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 satisfactory results. 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 %J pH,
0 20
40 60 70
trial I trial I1 trial I11 re1 reading re1 reading re1 reading 0.0
... a
2.2 3.4
0.0
...
2.2
0.0 1.2 2.1
3.4 ... 3.8 3. 7 ... 80 4.5 ... 4.5 100 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
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Anal. Chem. 1981, 53, 929-931
H2 + 2H+ t,2e2AgC1
(anodic reaction)
+ 2e- + 2Ag + 2C1-
(cathodic reaction)
T h e chloridation seems t o occur during the conditioning procedures. The readings obtained are linear with respect to the amount of hydrogein dissolved in solution. The response is essentially complete within 90 s. The lower detectable limit can be estimated as follows: Assuming Henry’s law holds, a solution saturated with. Hz contains 1.5 ppm of H2and gives a reading of 5.5 divisions on the panel. The lowest level of hydrogen attempted t o be read was 20% Hz which gave a reading of 1.2.
1.5 ppm 5.5
X 1.2 = 0.33 ppm
of H2
LITERATURE CITED (1) Sweet, W. J.; Houchins, J. P.; Rosen, P. R.; Arp, D. J. Anal. Blochem. 1980, 107, 337-340. (2) Clark, L. C., Jr.; Bargeron, L. M., Jr. Sclence 1959, 130, 709-710. (3) Clark, L. C., Jr. U.S. Patent 3380905, April 30, 1968. (4) Olsen, R. R.; Srlnivasan, V. S. Anal. Chem. 1977, 49, 853-857.
RECEIVEDfor review November 24, 1980. Accepted March 3,1981. This research was supported by the Faculty Research Committee, Bowling Green State University, Bowling Green, OH 43403.
Automatic Liquid Injector for Headspace Gas Chromatography Rein Otson Bureau of Chemical Hazards, Environmental Health Directorate, Health and Welfare Canada, Tunney‘s Pasture, Ottawa, Ontario K1A OL2, Canada
General aspects and practical applicatilons of headspace gas analysis have been discussed in detail (1, 2). Static headspace chromatographic techniques have been successfully applied in areas such as the analysis of food and beverages (3) and the analysis of water ( 4 , 5 ) . When a large number of samples must be analyzed, the technique requires time-consuming manual injection of headspace aliquots into the gas chromatograph unless an autornated headspace sample injector, such as the system of the Moldel F45 (Perkin-Elmer Corp., Norwalk, CT) is available. Since such an injector was not available when a survey of volatile compounds present in a large number of consumer products was initiated, adaptation of available equipment, a Model 8020 liquid autosampler (Varian Associates, Inc., Palo Alto, CA), to perform automated headspace analyses was investigated.
EXPERIMENTAL SEC’TION Gas Chromatography. All analyses were performed on a Model 5840 gas chromatograph (Hewlett-Packard Co., Palo Alto, CA) equipped with an OIV-17,SCOT stainless steel column (50 ft X 0.020 in. i.d., Perkin Elmer Corp., Norwalk, CT) attached to a Hewlett-Packard capillary column inlet system operated in the splitless mode and maintained at 180 “C. For hydrocarbon analyses, the column was1 attached to a flame ionization detector (FID) and for trihalomethane (THM) analyses it was attached to a 63Ni,electron capture detector (ECD). Both detectors were held at 300 “C, and after each injection the column oven temperature was maintained at 60 “C for 3 min and then raised at a rate of 8 OC/min to 150 “C where it was held for 6 min. Nitrogen gas, passed through Oxiclear (Labclear, Oakland, CA) and molecular sieve traps, was uiaed for column carrier gas, at 4 mL/min flow rate, and detector makeup gas, at 25 mL/min flow rate. Autosampler. A Varian Model 8020 autosampler was interfaced with the gas chromeitograph and was operated in the manner prescribed in the autosampler instruction manual. Zero grade nitrogen gas was used to operate the autosampler pneumatic system. Autosampler vials (2-mL capacity) sealed with Tefloncoated silicone disks and screw caps were used for preparation of samples for headspace analysis. A rinse volume of about 200 p L was used for both sample and rinse vials. Reagents. The purity of all organic reagents was determined by gas chromatography. Composite stock solutions of four hydrocarbons (Chem Service, Inc., West Chester, PA) in nitrobenzene (Fisher Scientific Co., Pittsburgh, PA) were prepared to contain hexane, benzene, toluene, and o-xylene in the concentrations shown for sainples 1-5 in Table I. Three additional stock solutions, respectively containing lo9 of the hydrocarbon
concentrations for samples 1, 3, and 5, were also prepared for quantitation of hydrocarbons in headspace aliquots. Aliquots of methanolic (Caledon Laboratories Ltd., Georgetown, Ontario, Canada) solutions containing 0.20 mg of four trihalomethanes (Chem Service, Inc.) per milliliter of solution were injected into trihalomethane (THM) free water (2)to obtain aqueous, composite stock solutions. The aqueous solutions contained chloroform, bromodichloromethane,chlorodibromomethane,and bromoform at the concentrations corresponding to samples A-E in Table 11. Three composite solutions containing 5,10, and 50 ng/mL of each of the four trihalomethanes in hexane (CaledonLaboratories LM.) were also prepared for determination of THMs in headspace aliquots. A variety of consumer products, such as paints, paint removers, automotive and household cleaners, wood preservatives and sealants, and contact cements, were obtained. Procedure. Sample, blank, and rinse vials for headspace analyses were prepared by injecting 200-pL aliquots of composite hydrocarbon solutions, consumer products, or nitrobenzene and 250-pL aliquots of THM solutions, THM free water (containing 0.1% of methanol by volume), methanol, or hexane into sealed autosampler vials. The storage time between aliquot injection and headspace analysis and the ambient temperature at the autosampler were recorded. Sealed autosampler vials containing ambient laboratory air only were used for air blank analyses and air rinses. Headspace aliquots of 5 pL volume were used both for the autosampler injections and for the manual injections performed by means of a 1 0 - ~ LHamilton syringe. Results from gas chromatographic analysis of aliquots of the solutions were used to construct calibration curves. These plots of peak area against the calculated amount of compound injected allowed estimation of the concentration of organics in headspace aliquots.
RESULTS A N D DISCUSSION The Model 8020 autosampler principles of operation suggested that it could be used to sample and inject headspace above samples in the autosampler vials. It was shown that headspace rather then liquid was sampled and injected if less than 300 p L of consumer product was added to a vial. Volatile organic compounds in the consumer products were thus readily detected by the automated headspace chromatography technique. For some products, contamination of the autosampler transfer and injection system by compounds in the headspace caused a “memory effect”. The “memory” for volatile compounds was reduced when the contaminated system was rinsed with air from an empty, sealed vial. The analysis of some consumer products caused such high
0003-2700/81/0353-0929$01.25/0Published 1981 by the Amerlcan Chemical Society
