The columns tested were the following: (1) 6-foot X 1/4-inch silver nitrate (AgN03) column. A 6-foot, W-shaped column made of l/d-inch copper tubing and packed with a 25 wt. % mixture of 30 wt. % silver nitrate and 70 wt. % ethylene glycol on 80- to 100-mesh chromosorb. (2) 20-foot X l/r-inch polypropylene glycol (PPG) column. A 20-foot coiled column made of 1/4-inch copper tubing and packed with 30 wt. % polypropylene glycol (average molecular weight approximately 550) on 60- to 80-mesh C-3 firebrick. The column packing was obtained from W. H. Curtin & Co. Each of the pentene isomers was obtained from Phillips Petroleum Company in the form of pure grade (99yo min.) chemicals except for the pair of pentene-2 isomers in which case a pure grade mixture was obtained.
Helium was used as the carrier gas in all runs.
markedly in performance after repeated on-stream use.
RESULTS
LITERATURE CITED
Chromatograms for the polypropylene glycol and silver nitrate columns are presented in Figures 1 and 2, respectively. Retention-time data are assembled in Table I. Clearly, the polypropylene glycol column affords a useful means for separating all the isomers of peiitene in a short time. Although retention times are small in the silver nitrate column, 3 methylbutene-1 and 2 methylbutene-1 could not be resolved. This does not preclude its use for following double-bond shift isomerizations, but the silver nitrate column appears to decline rather
(1) Bednas, M, E., Russell, D. S., Can.J. Chem. 36,1272 (1958). (2) Fredericks, E. ?VI., Brooks, F. R., ANAL. CHEY.28, 297 (1956). (3) Knight, H. S., Zbid., 30,Q (1958). , (4) Polgar, A. G., Holst, J. J., Gi>ennings, S.,Ibid., 32,336 (1960).
DELBERT &I. OTTMERS~ GEOFFREY R. SAY* HOWARD F. RASE Department of Chemical Engineering University of Texas Austin, Texas Present address, Esso Research and Development, Baytown, Texas. Present address, Esso Research and Development, Baton Rouge, La.
Quantitative Analysis of Helium-3 by Gas Chromatography SIR: The separation and quantitative analysis of permanent gases by gas chromatography using adsorption columns is well known (4). Since 1956, techniques have been known for the separation of the isotopes of hydrogen ( 7 ) , although the quantitative analysis reported by Phillips and Owens remains difficult (8). The quantitative analysis of the mass number three isotope of helium is reported in the present work. EXPERIMENTAL
The apparatus has been described previously (3). An 8-ft. long 0.180-inch i.d. stainless steel column filled with a 24to 50-mesh Linde 5A Molecular Sieve is operated a t 100' C. The coLumn is activated by heating it to 220' C. for 1 hour while He4 gas flows through it. The thermal conductivity detector is operated as a bridge circuit with a constant current of 400 ma. The bridge imbalance is measured directly on a 1-mv. full scale potentiometric recorder and may be attenuated by up to a factor of 1000. He4 carrier gas is
4
.
p
o
The elution of He3 as a distinct peak under the conditions just described is shown in Figure 1 for a sample pressure of 0.49 p.s.i.a. and a He3partial pressure of 0.26 p.s.i.a. A nearly symmetrical elution peak is obtained indicating a linear adsorption isotherm. The introduction of He3 a t varying pressures and concentrations in He4 allows a calibration curve of output voltage us. volume-
I
0.10
I
0.m
I
OJO
i
aa
1
0.50
I 0.w
1
a70
0 0.0
B R I D E IMBALANCE, MILLIVOLTS
Figure 1.
Elution of Hea
Column temperature, 100' C.; flow rate of He4 carrier, 24 atm. ' 0 C. cc./min.; bridge current, 400 ma.; attenuation, 5. Peak is negative with 2 peak shown a t attenuation of 1. To i s time of sample respect to 0
introduction
tric concentration to be obtained. The slope of this linear curve is 0.000196 mv. per p.p.m. (volume) a t the above column conditions. In the same units the sensitivities of O2 and N2 for this column a t the above conditions are 0.00192 and 0.00168 mv. per p.p.rn., respectively. The sensitivity of the column to H2is 0.0000624mv. per p.p.m. The separation of Hz and He3 peaks is indicated in Figure 2. This chromatogram was obtained a t a column temperature of 40' C. Analyses are possible when the H2-He3 ratio is as large as 1000. The gas chromatographic analysis of He3 is sensitive enough to detect small leaks in systems containing He3 as well as to monitor the purity of He3 to such likely contaminants as the permanent gases.
i
CHROMATOGRAPH CONDITIONS T. IDQ.0 I *4OOrno P*2Spsig
p
o
RESULTS
H:P.0.49 piin CONCENTRATION-OF Hfa533
0,
I14!
supplied a t a flow rate of 34.6 ml. (1 atm., 0' C.) per minute. Small volumes (about 5 std. cc.) of He3-He4 mixtures were obtained. A mass spectrograph analysis had been made of the He3-He4 ratio. The sample volume to the chromatograph is 1 cc. and the sample inlet pressure is measured with a Wallace and Tiernan gauge.
'
I
o
TO
I ai0
0.~0
o . ~
.I,
o . ~
a~
a
D
BRIDGE IMBALANCE, MILLIVOLTS
Figure 2. Separation of eluted peaks of Hz and Hea at column temperature of 40" C. and other conditions as described in text Large hydrogen concentration can b e inferred from reversed height ( 4 )
VOL. 38, NO. 1, JANUARY 1966
0
peak
149
DISCUSSION
A measurement of the width of the peak a t half the total height and the elution time allows a calculation of the number of theoretical plates in the column analogous to the plates in distillation theory (6). For the above column conditions the number of theoretical plates is measured to be 450. No attempt has been made to optimize the flow rate of carrier gas. A measurement has been made of the heat of adsorption in a manner similar to that used by Greene and Pust ( 1 ) Data are obtained a t column temperatures of 40°, loo”, and 190” C. and a carrier gas flow rate of 25.3 std. cc. per minute. The heat of adsorption Had is related to the temperature variation of corrected retention times as
LITERATURE CITED
where V,W is the product of the limiting retention volume, V,, and the mass of adsorbant in the column, W ( 5 ) . The result of these measurements yields Hod = 0.10 =t0.02 kcal. per gram mole for He3 on Molecular Sieve 5A. No direct comparison of this result is available, though Hoffman, Edeskuty, and Hammel (2) have determined the heat of adsorption using measured adsorption isotherms a t T = 3.00” K. to be 0.045 kcal. per gram mole on charcoal. ACKNOWLEDGMENT
The author thanks F. J. Edeskuty for discussions during the course of this work and E. F. Hammel for commenting on the manuscript.
(1) Greene, S. A., Pust, H., J. Phys. Chem. 62, 55 (1958). (2) Hoffman, C. J., Edeskuty, F. J., Hammel, E. F., J. Chem. Phys. 24, 124 (1956). (3) Liebenberg, D. H., Edeskuty, F. J
“Advances in Cryogenic Engineering,;’
K. D. Timmerhaus, ed., Vol. 9, p. 430, Plenum Press, New York, 1964. (4) . , Littlewood. A. B.. “Gas Chroma-
tography,” Chap. 11,’ Academic Press, New York, 1962.
(5) Ibid., p. 30. (6) Ibid., p. 129. ( 7 ) Ibid., p. 378. (8) Phillips, T. R., Owens, D. R., P. G. Rept. 419(CA), United Kingdom Atomic Energy Authority, 1963. DONALD H. LIEBENBERG Los Alamos Scientific Laboratory University of California Los Alamos, N. M. WORKperformed under the auspices of the U. S. Atomic Energy Commission.
Experimental Evaluation of Cyclic Stationary Electrode Polarography for Reversible Electron Transfer SIR: Numerous investigators have compared experiment with theory for reversible electron transfer and single cathodic scan stationary electrode polarography (2, 4, 10). Anodic single scans for reversible amalgam oxidation a t a hanging mercury drop electrode also have been compared with theory (3, I S ) . Although single scan stationary electrode polarography has many important applications, cyclic triangular wave experiments possess several advantages over the single scan technique; for example, the potentials a t which oxidation and reduction occur are observed directly, and this provides a simple estimate of reversibility ( 7 ) . Nevertheless, to our knowledge no comparisons between theory and experiment have been reported for the complete cyclic experiment. I n addition, some useful correlations predicted on theoretical grounds for the cyclic methods (9) have not been observed experimentally. Also, the effects of spherical diffusion for cyclic stationary electrode polarography have not been discussed. Therefore, we report a comparison with theory for reduction of ferric oxalate complex at a hanging mercury drop electrode. EXPERIMENTAL
Apparatus. Measurements were made with a three-electrode potentiostat which has been described previously (7, 8 ) . The detector was a potentiometric recorder (Leeds & Northrup, Nodel G), and the current measuring load resistor was isolated electronically from the potentiostat with a current 150
ANALYTICAL CHEMISTRY
follower (12). The triangular wave voltage was derived from an electronic function generator which provided either triangles or the first arm of a triangle followed by a constant potential ( 8 ) . The scan rate used was 25.0 mv. per second, and the amplitude of the triangular wave was always 0.35 volt. Switching potentials were varied by changing initial potential, which in turn was set with a portable potentiometer (E. H. Sargent). The same function generator also provided square waves for the potentiostatic experiments to determine diffusion coefficients. Conventional polarograms were recorded with a Sargent Model XXI Polarograph. The working electrode was a hanging mercury drop of radius 0.0712 cm. The cell assembly was of conventional design and is described elsewhere ( 7 ) . Materials. The test solution contained 1.01 X l O - 3 M iron(III), 0.2M oxalic acid, and 0.2M potassium oxalate. The measured pH was 2.50. Experiments were performed at ambient temperatures of 23” to 25” C. RESULTS AND DISCUSSION
Theory. Theoretical cyclic stationary electrode polarograms have been presented previously (9). These calculations applied only to linear diffusion except for the first cathodic scan where the spherical correction term given by Reinmuth (11) could be used. Therefore, t o apply these calculations to multicycle experiments involving hanging mercury drop electrodes, it was necessary to extend the spherical correction to include more than a single scan. By a simple modification of Reinmuth’s treatment, the current
i
= i(p1ane)
+ nFADoCo* (l/ra)@(at) (1)
where
All terms in Equations 1and 2 have been defined previously (9). Theoretical curves were calculated in the manner described previously (9), except the numerical method of Huber (6) was used. Determination of Diffusion Coefficient. To compare experimental stationary electrode polarograms with theory, the diffusion coefficient for the system under investigation must be known accurately. Although the diffusion coefficient for ferric oxalate has been evaluated polarographically, i t seemed desirable to redetermine it using a stationary electrode where the mass transport process was well defined. Analysis of potentiostatic current-time curves obtained with a hanging mercury drop electrode is a reliable electrochemical source of diffusion coefficients, and is the method we have used. Thus, potentiostatic current-time curves for ferric oxalate reduction were fit by a least squares procedure to the equation for spherical diffusion. Diffusion coefficients calculated from slope and intercept agreed within lo%, and in accord with the discussion of Alberts ( I ) , the slope value is reported here: 6.31 X 10+ sq. cm. per second. This agrees well with the value calculated from the Ilkovic equation and the dif-
