1126
Anal. Chem. 1980, 52, 1126-1 130
Rotated, Membrane-Covered Oxygen 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
permeability could be readily determined. In subsequent communications, we analyzed transient currents in this system after an abrupt step of either electrode potential ( 3 ) or of bulk solute concentration ( 4 ) . These transient techniques provide a direct method of estimation of the diffusion coefficient for solutes that are rapidly consumed at the electrode surface, and can be useful to determine either the electrochemical reaction rate or the rate of transport for solutes that are partially reaction-limited, provided that the rate of one or the other process is known. In these studies on transient response, the model solutes employed were oxygen, hydroquinone, and ferrocyanide for processes that are strictly diffusion-limited, and glucose for processes that are partially reaction-limited. In the present paper, we present steady-state results for oxygen and hydroquinone. Examples are given of the use of the electrode system for characterization of permeability of hydrophilic membranes, and some observations are reported that may help explain certain properties of membrane-covered microelectrodes commonly used in oxygen analysis.
The use of the membrane-covered, rotated disc electrode system is described for characterization of steady-state transport of oxygen and hydroquinone in hydrophilic membranes. The analysis takes into account the concentration boundary layer in solution which can make a significant contribution to total diffusional resistance for small molecules. The electrode system Is compared to conventional, nonrotated membranecovered microelectrodes commonly used in oxygen analysis.
Many practical separation processes are based on membranes. Such treatments as desalination of seawater, removal of toxic metabolic products from blood by hemodialysis, and concentration of proteins by ultrafiltration operate by selective permeability in membranes. In many cases, improvement of membrane selectivity or enhancement of solute permeation rate can result in improvement in the overall effectiveness of the process. However, it is frequently difficult to accurately characterize transport in membranes because of external mass transfer resistance in the solution adjacent to the membrane. Determination of true membrane permeability is complicated when the rate of mass transfer in the external concentration boundary layer is comparable to the rate of transfer within the membrane itself. This effect becomes most obvious when low molecular weight solutes are involved. One of our objectives is to develop biochemical-specific sensors that can be used to continuously monitor the concentrations of such metabolites as oxygen, glucose, and lactate. We are developing electrochemical sensors that operate by selective transport of the solute across a membrane to a metal electrode where it reacts electrochemically to generate current. In development of this type of device. a challenging problem is the design of membranes that have acceptable permeability and selectivity for t h e solute of interest. Thus, accurate characterization of transport in membranes, taking into account the concentration boundary layer in solution, is essential. I n a previous paper ( I ) ,we described a novel, membranecovered rotated disc electrode that can be useful for characterization of transport in hydrophilic gel membranes. In this system, a disc working electrode and a reference electrode were both mounted on a rotating shaft and covered by a hydrophilic membrane. With the use of high conductivity background electrolyte, and by proper positioning of the nonrotating counter electrode, this arrangement resulted in a uniform current distribution across the disc so that the electrode surface was uniformly accessible for the electrochemical reaction. Using ferrocyanide as a model solute that is rapidly consumed a t the electrode, the following effects of mixed membrane and solution mass transfer resistance were demonstrated. At low rotation rates, where the concentration boundary layer in solution is large, the diffusion current approached the Levich current (2) that would prevail in the absence of the membrane. At high rotation rates, however, the concentration boundary layer in solution is relatively small, and the diffusion current was dependent on the properties of the membrane. Under these conditions, membrane 0003-2700/80/0352-1126$01 O O / O
PREVIOUS WORK Oxygen reduction a t platinum cathodes has been studied extensively (as reviewed in (5)). In acidic medium, the reduction most likely occurs by the following two-step mechanism (6):
O2 + 2H+ + 2e
H 2 0 2+ 2H+ 2e
-
-
H202 2H20
Oxygen may also undergo a simultaneous side reaction resulting in formation of a platinum oxide surface complex that partially inhibits the reduction process (7). The intermediate hydrogen peroxide is formed at all current densities but is only detectable in solution at current densities of A/cm2 or greater. In basic medium, where hydrogen peroxide is less stable, the equivalent of four electrons per molecule of oxygen is more easily obtained a t low over-potentials. It has been observed that cathodic electrode prepolarization is necessary in order to obtain reproducible current-potential curves (see 5). T h e electrode should be polarized a t A/cm2 until a steady potential is attained, thereby generating a steady-state concentration of hydrogen peroxide in solution and reductively consuming the platinum oxide film. Oxygen reduction then proceeds readily on the oxide-free surface. T h e process is similar a t physiological p H 15). Cathodic oxygen reduction has been employed extensively as a basis for oxygen measurement (8). For many applications, an oxygen permeable, hydrophobic membrane is used t o separate the working and combination reference/counter electrodes from the medium in order to restrict transport of water soluble species that may poison the electrode surface (9). Nevertheless, in spite of widespread use, there are some unresolved problems associated with this type of sensor. Experienced investigators report that sensors may exhibit significant flow artifacts, drift, and lack of reproduciblity ( I O ) . For example, it is recommended (8, 10, and references therein) C
1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7 , JUNE 1980
t h a t oxygen electrodes be allowed t o "age" under operating conditions after fabrication prior t o use, during which time the oxygen current decays rapidly. As a result, it becomes difficult, if not impossible, t o determine the oxygen concentration directly from the resulting electrode current bused on the number of electrons inrolced in the process. Rather, the magnitude of the electrode current is virtually always reported in relative terms and the concentration determined by arbitrary calibration of t h e observed currents with standard solutions of known oxygen concentrations. Although not widely appreciated, this practice complicates the quantitative experimental verification of various mass transport models (e.g., 11-1 3 ) t h a t have been proposed for the response of oxygen sensors. These considerations suggest that details of the mechanism of sensor operation are not clear, and that further studies with a different perspective may be advantageous for developing more reliable sensors.
1127
'1 I---
2 5 01
EXPERIMENTAL Equipment. The apparatus and construction of the membrane-covered, rotated disc electrode have been described elsewhere ( I ) . The only modification was the use of a platinized platinum electrode in certain experiments. The platinization procedure (14) resulted in an electrode surface with an initial roughness factor (the ratio of true surface area to geometric surface area) of approximately 2000, as measured by the galvanostatic hydrogen stripping method (14). During the first few days after platinization, the surface area decreased exponentially to a value of 650, and thereafter remained remarkably stable for several months provided that the deposit was not mechanically disturbed. The membrane, Bemberg Cuprophane PT-150, has been characterized previously by rotated disc electrode techniques (1, 3, 4 ) and by conventional methods (15). Thickness of the hydrated membrane was determined with a precision micrometer. Membrane ionic resistivity has been measured previously ( I ) and was found to be negligible compared with membrane resistivity to the solute. Studies with Oxygen. Solutions were prepared from analytical grade reagents and distilled, deionized water. Studies were conducted at 37 "C with a supporting electrolyte consisting of 0.1 N KC1 in lo-* M sodium phosphate buffer, pH 7.3. The solution was equilibrated with premixed gases of 0,0.98, 3.02,5.18, and 7.98% oxygen, respectively, each containing 5.0% COz,with the balance nitrogen. Gas mixtures were assayed by chromatography and certified correct to within 0.01%. Oxygen concentrations in solution were calculated with the use of standard solubility tables. An electrode prepolarization procedure was employed a t the beginning of each experiment for reproducible activation of the electrode surface. Using a previously described method (16),the electrode was first polarized at +900 mV vs. the saturated silver, silver chloride electrode (SSCE) for 2 min to oxidize the electrode surface, then polarized at -300 mV for 3 min t o reduce the oxide layer. The measurement potential, -200 mV, was subsequently applied and ample time allowed for the system to reach steady state before recording the diffusion current. With this activation procedure, adsorbed species were anodically oxidized and a surface oxide simultaneously deposited. The oxide was then cathodically reduced, exposing a bare platinum surface upon which reaction could proceed a t a maximal, diffusion-controlled rate. This activation procedure has been reported to be effective for obtaining highly reproducible glucose oxidation currents ( 3 , 4 , 16) and is consistent with the requirement of cathodic prepolarization for obtaining steady, reproducible oxygen currents. The conditions are also not extreme enough to cause formation of bubbles under the membrane. It was necessary to activate the electrode only a t the beginning of a series of measurements, since the oxygendependent current remained stable for several hours after a single prepolarization treatment. Studies with Hydroquinone. Experiments were conducted at M solute concentration with a background electrolyte of 0.1 N KC1, containing 5 X M H2S04,equilibrated with pre-purified nitrogen gas. The measurement potential was +700 mV and electrode prepolarization treatments were not necessary.
50 0
10
20
30
40
50
W '(radt'seci
Figure 1. Current density at the bare electrode as a function of w ' ' ~ for several oxygen concentrations. Lines were determined by linear regression analysis. The dashed line represents current density in the absence of oxygen
A Luggin capillary was employed with the reference electrode in all experiments without the membrane.
RESULTS A N D DISCUSS10 N Experiments were first conducted without a membrane using either a shiny or platinized platinum electrode to determine if surface roughness had any effect on mass transfer in diffusion-limited processes. Over the range of rotation rate employed here, the diffusion current of hydroquinone was identical with either type of electrode, suggesting that surface roughness need not preclude application of the Levich analysis. An example of the steady-state results for oxygen reduction a t the platinized electrode without the membrane are given in Figure 1. T h e data points correspond t o the limiting current density a t different oxygen concentrations for individual values of the square root of angular rotation rate. T h e solid lines, obtained by a least squares fit, intercept the ordinate a t a slightly anodic current density of 3.0 X lo4 A/cm2. Measurements attempted a t rotation rates of' less than approximately 0.5 rev/s (wl/' = 2.0 (rad/s)l/*) showed considerable deviation from linearity due to natural convection. The background current density obtained in the absence of oxygen (identified as "Buffer Only") was independent of rotation rate and equal to the anodic intercept of the oxygen current density slopes on the ordinate. With the background current taken into account, the net current corresponded to the Levich current ( 2 ) given by
iL = Q.62nFnR2D2i3u'/'ul/*C~
(3)
where n is the number of electrons involved, F is the Faraday constant, R is the electrode radius, D is the molecular diffusion coefficient in solution, u is the kinematic viscosity, (L! is the angular rotation rate, and CB is the bulk solute concentration. Assuming an overall process involving four electrons, a diffusion coefficient of 2.32 X 10-5 cm2/s was calculated for oxygen. This value is consistant with previously reported values determined by other techniques (2, 5 ) Figure 2 shows the results of the same experiment with a membrane in place. Here, experimentally observed oxygen current densities corresponding to various concentrations and
1128 Table
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
I. Mass Transfer Properties in Cuprophane
solute and conditions oxygen; 0.1 N KC1, 0.01 M phosphate buffer, pH 7.3, 37 " C (mol wt = 3 2 ) hydroquinone; '0.1 N KC1, 5x M H,SO,. 303c (mol w t = 110) ferrocyanide; ( 1 ) ; 1 N KCl, 0.01 M phosphate buffer, pH 7.3, 30 " C (mol wt = 212)
R*, min/cm 12.0
prn 9
D,
cm/s x I O 4 13.9
cmz/s x I O 6 23.2 27.6 ( 2 )
26.2
6.37
DefflD
15.8
0.14
4.2
0.07
11.6
0.15
8.5' ( 2 )
- I
a
Deffr cm2/sx l o 7 34.7
1.67
99.7
6.3
15 "C. I
1
ROTATION RATE ( r e v / s e c i
I I
01
-7 0
10.
I
I
1
1
10
20
30
40
50
W12[rad sec)12
Flgure 2. Diffusion current density at the membrane-covered electrode as a function of wl" for several oxygen concentrations
0 1
1
05
1
15
20 25 30 35
2% ERRORLIMITSSHADED
./
01 0
10
-
MEMBRANE CUPROPHANE PT-750 (BACKGROUND CURRENT SUE TRACTED) 1
I
1 .o
1
I
2.0
1
1
3.0
I
4.0
1
5.0
W"2 (rad/sec)''z
rotation rates are compared to theoretical curves calculated from the following equation ( I ) , id
= iL
[2 1 1
(4)
l+-
D P, = gd
is the concentration boundary layer thickness (2)
6d -- 1.6101/3v1'6w-1,2
without membrane. The shaded region represents an estimated f2% error limit in determination of permeability of this membrane sample
the membrane resistivity t o the solute R,, from which the membrane permeability was determined from the relationship
where P, and P, are the solution and membrane permeabilities, respectively. T h e solution permeability or mass transfer coefficient is given by
where
Figure 3. Current density normalized by oxygen concentration plotted as a function of wl/'. iLlaR2C, is the normalized Levich current density
(6)
T h e membrane permeability is given by the equation
where Deffis the effective diffusion coefficient, which is a product of the solute partition coefficient and diffusion coefficient in the membrane, and 6, is the membrane thickness. A value for membrane permeability was determined by plotting id-' vs. u-lI2,with the points fit by linear regression analysis, and extrapolating to infinite rotation rate a t u-'l2 = 0 where mass transport is totally membrane-limited. The resulting intercept on the id-l axis was used in calculation of
leading to the calculated values for the theoretical curves. In Figure 2, the theoretical curves were displaced downward to intercept the ordinate a t the value of the anodic background current density. An important observation is t h a t the steady-state background current, in addition to being independent of rotation rate, is quantitatively equivalent in the presence or absence of the membrane. Figure 3 summarizes the steady-state properties of the membrane-electrode system. T h e oxygen current density normalized by concentration is plotted with the background current density subtracted, as a function of the square root of rotation rate. T h e linear Levich current density with the background current density subtracted is also plotted. As shown previously ( I ) , theory predicts that the diffusion current with a membrane in place should approach the Levich current a t low rotation rates, but is determined by membrane permeability a t high rotation rates. The data obtained a t high rotation rates, where the effects of natural convection are small, were consistent with this theory. The area shaded represents the maximal discrepancy in current density t h a t
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
would result from an error of &2% in estimation of membrane permeability, suggesting that the precision attainable with this method can be quite high. The same method was employed for determination of the membrane permeability to hydroquinone. I n Table I, values for R,, P,, D, Deff,and the ratio D,ff/D are summarized for oxygen, hydroquinone, and ferrocyanide (the latter being taken from ( I ) ) . The values of the diffusion coefficient and permeability decrease predictably with increasing molecular weight. Although results for this membrane have not been previously determined with these solutes (except by our other membrane-covered, rotated disc electrode methods ( 3 , 4 ) )the , permeability to several other biochemicals representing a wide range of molecular weight has been reported (15). The present results are consistent with those of the previous study when compared on a basis of interpolation with respect to molecular weight, and we suspect that the present results may be more accurate, especially for lowmolecular weight solutes. Nevertheless, the format of this table illustrates an important limitation of the steady-state technique: by use of this method alone, it is not possible to resolve the effective diffusion coefficient into its components, the partition coefficient and the diffusion coefficient in the membrane. This is admittedly not necessary for many applications, but in certain instances, control of the effective diffusivity by independent adjustment of these parameters through novel membrane fabrication techniques may be advantageous. The transient techniques reported elsewhere (3, 4 ) allow direct determination of the diffusion coefficient in t h e membrane and the permeability in a single experiment, from which the partition coefficient can be calculated. and are therefore complementary to the present method. Although use of the platinized electrode had no effect on the Levich current for hydroquinone, there was a significant effect for oxygen. Irrespective of the presence of the membrane, with a platinized electrode the oxygen current density a t a given rotation rate rapidly reached steady state within minutes after the prepolarization treatment and remained at that value. In contrast, under the same conditions with shiny platinum, the oxygen current density continued to decay for a much longer period after the prepolarization treatment, eventually reaching an apparent steady state at a substantially lower value. Thus, the Levich current corresponding to four electrons was not observed for oxygen on shiny platinum. Certain other investigators have reported similar results (see 5) and there may be a plausible explanation for this observation: a slow passivation process may have occurred in parallel with the oxygen reduction process, probably involving the adsorption of impurities or the formation of an adsorbed oxide layer. This would decrease the effective geometric surface area of the smooth electrode, possibly bringing the process under reaction control. Such a partially poisoned electrode may not be sufficiently active for rapid reduction of the peroxide intermediate, thereby decreasing the number of electrons involved in the overall process. By comparison, the platinized electrode may have an adequate reserve surface area to postpone these effects. T h e nature of the background current is also of interest. The observation that this current is independent of rotation rate and not altered a t steady state by the presence of a membrane suggests t h a t the responsible process does not involve a diffusible reactant or impurity. A background current of similar magnitude was not observed with the same buffer when equilibrated with C02-free nitrogen, indicating t h a t an adsorbed, anodicably oxidizable complex of carbon dioxide may be partially responsible. Giner (17)has described a complex referred to as “reduced COP”,which has been observed in studies with physiologic buffer equilibrated with COz,
1129
but only significantly after prepolarization at potentials of -500 mV or lower. The composition of the supporting electrolyte used in this study was different (phosphate buffer containing KC1 vs. Krebs-Ringer bicarbonate buffer) and may account for the discrepancy. The important point for analytical purposes is that the background current was stable and relatively small. As an incidental word of caution, we have found that certain widely-used practices (8, 10) for obtaining a background current for oxygen sensor calibration may be detrimental. For example, opening the circuit as a substitute for measuring the current without oxygen causes uncontrolled electrode polarization which can result in erratic current after the circuit is again established, and the use of sulfite ion in direct contact with the electrode to scavenge oxygen from the solution causes electrode poisoning. We found excellent reproducibility of the diffusion current. Individual experiments carried out repeatedly over a period of more than one year resulted in measurements that were consistent to within 10%. I t is likely that much of this error can be attributed to variations in permeability within a given sheet of membrane, which have been previously estimated to be approximately 5% (15). The finding that a highly stable and reproducible oxygen electrode can be obtained is contrary to the common experience of many investigators with nonrotated oxygen sensors (10,12). Comparison of the two-electrode systems may suggest some reasons for this discrepancy. One of the most obvious differences is the method of maintaining the potential of the cathode. Our rotated electrode system employs a threeelectrode configuration in which the potential a t the disc electrode is determined with respect to a high impedance reference electrode, while the ionic current passes through the membrane between the disc electrode and a third, stationary counter electrode. This conventional three-electrode configuration is useful to maintain stability and prevent polarization of the reference electrode when a working electrode of relatively large geometric area is employed. For the nonrotated sensor, a two-electrode configuration is commonly used in which the second electrode serves as both a reference electrode and a counter electrode. The second electrode must be much larger than the working electrode so that the current, which is limited by the geometric area of the cathode, may pass without significantly polarizing the second electrode, and thereby compromising its function as a stable reference electrode. Obviously, any drift in the potential of the second electrode caused by this or other mechanisms may alter the diffusion current if the potential of the working electrode is moved from the diffusion-limiting region. Moreover, the cathodic deposition of silver cations that may be released from a polarized Ag/AgCl reference electrode has been suggested as a mechanism of slow loss of sensitivity to oxygen (see 10). Thus, reference electrode polarization, which may be significant with the two-electrode configuration for cathodes of relatively large geometric area, should be avoided. Beside the possibility of an unstable reference electrode, the current distribution in the vicinity of the oxygen cathode may not be the same for both electrode systems. In our rotated electrode, a uniform distribution of current across the disc surface can be obtained by properly positioning the counter electrode and by the use of a high conductivity background electrolyte. This, and the fact that the rotated disc is also uniformly accessible to the diffusion of the reactant, ensures that the electrochemical reaction proceeds a t a uniform, diffusion-limited rate a t all points on the surface of the disc. For the two-electrode configuration, the second electrode, which functions as the counter electrode, is frequently made in the form of a disc or concentric ring located near the edge of the cathode. Without a membrane, this configuration alone
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
could lead to a greater current density a t the edges of the cathode than a t the center, depending of course, on the cathode dimensions and on the conductivity of the electrolyte. The nonuniform current distribution would be further accentuated with a membrane in place, since the current flux would be restricted to the small volume between the surface of the hydrophobic membrane and the electrode. The consequence of this nonuniform current density is that oxygen reduction may not take place a t a constant rate across the electrode surface, even though the electrode is apparently held a t the diffusion-limiting plateau potential. Under such conditions, the oxygen diffusion field may be poorly defined. Previous transport models for oxygen electrodes have not taken this effect into account. If the electrochemical reaction is nonuniform across the electrode surface, undesirable side reactions may occur simultaneously with oxygen reduction, producing adsorbed species that decrease the catalytically effective electrode area with time and, hence, the sensitivity to oxygen. This may occur in addition to more obvious mechanisms of instability such as alterations in the permeability of the membrane with time. Although comparison of the two electrode systems is useful for understanding the operation of the oxygen sensor, it is not clear that all of the advantageous features of the rotated electrode system can readily be incorporated in present sensor designs. The need to employ hydrophobic membranes in the sensor system to prevent electrode poisoning is a major limitation to obtaining a more uniform current distribution. Nevertheless, the use of cathodes with moderate roughness factors may help extend current linearity at hyperbaric oxygen partial pressures where current may otherwise be nonlinear. A rough surface, if mechanically stable, may also provide reserve surface area to forestall the effects of poisoning. The use of a prepolarization treatment may allow some improvement in reproducibility of the current, but may not be maximally effective because of the nonuniform current distribution in effect during the treatment. Better definition of the solute flux to the sensor may be beneficial to help reduce flow artifacts, but improvement would be limited to some extent
because of the ill-defined current distribution. Finally, employment of the three-electrode configuration may produce improvement where cathodes of large geometric area are used if reference electrode drift is found to be a problem.
CONCLUSIONS A highly stable, membrane-covered rotated disc oxygen electrode system is described that can be useful in characterization transport of oxygen and certain other small molecules in hydrophilic membranes. This electrode may be useful to facilitate the development of novel membranes t o be employed in nonrotated chemical-specific sensors. Although this electrode system was not intended for use directly as a sensor, an analysis of its unique features may lead to a better understanding of the performance of nonrotated oxygen sensors.
LITERATURE CITED D. A . Gough and J. K. Leypoldt, Anal. Chem., 51, 439 (1979).
V. G. Levich, "Physicochemical Hydodynarnics", Prentice-Hall, Englewood Cliffs, N.J., 1962. D. A. Gough and J. K. Leypoldt, J . Electrochem. SOC.,in press. D. A . Gough and J. K. Leypoidt. AIChE J . , in press. J. P. Hoare, "The Electrochemistry of Oxygen", Interscience, New York,
1968. L. Myuller and L. N. Nekrasov, J . Elecboanal. Chem., 9, 282 (1965). J. P. Hoare, J . Electrochem. SOC..125, 1768 (1978).
I. Fatt, "Polarographic Oxygen Sensor", CRC Press, Cleveland, Ohio, 1976. L. C. Clark, Jr., Trans. Am. SOC.Arfif. Inter. Organs, 2, 4 1 (1956). Y . H. Lee and G. T. Tsao, Adv. Eiochem. Eng., 13, 35 (1979). W. Grunewald, Pflugers Arch., 320, 24 (1970). J. M. Hale and M. L. Hitchman. J . Electroanal. Chem., 107, 281 (1980). V. Linek and V. Vacek, Biotech. Eioeng. 18. 1537 (1976). L. Marincic, J. S. Soeldner. C. K . Colton, J. Giner, and S. Morris, J . Electrochem. SOC.,126, 43 (1979). C. K. Colton, K. A. Smith, E. W . Merrill, and P. C. Farrell, J . Eiomed. Mater. Res., 5 , 459 (1971). H. Lerner, J. Giner, J. S. Soeldner, and C. K . Colton, J . Electrochem. SOC.,126, 237 (1979). J. Giner, Electrochem. Acta, 8, 857 (1963).
RECEIVED for review September 27, 1979. Accepted March 28, 1980. We gratefully acknowledge support from the Academic Senate Research Fund of the University of California, San Diego, and The Kroc Foundation. J.K.L. was supported by NIH grant HL-10881 to B. W. Zweifach.
