148
Anal. Chem. 1984, 56, 148-152
Amperometric Enzyme Electrode for Determination of Glucose Based on Thin-Layer Spectroelectrochemistry of Glucose Oxidase HBlbne Durliat and Maurice Corntat* Laboratoire de Chimie-Physique et Electrochimie, Laboratoire associd a u C.N.R.S. No. 192, Universit; Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse Cedex, France
Constant potential electrolysis and sweep voltammetry performed wlth an optlcally transparent thln-layer electrode allows us to study the electron transfer between platlnum and glucose oxldase (GO) and between platlnum and flavlne adenine dinucleotide (FAD), the prosthetic group of the enzyme. The system FAD,,/FAD,,d appears quasl-reverslble wlth the partlclpatlon of an lntermedlate In the electron transfer. Repetltlve potentlodynamlc perturbations of the platlnum electrode are necessary In order to brlng about a dlrect electron transfer between the metal and GO wlthout addltlon of redox medlators to the solutlon or modlflcatlon of the surface by graftlng. Molecular actlvltles are preserved after electrolysis performed for several hours and It Is posslble to propose an amperometrlc enzyme electrode based on the reoxldatlon of the reduced form of the enzyme by constant potentlal electrolysis.
The control of the electronic transfer between a biological molecule functioning physiologically as an electron transfer agent and an electrode is important for several applications: development of in vivo sensors, electroenzymatic reactors, or bio fuel cells, coulometric assays of enzymatic solutions without oxidoreduction mediators. It is also important on a fundamental level for a better understanding of intermolecular or intramolecular electron transfer mechanisms and of the estimation of the corresponding rate constants. Several investigations of the electron transfer between glucose oxidase molecules and eledronic conductors have been done and may be schematically grouped under two headings: the studies devoted to mercury-enzyme interfaces with the use of differential pulse voltammetry or alternating current polarography (1,2) and, the studies dealing with the possibility of electron transfer between a solid electronic conductor and the biomolecule adsorbed or fixed on the surface. There, the results about carbonaceous electrode surfaces on which glucose oxidase has been covalently attached are found. If the direct electron transfer turns out to be impossible with a carbodiimide modified glassy carbon electrode (3) it is possible on a cyanuric chloride modified graphite electrode ( 4 ) . On the other hand it should be observed that electrochemical reaction rate obtained with intact glucose oxidase extracted either from Penicillium notatum or Aspergillus niger on a gold minigrid electrode modified by polymeric methyl viologen is relatively high (2). In previous work (5), using thin-layer spectroelectrochemistry, we have shown the possibility of a direct electron transfer between platinum and various large biological molecules (horse heart cytochrome c, aerobic yeast lacticodehydrogenase), pointing out that reagentless amperometric enzyme electrodes are working (6). The same approach is done here in the case of the interface platinum/GO; the results are compared with those obtained with FAD. The consequence of this study is
the proposal of an enzyme electrode allowing the assay of glucose even in anaerobic medium.
EXPERIMENTAL SECTION Apparatus. The working electrode is a platinum grid inserted between two microscope slides glued together so as to make an optically transparent thin-layer electrode similar to the one described by De Angelis and Heineman (7). The electrolytic cell, its support, and the optical and electronic equipment have already been described (5). All the potentials are referred to a saturated calomel electrode. Reagents. All the solutions were prepared from analytical grade reagents from various manufactures and doubly distilled water. The pH 5.3 buffer is a mixture of citric acid 0.055 M and monosodium phosphate 0.13 M brought to the appropriate pH by sodium hydroxyde. The pH 9.3 solution is a mixture of sodium pyrophosphate 0.05 M and chlorhydric acid. Glucose oxidase extracted from Aspergillus niger (grade VII) and flavin adenine dinucleotide from Sigma Chemical Co. were used without further purification. Procedure. Before each experiment the thin layer is filled with the buffer solution and the electrode is submitted for several hours to linear potential sweeps at the rate of 4.2 mV/s in the range of -0.7 V to +0.9 V. After this pretreatment the thin layer is filled with the biomolecule solution and the electrode kept for an hour at -0.2 V in order to reduce dissolved oxygen. This procedure allows an easy filling of the thin layer without use of a glovebox under nitrogen atmosphere. Moreover it allows the use of a very simple kind of thin layer. Linear Potential Sweep. The potential sweep rate is 0.4 mV/s and the current-potential curve and the variation of the absorbance vs. time are both recorded. The wavelength selected for FAD is 450 nm, maximum of absorption for the FAD oxidized form (extinction coefficient 11.3 mM-' cm-' (8). In the case of glucose oxidase the following known extinction coefficients were used (9): 14.0 mM-' cm-l at 455 nm for oxidized enzyme, 1.2 mM-' cm-' at 455 nm for fully reduced enzyme, 4.1 mM-' cm-' at 455 nm for semiquinonic intermediate, 4.1 mM-' cm-' at 570 nm for the intermediate at pH 5.3. Constant Potential Electrolysis. The electrode is kept at a constant potential while current and absorbance are simultaneously recorded until the absorbance becomes constant and the current is equal to the residual current. Absorbance measurements are made from time to time in order to protect the enzyme from too much exposure to light. Molecular Activity of GO. The GO catalyzes the oxidation of a glucose solution by oxygen. The formed hydrogen peroxide oxidizes the o-dianisidine in a reaction catalyzed by a peroxidase, leading to an absorbance variation for the 436-nm wavelength. Molecular activity of the GO is deduced from this measurement when expressed by time and enzyme concentration units. Enzymatic Glucose Electrode. A platinum disk 7 mm2 in surface area is covered by a semipermeable membrane enclosing a reaction chamber of a few tens of micrometers and incorporated in a potentiostatic circuit, The potential is maintained at a constant value while current is recorded vs. time. GO concentrations used in the reaction chamber are approximately 0.1 mM. The cell may be held under nitrogen atmosphere, with estimation of the oxygen concentration by means of an electrochemical oxygen analyzer (Beckman 260).
0 1984 American Chemical Society 0003-2700/84/0356-0148$01.50/0
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
149
0.5
12
8
>
I
I
-0.30
a p p l i e d p o t e n t i a l , V , v s SCE
-0.35
V applied
0.5
p o t e n t i a l , V . v s SCE
Flgure 1. Curve I(€)and A450nm(E)for a FAD solution: 0.82 mM; sweep rate, 0.42 mV/s; pH 5.3.
RESULTS AND DISCUSSION Flavin Adenine Dinucleotide (FAD). Figure 1 shows the current-potential and the absorbance-potential curves obtained at pH 5.3 with a FAD concentration of 0.82 mM and a potential sweep rate of 0.42 mV/s. The absorbance is measured for a wavelength of 450 nm and the variation observed during a sweep indicates that all the FAD molecules in solution in the thin layer are oxidized and reduced (during an anodic or a cathodic sweep). Constant potential electrolysis have been performed for potentials in the range of -0.20 to -0.45 V and the concentrations of oxidized FAD and reduced FADH2 forms of flavin adenine dinucleotide were determined by absorbance measurement when the current intensity is practically zero. A linear relationship exists between the applied potential assumed to be the equilibrium potential in these conditions and the logarithm of the concentration ratio FAD/FADH2 (typical correlation coefficient, 0.9998). The potential corresponding to (FAD) = (FADH2)varies between -0.30 and -0.35 V and the slope between 0.043 and 0.032 when the temperature is 20 "C over several experiments. These intercept values have to be compared with the set of E a values determined by potentiometric titrations and reported in ref 10: -0.445 V at pH 7.8 and 30 "C; -0.465 V at pH 7.0. The apparent value (n)of the number of electrons exchanged ranges between 1.5 and 1.8. Such a deviation with regard to the theoretical value has already been observed by Braun (11) during the polarographic study of the reduction of FAD. This author implicates the reoxidation of reduction products by dissolved oxygen and proposes a pseudo-firstorder rate constant of 1.1 x lo4 s-l. Such a possibility has to be discussed in the case of thinlayer spectroelectrochemistry. First of all the oxygen initially present in the thin layer is electrochemically reduced before each experiment and the peak associated with this reduction is never observed on the I ( E ) curves. On the other hand, although the spectrophotometer chamber is under nitrogen atmosphere, it may be supposed that traces of oxygen are present in the cell where the thin-layer electrode dips. The calculation of the oxygen concentration profile (12) in the case of oxygen diffusion coupled with a pseudo-first-order reaction
Flgure 2. Oxidation curve I(€)for a FAD solution (0.50 mM; sweep rate, 0.42 mV/s; pH 5.3): (-) experlmental curve; (curve 1) calculated wlth a disproportionation process, eq 1 and K = 1.4; (curve 2) case of monoelectronic transfer without disproportionation; (curve 3) case F; R = of bielectronic transfer wlthout disproportionation; C = F; E'" = -327.5 mV; r = 0.42 mV/s. 3100 Q ; C = 1.8 X
shows that for the system used in this study (5) and in the span of a few minutes, the oxygen level has not yet reached the working electrode. It seems that another assumption may be the presence of an intermediate in the electronic transfer. The theoretical study of a two-step electrochemical reaction associated or not associated to a chemical reaction in sweep voltammetry performed either with a thin-layer electrode or carbon paste electrode was already presented (13,14). The sight of experimental and calculated curves leads us to consider a bielectronic transfer occurring in two consecutive steps taking into account a disproportionation according to the system
R + S + e-
El'"
S + 0 + e-
E2'"
2s + 0
+R
K = (S)2/(0)(R)
and taking into account the uncompensated resistance R between the working electrode and the Luggin capillary and the electrode capacity C. In this case it has been shown (13) that intensity and potential are correlated by means of the equation
di d E R
i - ( E - Ri) FQrf(P)/RT+ Cr
In this equation Q is the amount of charge deduced from the surface area under the curve I(E),r is the potential sweep rate, and
4 + K1/2(P1/2 + P1l2) fv)= 2(Kl/Z + p1/z + p-l/2)2 with
and
E'" = (Elr0+ E i 0 ) / 2 Numerical integration of eq 1 is made by use of Runge-Kutta method with a forth-order approximation. Figure 2 shows an example of experimental results compared with calculated
150
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
I '
applied
Figure 3. Curves I ( € ) for GO solution (0.59 mM; sweep rate, 0.42 mV/s; pH 5.3): (1) first scan; (2) after 10 min between -0.7 and 4-0.9 V
potential .V.vsSCE
Figure 4. The same as Figure 3 but for pH 9.3.
at the sweep rate of 4.20 mV/s; (3) after 4 h of the same treatment.
curves for the case of a disproportionation (curve I), a monoelectronic transfer (curve 2), and a bielectronic transfer (curve 3) without disproportionation. A good fit is obtained only for a two-step bielectronic transfer with a disproportionation constant equal to 1.4. Due to some irreproducibility in experimental results, the parameters may be slightly different. Other experiments including pH influence and performed by other electrochemical methods are actually carried out to elucidate the electron transfer mechanism. In thin-layer experiments it is possible to completely reduce FAD before reducing the solvent. The system FAD/FADH2 seems to be more reversible than previously reported (13)by cyclic voltammetry where the potential difference between anodic and cathodic peaks is about 0.2 V. The present results lead to an electron transfer constant higher than lod4cm/s. Glucose Oxidase (GO). Figures 3 and 4 show the current-potential curves obtained by using a potential sweep rate of 0.42 mV/s with GO in the oxidized form for pH 5.3 and 9.3. Curves 1 are the first cycle recorded between -0.2 and -0.45 V and do not show a peak for the GO reduction. The recording of the curves is accompanied by a decrease of absorbance measured at 455 nm up to a minimum value. So the relatively high currents observed are attributed both to the solvent and to the GO molecules. Curves 2 are registered in the same conditions after some scans performed between -0.2 and -0.45 V at the sweep rate of 4.20 mV/s, and only a small wave appears during the backward scan. After 4 h of the same treatment, curves 3 are observed pointing out a reduction peak at -0.40 V, an oxidation peak at -0.36 V for pH 5.3, a plateau at about -0.38 V, and an oxidation peak at -0.36 V for pH 9.3. Several sweeps performed consecutively do not show evolution of the current-potential cupves. During the electrode treatment and during these sweeps, absorbance at 455 nm does not vary significantly. This is in good agreement with the
applied
potent8aI
Y
v5
5CE
Figure 5. A 455nm(E) during constant potential electrolysis: (0.385 mM, pH 9.3; (-) GO 0.448 mM, pH 5.3.
--)
GO
concentration of the electroactive form involved in a sweep. As a matter of fact the amount of charge deduced from the peak area of curve 3 indicates that about 10% of the GO molecules present in the thin layer are now electrochemically transformed. On the other hand the peak current intensities are linear functions of the potential sweep rate whereas the amounts of charge under a peak are independent of this parameter in the range 0.4-16.7 mV/s. This last behavior is typical of an adsorption phenomenon. However, it is possible to reduce or oxidize all the molecules in the thin layer by constant potential electrolysis performed at a judiciously chosen potential. Constant potential electrolysis is performed with solutions where GO is initially in the oxidized form. The first potential applied is -0.2 V maintained over a long period in order to reduce the oxygen dissolved in the thin layer. For each applied potential, absorbances are periodically measured at 455 nm and 570 nm until they are constant at 15-min periods. In the
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
.
\ \
\
r
\
i C
0.3
0 4-
m
+ C
al
u
C
0 Y
0.1
0 0
’
20;
60
80
I
1
t i m e , min
Flgure 6. Concentrations of various forms during constant potential electrolysis, (GO 0.45 mM; pH 5.3;E = -0.29 V): (0)GO oxidized form; ( 0 )GO intermediate form; (W) GO reduced form.
potential range corresponding to the electron transfer the potential increment is about 4 mV. Figure 5 shows an abrupt absorbance decrease from its maximum to its minimum values for a few millivolt potential variation. This phenomenon is observed at the two pH values used with 30 mV for pH unit variation. The A(E) curves indicate that the system is not a t equilibrium and it is not possible to propose a Nernst type relation. T o be complete the time required for the absorbance variation is about 100 min. Assuming that the electron transfer between the oxidized form (GOO) and the reduced form (GOR) of the GO occurs by means of an intermediate (GOs) it is possible to calculate the various concentrations of GOO,GOR, and GOs during a constant potential measurement, absorbance measurements performed at 455 nm and 570 nm. Indeed the sum of the three forms as well as the extinction coefficients of each species at 455 nm are known (9). On the other hand GOs has an absorbance maximum a t 570 nm a t which the two other forms do not absorb. Such a result is shown on Figure 6 for an applied potential of -0.290 V and pH 5.3. The C ( t ) curves depend largely on the electrode pretreatment and we did not try to deduce rate constants from these results. A better reproducibility and a better control of the electrode surface are necessary in order to make spectroelectrochemistry a quantitative tool adapted to the study of intramolecular electron transfer in oxido-reduction enzymes. Nevertheless the present study confirms the proximity of the standard potentials for the GOo/GOs and GOs/GOR systems already observed in potentiometric titration experiments (9,15-17) and brings another example of the usefulness of spectroelectrochemistry in the elucidation of complex processes. Complete reoxidation of GO solutions is very slow in the potential range above, but for a 0.5 V applied potential, oxidation is finished in about 1 h. Electrolysis has also been done on GO solutions 0.250 mM in classic electrochemical cells, with platinum working electrode a t the potential of -0.55 V. Complete reduction of solutions 1.5 cm3 in volume occurs in about 3 h when carried out under an argon atmosphere with evolution of the solution from yellow to colorless through a green intermediate. On the other hand the spectrum does not show any modifications in the range 300-600 nm after several
151
successive reductions and oxidations. Enzyme molecular activity determinations before electrochemical reduction and after reoxidation by the oxygen of the electrolyzed solution give the same result, showing that the application of an electric field does not denature the enzyme. As far as we know this is the third example of direct electronic transfer between GO solution and electrodes. The first example has been obtained on mercury by using differential pulse polarography (1);a very small peak is observed on the polarogram, using native molecule solutions. A proteolysis occurring during the steps of purification leads to the observation of a well-defined peak centered on the standard potential. The transfer seems to be facilitated by the strong adsorption of the protein on mercury, the interaction mercury-disulfide favoring the flattening of the molecule on the surface. With GO adsorbed on grtlphite, by the same electrochemical method, a peak appears at -0,63 V attributed to reduction of GO in solution whereas a shoulder a t -0.50 V is due to the reduction of adsorbed GO molecules ( 4 ) . The covalent attachment of glucose oxidase on cyanuric chloride modified electrodes leads to a voltammogram with a welldefined peak centered a t -0.53 V. In this work it has not been possible to distinguish between the electron transfer occurring between platinum and GO adsorbed molecules or GO in solution. Moreover the peaks for oxidation and reduction of GO or FAD are centered at the same potential. Perhaps this means that the electronic transfer occurs because of some FAD molecules acting as electrochemical mediator. The conservation of the molecular activity for the enzyme in solution after electrolysis indicates that if this hypothesis is true, only a small ratio of FAD molecules is involved in the electron relay. Application to Glucose Assay by an Enzymatic Electrode. Numerous enzyme electrodes using the reaction between glucose and oxygen catalyzed by GO have been proposed (18). They differ in the mode of enzyme fixation: grafting on a membrane (19) or od carbonaceous electrodes (20,21) and correticulation in a gel (22). They also differ from one another in the method of detection, either potentiometric (23,24)or amperometric, with the measurement of dissolved oxygen concentration or oxidation of hydrogen peroxide formed in the enzymatic reaction. The present results show that it is possible to oxidize GO electrochemically from 0.00 V; the use of higher potentials allows the increase of the rate of reoxidation and we have applied potential in the range 0.40-0.50 V to platinum electrode covered by a thin layer of enzymatic solution maintained in contact with the metal by an semipermeable membrane. In this potential range both hydrogen peroxide and glucose oxidation (25)rates are low. In so far as molecular activity remains constant under applied electric field, it is possible to propose an enzyme electrode based on the constant potential regeneration of the enzyme oxidized form. Such an electrode would allow the assay of glucose in an anaerobic medium. Working at a potential lower than amperometric electrodes oxidizing hydrogen peroxide, it would present fewer interferences with organic molecules able to be electrochemically oxidized. Figure 7 shows an example of the response obtained when an electrode containing 0.08 mM GO in the reaction chamber with a 0.10 mm thick cellophane membrane and a 0.04 mm deep reaction chamber is dipped in a glucose solution 3 mM a t pH 7.2. This curve is compared with residual current intensity obtained without GO in the reaction chamber. The steady state is obtained in about 6 min; current intensity in this steady state is proportional to glucose concentration between 0.01 and 7 mM with a slope of 0.3 x lo4 A/mM. With the geometrical characteristics used and not optimized,
152
Anal. Chem. 1984, 5 6 , 152-156
because the purpose of this work was to demonstrate that the control of the electron transfer at the metal solution interface enables the development of a reagentless glucose electrode. Registry No. D-Glucose, 50-99-7; glucose oxidase, 9001-37-0; flavin adenine dinucleotide, 146-14-5;platinum, 7440-06-4. 4:
x
LITERATURE CITED
+
(1) Scheller, F.; Strnad, G.; Neumann, 8.; Kuhn, M.; Ostrowski, W J. Electroanal. Chem. 1979, 6 , 117-122. (2) Scheller, F.; Strnad, G. Adv. Chem. Ser., in press. (3) Bourdillon, C.; Bourgeois, J. P.; Thomas D. J. Am. Chem. SOC.1980, 102, 4231-4235. (4) Ianniello, R. M.; Lindsay, T. J.; Yacynych, A. M. Anal. Chem. 1982, 54, 1098-1101. (5) Durliat, H.; Comtat, M. Anal. Chem. 1982, 5 4 , 856-861. (6) Durliat, H.; Comtat, M. Anal. Chem. 1980, 5 2 , 2109-2112. (7) De Angelis, T. P.; Helneman, W. R. J. Chem. Educ. 1976, 5 3 ,
C
0 L
¶
u
594-597.
(8) Witby, L. G. Blochem. J. 1953, 5 4 , 437-442. 10
5 t i m e , rnin
Figure 7. Typical response curve for an enzymatic glucose amperometric electrode (GO in reaction chamber, 0.1 mM; E = -I-0.45V): (-) glucose 3 mM; (- - - ) residual curve.
the detection level i s about 0.1 mM. Several hundreds of assays have been realized in this concentration range with constant intensities and response time on periods of a week. Many enzymes have been used during 1 month, thus corroborating the low influence of the electric field on the enzyme denaturation. Among these assays a high number has been performed under nitrogen atmosphere, the absence of dissolved oxygen being controlled by means of an electrode in the limit of sensitivity of the commercial oxygen electrode. The results are the same as those performed under air. The difference between glucose concentrations determined on the one hand by the enzyme electrode and on the other hand by an enzymatic method is less than 5 % . The reduction of the depth of the reaction chamber was not used to optimize this sensor,
(9) Stankovitch, M.; Schopler. C.; Massey, V. J. Blol. Chem. 1978, 253, 4971-4979. (10) Dryhurst, G. "Electrochemistry of Biological Molecules"; Academic Press: New York, 1977; Chapter 7. (11) Braun, R. D. J. Electrochem. SOC.1977, 124, 1342-1347. (12) Danckwerts, P. V. "Gas Liquid Reactions"; McGraw-Hill: New York, 1970. (13) Vallot, R.; N'Diaye, A.; Bermont, A.; Jakubowicz, C.; Yu, L. T. Electrochim. Acta 1980, 2 5 , 1501-1512. (14) Plichon, V.; Laviron, E. J. Nectroanal. Chem. 1976, 71, 143-156. (15) Gorton, L.; Johansson, G. J. Electroanal. Chem. 1980, 113, 151-158. (16) Svoboda, B.; Massay, V. J. B i d . Chem. 1966, 241, 3409-3416. (17) Duke, F. R.; Kust, R. N.; King, L. A. J. Nectrochem. SOC.1969, 116, 32-34. (18) Guilbault, G. G. "Handbook of Enzymatic Methods of Analysis"; Marcel Dekker: New York, 1976. (19) Thevenot, D. R.; Sternberg, R.; Coulet, P.; Laurent, J.; Gautheron, D C. Anal. Chem. 1979, 59, 86-100. (20) Kamin, R.; Wilson, G. S. Anal. Chem. 1980, 5 2 , 1198-1205. (21) Ianniello, R. M.; Yacynych, A. M. Anal. Chem. 1981, 53, 2090-2095. (22) Romette, J. L.; Froment, 13.:Thomas, D. Clin. Chim. Acta 1975, 95, 249-253. (23) Nagy, G.; Von Storb, L. H.; Guilbault, G. G. Anal. Chlm. Acta 1973, 6 6 , 443-447. (24) Liu, C. C.; Weaver, J. P.; Chen, A. R. Bloelectrochem. Bioenerq. 1981, 8, 379-386. (25) Marlncic, L.; Soeldner, J. S.; Colton, C. K.; Giner, J.; Morris, C. J. Electrochem. SOC. 1979, 126, 43-49.
RECEIVED for review March 7,1983. Accepted October 3,1983.
Ion-Selective Electrodes for Octyl and Decyl Sulfate Surfactants Gordon C. Kresheck* and Ioannis Constantinidis Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115 Surfactant Ion selectlve electrodes were prepared which gave near-Nernstlan behavior In aqueous solution for sodlum octyl sulfate at 25 OC and sodlum decyl sulfate at 20, 25, 30, and 35 OC. The selectlvlty of both electrodes to other alkyl sulfates was determined. The electrode response varied In golng from water to pH 7.4 In 0.01 M Trls-HCI buffer with and wlthout 0.1 M NaCl or 6 M urea. The blndlng of octyl and decyl sulfate to poly(vlnylpyrro1ldone)(PVP) of dlfferent molecular weights, concentratlon, and temperature was also studied. Flnally, It was shown that slmllar results are glven by using equlllbrium dlalysls or potentiometric titrations with the surfactant electrodes for the blndlng of decyl sulfate to bovine P-lactoglobulin.
Procedures for the preparation of surfactant ion selective electrodes which are simple to use and allow the rapid de-
termination of surfactant ion concentrations for use in physical chemical as well as analytical studies were described by Birch and Clarke (I, 2). This work focused on the use of dodecyl sulfate electrodes. A slightly different electrode system was later developed to study premicellar aggregation and the degree of counterion binding for two cationic surfactants and one anionic surfactant (3). We have confirmed the nearNernstian behavior of the more recent electrodes as well as their potential use for surfactant binding studies with several types of polymers (4,5). However, during the course of these studies occasional unexplained electrode responses were noted, and it was decided to undertake a systematic study of the behavior of octyl and decyl sulfate electrodes under conditions which might be of interest to chemists in view of their potential utility as originally demonstrated by Birch and Clark ( 1 , 2 ) for the dodecyl sulfate electrodes. The results of these studies will be reported.
0003-2700/84/0356-0152$01.50/00 1984 American Chemical Society
