Anal. Chem. 1982, 54 , 2307-2310 2307

Allyl Alcohol as Masking Agent in Determination of Copper( I ) and Copper( I I ) in Aqueous Mixtures

Gerard0 Ulibarri D. and Tetsuya Ogura

Departmento de QGmica, Universidad Aut6noma Guadalajara, A.P. 1-440 Guadalajara, Jalisco, Mexico

Jau-Hwan Tzeng, Nelson Scott, and Quintus Fernando*

Department of Chemistty, LJniversity of Arizona, Tucson, Arizona 8572 1

The concentratloris of copper( I ) and copper( I I ) in mixtures Containing the two ions can be determlned by an EDTA ti- tratlon. The copper( I ) is first masked by allyl alcohol durlng the course of the tltratlon of the copper( I I ) wlth EDTA; the copper( I ) Is next oxidized by hydrogen peroxlde to copper- (11) whlch Is determlned tnrimetrically, as before, wHh EDTA. The presence of dissolved oxygen affects the accuracy of the tltrlmetrlc method. Low concentratlons of copper( I ) (- lom5 M) In mixtures contalnlng copper( I ) and copper( I I ) can be stablllzed by allyl alcohol and determlned spectrophotomet- rically wlth 2,9-dimethyl-l,lO-phenanthrollne. Evidence Is presented for the Vormatlon of a ternary complex of copper( I) , 2,9-dlmethyl-l,lO-phenanthroline, and EDTA.

Aqueous solutions containing a mixture of copper(I1) am- mines are reduced by SOz in the presence of an excess of NHB to copper(1) containing species via a series of complex reac- tions. Acid solutions of copper(I1) are also reduced to c o p per(1) a t the surface of copper metal in the presence of a variety of water-soluble unsaturated organic compounds that can form stable complexes with copper(1). Important infor- mation concerning the kinetics of these homogeneous and heterogeneous reactions can be obtained by the determination of the concentration of copper(1) in solution as a function of time. The continuous monitoring of either the copper(1) or copper(I1) concentrations in these solutions is difficult and subject to several interferences. The reaction rates can be followed, however, by the analysis of the reaction mixture at predetermined time intervals. The concen1,ration of copper(1) can be determined in aliquots that are withdrawn from the reaction mixture. The exclusion of copper metal from these solutions would ensure that no further conversion of copper(I1) to copper(1) would occur in the course of the analysis. In a homogeneous reduction process, the reduction of copper(I1) to copper(1) can be stopped for the duration of the analysis by lowering the pH of the solution. Under the conditions in which many of the kinetic studies are carried out, the rate of formation of copper(1) is slow, and during the initial stages of the reaction, the ratio of copper(I1) to copper(1) in solution is about lo4. The analytical method should therefore be capable of determining at least lod M copper(1). The method should also be free of interferences that may arise from the presence of a wide range of concentrations of copper(I1) as well as excess reducing agents and unsaturated organic ligands. Most of the methods that have been reported for the deter- mination of copper(1) in solution cannot be used in these kinetic studies. The presence of unreacted reducing agents precludes the use of iodometric methods and other methods that are based on redox reactions. An alternative approach is to measure the decrease in concentration of copper(I1) rather than the increase in the concentration of copper(I), as a

function of time. The copper(I1) ion selective electrode that is routinely employed for the determination of copper(I1) in aqueous solutions cannot be used in the presence of copper(1) or in the presence of organic ligands that form complexes with copper(1) (1,2). The electron spin resonance (ESR) technique appeared to have more promise since it has been used for the quantitative determination of paramagnetic ions such as copper(I1) (3). Attempts to determine copper(1) in the com- plex reaction mixtures by the ESR method gave poor repro- ducibility especially during the initial stages of the reduction reactions, where the method involved the measurement of small differences between two large concentration terms.

A simpler and more successful solution to this problem was achieved by the combined use of a masking agent and a se- lective complexing agent. For example, copper(1) could be masked and prevented from participating further in redox reactions, while copper(I1) that is present in the same solution is determined with a selective complexing agent. An additional advantage is that the conversion of copper(1) to copper(I1) by traces of dissolved oxygen in solution can be minimized by stabilizing the copper(1) with the masking agent. Alter- natively, copper(I1) in solution could be masked and copper(1) determined by selective complexation. We have adopted both general approaches with some modifications, for the deter- mination of copper(1) as well as copper(I1) in solutions con- taining a mixture of the two species. My1 alcohol is extremely water soluble and is an effective masking agent for copper(1). When present in sufficiently high concentration, allyl alcohol masks copper(1) during the titrimetric determination of copper(I1) with EDTA. When the end point is reached, the allyl alcohol complex of copper(1) is oxidized to copper(I1) by the addition of hydrogen peroxide and the determination is completed by titrating the copper(I1) that is formed, with EDTA. Details of this procedure are given below. In the alternative method, copper(1) is determined spectrophoto- metrically with 2,9-dimethyl-l,lO-phenanthroline (neo- cuproine) (4) . It is not necessary to mask the copper(I1) if no further conversion of copper(I1) to copper(1) is occurring in solution. Copper(I), however, is quite sensitive to traces of dissolved oxygen and the formation of a copper(1)-allyl alcohol complex suppresses the oxidation of copper(1) to copper(I1) but does not affect the extent of formation of the copper(1)-neocuproine complex. The experimental procedure for the spectrophotometric determination of low concentra- tions of copper(1) in the presence of a large excess of copper(I1) is described below.

EXPERIMENTAL SECTION Reagents. Allyl alcohol and 2,9-dimethyl-l,lO-phenanthroline,

or neocuproine (Aldrich Chemical Co.), were used without further purification. Reagent grade 95% ethanol was purified by frac- tional distillation of a mixture of the ethanol, NaOH, and alu- minum metal to remove any aldehydes present in the ethanol.

0003-2700/82/0354-2307$01.25/0 @ 1982 American Chemical Society

2308 ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982

Table I. Determination of Copper(1) and Copper(I1) in Aqueous Mixtures calcd, M found, M calcd, hi found, M calcd, M found, M calcd, M found, M

Cu(1) in solutiona 0.0970 0.1013 0.0970 0.1013 0.0970 0.1013 0.1649 0.1759

CU(II) present b in 0.0046 0.0053 0.0055 0.0054

Cu(I1) formedb by the 0.0970 0.1046 0.0970 0.0946 0.0970 0.0942 0.1649 0.1643

Cu( 11) added to initial 0.1982 0.2069 0.1982 0.1945 0.1982 0.1906 0.1982 0.2054

concn of allyl alcohol (M) 0.1017 0.1017 0.4625 0.9538

masked by allyl alcohol

the Cu(1) solution

oxidation of Cu(1) with H202

solution containing Cu(1)

a Determined titrimetrically with cerium(1V). b Determined titrimetrically with EDTA; the values in rows 3 and 4 are corrected for the copper(I1) that is found in the copper(1) solutions. The relative standard deviation of the EDTA titration is 2.6%.

Granular copper metal (C.P., 20-30 mesh, J. T. Baker Chemical Co.) was used for the preparation of standard solutions. All other compounds were of reagent purity.

Preparation of Standard Solutions. A standard solution of copper(I1) was obtained by dissolving copper metal in a 1:l HNOB solution. A 70% HCIOl solution was added and the re- sulting mixture heated until no more fumes from the oxides of nitrogen were evolved. The solution was diluted with distilled water to give a standard stock solution of copper(I1).

Solutions containing known concentrations of copper(I1) were also prepared from Cu(ClO& (G. F. Smith Chemical Co.) dissolved in distilled water and standardized with EDTA, between pH 7 and 8, with murexide as indicator (5).

A standard solution of copper(1) was prepared by the complete reduction of a solution containing a known concentration of copper(I1). A 10% solution of NH,OH.HCl was added in excess to a solution of copper(I1) maintained at a pH between 5 and 6 with an acetate buffer. A series of standard solutions of copper(1) was prepared in this manner and used to obtain a calibration curve (absorbance vs. concentration) for the neocuproine-copper(1) complex.

Standard solutions of copper(1) were also prepared by the reduction of copper(I1) chloride in 0.10 M HC1. An excess of allyl alcohol was added and the reduction was carried out by the addition of copper metal. The mixture was stirred for about an hour with a magnetic stirrer until the blue color disappeared completely. Standard solutions containing copper(1) were freshly prepared in this manner for each experimental determination. Mixtures containing known concentratins of copper!T) and cop- per(I1) were prepared by the addition of a measured volume of a standard copper(I1) solution to the solution containing the copper(1)-allyl alcohol complex. These mixtures are stable in the absence of oxygen and the copper(1)-allyl alcohol complex does not undergo disproportionation. If copper metal is excluded from the system there is no tendency for the copper(I1) to be reduced to copper(1).

Standard solutions containing copper(1) may also be prepared dissolving solid copper(1) chloride in a solution of neocuproine in ethanol. The copper(1) concentration in the solution can be calculated from the absorbance of the solution at 450 nm and the molar absorptivity of the copper(1)-neocuproine complex.

All experiments involving solutions containing copper(1) were carried out in a nitrogen atmosphere. Oxygen readily oxidized copper(1) to copper(I1) species and had to be rigorously excluded from these solutions. An alkaline pyrogallol solution was used to remove the residual oxygen in the nitrogen gas that was used in this work.

Titrimetric Determination. All titrations were carried out in a stoppered 150-mL titration vessel equipped with a magnetic stirrer. An inlet tube and outlet tube for nitrogen gas were inserted through holes in the stopper. Additional holes were made in the stopper for the insertion of the buret tip and a tube capped with a rubber septum through which the copper(1) containing solutions were added with the aid of a microsyringe. The copper(1) added to the titration vessel was calculated from the difference in the weight of the microsyringe.

The titration vessel was purged with oxygen-free nitrogen, the copper(1) containing solution and -1 M allyl alcohol were added, and the pH of the solution was adjusted to a value of 8.0 by the

addition of dilute ammonia. Nitrogen gas was bubbled through the solution for at least 10 min, and the copper(I1) in the copper(1) solution titrated with a standard EDTA solution (0.01-0.05 M) with murexide as indicator. The solution in the titration vessel was blanketed with nitrogen gas during the course of the titration. After the end point was reached, about 1 mL of 10% hydrogen peroxide was added, the pH of the solution readjusted to 8.0, and the copper(I1) formed by the oxidation with hydrogen peroxide, titrated with EDTA with murexide as indicator.

Spectrophotometric Determination. A series of standard solutions containing copper(1) (1.6 X lo4 M to 1.6 X M) were prepared as described above, by the reduction of a standard solution of copper(I1) with an excess of 10% NH,OH-HCl solution. An excess of neocuproine (10 mL of 0.1% neocuproine in 95% ethanol) was added and the absorbance of the yellow solution measured at 450 nm. A linear calibration curve was obtained in this concentration range and the molar absorptivitity of the copper(1)-neocuproine complex was found to be 7770 cm-l M-*. This value is unaffected by the presence of allyl alcohol (-0.5 M). The precipitation of ion pairs, formed with the complex Cu(neocuproine),+ and anions such as C104-, was prevented by maintaining at least a 40% v/v concentration of ethanol in so- lution.

Mixtures containining copper(1) and copper(I1) that were ob- tained, either by the partial reduction of a solution of copper(I1) with a deficiency of NH20H.HC1 or by the incomplete conversion of copper(I1) to copper(1) in the presence of allyl alcohol, perchloric acid, and copper metal, were analyzed spectrophotometrically in a similar manner. All solutions were at least 0.5 M in allyl alcohol and were manipulated in a nitrogen atmosphere. A measured volume of the mixture containing copper(1) and copper(I1) was added to 10 mL of 0.1% neocuproine in 95% ethanol. After appropriate dilution, the absorbance of the solution, which con- tained about 50% v/v ethanol, was measured at 450 nm. The concentration of copper(1) was calculated from the calibration curve.

RESULTS AND DISCUSSION The concentrations of both copper(1) and copper(I1) in

mixtures containing 0.1-0.2 M copper(1) and 0.2 M copper(I1) can be determined titrimetrically with EDTA (0.04 M). The copper(1) is masked by allyl alcohol and the copper(I1) in solution is titrated with EDTA with the aid of murexide as indicator. In the next step copper(1) is oxidized to copper(I1) with hydrogen peroxide and the additional copper(I1) formed is determined tritrimetrically with EDTA. The effectiveness of the masking action of allyl alcohol can be determined by the addition of a known concentration of copper(I1) to the solution containing the copper(1) complexed with the allyl alcohol, followed by the titrimetric determination of the total copper(I1) in solution. Table I shows one set of results of a series of determinations that were carried out in the sequence described above. The values in the first row represent the initial concentrations of copper(1) in solution that were de- termined independently by a ceric titration. The small amount of copper(I1) that was present in each solution was determined by the EDTA titration (Table I, row 2); the dis-

ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982 2303

The allyl alcohol stabilizes the copper(1) in solution and prevents its disproportionation into copper metal and cop- per(I1) and neocuproine readily displaces the allyl alcohol to form the stable copper(1) complex.

The spectrophotometric method can be used to determine M copper(1) in the presence of 0.1 M copper(I1)

with a reproducibility of about 1%. Errors caused by the presence of traces of dissolved oxygen are magnified if at- tempts are made to determine copper(1) concentrations less than low5 M. The presence of oxidizable impurities must be scrupulously avoided in the presence of high copper(I1) con- centrations (>0.1 M). Oxidation reactions caused by cop- per(I1) may be prevented from occurring by masking the copper(I1) with a complexing agent such as EDTA. The values of the conditional constants given above indicate that it should be possible to determine copper(1) spectrophotometrically with neocuproine, in the presence of copper(I1) that is masked by EDTA. Unexpected results were obtained in solutions con- taining copper(I), copper(II), EDTA, and neocuproine. When a mixture containing EDTA and neocuproine in a 1:2.5 molar ratio was added to a solution containing a mixture of copper(1) and copper(II), the absorbance of the solution at 450 nm was lower than the absorbance of an identical solution containing a mixture of copper(1) and copper(I1) to which only neo- cuproine was added. A further decrease in the absorbance was observed when the EDTAneocuproine ratio was increased from 1:2.5 to 3:2.5. The presence of EDTA in solution probably prevents the conversion of all the copper(1) into the Cu(neocuproine)2+ complex and thereby decreases the mea- sured absorbance at 450 nm. The following series of exper- iments was designed to clarify this anomalous result. The absorbance of a solution that was 1.34 X M in copper(1) and 5 X M in neocuproine was measured at 450 nm and found to remain unchanged with time for a t least 24 h. In the presence of 0.05 M allyl alcohol, the absorbance of the solution remained unchanged for a period of 10 days. The same experiment was carried out with a solution that was 1.34 X M in copper(I), 5.0 X M in neocuproine, and 1.0 X M in EDTA; the EDTA solution was added after the copper(1)-neocuproine complex was formed. The initial ab- sorbance of the solution at 450 nm was the same as that obtained in the previous experiment, but decreased with time until a constant absorbance value that was about 6% lower than the initial value was reached in about 3 days. If the experiment was repeated with the same initial concentrations of copper(I), neocuproine, and EDTA, the only difference being that the neocuproine and EDTA solutions were added simultaneously (in the form of a mixture) to copper(I), the initial absorbance was 20% lower than that obtained in all the previous experiments. This absorbance increased slowly with time, and after about 2 weeks approached the constant absorbance value that was obtained in the earlier experiment. The presence of 0.05 M allyl alcohol in these solutions merely decreased the time required to reach the equilibrium absor- bance values. The results of these experiments led to the conclusion that neocuproine and EDTA form a mixed ligand complex with copper(1) and that the molar absorptivity of the mixed ligand complex is less than that of the copper(1)- neocuproine complex. The formation of this type of mixed ligand complex has been suspected by other workers. It has been reported for example that the color of the Cu(neo- cuproine)z+ complex developed slowly and incompletely in the presence of EDTA (9). Recently, ternary complexes containing copper(I), neocuproine, and ligands containing oxygen donor atoms such as acetylacetone, acetate, and oxalate ions have been synthesized (10). It should be possible, therefore, to synthesize similar ternary complexes of copper(I), neocuproine, and aminopolycarboxylic acids and to prove that these com-

to

proportionation of the copper(1) during the course of the titration was suppressed by the addition of allyl alcohol (0.10-0.95 M). Itow 3 in Table I shows the values obtained by EDTA titration for the total copper(I1) formed by the complete conversion of copper(1) to copper(I1) with hydrogen peroxide. After the addition of copper(I1) to each of the initial solutions containing copper(I), the values for copper(I1) de- termined by the EDTA titration (row 4) show that it is pos- sible to carry out this titration in the presence of considerable concentrations of copper(1) that is masked by allyl alcohol.

The values obtained for the copper(1) concentration in these solutions by the ceric titration (row 1) are consistently high (standard deviation 3.4%). This may be attributed either to the oxidation of allyl alcohol or to the oxidation of an impurity in the allyl alcohol by the cerium(1V) ions. The copper(1) in solution is always accompanied by a small amount of cop- per(I1) which is probably produced by the oxidation of cop- per(I), or its allyl alcohol complex, by the residual oxygen which cannot be removed from these solutions. It is impos- sible, therefore, to prepare accurately standard mixtures of copper(1) and copper(I1) in which the concentration of cop- per(1) is very low (-lo4 to M). The spectrophotometric method with neocuproirne can be used to determine copper(]) that is stabilizedl by allyl alcohol and present a t these low concentrations in mixtures in which the copper(I1) concen- tration is very high (-0.1 M). The accuracy of the spectro- photometric method, however, cannot be verified independ- ently, by the use of standard mixtures containing low4 to M copper(1) and -lo-' M copper(I1). The standard deviation of about 50 replicate spectrophotometric determinations of copper(1) in this concentration range was 1.2%.

Allyl alcohol forms a stable 1:l complex with Cu' (log 81 = 4.7 at an ionic strength of 0.1 (6)) but does not form a complex with Cu2+. The formation constant of the EDTA complex of Cu2+ is greaher than the formation constant of the EDTA complex of Cu+ by a factor of 10l1 (7). In the presence of 0.5 M allyl alcohol and an ammonia buffer a t a pH of 8 ([NH,] i= lo-, M) , the conditional formation constant of the Cu2+-EDTA complex is decreased by a factor of about lo4, but the conditional formation constant of the Cu+-EDTA complex is decreased even further, by a factor of about lo7. The net result is that the ratio of the conditional formation constants of the Cu2+-EDTA complex and the Cu+-EDTA complex is 1014. This factor can become larger if Cu+, NH3, and allyl alcohol form mixed ligand complexes. The copper(]) therefore, can be effectively masked by allyl alcohol that is present over a wide concentration range, during the course of the copper(I1) titration with EDTA in an NH3-NH4+ buffer. The presence of impurities that can be oxidized by copper(I1) and the presence of dissolved oxygen in the solutions that are titrated affect the accuracy of the titration. The extreme difficulties that were encountered in the removal of oxygen from these solutions indicate that the copper(1)-allyl alcohol complex may be capable of forming an addition complex with oxygen. Experiments are being carried out to test the validity of this hypothesis.

Low concentrations of copper(1) to M) can be determined in the presence of -IO-' M copper(I1) by selective complexation with 2,9-dimethyl-l,lO-phenanthroline or neo- cuproine. The yellow neocuproine complex of copper(1) (log pz = 19.1 (8)) has an absorption maximum at 450 nm; the copper(I1) complex of neocuproine (log pz = 11.7 (8)) does not interfere with the spectrophotometric determination of cop- per(1) with neocuproine. If the spectrophotometric deter- mination is carried out at a pH of 6 in the presence of 0.5 M allyl alcohol, the conditional formation constant of the Cu+-neocuproine complex (log pz = 14.3) is greater than that of the Cuzf-neocuproine complex by a factor of a t least lo3.

2310 Anal. Chem. 1902, 54, 2310-2314

plexes are stable in aqueous solution.

LITERATURE CITED (1) Blaedel, W. J.; Dinwiddie, D. E . Anal. Chem. 1974, 46, 873. (2) Westall, J. C.; Morel, M. M. F.; Hume, D. N. Anal. Chem. 1979, 51,

1792. (3) Gilbault, G. G.; Meisel, T. Anal. Chem. 1969, 41, 1100. (4) Smith, G. F.; McCurdy, W. H., Jr. Anal. Chem. 1954, 26, 371. (5) Flaschka, H. A. “EDTA Titrations”; Pergamon Press: New York, 1959. (6) Manahan, S. E . !norg. Chem. 1966, 5 , 482. (7) Anderegg, G. Critical Survey of Stablllty Constants of EDTA

Complexes”; Pergamon Press: Oxford, 1977.

(8) James, R. 5.; Wllllams, R. J. P. J . Chem. SOC. 1961, 2007. (9) Larsen, E . E . Anal. Chem. 1974, 46, 1131. (IO) Kwlk, W. L.; Ang, K. P. J . Chem. SOC., Dalton Trans. 1961, 452.

RECEIVED for review June 1, 1982. Accepted July 23, 1982. This work was supported under the U.S.-Latin American Cooperative Science Program, by the Consejo Nacional de Ciencia y Technologia in Mexico (Grant NSF/PT/79/0111) and by the National Science Foundation (Grant INT.8000529) in the U.S.A.

Electrochemical Pretreatment of Glassy Carbon Electrodes

Royce C. Engstrom

Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069

The effects of preanodlzatlon and precathodlzatlon on the electrochemlcal oxldatlon of hydroqulnone, ferrocyanide, and hydrazine were studied at glassy carbon electrodes. Pre- treatment was characterized with respect to the sequence of voltages applied to the electrode, the amplltude of applied voltage, and the duration of pretreatment. For all three electroactlve species, preanodization at a voltage greater than 1.5 V vs. SCE was requlred to activate a freshly pollshed electrode. Ferrocyanide and hydrazlne also required preca- thodlzatlon to remove an inhibitory layer formed durlng preanodizatlon. I n all cases, pretreatment resulted in a substantlal Improvement In the half-wave potential of the voltammetric wave and In the reproduclblllty of the wave. The results are Interpreted in terms of three dlstlnct electrode surface condltlons.

Reports involving the use of carbon electrodes often describe pretreatment procedures that were found necessary to observe reproducible and well-defined electrochemical behavior. Taylor and Humffray noted that dipping a freshly polished glassy carbon electrode in chromic acid produced a significant increase in the rate constant for the iron(II1)-iron(I1) system ( I ) . Blaedel and Jenkins found that glassy carbon electrodes required “preconditioning” and “pretreatment” (2, 3). Pre- treatment, performed immediately before an experiment, consisted of applying 1.35 V and -1.35 V for 2 min each. Preconditioning, performed at any time in the history of the electrode, consisted of cycling the electrode repeatedly between those two voltages. After such treatment, the half-wave po- tential for the oxidation of dihydronicotinamide adenine dinucleotide (NADH) became less anodic by approximately 0.3 V. Similar pretreatment was used by Moiroux and Elving in their study of the same system (4). Exposure of pyrolitic carbon to radio frequency plasma in an oxygen atmosphere also resulted in a substantial improvement in the oxidation potential for NADH (5). Pyrolytic carbon film and glassy carbon electrodes showed improved catalytic ability toward the ferricyanide-ferrocyanide system following application of an 8-V (peak-to-peak) signal at a frequency of 70 Hz (6). The basal plane of pyrolytic graphite was shown to have advantages as an electrochemical detector for high-performance liquid chromatography, provided the electrode was preanodized in citrate-acetate buffer (7). Recently, electrochemical pre-

0003-2700/82/0354-23 10$0 1.2510

treatment of pyrolytic carbon fiber electrodes was found to cause a substantial decrease in the peak potentials for the oxidation of dihydroxyphenylacetic acid and ascorbic acid (8). A 7 0 - H ~ signal with an amplitude of a t least 2.6 V yielded well-resolved peaks for the otherwise overlapping oxidations of the two compounds.

While the above pretreatment procedures were arrived at largely by trial and error, most of the workers suggested that pretreatment involves the oxidation-reduction of functional groups on the carbon surface. Chemical evidence presented in the 1950s suggested the existence of quinone, phenol, lactone, chromene, and carbonium groups on graphite (9, 10). Polarography (11,12) and controlled-potential coulometry (13) of carbon suspensions resulted in the observation of a well- defined electrochemical process on the carbon surface, which was attributed to the reduction of a quinone group. Spec- troscopic examinations using internal reflectance infrared spectrometry indicated the presence of carbonyl groups on the surface of graphite, with the possibility of a cyclic anhy- dride (14-16). The work of Miller et al. resulting in a chiral electrode involved the introduction of titratable acid groups onto graphite through heating in air (17 ) , the groups being concentrated on the edge surface of the graphite microcrys- tallites (18). Other work involving chemical modification of electrodes apparently involved chemical bonding through either quinone or carboxyl functionalities (19-21). Current- potential curves taken of graphite electrodes yielded three anodic and three cathodic peaks (22). The peaks were at- tributed to the oxidation-reduction of quinone-hydroquinone groups in differing environments on the electrode surface. Similar work with glassy carbon showed two anodic and cathodic processes, one of which was thought to involve a surface functional group and the other the chemical adsorption of oxygen (23). The differential capacitance of glassy carbon and the edge orientation of pyrolytic carbon showed potential dependences suggestive of the presence of surface functional groups (24).

The work reported here concerns glassy carbon, a material reported to contain both trigonally and tetrahedrally bonded carbon in an arrangement of tangled aromatic ribbons with varying degrees of cross-linking (25,26). The glassy carbon surface has some of the characteristics of edge-oriented py- rolytic carbon, based on capacity measurements (24). The present work was undertaken as a result of observations made during studies of electrode fouling, where it was noted that

1982 American Chemical Society