the narrow end. To render the needles water repellent, the following silicone surface treatment is carried out. The needles are dipped in a solution of Dow Corning 1107 Fluid, allowed to stand upright on a pad of cellulose tissue to drain, and then centrifuged with a similar pad at the bottom of the centrifuge tube. The coating is heat-cured at 175 "C for 15 min. To make the pycnometer, the plunger of a syringe is retracted 2 cm from the Luer fitting; with the syringe held horizontally, the wide end of a needle is inserted through the fitting to point B (see Figure 1). The syringe is slowly rotated with the tip of
Flgure 1. Construction of pycnometer. A glass needle (cut off at point A) is inserted into a syringe as far as point B and sealed in piace by heating at point C.
the fitting (point C) above a small Bunsen burner flame which melts and ignites the plastic. The syringe is moved away from the burner, and as the molten zone approaches the wide end of the Luer fitting the flame from the ignited plastic is blown out. Rotation is continued as the plastic cools; when the plastic has become sufficiently viscous, the seal is drawn out slightly by holding the pycnometer vertically by the tip of the needle until the plastic is hard. Sometimes, when the plastic is burning, the needle moves into the barrel of the syringe. This is rectified by
0003-2700/86/0358-3269$01.50/0 0 1986 American Chemical Society
