hydroxyl radical are 3.3 X 109, 3.0 X lo9, and 2.1 X lo6, respectively. Thus a diffusing OH. should be consumed entirely by benzene or toluene when generated in mixtures of those hydrocarbons with acetonitrile. We have accordingly searched for the formation of the coupling products biphenyl and bibenzyl in exhaustive electrolysis of Bu4NOH in mixed solvents of benzene/acetonitrile (40/60)(v/v) and toluene/acetonitrile (40/60)(v/v). In electrolyses conducted a t +1.1and f1.7 V, no coupling products were observed by thin layer chromatography. In these electrolyses, as well as those preceding (no trapping substrate), we observed a darkening (brown coloration) of the reaction mixture during the electrolysis. While the negative product findings in this investigation are somewhat unsatisfying, a partial interpretation of the anodic hydroxide oxidation in acetonitrile is possible, and follows. We regard anodic current peaks I and I1 and cathodic current peak V as an ensemble of Pt “oxide-forming” and “oxide-reducing” waves. The two anodic waves yield only surface-limited quantities of anodic electrolysis products and so, in a sense, are incidental to anodic current peaks I11 and IV, which appear to constitute the steps in which bulk quantities of hydroxide ion are consumed. We unfortunately do not know the chemical identity of the product(s) of the bulk electrolysis. They appear to be species of higher molecular weight than would be readily detected by our extensive chromatographic examination, and may be polymeric. As for the nature of the electrode reaction which is initiated in waves I11 (and IV?), the coulometric n = 1 result indicates the initial product must be (Pt-OH)” or (OH-)soln.The hydroxyl radical trapping experiment demonstrates that a freely diffusing hydroxyl radical containing the aromatic hydrocarbon in its solvation shell is not generated. The hydroxide ion reactant on the other hand probably does not contain the aromatic hydrocarbon in its solva-
tion shell as it arrives a t the electrode surface. Then, if the decay of the hydroxyl radical oxidation product exceeds the rate of acetonitrile solvation shell relaxation, no trapping of the radical by the aromatic hydrocarbon would be observable. Such a fast decay reaction is unexpected from pulse radiolysis kinetic data for hydroxyl radicals produced in solution ( 2 ) , which in turn suggests that the hydroxyl radical decay reaction is catalyzed by the electrode surface and, in fact, occurs a t the electrode surface. Whether the surface-catalyzed decay of the hydroxyl radicals is occurring by modes of chemical reactivity not available to the solution radical species is an interesting question. Exploring this question requires knowledge of the ultimate electrolysis products, frustratingly unknown.
LITERATURE CITED (1)A. D. Goolsby and D. T. Sawyer, Anal. Chem., 40, 83 (1968). (2)M. Anbar and P. Neta, lnt. J. Appl. Radiat. Isotopes, 18, 493 (1967). (3)M. Fleishmann and F. Goodridge, Discuss. Faraday SOC., 45, 254 (1968). (4)G. Atherton, M. Fleishmann, and F. GoodridQe, Trans. Faraday SOC., 63. 1468 (1967). (5)R. P. Van Duyne and C. N. Reilley, Anal. Chem., 44, 142 (1972). (6)L. Fox, Ph.D. Dissertation, University of North Carolina, Chapel Hill, 1972 . _ (7)W. S. Woodward, T. H. Ridgway. and C. N. Reilley. Anal. Chem. 45, 435 (1973) (8)J. J. Lingane, “Electroanalytical Chemistry,” 2nd ed., Interscience, New York. N.Y.. 1958. (9)D. T.’Sawyer, Personal Communication, 1974. (IO)A. I. Popov and D. H. Geske, J. Amer. Chem. SOC., 80, 1340 (1958). (11) H. Angerstein-Kozlowska, B. E. Conway, and W. B. A. Sharp, J. Electroanal. Cbern. hterfacial Electrochem., 43, 9 (1973). (12)B. E. Conway and S. Gottesfeld, J. Chem. Soc., Faraday Trans. 1, 69, 1090 (19733. (13)L. C. Portis, J. C. Roberson, and C. K. Mann, Anal. Chem., 44, 294 (1972). I
- I
RECEIVEDfor review August 16, 1974. Accepted October 16, 1974. We acknowledge support of this research in part by National Science Foundation Grant GP-38633X and Materials Research Center, U.N.C., under National Science Foundation Grant GH-33632.
Voltammetric Deposition and Stripping of Selenium(1V) at a Rotating Gold-Disk Electrode in 0.1M Perchloric Acid Richard W. Andrews and Dennis C. Johnson’ Department of Chemistry, Iowa State University, Ames, Iowa 500 10
The electrodeposition of Se(lV) at a Au electrode in 0.1 M HC104 is concluded to produce Se in three distinct states of activity, and three anodic stripping peaks are observed for large quantities of deposited Se. Approximately a monolayer is initially deposited which is apparently stabilized by one-dimensional interaction with the Au electrode surface. The formation of a bulk deposit of Se produces a large activity gradient which is the driving force for irreversible diffusional transport of Se into the Au electrode forming a AuSe alloy of unknown stoichiometry. Application of stripping voltammetry for determination of trace Se(lV) in 0 . 1 M HC104 is possible if the total deposit does not exceed the equivalent of one monolayer. The detection limit of the technique is approximately 0.04 ppb Se(lV) in 0.1 M HCIOd. Author to whom correspondence should be addressed. 294
ANALYTICAL CHEMISTRY, VOL. 47, NO.
The physiological significance of selenium has been extensively documented; Se is an essential nutrient a t trace levels ( I ) and toxic when ingested in exces:, ( 2 ) .The beneficial role is reported to involve a synergistic relationship with vitamin E ( 3 ) . When Se is present in animal feeds a t concentrations less than 0.1 ppm, deficiency symptoms develop. These include white muscle diseme, stiff lamb disease ( 4 ) , high embryonic mortality in ewes ( 5 ) ,hepatosis dietetica in weaning pigs ( 6 ) , and kwashikor in Guatamalan children ( 7 ) . The consequences of Se deficiency are sufficiently severe that the American Feed Manufacturers Assoication has petitioned the Food and Drug Administration to allow the supplementing of chicken, turkey, and swine feeds to adjust the Se concentration to 0.1-0.2 ppm (8).When Se is present in feeds a t concentrations exceeding 5 ppm, chronic selenosis develops manifesting itself as
2, FEBRUARY 1975
blind staggers, “alkali disease,” or, in acute cases, death (9). The carcinogenic nature of Se a t high levels is unresolved ( 1 0 ) . We are investigating the electroanalytical determination of Se a t trace levels in a variety of samples by application of voltammetric stripping. We report here the results of a fundamental study a t a rotating Au-disk electrode (RAuDE) in 0.1M HClO4 and an analysis of National Bureau of Standards SRM 1577 Bovine Liver. Bovine liver was chosen for analysis because of its availability as a standard reference material and because the one metabolic pathway which has been elucidated takes place in liver tissue (11). Lingane and Niedrach (12, 1 3 ) described the polarographic behavior of the various oxidation states of Se in aqueous solutions, and Christian et al. ( 1 4 ) were able to polarographically determine as little as 0.2 pg Se in 1- to 2g samples following extraction of the Se(1V)-diaminobenzidine complex. Vadja (15) developed a cathodic stripping method in which deposited Se was stripped as H2Se a t a Hg electrode; he applied the method to the determination of Se(1V) to 8 ppb in 0.2M HC104. Agasyan, Yurchenko, and Agasyan (16 compared the cathodic current-potential curves for Se(1V) a t various solid electrodes: Pt, Au, W, and Au amalgam. They observed only one wave a t a Au electrode in 1M HCl and developed a controlled potential coulometric method for Se(1V) and Te(1V). Devynck and Tremillon ( 1 7 ) applied a Au microelectrode to the measurement of the ionization constant of SeOC12. Gato and Ishii (18) used a Au wire electrode for a chronopotentiometric study of the reduction of Se(1V) in concentrated phosphoric acid. Two waves were reported; the first was concluded to correspond to reduction of Se(1V) to Se(0) and the second for reduction of Se(0) to Se(-TI). No report of the voltammetric stripping of Se(1V) a t a Au electrode was located. Rotating disk electrodes and voltammetric stripping techniques have been applied to the determination of many metal cations a t trace levels including Cu(I1) and Ag(1) ( 1 9 ) , Cu(II), Pb(II), Cd(II), Tl(I), In(III), and Bi(II1) (20), Ag (21, 2 2 ) , and Hg(I1) ( 2 3 ) .Difficulties frequently experienced a t solid metal electrodes are the appearance of multiple stripping peaks and the stripping of a portion of the deposited metal in a region of the potential axis where the noble metal electrode is anodized. These phenomena are associated with the stabilization of the deposited metal by strong interactions with the electrode material.
EXPERIMENTAL A p p a r a t u s a n d Instrumentation. The rotating disk electrodes and a PIR rotator were obtained from Pine Instrument Co. of Grove City, Pa. A rotating Au-disk electrode (RAuDE) was used for the majority of work described here with a geometric area of 0.6L53cm). The rotating glassy carbon-disk electrode had an area of 0.462 cm2,T h e three-electrode potentiostat was the disk portion of the RDE2 Ring-Disk Potentiostat from Pine Instrument Co.: which was constructed from operational amplifiers. The 200-ml all-glass cell was constructed with a separate compartment for the Pt-wire counter electrode separated from the main chamber by a fine fritted glass disk. The SCE reference electrode was inserted in a chamber connected to a glass Luggin capillary by a ground glass stopcock. The cell was soaked in hot, concentrated nitric acid between uses. Reagents. All solutions were prepared from Baker Analyzed Reagents except as noted. The supporting electrolyte was 0.1M HC104 made from G. F. Smith reagent grade acid. The water was triply distilled with demineralization following the first distillation, and the second distillation was from alkaline permanganate solution. Stock solutions of Se(IV) were prepared by dissolving Se metal in a minimum of concd nitric acid and diluting with water to give 0.100M Se(1V). This stock solution was stored in polyethylene
bottles. Subsequent solutions were made by dilution of the stock dispensed with Gilmont micrometer burets. A gold plating solution was prepared by dissolving Engelhard metal in a minimum of aqua regia and diluting with water and nitric acid. National Bureau of Standards SRM 1577 Bovine Liver has the following certificate listing of composition: as wt 46, 10.6 f 0.6 N , 0.97 f 0.06 K, and 0.243 f 0.013 Na; as ,ug/g, 270 f 20 Fe, 193 f 10 Cu, 130 f 10 Zn, 18.3 f 1.0 Rb, 10.3 f 1.0 Mn, 1.1 i 0.1 Se, 0.34 f 0.08 Pb, 0.21 f 0.04 Cd, and 0.016 f 0.002 Hg. Procedures. Standardization of Se(IV) stock solutions was by an indirect iodometric method (24 ). Results of the standardization were in agreement with the concentration calculated on the basis of the weight of Se taken. Pretreatment of the RAuDE involved polishing the electrode surface with 0.3 pm Buehler alumina on a Buehler AB Microcloth lubricated by water. The electrode was washed with a wet cotton swab and rinsed with water to remove alumina. At the start of each experiment, the electrode was inserted into the rotator, rotated in the test solution, and the disk electrode potential, E d , cycled between the prescribed limits until the current-potential curve was reproducible while deaerating with N2. T h e glassy carbon electrode was plated with Au prior t o use by the procedure described in Reference 23. The complete procedure for analysis of bovine liver is as follows: 1) Weigh lyophilized sample of approximately 300 mg. 2) Transfer to conical flask and add 5 ml of 1:l HC104:HN03. Connect reflux and heat. When dissolution is complete, remove reflux and heat to perchlorate fumes. 3) Cool and dilute to 100 ml. 4) Transfer to cell and deaerate for 5 minutes while scanning the disk potential between +1.2 and -0.30 V. Rotate electrode a t 377 radiandsecond. 5 ) Deposit for 10 minutes a t -0.30 V. 6) Scan the disk potential to +1.20 V and record I us. Ed curve. 7) Add 0.40 pg Se as Se(1V) and repeat steps 5 and 6. 8) Add an additional 0.40 pg Se as Se(IV) and repeat steps 5 and 6. A blank determination is necessary and is to be conducted according to the above procedure but with the absence of sample. All integrations of peak areas were made by a Keuffel and Esser Compensating Planimeter. Electrical currents given here are in pA, and potentials were measured and are reported in V cs. SCE. Electrical charge is reported in pC.
RESULTS AND DISCUSSION Current-Potential Curves for Se(1V). The voltammetric stripping of Se from a RPtDE in 0.1M HC104 was studied briefly and determined to occur simultaneously with formation of platinum oxide a t the electrode surface (25). Consequently, a Pt electrode was judged unsatisfactory for determination of Se by voltammetric stripping techniques a t trace levels. Current-potential (Id-E d) curves obtained a t the RAuDE for three values of bulk concentration of Se(IV), Cke,l\.r, are shown in Figure 1. The residual curve is also shown. The Au electrode surface in the absence of Se(1V) is anodized for Ed > 1.0 V, and the Au oxide film is reduced during the negative potential scan by a process producing the peak a t 0.8 V (26). The limiting cathodic process occuring for Ed < -0.4 V corresponds to evolution of HS (26). The anodic and cathodic waves obtained in the presence of Se(1V) are labeled for convenience in this discussion. Selenium(1V) is reduced a t the electrode surface by processes producing Waves A and B; the magnitude of both increases with increasing C &.(I\’). The limiting current plateau for Wave B varies proportional to C$ecIv, in the concentration range represented and is concluded to result from the convective-diffusional deposition of Se(1V). The height of Peak A is proportional t o the rate of potential scan, and the process is concluded to be surface-controlled. Furthermore, the peak height exceeds the limiting current of Wave B for c $ e , ~ 5 sec. The formation of bulk Se and, hence, the intermetallic AuSe compound cannot occur until the formation of the layer of adsorbed Se is virtually complete. The plot of total charge for the three anodic processes reaches a limiting value a t Tdep N 600 sec for the flux of Se(1V) in this experiment. The greatest analytical utility of the voltammetric stripping of Se at the Au electrode is restricted to trace levels and short T d e p . For these conditions, only Peak K is observed. For many analytical situations, this can be achieved by decreasing rotational velocity and/ or T d e p . A calibration plot showing the area of Peak K plotted against Ck,,~v,is given in Figure 6 for the experimental
A N A L Y T I C A L CHEMISTRY, VOL. 47, NO. 2, FEBRUARY 1975
297
~~~
~
Table I. Determination of Se in NBS SRM 1.577 (Bovine Liver) Blank
Se found, ug
1
2 3
4 5
Average
1.0
- 0.2
0.2
0.6
E
( V vs.SCE)
d
Figure 7. Stripping of S e a t a Au-plated, glassy carbon RDE 0.1M HCIO.,. 400 revlmin, 2.0 V/min. and 1 X IO@M Se(lV); Tdep: ( a ) Residual, OM Se(lV), ( b ) 0 sec, ( c ) 10 sec, ( d )30 sec, ( e ) 60 sec, ( f ) 120 sec, (9)300 sec, ( h )600 sec
Sample
Sample wt, g
1 2 3 4 5
0.4882g 0.34548 0.28028 0.35038 0.4103g Average Corrected for blank Certificate value
0.042 0.066 0.066 0.068 0.077 0.064
i
0.01
S e foundlg sample, rrg
1.15 1.22 1. 18 1.15 1.19
1.18 1.12 1.10
* i
0.03 0.03 0.10
showed no appreciable interference from these cations (25). Hg(I1) is readily deposited a t a Au electrode and is stripped in the potential region of Peak K where the monolayer of Se is observed to strip. Traces of Hg(I1) are conveniently removed from HC104-HN03 digestion mixtures by boiling which results in the volitilization of the Hg, probably as a chloro complex ( 3 4 ) . A potentially serious interference from Cu(I1) was investigated; note that Cu is present in the NBS bovine liver a t a concentration of about 170X that of Se. The area of Peak K was measured as a function of C $ u ( ~for ~ l C&,Ivl = 1 X 10-7M and T d e p = 5.0 min with E d e p = -0.30 v. Under these conditions, Se is deposited only in the adsorbed state; Cu is stripped a t Ed = 0.1 V, and there is no evidence of multiple stripping peaks for Cu or Se. Negligible decrease of the area for Peak K occurred for CFu(~1) < 30 C & ( I ~This ). fact is amazing when one considers that Se is codeposited with Cu, and yet the mobility is such that no Se is physically removed from the electrode by the rapid anodic removal of the large quantity of Cu. Instead, the small quantity of deposited Se finds its way to the Au surface and is stripped from the adsorbed state. The area of Peak K gradually decreases when CFu(Il,is increased above 30 C & r ~ v iThe . standard addition technique is still applicable, however, so long as the degree of interference is the same for the unknown and the unknown plus standard addition. Determination of Se in NBS Bovine Liver. Several procedures were tried for dissolution of the samples. Digestion with a H2S04-HC104 mixture was not satisfactory because considerable charring resulted, and analytical results were very low. I d - E d curves obtained following digestion under reflux in HNOa with or without added HClOj showed a large anodic wave with E 112 0.85 V which is identical to the anodic wave for NOn- in HClOJ. Patriarche ( 3 5 ) used a "02 digestion of urine for iodometric determination of Se and also observed production of "02. He recommended addition of urea to destroy "02. This did not eliminate the " 0 2 in our experiments. Successful digestion was achieved when the sample in a 1:l "0.1HC104 mixture was boiled to HC104 fumes. No anodic wave for HNOz was obtained in the resulting solution, and no interference by the Hg(I1) from the sample was observed. This dissolution procedure was used for all determinations reported below. The anodic I d - E d curve for the solution of bovine liver following a 10-min deposition a t -0.30 V is shown in Figure 8. Changes of current sensitivity were made during the re-
I
I
I
IO
I
1
I
I
I
02
36
I
I
-a2
E (VVSSCE)
Figure 8.
6-4 curves for NBS bovine
liver
3600 revlmin. 5 0 V/min. rdep = 10.0 min, 6,dep = -0.30 V, and 350-mg sample; Sensitivity; ( A ) 500 pA/unit, ( s ) 50 pA/unit, (05 pA/unit. Curves: ( 1 ) sample, (2) sample
+ 0.40 pg Se(lV), and (3) sample + 0.79 pg Se(lV)
conditions when Peaks J and L are not obtained. The plot is linear and obviously useful for analyses. The detection limit for this analytical technique ( i e . , &50% relative uncertainty) is approximately 0.04 ppb. Gold-Film Electrode. Extensive commercial use is presently made of stripping analysis a t gold-film electrodes to minimize the cost of electrodes. The rotating glassy carbon-disk electrode was plated with approximately five equivalent monolayers of Au and tested for suitability in this determination. Typical I d - E d curves as a function of T d e p are shown in Figure 7. The results are virtually the same as obtained a t the Au-disk electrode. Interferences. The certificate values for the component concentrations of NBS SRM 1577 are given in the experimental section. Cu, Pb, Cd, and Hg are potential interferences because they can be deposited a t a Au electrode. Studies of the deposition and stripping of Se(1V) a t 1 X 10-'M in the presence of a 1 O X excess of Pb(I1) and Cd(I1) 298
*
A N A L Y T I C A L C H E M I S T R Y , VOL. 47, NO. 2, F E B R U A R Y 1975
cording to show the relative magnitudes of the anodic Cu and Se peaks. The curves in the vicinity of Peak K are also shown for successive runs following standard additions of Se(1V). The increase in area of Peak K with standard addition was used to calculate the quantity of Se present in the sample assuming linear response to Se(1V). The results, corrected for the blank, are summarized in Table I. The uncertainties given are standard deviations. Agreement with the certificate value is excellent.
LITERATURE CITED H. A. Schroder, D. V. Frost, and J. Balassa. J. Chronic Dis., 23, 227 (1970). W. 0. Robinson, J. Ass. Offic. Agr. Chem., 16, 423 (1933). R. A. Passwater and P. A. Welker, Amer. Lab., 3 (5). 21 (1971). W. H. Allaway, "Proceedings Semi-Annual Meetings Nutrition Council, American Feed Manufacturers Association," Chicago, Ill., 1968, p 27. E. D. Andrews, W. J. Hartley. and A. B. Grant, N. Z. Vet. J., 16, 3 (1968). R. 0. Eggert, E. Patterson, W. T. Akers. and E. L. R. Stokstad, J. Animal Sci., 16, 1037 (1957). R. J. F. Burk. W. N. Pearsons, R. P. Wood 11, and F. Viteri, Amer. J. Clin. Nutr., 20, 723 (1967). Fed. Regist., April 27, 1973, p 10458. I. Rosenfeld and 0. A. Beath, "Selenium," Academic Press, New York, N.Y., 1964, p 145. Editorial, Nutr. Rev., 28, 75 (1970). W. G. Hoekstra, Fed. Proc., Fed. Amer. Soc. Exp. Bioi., 31, 691 (1972). J. J. Lingane and L. Niedrach, J, Amer. Chem. Soc., 70, 41 15 (1948). J. J. Lingane and L. Niedrach, J. Amer. Chem. SOC., 71, 303 (1949).
G. D. Christian, E. C. Knoblock, and W. C. Purdy, J. Ass. Offic. Agr. Chem., 48, 877 (1965). F. Vadja, Acta Chim. Acad Sci. Hung., Tomus, 63,257 (1970). L. B. Agasyan, A. G. Yurchenko, and P. K. Agasyan, Zh. Anal. Khim., 22, 229 (1967). J. Devynck and B. J. Tremillon. J. Electroanal. Chem., 23,241 (1969). M. Gato and D. Ishii, J. Nectroanal. Chem., 40, 303 (1972). G. W. Tindall and S. Bruckenstein, J. Electroanai. Chem., 22, 367 (1966). T. M. Florence, J. Nectroanal. Chem.. 27, 273 (1970). M. Kopanica and F. Vydra, J. Electroanal. Chem., 31, 175 (1971). R. E. Allen and D C. Johnson, Talanta, 20, 305 (1973). R. E. Allen and D. C. Johnson, Talanta, 20, 799 (1973). G. Narwitz, Anal. Chirn. Acta, 5, 109 (1951). R. W. Andrews, Unpublished Data. R. N. Adams, "Electrochemistry at Solid Electrodes," Marcel Dekker, New York, N.Y., 1969, p 23. W. M. Latimer, "The Oxidation States of the Elements and Their Potentials in Aqueous Solutions," 2nd ed., Prentice-Hall. Englewood Cliffs, N.J. E. Schmidt and H. R. Gygax. J. Electroanal. Chem., 13,378 (1967). V. A. Vicente and S. Bruckenstein. Anal. Chem., 45, 2036 (1973). G. W. Tindall and S.Bruckenstein, Anal. Chem., 40, 1051 (1968). G. W. Tindall and S. Bruckenstein, Nectrochim. Acta, 16, 245 (1971). S. H. Cadle and S. Bruckenstein, Anal. Chem., 44, 1993 (1972). Kh.2. Brainina, N. F. Zakhaschuk, D. P. Synkova, and I. G. Yudelevich, J. Nectroanal. Chern., 35, 165 (1972). T. T. Gorsuch, Analyst (London). 84, 135 (1959). G. J. Patriarche, Anal. Lett., 5, 45 (1972).
RECEIVEDfor review August 12, 1974. Accepted October 30, 1974. The support of the National Science Foundation through Grant GP-40646X is acknowledged.
A Model for the Amperometric Enzyme Electrode Obtained through Digital Simulation and Applied to the Immobilized Glucose Oxidase System Leroy D. Me11 and J. T. Maloy' Deparfment of Chemistry, West Virginia University, Morgantown, W. Va. 26506
Digital simulation has been used to model the steady-state current response of the amperometric enzyme electrode. Two types of calibration curves are predicted, depending upon whether the current is controlled by enzyme catalysis or by diffusion. Linear calibration curves are predicted at low substrate concentration; however, linearity extends to concentrations greater than Km for the enzyme in the case of diffusion-control. This behavior has been observed using a glucose oxidase electrode meeting diffusion-control criteria established by the simulation. Approach to steady-state data permits D for glucose in the membrane medium to be cm2/sec. Low apparent current estimated at 0.24 X densities observed are attributed to the inhibition of enzyme activity in the membrane and the reduction of effective electrode area by the membrane matrix; each is found to be reduced to ca. 1% of its theoretical value by the immobilization process.
Immobilized enzyme electrodes that are specific for a wide variety of substrates are used a t this time. In most cases, changes occurring as a result of the enzyme reaction are monitored potentiometrically (1-4 ). A frequently used substrate-enzyme combination for the study of immobil1
To w h o m correspondence should b e addressed.
ized enzymes and immobilized enzyme electrode systems is the glucose-glucose oxidase reaction sequence ( 5 , 6) Glucose
+
glucose oxidase
0,
*Gluconic acid
+
H,O,
(1)
The overall progress of the reaction can be monitored from the build-up of hydrogen peroxide in the vicinity of the electrode; sometimes an indicator such as iodide (which reacts with the hydrogen peroxide) is used (7-9).
H,O,
+
2H'
+
molvbdate(V1)
21-
* 1,
+ 2HZO
(2)
The subsequent decrease in iodide, or increase in iodine concentration serves as a means to monitor the overall extent of reaction. Recently, Guilbault and Lubrano (IO) and Brunsman ( I I ) introduced glucose sensitive electrodes which respond amperometrically to changes in substrate concentration by measuring the current produced by the oxidation of hydrogen peroxide a t a platinum electrode. Although the electrodes involved in these two studies were not identical, both groups report linear steady-state current response as a function of glucose concentration. I t is of particular interest to note that the calibration curves obtained by Guilbault and Lubrano exhibit linearity up to glucose concentrations in the vicinity of 20 mM. Of course, one would not
ANALYTICAL CHEMISTRY, VOL. 47, NO. 2, FEBRUARY 1975
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