ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
A N / E t O H / H 2 0 ) offers large (but still unpredictable) changes in D,L selectivity. These results will be reported at a later date.
LITERATURE CITED (1) N. H.C. Cooke, R. L. Viivattene, W. S. Wong, G. Davies, and B. L. Karger. J . Chromatogr., 149, 391 (1978). (2) B. L. Karger, W. S. Wong, R. L. Viavattene, J. N. LePage, and G. Davies, J. Chromtogr., to be published in the Proceedings of the 12th International Symposium on Chromatography, Baden-Baden, September, 1978. (3) J. N. LePage and B. L. Karger, unpublished results. (4) V . A. Davankov and Yu. A. Zolotarev, J . Chromatogr., 155, 285, 295, 303 11978). (5) B I L d e b i e , R. ~udebert,and C. Qubwom, Isr. J . Chem., 1 5 6 9 (1977). ~ (6) J. Gail and J. InczBdy, Talanta, 2 3 , 78 (1976). (7) G. H. Searle, Aust. J . Chem., 3 0 , 2625 (1977). (8) H. Nakazawa and H. Yoneda, J . Chromatogr., 160, 89 (1978).
435
(9) V. A. Davankov and A . V. Semechkin, J . Chromatogr., 141, 313-353 ( 1977). (10) H. Wenker, J . A m . Chem. SOC.,5 7 , 2328 (1935). (11) L. B. Clapp, J . Am. Chem. Soc., 7 0 , 184 (1948). (12) F. Vasileva, 0. Y. Laurova, N. M. Dyatiova. and V . G. Yashunskii, Zh. Obsheh. Khim., 3 6 , 724 (1966). (13) E. Bayer, E. Grom, B. Kaitenegger, and H. Uhmann, Anal. Chem., 48. 1106 (1976). (14) A. Wehrli, J. C. Hildenbrand, H. P. Keiler, R . Stampfli, and R. W. Frei, J . Chromatogr., 149, 199, (1978).
RECEIVED for review November 6,1978. Accepted December 18, 1978. T h e authors gratefully acknowledge the National Science Foundation for support of this work. Contribution 255 from the Institute of Chemical Analysis.
Effect of Oxygen on Composition of Light Hydrocarbons Evolved in Oil Shale Pyrolysis Mark V. Robillard, Sidney Siggia, and Peter C. Uden” Department of Chemistry, GRC Tower
I, University of
Massachusetts, Amherst, Massachusetts 0 1003
The changes in the absolute and relative levels of light alkane and alkene hydrocarbons evolved during the thermal degradation of oil shale have been studied with respect to pyrolysis atmosphere for milligram samples. Programmed temperature gas-solid chromatography on Spherocarb carbon molecular sieve gives effective resolution of alkane-alkene pairs up to the C6 compounds. n-Alkane formation is favored over that of 1-alkenes under inert pyrolysis, but the alkane/alkene ratios decrease as the atmospheric oxygen content is raised to the 10 % level. Thereafter they change relatively little up to 20 % oxygen atmosphere. Absolute amounts of alkanes evolved increase up to a 5 % oxygen level as do those of alkenes, which however exhibit the effect to a considerably greater degree. Absolute amounts of all the light hydrocarbon gases evolved decrease markedly above the 5 % oxygen level.
T h e continually increasing efforts to discover and develop alternative energy sources have led to a renewed interest in the world’s oil shale deposits as potential alternatives to crude oil resources. In the United States, the Green River oil shale deposits have been estimated to contain the equivalent of 600 billion barrels of obtainable oil in high grade deposits ( I ) , this being t h e equal of all presently known United States crude oil reserves. T h e major organic component of oil shale is known as kerogen and comprises u p to 15/20?& by weight of the shale. I t is a macromolecular organic material bound to the inorganic shale matrix and is not extractable by organic solvents. When oil shale is heated to 500-550 “C, degradation of the kerogen occurs, releasing smaller organic molecules. These organic vapors are collected and condensed as shale oil, this being the basis for the process known as “retorting”. T h e possibilities for using shale oil as a fuel material are real; however, its potential as a petrochemical feedstock is equally important. Shale oil has a large hetero-atom content as well as high olefinic and aromatic character; this greater variety of organic constituents of differing functionality may be a positive feature with regard to feedstock possibilities. If 0003-2700/79/035 1-0435$01.OO/O
oil shale is to be used thus, the question arises whether the retorting process is a viable means to produce commercially useful petrochemicals. It has now been reported that oil shale that is retorted by the entrained-solids method permits control of the composition as well as of the quantity of volatile products ( 2 ) . I t has also been found that the rate a t which oil shale is heated affects the nature of the oil produced ( 3 ) ; the slower the heating rate, the higher the paraffin and t h e lower in olefin content the shale oil becomes. Slower heating also results in increased naphtha and light distillates. Variations in retorting temperature result in changes in the amount of oil produced, the boiling range of the oil, and its chemical character ( 4 ) . T h e major retorting parameters that can affect the composition of shale oil are: shale particle size, retorting temperature and heating rate, the velocity of the flue gases, and the retorting atmosphere ( 3 ) ,but the effect of the latter has been the subject of little investigation. From the petrochemical point of view, the ability to control the character of the oil during the retorting process by means of temperature and atmosphere is a n important concept; by allowing a controlled stream of air to enter the retort system, it may prove possible to effect changes in the quantity and character of the resulting shale oil. Interest has arisen recently in the determination of alkane/alkene ratios of hydrocarbons present in shale oil. T h e ratio of 1-alkene to the n-alkane a t the same carbon number has been correlated linearly with the percent oil yield of the oil shale ( 5 ) . Also the ratios of acetylene and methane to ethylene generated have been employed in the characterization of oil shale (6). Through observation of changes in alkane/ alkene ratios, it may be possible to determine the effect of variations in the amount of oxygen in the shale pyrolysis atmosphere on the overall character of the resulting oil. T h e effect on the light hydrocarbon gases (C,-C5) released on pyrolysis of oil shale is discussed in this paper. T h e gas chromatographic analysis of such gases has been a challenging problem. Many liquid stationary phases and adsorbing media have been tried without total success. T h e use of complex substrate mixtures and extensive treatment 1979 American Chemical Society
436
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979 HELIUM
ATMOSPHERE
E
c
Figure 1. System used for t h e pyrolysis of oil shale and the analysis of the evolved hydrocarbons: (A) MP-3 pyrolysis furnace, (B) sample in quartz boat, (C) '/,-inch quartz pyrolysis tube, (D) pyrolysis atmosphere. (E) '/,,-inch stainless steel transfer line, (F) '/,-inch stainless steel trap at 0 "C, (G) Dewars, (H) vent, ( I ) Valco switching valve, (J) helium backflush, (K) trap heater, (L) '/,-inch stainless steel Porapak Q trap at -196 OC, (M) Varian 2760 gas chromatograph with 4 ft X '/, inch Spherocarb column
IC i
IO0
200
COLUMN TEMPERATURE,
of solid supports, as well as application of multiple columns, has often proved laborious and difficult. Gas -liquid chromatography is generally unsatisfactory due t o the wide range of component boiling points encountered and the subambient temperatures needed t o gain adequate separations. Both pure and modified alumina and silica substrates have been tried, b u t their main problem is their affinity for water, which ultimately results in changing retention times for hydrocarbons. n-Octane bonded Poracil C has been employed b u t it suffers from similar drawbacks. Porous polymer substrates have shown reasonable success, in t h a t separations can be accomplished above 50 "C, b u t complete resolution of t h e alkane and alkene is not always obtained. Guiochon ( 7 )and DiCorcia (8)have employed graphitized carbon black supports, modified with various liquid phases with good success. I n t h e study now described, short columns of unmodified Spherocarb, (Analabs Inc., North Haven, Conn.) a carbon molecular sieve, have been used for t h e separation and resolution of light alkane and alkene hydrocarbon gases utilizing temperature programming to 400 "C. T h e ease and reproducibility of column packing with this material has made this analysis simple and efficient.
EXPERIMENTAL Oil Shale. Green River oil shale (SGR-1grade) obtained from the Oil Shale Corporation (TOSCO) was used throughout the analysis. The partial elemental composition of the shale used was approximately: carbon (total) 28%, hydrogen 3.310,nitrogen 0.9% and sulfur 1.0'%. Samples were prepared by crushing and sieving several gram-size pieces, collecting only the 20/40 mesh material; this was then further crushed and sieved to a final mesh size of 80/100. Milligram range samples were weighed by microbalance into small quartz boats for pyrolysis. Pyrolysis. An MP-3 Thermal Chromatograph (Spex Industries, Inc., Metuchen, N.J.) (9) was used for oil shale pyrolysis. Small quartz boats containing the shale were placed in the quartz tube furnace, purged for 5 min under the test atmosphere and then heated from ambient to 600 "C at 40 "C per minute. The gas stream atmosphere flowing over the sample was varied from sample to sample from pure helium t o air (ca. %0%,oxygen) by means of a constant flow gas metering system. Various flows of air and helium were combined to give the desired level of oxygen and an overall final flow rate of 30 mL/min. The products from pyrolysis were swept to a trapping system through a ',lI6-inch stainless steel transfer line maintained at 300 "C. The trapping system consisted of a 12 inch X ' / a inch empty stainless steel tube held at 0 " C whose purpose was t o trap all of the higher boiling components and allow only lower boiling gases to continue to a second trap: this consisted of a similar tube packed
'C
Figure 2. Chromatogram of the light hydrocarbon gases pyrolytically released from oil shale under an inert helium atmosphere: (A) methane, (8)ethene, (C) ethane, (D) propene, (E) propane (F) 1-butene (G) n-butane, (H) 1-pentene, (I) n-pentane. Conditions: 4 ft X inch stainless steel Spherocarb column. Flow; 55 mL/min helium. Temperature program; 50-400 "C (& 20 OC/min with 80/100 mesh Porapak Q (Waters Associates) held a t liquid nitrogen temperature. The latter trap has been found to retain methane for more than 30 min under the noted conditions. A block diagram of the system is depicted in Figure 1. Gas Chromatography. After pyrolysis was completed, the Porapak Q trap was heated to 150 "C and the light gases backflushed to a Varian 2760 gas chromatograph equipped with flame ionization detection. Helium was used as backflush and carrier gas at a flow rate of 55 mL/min. The gas chromatographic column consisted of 4 f t X l / * inch stainless steel packed with 80/100 mesh Spherocarb and was maintained a t 50 "C until the methane eluted. The oven was then programmed to 400 O C a t 20 "Cimin and held until pentane eluted. Another quartz tube pyrolysis unit was constructed in similar fashion to the MP-3 unit, utilizing Valco high temperature valves and a pyrolysis furnace temperature programmer (PC 6011, V d e y Forge Instrument Co., Inc., Phoenixville, Pa.). This unit could be interfaced to a GC-Vapor Phase Infrared (GC-VIPR)system consisting of a Varian 2760 gas chromatograph and a Norcon 201 vapor phase infrared spectrophotometer (Norcon Instrument Co., South Norwalk, Conn.) (9). The total amount of hydrocarbons released per gram of oil shale was determined by injecting and trapping known volumes of standardized hydrocarbon mixtures (Supelco, Inc.) utilizing a gas tight microsyringe. Narrow symmetrical peaks enabled peak height to be used as a basis for quantitation.
RESULTS AND DISCUSSION Chromatograms of t h e light hydrocarbon gases released pyrolytically from oil shale under atmospheres containing pure helium and 10% oxygen in helium are shown in Figures 2 and 3, respectively. At each carbon number from 2 to 5, the peak attributable t o t h e 1-alkene is fully or almost completely resolved from t h e n-alkane eluting before it. Under t h e conditions of inert pyrolysis, t h e n-alkane is seen t o be t h e predominant species at each carbon number. However, as the oxygen content of the pyrolysis atmosphere is increased, t h e alkene t o alkane ratio increases, most notably for t h e ethene-ethane and propene-propane pairs. In addition, as oxygen content increases, other small component peaks become evident indicating a greater degree of fragmentation t o be occurring under oxidative conditions. This tendency is also
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
IO %
OXYGEN
ATMOSPHERE
437
Table I. Total Micrograms of ( a ) Alkane and (b) Alkene Produced per Gram of Oil Shale under Increasingly Oxidizing Pyrolysis Conditions ( a ) Micrograms of Alkane Produced per Gram of Oil Shale carbon = C, c 2
c3 c4
c,
He 1030 815 5 68 368 26
1%
5%
0
0:
0 2
0:
1500 868 681 440
1220 780 702
2420
31
36
3150 38 3 453 224 26
2
20%
10%
510
508 338 25
460
( b ) Micrograms of Alkene Produced per Gram of Oil Shale
cz c,
423 522 215 53
c4
C;
-____ 100
200
300
COLUMN TEMPCPATURE
400
513 719 309
91
620 828 381 106
439 47 5 130 5
531
67 2 310 85
HOLD
'C
Figure 3. Chromatogram of the light hydrocarbon gases pyrolytically released from oil shale under a 10 % oxygen atmosphere Peaks and conditions as in Figure 2
. w
z
Y
-
L
2 1 C5H12
/
5
/"
I
I _
0 1
5 %
OXYGEN
10
15
\ 1
R
PO
IN ATKOSPHERE
Figure 4. Plot of alkane/alkene ratios for carbon numbers C2 through C5 released from oil shale under increasingly oxidizing pyrolysis conditions
substantiated by a corresponding relative increase in methane production as is seen in Figure 3. Figure 4 shows alkane/ alkene ratios for carbon numbers 2 through 5 as a function of the percent oxygen in the pyrolysis atmosphere; data were obtained from peak height measurements and showed a relative standard deviation of approximately 3 % . Each ratio favors the alkane under inert pyrolysis conditions. The effect of small proportions of oxygen is to cause an immediate decrease in the alkane/alkene ratios. These ratios continue to decrease as the level of oxygen in the pyrolysis atmosphere increases, to about the 10% level. Atmospheres containing oxygen levels above 10% have little further effect on the alkane/alkene ratios for some pairs or appear to cause slight reversals of the alkane/alkene ratio trend for other pairs. A quantitative study of absolute amounts of alkane and alkene produced was then undertaken to determine what
10
15
20
% OXYGEN I N ATMOSPHERE
Figure 5. Plot of total micrograms of alkane produced per gram of oil shale under increasingly oxidizing conditions
changes were responsible for the variation in alkane/alkene ratios. Known volumes of a standard hydrocarbon gas mixture with components a t the 100-ppm level were thus injected into the pyrolysis system without heating, trapped, a n d chromatographed under the same conditions as for the shale pyrolysis. T h e total quantities of hydrocarbons produced under the various atmospheres were then determined from the standard curves, the results being given in Table I. T h e quantity of alkane from ethane to pentane increased on the average 20-30% as the pyrolysis oxygen content was raised to 1 to 5 % . Above 5% oxygen, the absolute quantities of alkane produced decreased significantly and a t 20% oxygen levels, less alkane was invariably produced t h a n under the inert conditions. A similar trend is observed for the alkenes, except that the relative increase in amounts of alkene produced are much greater than seen for the alkanes. This factor accounts for the initial trend in the alkane/alkene ratios. Again, above 5% oxygen atmosphere levels, the amounts of alkene produced begin to decrease until a t the 20% oxygen level, less alkenes are generally produced than under the inert conditions. T h e trends in total quantities of Cz-C5 hydrocarbons produced are indicated in Figure 5 for the alkanes and in Figure 6 for the alkenes with appropriate precision ranges being indicated.
438
ANALYTICAL CHEMISTRY, VOL. 51, NO.
3, MARCH 1979
tained by vapor phase infrared spectral monitoring of the evolution thermogram. Clear evidence was found for the presence of carbon dioxide and water for pyrolysis in the presence of higher proportions of oxygen, throughout the evolution profile. These were not observed below 500 " C under inert gas pyrolysis conditions. This indicates that partial combustion of organics is occurring during pyrolysis explaining the decrease in both alkanes and alkenes found at higher oxygen levels. It might be expected that for gas phase oxidations, the alkenes would have reacted a t faster rates than the alkanes; as this was not seen experimentally, i t is likely that the oxidation reactions occur as surface-gas phase processes coupled into the kerogen degradation processes.
% OXYGEN
I N ATMOSPHERE
Figure 6. Plot of total micrograms of alkene produced per gram of oil shale under increasingly oxidizing conditions
I
100
200
300
PYROLYSIS
400
I
500
TEMPERATURE,
600
700
O C
Figure 7. Flame ionization detection thermogram of the organics pyrolytically released from oil shale under an inert helium atmosphere
T h e temperature range over which evolution of organics took place under different atmospheres was then investigated. In this study, all of the evolved pyrolysis products were passed into a flame ionization detector without prior separation as they were continually released from the oil shale. A typical resulting evolution thermogram is shown in Figure 7 for pyrolysis in the absence of oxygen. T h e integrated flame response which indicates the amount of organic material being produced under the helium atmosphere shows maximum release rate at around 465 "C. As the level of oxygen in the pyrolysis atmosphere was increased, the temperature at which maximum organic evolution took place decreased to around 330 "C for pyrolysis in air (20%, oxygen). The total integrated area under the evolution thermogram also decreased continually with increased oxygen level showing again that diminishing amounts of organics were being produced. I t should be noted that with regard to the temperatures quoted, the actual surface temperature at which reaction occurs may differ considerably because of surface reaction and could be markedly higher than those noted. Further evidence of the change in pyrolysis products with atmosphere, both quantitatively and qualitatively, was ob-
CONCLUSIONS The light hydrocarbon gases evolved upon oil shale pyrolysis may be readily resolved with respect to alkane and alkene content by gas-solid chromatography on packed Spherocarb carbon molecular sieve columns. Hydrocarbons as high as five and six carbon species may be quantitatively separated in this mode with good precision, and both changes in ratios and absolute amounts evolved measured with changes in the oxygen content of the pyrolysis atmosphere. T h e effect of oxygen is manifested in three different ways with regard to the nature of the pyrolytic release of light gases. T h e relative proportion of alkene to alkane for a given number increases with atmospheric oxygen content to around the 10% O2 level but thereafter remains fairly constant or may decrease in some cases. T h e total quantity of light hydrocarbons released increases significantly under slightly oxidizing conditions but decreases above the 10% level as combustion becomes significant. Parallel evidence exists for the increased fragmentation of kerogen in the presence of oxygen, to give more methane and possibly other small nonhydrocarbon molecules. Thirdly, an effect of oxygen is to lower the temperature a t which the maximum level of organic gas evolution occurs. This observation may represent another thermal effect, namely that gas yields are strongly dependent upon heating rate. T h e higher oxygen levels will result in higher surface temperatures due to oxidative kinetics. Separate analysis for carbon monoxide and carbon dioxide levels would aid in elucidating these effects further. While it must be noted that the pyrolysis system used in this study may not replicate conditions present in a large scale retorting process, particularly as regards particle size and atmospheric gas velocities, it has been shown ( I O ) that, in general, gas chromatographic profiles of shale oil produced in this pyrolysis system are closely similar to those for commercial samples. The reproducibility gained in this study and the general effects noted do serve to indicate trends in the content of shale light gases as pyrolysis atmospheres are varied. Further details of these changes are under present investigation, notably with regard to changes in the composition of liquid oil fractions. LITERATURE CITED (1) S. Siggia and P. C. Uden, Ed.. "Analytical Chemistry Pertaining to Oil Shale and Shale Oil", Report of a Conference Workshop of the National Science Foundation, Washington, D.C., June 24-25, 1974. (2) H. W. Sohns, E. E. Jokkola, R. J. Fox, F. E. Brantley, W. G. Collins, and W. I. R. Murphy, Ind. f n g . Chem., 47, 461 (1955). (3) H. B. Jensen, R. E. Poulson, and G. L. Cook, Prepr. Div. FuelChem., A m . Chem. SOC.,15(1), (1971) (4) S. S. Thien. J. F. Brown, H. 8 . Jensen, P. R. Tisot, N. M. Melton, and W. I. R. Murphy, Ind. Eng. Chem., 47, 464 (1955). (5) T.T. Cobern, R. E. Bozak, J. E. Cbrkson, and J. H. Campbell, Anal. Chem., 5 0 , 958 (1978). (6) R. L. Hanson, D. Brookins, and N. E. Vanderborgh, Anal. Chem., 48, 2210 (1976). (7) C. VidaCMadjar, S. Bekassy. M. F. Gonnord, P. Arpino, and G. Guiochon, Anal. Chem., 49, 768 (1977). (8) A. DiCorcia and R . Samperi, J . Chromatogr., 107, 99 (1975). (9) P. C. Uden. D. E. Henderson, and R. J. Lloyd, J . Chromatogr., 126, 225 (1976).
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979 (10)
F. P. DiSanzo, M. V. Robillard, S. Siggia, and P. C. Uden, unpublished observations.
RECEIVED for review October 25, 1978. Accepted January 2,
439
1979. This work was supported by the National Science Foundation through grant CHE74-15244, a n d through Research Instrument grant G P 42542 to the University of Massachusetts.
Membrane-Covered, Rotated Disc Electrode David A. Gough" and John K. Leypoldt Department of Applied Mechanics and Engineering Sciences, Bioengineering Group, University of California at San Diego, La Jolla, California 92093
A rotated disc electrode that can be used to evaluate mass transport in membranes is described. I n this design, both a disc electrode and reference electrode are mounted in a rotating shaft over which the membrane is placed. A transport model is given for solutes that diffuse through the membrane and undergo simple reaction at the electrode surface. With membrane in place, the diffusion current at low rotation rates approaches the theoretical (Levich) current that would be seen without membrane. At high rotation rates, however, the current is dependent on membrane permeability.
In recent years, there has been a growing interest in the development of chemical-specific electrodes for monitoring chemical concentrations ( 1 - 4 ) . Sensors for a variety of chemicals have been reported, and some have reached a relatively practical state of development for certain monitoring applications (e.g., 3, 4). One type of electrochemical sensor that has received some attention is the amperometric electrode, in which current is produced when electrochemically active species react a t the electrode surface. In this type of sensor, selectivity for the chemical of interest is promoted by operating at a particular electrode potential (5)and by placing a semipermeable membrane between the electrode surface and the sample. In addition, immobilized catalysts can be incorporated in the membrane to extend the range of chemicals that can be monitored. In this way, certain chemicals for which selectivity is not readily obtainable on the basis of transport or electrode potential alone can be converted to other measurable species (6). A variety of potentially useful sensors based on these electrode designs is conceivable. However, the amperometric-type sensor has had relatively limited application in biochemical monitoring situations because of a n inadequate understanding of the complex physicochemical phenomena involved in its operation. An example is the lack of information about the electrochemical reaction. The mechanisms of reaction of most organics at solid electrodes have not been thoroughly studied. These processes at the electrode surface may include: formation of multiple intermediates, adsorption a n d electrode poisoning, electrochemical reactions in which the number of electrons exchanged is variable, or competitive reactions of other biochemicals that are not completely excluded by the membrane ( 5 ) . Mass transfer is another example. Chemical transport in the external medium to the membrane surface must be quantitatively defined. T h e membrane must be designed to provide selectivity for the biochemical of interest and an acceptable response time. For diffusants that saturate the electrode 0003-2700/79/0351-0439$01 .OO/O
surface, membrane permeability must necessarily be limited in order to promote linearity of the current with concentration. .4dditionally, for sensors with catalytically active membranes, account must be taken of such properties as kinetics, catalyst loading, and catalyst inactivation. A rotated disc electrode on which a membrane could be mounted would offer some unique advantages for sensor development. With such an electrode, a well-defined transport regime could be established that would be analogous to that of the classical rotated disc electrode first described by Levich ( 7 ) ,in which the disc surface is uniformly accessible to reactant and the diffusion boundary layer thickness precisely determined. At low rotation rates, the diffusion boundary layer would be relatively large, so that transport through solution would be a major contribution to total diffusional resistance. Under such conditions, the current would be proportional to the square root of rotation rate. At high rotation rates, however, where the diffusion boundary layer would be small, the resistance of the solution would be negligible making the diffusion current independent of rotation rate and determined by the properties of the membrane. Thus, transport in the membrane could be evaluated quantitatively without complications of external mass transfer. This feature would be particularly useful where the membrane is permeable enough to the solute that transport in membrane is comparable to transport in solution, or where carefully defined conditions are essential for distinguishing the effects of diffusion from chemical reaction in catalyst-containing membranes. This electrode could therefore be used for characterization of membranes that are to be used in nonrotating sensors or, as appropriate, employed directly as a sensor in the rotated configuration. Two previous investigations with a membrane-covered, rotated disc electrode have been reported. Chien, Olson, and Sokoloski (8) employed fixed potential voltammetry and a carbon rotated disc electrode with a Millipore VC membrane to qualitatively study transport of p-nitrophenol. T h e other electrode was a stationary combination reference/counter electrode. These investigators found that, at steady state, the limiting current was approximately linear with the square root of rotation rate a t low rotation rates (although unexplainedly lower than the Levich current), and independent of rotation rate a t high rotation rates. No theoretical treatment was proposed to describe the effect of rotation rate on current. Determinations of membrane permeability were made at high rotation rate by injecting a known amount of the reactant and measuring the time to establish a new steady state. In a subsequent publication by the same authors (9), this system was used to determine the relative concentrations of bound 1979 American Chemical Society
