Anal. Chem. 1981, 53, 925-928
925
Pyrohydrolytic-Ion Chromatographic Determination of Fluorine, Chlorine, and Sulfur in Geological Samples Keenan L. Evans, Jarnes G. Tarter, and Carleton B. Moore* Department of Chemistry, .Arizona State University, Tempe, Arizona 85281
In a previous paper (I),we have detailed a combustion-ion chromatographic method for the determination of chlorine in silicate rocks. This paper describes a modification of that method to include the determination of fluorine and sulfur in the same sample. The significant role otf chlorine in geochemical processes was noted earlier (I). Fluorine and sulfur are greatly enriched in a large number of important ore deposih, and Boyle states that along with boron, fluorine and sulfur are the most universal indicators of mineralization (2). Other workers have documented the utility of fluorine, chlorine, and sulfur as indicator elements in many different types of mineral deposits worldwide (3-5). Furthermore, recent, reliable data on the abundance of fluorine and chlorine in meteorites are scarce and highly variable (6). The need for a reliable, low-cost method of fluorine, chlorine, and sulfur determination capable of handling large numbers of samples with relative ease is thus well established. Existing methods for the determination of chlorine in geological samples were dislcussed in an earlier work (1). Recent techniques for fluorine (determination inlclude photon activation analysis (3, basic fusion-spectrophotometry (8), pyrohydrolytic-spectrophotometry (9, IO), basic fusion-ion-selective electrode (1I), and pyrohydrolytic-ion-selectiveelectrode (12-14) techniques. All of the techniques mentioned for fluorine seem to have detection limits, accuracy, and precision necessary for most geological sample investigations. However, photon activation analysis is rather expensive and compared to the proposed technique the others are timeconsuming requiring substantially more sample preparation. Methods for sulfur determination include X-ray fluorescence (15), combustion--iodometric titration (16, 17),and combustion-infrared spectrophotometry (18). Practical determination limits for sulfur by X-ray fluorescence are around 50 ppm and for the combustion methods 30 and 10 ppm, respectively, with the latter two technique13being specific for sulfur only. The pyrohydrolytic-ion chromatographic method described here is capable of determining all three elements from the parts-per-million level to
standard
F content
2
NBS-91
5.7 2%‘“
8
U 01
.“ P CI
no. of % sample Sam- recovsize, mg ples ery 5-10 20 40 40
4 5 6 6 2 2 2 2 5 4
80
BCR-1 AGV-1 ZGI-BM DTS-1 a Sn
Fe
Cu
Fs
SnCu F r C u
VP5
F r C u Fc-Sn Fe-Sn
Fe-Sn
VZOS
v205
US08
470ppmb 435ppmb 250ppmb 15ppmb
NBS certified value.
Flgure 1. Relative fluorine recoveries of various sample-accelerator
combinations.
standard
8
E22 ..7700-
f3 €98
0
u
0
d
82.3 81.0 87.6 77.8 83.4 82.6 79.5 79.8 84.1 83.3
4.2 3.4 3.6 3.4
2.8 10.1
Reference 20.
Table 11. Chlorine Recovery from Standard Samples
SAMPLE-Accelerator Combinations
80 -
100 150 150 20-200 300
% RSD
ZG1-BM AGV-1, 110/11 BCR-1, 80/5 DTS-1, 44/30 ‘“Reference1.
@
amt of Cl,ppm this work previous worka 67 122 69.3 9.5
68.5 120.5 67.8 9.3
@
0
Table 111. Sulfur Recovery from Standard Samples standard sulfur content % recovery
n
NBS-153a NBS-13f NBS-16e BCR-1 Allende
BURN TIMES
5 MINUTES
0 10 MINUTES
n I
0.4
I
15 MINUTES
@ 20 MINUTES
I
1
1
0.8 1.2 1.6 2.0 O2 Flow Rate (hrnin.)
I
I
2.4
2.8
a
NBS certified value.
85.3 79.1 79.5 85.7 87.9
Reference 17.
Reference 16.
Table IV. Determination of Sulfur in NBS Steels
Figure 2. Variation of fluorine recovery with O2flow rate and burn time.
greater than 15 min. Addition of V205or UBOspowder to the sample mixture increased recoveries only slightly, but along with trapping of a postsample 5-min blank as a combustion tube purge with each sample, the addition of these powders enhanced our relative standard deviation to an average of f5%. Under these fluorine-optimized conditions chlorine recovery was 100 f 5% and sulfur recovery was 85 f 6% for most samples. Precision Estimates, Working Curves, and Detection Limits. Detection limits were practically determined by the precision of the blank values. A series of four to six blanks was trapped with each series of samples, and detection limits were defined as the amount of fluorine, chlorine, or sulfur necessary to give a concentration in the trapping solution three standard deviations above the average blank value. Under the optimum operating conditions detection limits commonly averaged 1 pg, 0.9 pg, and 5 pg absolute for F, C1, and S, respectively. Fluorine and chlorine concentrations of the trapping solutions were calculated from the same chromatogram by comparison to a working curve constructed by plotting concentrations vs. relative peak heights of a series of appropriately diluted solutions of NaF and NaCl in the standard eluent. All these initial chromatograms were run by using a 3 X 250 mm separator column except when there was concentration difference greater than 2 orders of magnitude between fluoride and chloride, in which case it was necessary to use the 3 X
70 ppma 160 ppma 290 ppma 473 ppmb 2.10%c
“unknown” 1(NBS-361)
2 (NBS-163) 3 (NBS-8j) 4 (NBS-361) 5 (NBS-361) 6 (NBS-129~) 7 (NBS-163) 8 (NBS-129~) 9 NBS-8.i) 10 (NBS-129~) 11(NBS-163) 1 2 (NBS-8j)
amt of S, ppm NBS method method value A B 170 270 770 170 170 2450 270 2450 770 2450 270 770
180 270 6 80 180 170 2560 260 2520 770 2460 2 50 7 50
130 195 700 130 105 2960 210 2910 800
2820 200 820
500 mm column to achieve sufficient separation of the two peaks. Enough 3.0% HzOzwas then added to the trapping solution to oxidize all the sulfur species to SOf, and a chromatogram to determine total SO:- concentration was run. Impurities in the HzOzsolutions caused interference peaks in the area of the fluoride and chloride peaks thus preventing determination of all three species in a single ion chromatogram. Recovery data for a number of standard samples are listed in Tables 1-111. Fluorine and sulfur recoveries varied by a few percent from day to day but were consistent on any one day. A series of NBS-91, GSP-1, and ZGI-BM standards for fluorine and a series of NBS standard steels with certified sulfur values were run with each set of samples. Plots of
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
Table V. Analysis of Standard Rocks standard split Fa
Cla
Sa
343 455 35 35 335 485 30 319 490 326 479 35 481 35 307 479 35 326 < 50 34 2 440 10 325 492 QLO-1 288 40 202 40 (quartz latite) 280 189 35 189 291 40 191 26 9 40 av 282 193 lit. valueb 256 192 < 50 13 lit. valueC 271 2 26 111 BHVO-1 4711 399 77 112 75 (basalt) 5719 390 114 75 23/30 389 110 47/28 390 75 17/11 398 82 112 112 77 av 393 lit. valueb n.d. < 50 90 lit. valueC 380 101 119 SDC-1 87/14 634 685 43 44 (mica schist) 110/11 626 660 av 630 672 43.5 lit. valueb 621 39 480 lit. valueC 587 40 671 2/20 996 STM-1 65 438 410 (nepheline syenite) 4/25 914 60 av 955 4 24 62 lit. valueb 901 420 < 50 lit. valueC 890 408 22 SGR-1 31/20 2270 87 1.69% 2300 96 (Green River shale) 30125 1.60% av 228B 92 1.64% lit. valueb n.d. 1.89% 44.6 lit. valueC 1869 7800 1.61% sco-1 35/31 794 46 620 30125 762 (Cody shale) 61 2 52 av 778 49 616 lit. valueb 779 600 68 lit. valueC 749 608 363 MAG-1 5414 901 3.00% 3600 2.94% 3800 (marine sediment) 65/31 864 2.97% 3700 av 882 lit. valueb n.d. 3.12% 4600 lit. valueC 737 3.09% 3940 SDO-1 0783 808 4.87% 84 (Devonian shale) 0550 780 4.61% 82 av 794 4.74% 83 a All values in ppm unless otherwise noted. Average of two each from three (different splits. Reference 10 for F and reference 1 5 for Cl and S. Average of five samples from a single split, reference 14 for F and C1; sulfur values are from reference 18. RGM-1 (rhyolite obsidian)
411 5 10125 3713 17/26 15/22 av lit. value lit. valueC 45/15 20112 2/22 42/14
927
0.9939, respectively. The calibration plots for sulfur were constructed in one of two ways. In method A total sulfur was varied by using a series of different NBS steels of varying sulfur content and maintaining a constant sample size of 100 mg. In method B total sulfur was varied by using different sample weights of a single NBS steel. Triplicate samples of several standards were treated as unknowns, burned in random order on the induction furnace, and analyzed by each of the two methods. The results shown in Table IV reveal that method A yields the more accurate results over the range of sulfur concentrations studied and is thus the method of choice. For high sulfur samples (>1%) aliquots of Allende meteorite (2.1% S) were burned as standards. Analysis of Standard Rocks. A recent series of USGS standards and one new proposed standard, SDO-1, were analyzed by the proposed method. Our results, listed in Table V, are compared with the values given in the original publication introducing these rock standards (10, 15) and to more recent determinations by Troll and Farzaneh (14) and by Terashima (18). Each split was analyzed in triplicate with the samples from each standard being burned and analyzed in random order. In general our agreement with the literature values is good. All three data sets listed for fluorine in Table V are compatible with correlation coefficients of 0.996 and 0.998 between our values and those listed in ref 10 and 14, respectively. The chlorine values listed by Troll and Farzaneh (14) appear to be anomalously high for the two shale samples, SGR-1 and SCo-1, when compared to both our data and those of Fabbi and Espos (15). However, the agreement between the three data sets is quite good for the remaining six standards, and a correlation coefficient of 0.999 exists between our data and that of Fabbi and Espos (15) for all eight standards. A much greater disparity exists between the three data sets for sulfur. Fabbi and ESPOS,who utilized an X-ray fluorescence technique, only report less than values for four of the standards and of their remaining values only that for SCo-1 agrees well with the other two data sets. The combustion-IR method of Terashima (18) yields lower values than our method for the three low-sulfur samples, RGM-1, QLO-1, and STM-1, but the agreement for the remaining five standards is quite remarkable. Which sulfur values are the more accurate may be debatable but those of Fabbi and Espos (15)appear to be somewhat suspect when compared to the other two data sets. The precision of our method indicates that it should give very good relative sulfur values for any series of similar samples.
ACKNOWLEDGMENT The authors wish to thank F. J. Flanagan of the United States Geological Survey for supplying many of the rock standards analyzed and Sidney Abbey of the Geological Survey of Canada for his helpful review of this manuscript.
LITERATURE CITED micrograms of fluorine in the standard vs. micrograms of fluoride in solution and micrograms of sulfur in the standard vs. micrograms of SO -: in the trapping solution were prepared as working curves for each set of samples. No separate standards were run for chlorine, but the chloride from the fluorine standards served to check the consistency of our chlorine recovery. A 12-point calibration curve from 5 to 2500 pg of fluorine yielded an overall correlation coefficient of 0.9996 with groups of five standards of 5-75,30-350, and 200-2500 pg of F having correlation coefficients of 0.9983,0.9998, and 0.9993, respectively. Recoveries ranged from 78 to 90% with no apparent correlation to absolute sample size. Working curves of micrograms of sulfur in NBS steels vs. SO-: in solution prepared from five standards ranging from 65 to 1200 and from 9 to 200 pg of sulfur gave correlation coefficients of 0.9996 and
Evans, K. L.; Moore, C. B. Anal. Chem. 1960, 52, 1908-1911. Boyle, R. W. Geo. Surv. Can. Prof. Pap. 1974, 74-45, 35-36. La Londe, J. P. Geol. Sum. Can. Prof. Pap. 1973, 73-28, 1-50. Stollery, G.; Borcsik, M.; Holland, H. D. €con. Geol. 1971, 66, 361-367. Kesler, S. E.; Van Loon, J. C.; Moore, C. M. Can. Mineral. Metall. Bull. 1973, 66, 56-60. Mason, B. Geol. Surv. Prof. Pap. ( U . S . ) 1979, 440-8-7, 25, 26. 37-39. Reed, G. W. Geochlm. Cosmochim. Acta 1964, 28, 1729-1743. Terashima, S. ChishrYsee Chosasho Geppo 1974, 25, 175-179. Jeffery, P. G. "Chemlcal Methods of Rock Analysis"; Pergamon Press: New York, 1975; pp 243-245. Machacek, V.; Rubeska, I.; Slxta, V.; Sulcer, 2 . Geol. Surv. Prof. 1978, 840, 73-77. Pap. (U.S.) Bodkln, J. B. Analyst (London) 1977, 102, 409-413. Clements, R. L.; Sergeant, G. A.; Webb, P. J. Ana/yst (London) 1971, 96, 51-54. Farzaneh, A.; Troll, G. Geochem. J. 1977, 1 1 , 177-181. Troll, G.;Farzaneh, A. Geostandards Newsl. 1980, 4 , 37-38. Fabbi, B. P.; Espos, L. F. Geoi. Surv. Prof. Pap. ( U . S . ) 1976, 840, 89-93.
926
Anal. Chem. 1961, 53, 928-929
(16) Moore, C. B.; Lewis, C. F.; Nava, D. In "Meteorite Research"; Millman,
Ed.; Reidel: Dordecht, Holland, 1969: pp 738-748. (17) Crlpe, J. D. "Total Sulfur Content and Distributlon in the Lunar SamDies". unDublished doctoral thesls. Arizona State Universitv. Tempe, AZ, 1976. (18) Terashima, S. Geostandards News/. 1979, 3, 195-198. (19) Warf, J. C.; Cline, W. D.: Tevebaugh, R. D. Anal. Chern. 1954, 26, 342-346.
(20) Flanagan, F. J. Go/. Surv. Prof. Pap. ( U . S . ) 1978, 840, 171.
RECEIVED for review December 29,1980. Accepted February 13,lg81*This research was supported in Part by NASA Grant NGL-03-001-001.
Adaptation of Clark Oxygen Electrode for Monitoring Hydrogen Gas Vakula S. Srinivasan" and Gary P. Tarcy Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403
There is a need for a convenient device for monitoring hydrogen in solutions, for example, in the anaerobic production and consumption of hydrogen by certain bacterial cultures (I). Clark and Bargeron developed a platinum potentiometric device for the detection of hydrogen in intravascular studies ( 2 ) . A device in which the voltage was not a function of the concentration of hydrogen, but could be used to activate a warning system, was later mentioned by Clark (3). We want to report the modification of the Beckman oxygen monitoring system that is capable of quantitatively responding to hydrogen. The instrument uses a Clark oxygen electrode (in fact, it is an electrochemical cell) in which 0.53 V is applied between a silver anode and a rhodium cathode. A Teflon membrane allows the gas to diffuse across it but keeps the solutions (0.1 N KC1 in O2electrode and the sample solution) separated. The instrument panel is graduated in arbitrary units which can be calibrated. The instrument was capable of responding to hydrogen if the following changes were made: (i) reversing the polarities of the rhodium and silver electrodes, (ii) converting the rhodium electrode to platinum electrode by electrodepositing platinum on rhodium, and (iii) vapor depositing palladium on the Teflon membrane to eliminate interfering gases, since only Hzdiffuses well through Pd.
EXPERIMENTAL SECTION A conditioning procedure described below was found to give satisfactoryresults. The rhodium surface was polished rigorously with 3 fi Alpha polish and rinsed with distilled water. By use of a platinum foil as the anode and a saturated platinum chloride plating solution, a dense uniformly black platinum was deposited on the rhodium surface. A standard 1.35-Vmercury cell was used for this purpose with a plating time of about 20 min. The rinsed electrode was then connected to the Beckman meter opposite to that used as oxygen electrode and allowed to stand in a stirred Hz saturated phosphate buffer solution (pH 6.93) for 5 min. The polarity was reversed (i.e., Pt, negative) and the electrode was allowed to stand in an air-saturated solution for 15 min. Once again the polarity was reversed (Pt, positive) and allowed to sit in the hydrogen saturated phosphate buffer solution for 24 h, with the Teflon membrane on it. This electrode without the palladium-deposited membrane on it responded well but not steady at the beginning of the 24-h period and much more steady at the end of 24 h. We modified the above procedure with the Teflon membrane, vapor deposited with palladium. The deposition was done by the standard method ( 4 ) . After the electrode was conditioned as above, the ordinary Teflon was replaced with a palladium-coated Teflon membrane and allowed to soak in a H2-saturatedbuffer solution for 24 h. The membrane was removed and the electrode was polished very lightly with 3 fi Alpha polish. After the electrode was covered once again with the Pd-Teflon membrane, the electrode was allowed to sit in Hz-saturated buffer solution for 24 h and then was ready for use. If the electrode modification was carried through only the first part, the response was higher but was not very steady. With the
Table I. Variation of Percent Pressure of Hydrogen with Relative Reading for Electrode Carried through the First Part %
0 20
40
re1 reading
% PH,
re1 reading
0 1.6 3.7
60 80 100
7.2 9.2
5.0
Table 11. Variation of Percent Pressure of Hydrogen with Relative Reading for an Electrode Carried through the Entire Procedure trial I trial I1 trial I11 re1 reading re1 reading re1 reading %J pH, 0
0.0
20
... a
60 70 80 100
3.4
40
2.2
...
0.0
...
2.2 3.4 3.8
0.0 1.2 2.1
... 3. 7 ...
4.5 4.5 5.5 5.4 5.5 a The experiments corresponding to these values were not performed. entire procedure the response was lower but extremely steady. The electrode was useable for about 1month before the electrode had to be reconditioned. The response of the electrode to hydrogen was measured by allowing the electrode to sit in a stirred phosphate buffer solution (pH 6.93) over which was kept a mixture of hydrogen and argon. The hydrogen and argon were mixed in a vacuum system. The partial pressures of the hydrogen and argon were measured with a mercury manometer. The storage bulbs were of approximately 1L capacity whereas the cell was of 100 mL capacity. The aqueous solutions were equilibrated with the mixtures for 12 h before use.
RESULTS AND DISCUSSION Table I shows the response of the electrode carried through first treatment. Although the response appears linear, all readings were estimates. Table I1 shows the response of the electrode carried through entire modification and in this case the response was very steady. Trial I and trial I1 were done by using the same atmospheric storage bulbs, and the measurements were made after exposure to the varying concentrations of the mixture. Trial111 was made with five different storage bulbs, each containing a different partial pressure of hydrogen which had come to equilibrium with the buffer solution. The electrode was immersed into the solution and the readings made 90 s later. The results show that in 90 s the electrode response is essentially complete. The hydrogen amperometric electrode works on the following reactions:
0003-2700/81/0353-0928$01.25/00 1961 American Ctpmical Society
