GRAPHITE PASTE ELECTRODES EFFECTS OF ... - Science Direct

J. Electroanal. Chem., 143 (1983) 89-102

89

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

GRAPHITE P A S T E E L E C T R O D E S EFFECTS OF P A S T E C O M P O S I T I O N A N D S U R F A C E STATES O N E L E C T R O N - T R A N S F E R RATES

M A R G A R E T E. RICE, Z B I G N I E W G A L U S * and R A L P H N. A D A M S **

Department of Chemistry, University of Kansas, Lawrence, KS 66045 (U.S.A.) (Received 26th April 1982; in revised form 15th June 1982)

ABSTRACT A new understanding of the nature of electrode reactions at graphite paste electrodes has been obtained by studying electron-transfer rates of redox systems under conditions of carefully controlled electrode composition and pretreatment. Dry graphite gives electron-transfer rates which give an almost Nernstian response and approach those obtained with platinum. The addition of any pasting liquid decreases these rates materially. Both electrochemical and chemical oxidative pretreatments of such pastes increase the electron transfer in the direction of the "dry" graphite limit. This effect appears to correlate with the increased hydrophilic nature of the electrode surface and concomitant removal of organic layers from the electrode surface.

INTRODUCTION

Graphite paste electrodes, composed of a matrix of carbon or graphite particles with water-immiscible, non-conducting liquids, have been employed over the past 20 years in electroanalytical chemistry. Their practical utility developed despite the fact that virtually nothing was known about the fundamental nature of charge transfer at these surfaces. Our recent interest in using miniaturized versions of these electrodes for in vivo neurochemical applications demanded a better understanding of electron-transfer processes at graphite paste surfaces. The studies summarized herein were initiated for this purpose. There are many carbons and graphites which might be examined, but practical experience suggested two which represent those most widely used for paste electrodes. The first was GP-38, a multicrystalline graphite powder with an average particle diameter between 10 and 20 Fm. It is the present equivalent of the graphite formerly designated Acheson 38 which was used for years in carbon pastes with highly satisfactory results. The second material chosen was UCP-1-M, a highly * On leave from the Department of Chemistry, University of Warsaw, Warsaw, Poland. * * To whom correspondence should be addressed. 0022-0728/83/0000-0000/$03.00

© 1983 Elsevier Sequoia S.A.

90 purified graphite with particle size of 1 /~m. Used in paste configurations, this material gives extremely low residual currents and has been especially useful for liquid chromatography detectors [1,2] and for in vivo electrode formulations [3]. These two graphites were combined with a wide range of pasting liquids as indicated later. In a similar sense, two redox systems were chosen as most useful for the present investigation: the well-characterized ferricyanide/ferrocyanide pair, and the oxidation of 3,4-dihydroxyphenylacetic acid (DOPAC). The latter is a principal metabolite of the central nervous system neurotransmitter, dopamine, and is of interest for in situ brain electrochemistry. The overall oxidation pathway for DOPAC is just the quasireversible catechol/o-quinone redox couple which has been examined previously [4]. For brevity, the ferricyanide/ferrocyanide and D O P A C - q u i n o n e / D O P A C redox test systems are referred to herein as simply ferrocyanide and DOPAC. The aim of the present studies was not to obtain exact measurements of charge-transfer parameters. Rather, by measuring trends and variations in these values as a function of experimental variables like paste composition and electrochemical pretreatment, it was hoped to understand better how such electrodes work. This is especially pertinent since a variety of electrical and chemical pretreatment techniques have already been applied in neurochemical studies, with little understanding of their effect on electrode surface modification and concomitant change in electrochemical response. EXPERIMENTAL Materials and reagents The GP-38 graphite powder was obtained from Union Carbide, Carbon Products Division, Cleveland, OH 44101, and UCP-1-M from Ultra Carbon, Bay City, MI. The BET surface areas, kindly measured by Mr. Ed Carroll, Central Research, E.I. Dupont & Co., were 7.85 m 2 g - 1 (GP-38) and 6.33 m 2 g - 1 (UCP). These graphites were used as received or, in some cases, were chemically oxidized before being mixed into paste formulations. The inert pasting liquids were high-purity, straight-chain hydrocarbons--hexane, octane, decane, dodecane, tetradecane, hexadecane, octadecane (all from Sigma) and heneicosane (Alfa). The octadecane and heneicosane (C2~) have melting-points of 28°C and 41°C, respectively, and were melted prior to making pastes. Hexane and octane pastes were prepared just prior to use to prevent excessive loss of the hydrocarbons by volatility. The mineral oil, Nujol, was also included. All paste compositions are expressed in terms of the weight/weight ratio of graphite : pasting liquid. All chemicals were reagent grade and solutions were freshly made in twice-distilled water.

91

Electrochemical measurements The electroanalytical techniques were completely conventional. Cyclic voltammetry or rotated disk (RDE) studies were carried out with a P.A.R. 174 or 174A polarographic analyzer. The R D E was calibrated stroboscopically. All potentials were measured vs, SCE at 25 _+ 0.1°C in a constant-temperature bath. Formal potentials ( E °') were estimated from the relationship E ° ' = ½(Ep, a + Ep,c), where Ep, a and Ep, c are the anodic and cathodic peak potentials, respectively. The heterogeneous rate constant, k s, can be evaluated from current-potential curves and serves as an index of the ease of electron transfer at various stationary electrode surfaces. In the method of Nicholson, k s is derived from the peak separation of anodic-cathodic peak voltammograms [5]. In the present case, frequently only the trends in k s with electrode pretreatment or paste composition were of interest, and the data are reported in terms of the v a r i a t i o n A E p = (Ep, c - Ep.a). More precise evaluations of k s were obtained by treating anodic voltammograms at the R D E by methods outlined by Galus [6]. The parameter kb,h, the backward (anodic) heterogeneous rate constant, was evaluted at several potentials on the anodic wave. A plot of log kb, h VS. E gives ks, the heterogeneous rate constant at the formal potential E °'. The product/3n, where/3 is the anodic transfer coefficient, is obtained from a slope of this plot. Our only interest in determining /3n was to illustrate that this parameter did not change appreciably with electrode pretreatments. A marked change of fin with pretreatments would have suggested a serious change in overall electrode mechanism. RESULTS

The effect of graphite paste composition on electron-transfer rates Electrochemical characterization of dry graphite powders To provide a " b e n c h m a r k " for the manner in which organic liquids modify the electrochemical properties of graphite, dry GP-38 and U C P were first examined. The dry powders were packed into an electrode well. This configuration is too unstable to rotate but, with care, cyclic voltammetry experiments can be carried out. As is well known, dry graphites have very high capacitive currents (indeed, one of the original reasons for making paste electrodes was to lower the residual current). However, these dry electrodes allow evaluation of the electrochemical behavior of test systems on graphites contaminated only by surface states inherent in their preparation. Dry GP-38 gave practically diffusi0nal controlled behavior for the ferricyanide system. The E °' was +0.233 V vs. SCE, almost identical to that obtained with platinum. The peak separation, AEp, was 56 mV with pure diffusion control, 57 mV for the one-electron process. The D O P A C system gave E °' = + 0.536 V compared to + 0.534 V on platinum; however, t h e AEp of 40 mV was considerably greater than the 28.5 mV for a Nernstain two-electron reaction.

92

The dry U C P showed very marked adsorption of both test systems. This was shown by unusually sharp peaks on the cyclic voltammograms and AEp values of only 30 mV (ferrocyanide) and 29 mV (DOPAC). The ferrocyanide E 0, was almost identical to that obtained on dry GP-38 surfaces, but that for D O P A C was shifted anodically to + 0.533 V, suggesting stronger adsorption of the reductant f o r m . Thus, the oxidation of ferrocyanide and D O P A C on these dry graphites is almost identical to that on platinum and almost Nernstian in behavior (although the results on U C P are complicated by strong adsorption). The data in the following sections will show that mixing dry graphites with any pasting liquid always decreases the electron-transfer rates. However, with advantageous pretreatments the pastes again can be made to approach the more rapid kinetics observed at dry surfaces. Owing to the complications of adsorption on UCP, most of the pretreatment experiments were carried out with GP-38.

Effect of graphite paste cons&tency on charge-transfer rates It is well known that qualitatively the shapes of cyclic voltammograms at carbon paste electrodes become more non-Nernstian if the ratio of graphite/liquid is decreased. It was necessary in this work to find an optimum "wetness" so that the properties of different pasting liquids could be compared. For this purpose, the D O P A C system in 1.0 M HC104 was studied at various compositions of GP-38 and hexadecane. Graphite : hexanedecane ratios from 10 : 1 to 1 : 1 were examined and k S was evaluated from the Ep in cyclic voltammograms. The rate constants varied from 1.4 x 10 -3 c m s - 1 for the driest paste to 9.1 x 10 -5 c m s - 1 for the more fluid 1 : 1

- 2.75

+ t-

-3

/f+ +

/

-3.25

/ 8

/

/

/

/ -35

/

.< rY

/

I

(.9 -3 75 o

I

I

I I I

-4

+

i

I

i

I

- 4"2"50

1

2

3

4 GP-38

I

i

i

I

5 6 7 8 : HEXADUCANE: RATIO

I

9

O

Fig. 1. Log rate constant vs. GP-38 : hexadecane ratio. The influence of the ratio of graphite to pasting liquid on the rate constant of the oxidation of DOPAC (5.0× 10 4 M) in 1.0 M HC104. Rate constants were calculated from the AEp in cyclic voltammograms.

93

mixture. These data are summarized in Fig. 1, and it can be seen that the rate constant is essentially independent of composition when the ratio of GP38 : hexadecane is > 2 or 3 : 1. The rate constants at UCP pastes were slightly more sensitive to composition. From a practical viewpoint, graphite:liquid ratios of 2: l make pastes which are easy to pack as electrodes and can be polished to smooth surfaces. This value, where rate constants are reasonably independent of composition, was chosen for all subsequent studies unless otherwise noted.

Effect of hydrocarbon paste liquids The rate constants for ferrocyanide and D O P A C were examined by both cyclic voltammetry and R D E methods, using a wide variety of pasting liquids mixed with GP-38 (2 : 1 composition). As is evident in the next section, the electrode history can have a marked effect on the results, so a new electrode surface was prepared for each of several scans used for the measurements. Care was taken that no potential was applied to the electrode before the scan was initiated. Furthermore, all potential scans were reversed at potentials ~ 200 mV more positive than the anodic E p . Table 1 summarizes the results. The kinetic data from the R D E experiments are believed to be the more reliable. In the cyclic voltammetry evaluations, the assumption is that transfer coefficients are ca. 0.5. Those for the ferrocyanide are in the usual acceptable range (0.3-0.7), but those for D O P A C are not. Hence, the greater discrepancy between the two methods of evaluating k s appears for the D O P A C system, as might be expected. In fact, both methods give acceptably close values. As mentioned earlier, the absolute values are of lesser interest than the variations as a function of paste composition. These trends are seen graphically in Fig. 2A, B. In

TABLE 1 T h e effect o f h y d r o c a r b o n p a s t i n g liquid o n r a t e c o n s t a n t P a s t e liquid

Hexane Octane Decane Dodecane Tetradecane Hexadecane Octadecane Heneicosane Nujol

F e r r o c y a n i d e a , 103 k s / c m s

1

D O P A C b, l0 3 k J c m

s

1

CV

RDE

fin

CV

RDE

fin

2.8 1.8 1.6 1.1 0.59 -

3.1 2.7 2.3 1.9 1.7 1.4 0.47 0.19 0.11

0.33 0.36 0.32 0.28 0.33 0.33 0.36

4.7 2.9 2.6 1.6 1.0 0.76 0.035 0.012 0.014

4.2 1.2 0.68 0.37 0.30 0.23 0.045 0.038 0.018

1.04 1.07 1.02 0.99 1.05 1.09 0.98 1.20

" In 1 M KC1, E ° ' = 0 . 2 3 2 V vs. SCE. b In 1 M H C 1 0 4 , E ° ' = 0.525 V vs. SCE. ' E x t r a p o l a t e d values.

94

~r ~

+~

z -3.0 M,J u

N

N

N.

$,N

LO

\

~ -3.5 0

\ \

o

-4.0

6

8

~

,

,

i

,

J

10

12

14

16

15

20

+

÷\\

\+

-2.5

-3.0

O U

-3.5

\

Ld -4.0

0

o,

k +'~__

-4.5

-5.0 6

\ ÷

i

i

J

i

i

i

i

8

10

12

14

16

18

20

HYDROCARBON

CHAINLENGTH

Fig. 2. (A) T h e influence of the number of carbon atoms in the straight chain hydrocarbon pasting liquid o n the rate constant of F e ( C N ) 6 3 (1.0 X 10 - 3 M ) in 1.0 M KC1. (B) C h a i n length influence o n electron transfer for D O P A C ( 5 . 0 X 10 - 4 M ) in 1.0 M H C 1 0 4. T h e ( + ) represents data calculated from R D E , while the (O) represents cyclic voltammetry data.

both systems the rate constants decrease only slightly with increasing hydrocarbon chain length, until there is a pronounced drop between hexadecane and heneincosane, which are liquid and solid, respectively, at 25°C. The slowest charge-transfer rates for both systems are seen with Nujol. In neither system was there any particular trend in transfer coefficient with hydrocarbon chain length, from which it can be supposed that there is no overt change in electrode mechanism. The formal potential for the D O P A C system shifted towards negative potentials as k s decreased (with increasing hydrocarbon chain length). This is due to the marked asymmetry of the anodic and cathodic i - E curves and was evident in the corresponding Tafel plots. The limiting currents in the R D E curves were fairly constant for all pastes made from dodecane or longer chain length hydrocarbons, but increased somewhat with compounds below dodecane.

95

Chemical oxidation of graphites Samples of dry GP-38 and U C P (3-5 g) were oxidized at ambient temperature for 2 - 2 4 h with strong oxidants such as 30% hydrogen peroxide, dichromate and Ce(IV) in 1 M perchloric acid, and persulfate in 1 M sulfuric acid (Ag ÷ catalyst). The persulfate oxidation was carried out at 80°C and the initial 60 g of persulfate (10 g graphite) was supplemented by several 5 g additions over a period of 20 h. All oxidized graphites were thoroughly washed until the rinse p H was neutral, then dried. Pastes prepared from these oxidized graphites showed no large cathodic currents at potentials of 0.0 to - 0 . 2 V, which would have indicated the presence of unremoved oxidant. Pastes were made from the oxidized graphites using Nujol (2 : 1). Only fresh electrode surfaces were used for cyclic voltammetry to evaluate the effect on AEp. A n increase in k s with oxidized graphites (for both the ferrocyanide and D O P A C systems) is reflected in a considerable decrease of AEp. F o r the ferrocyanide system in 1 M KC1, k s values tend to increase roughly in order to the strength of the oxidant measured b y the approximate formal potentials, as seen in Table 2. (Exact correlation is not to be expected since the formal potentials are only estimated and no control of redox ratio, etc. was attempted in these practical batch oxidations.) All graphites and carbons are known to have oxygen-containing functional groups on their surfaces. The exact composition of the mixture of carbonyl, quinoid, carboxylate and carbon and oxygen radical species on the surface is a complicated function of the manufacturing and pretreatment processes [7,8]. Most of these surface functionalities tend to increase the hydrophilic nature of the surface. It is interesting to note that, as received, GP-38 readily mixes and wets with water, whereas U C P will not wet but floats on the surface of aqueous solutions. F r o m this it can be suggested that U C P contains fewer inherent oxygenated surface groups. This suggestion is further supported by the typically lower k s values seen for pastes m a d e with U C P c o m p a r e d to GP-38 pastes. Consequently, one would expect that chemical oxidation of U C P should produce a greater shift in k s values than its

TABLE 2 Changes in electron-transfer rate for ferrocyanide oxidation with chemical oxidation of GP-38 graphite Oxidant

Oxidant formal potential ~/V (vs. H2)

AEp h/mV

None 30% H 2 0 2 Cr(Vl) in 1 M HC104 Ce(IV) in 1 M HCIO4 $202- (Ag+ )

+0.88 + 1.33 + 1.70 + 2.65

234 139 151 128 69

ks/cm s l 2.9X 10-4 1 . 0 x 10 - 3 8 . 6 x 10 - 4 - -

1.2x 10-3 1 X 10- 2

a Approximate value estimated from "standard'potentials. I, From cyclic voltammetry of l0 -3 M ferrocyanide in 1 M KC1, fresh GP-38 :Nujol surface each run.

96 c o u n t e r p a r t GP-38. This was i n d e e d f o u n d to b e the case; AEp shifts after o x i d a t i o n were greater for U C P t h a n for GP-38.

The effect of electrochemical pretreatments on electron-transfer rates The influence of reversing potential in cyclic voltammetry E a r l y in these studies it b e c a m e a p p a r e n t that the reversing p o t e n t i a l of cyclic v o l t a m m o g r a m s h a d an effect on k s (evaluated from the p e a k separation). T y p i c a l results are seen in T a b l e 3 for GP-38 pastes with three different p a s t i n g liquids. T h e o x i d a t i o n of f e r r o c y a n i d e in a s u p p o r t i n g electrolyte of 0.9 M KC1 a n d 0.1 M HC1 was used in the test system. T h e same electrode surface was used for each series. It is clear that as the reversing p o t e n t i a l is allowed to b e c o m e increasingly m o r e positive, small b u t real increases in k s are observed. Qualitatively similar effects were seen with b o t h types of g r a p h i t e using different p a s t i n g liquids a n d c o m p o s i t i o n ratios. These results can only b e used in a qualitative sense, since the surface carried the h i s t o r y of previous scans. Nonetheless, they are i m p o r t a n t from the s t a n d p o i n t that m a n y electrochemical studies are carried o u t u n d e r similar c o n d i t i o n s where the s a m e surface is used for e x t e n d e d p e r i o d s of time. It is clear that for the m o s t r e p r o d u c i b l e results with g r a p h i t e electrodes one should use a new surface for each scan, a n d the scanning direction should be reversed or the scan e n d e d at the same p o t e n t i a l each time.

The effect of constant potential anodic pretreatments F o r a m o r e q u a n t i t a t i v e evaluation, fresh graphic p a s t e surfaces were held at d e s i g n a t e d p o t e n t i a l s for 1 min a n d then the cyclic v o l t a m m o g r a m was i n i t i a t e d in the oxidative direction. Typical results are seen in Fig. 3, where AEp is p l o t t e d vs.

TABLE 3 Influence of reversing potential on k s of ferrocyanide system

Reversing

103 ks/cm s- :

potential/V vs. SCE

Paste composition a

+0.50 +0.60 +0.70 +0.80 +0.90 +1.00 +1.10 +1.20

Hexadecane

Octadecane

Nujol

3.5 3.3 3.3 3.5 4.9 4.4 5.2 -

2.1 2.3 2.4 2.7 2.6 2.7 2.7 -

0.55 0.60 0.61 0.63 0.78 0.95 1.2 1.4

"Composition ratio of all pastes GP-38 : liquid, 2 : 1 scan rate 10 mV s- 1.

97

700: - --

600

\ x\ \

\

500

~\

\\

\x

4oq m 300-

+

0

"~

\

\

+\

"0~

\ ~-+~.

100

o' .7

o' 8

09

\

v*'~'~.q)

200

0.6

\

1'

PRETREATMENT

' 11

' 1.2 POTENTIAL

1.'3 /

\

\... "0. -~

+

+

' 1.4

i 15

\ \

x

"" "0~

' 16

' 17

V vs. SCE

Fig. 3. The change in AEp of cyclic voltammograms of 10 4M DOPAC in 0.1 M phosphate buffer, pH 7.4, as a function of electrode pretreatment for 1 rain at the indicated potential. ( × ) Electrode surfaces of G P - 3 8 / N u j o l ; ( ~ ) GP-38/hexadecane; ( + ) represents G P - 3 8 / d e c a n e . All pastes were 2 : 1 GP-38 : liquid.

the 1 min pretreatment potential. The most dramatic changes in AEp are observed with the DOPAC system in pH 7.4 buffer. As Fig. 3 shows, pretreatment potentials up to about + 1.1 V produce only slight decreases in AEp. At higher potentials, however, a sharp drop in AEp is observed for three different GP-38 pastes. Equivalent results were obtained for GP-38 and UCP pastes with various liquids and DOPAC solutions at other pHs. The ferrocyanide oxidation is more Nernstian even without pretreatment, but shows similar effects. Since fairly rapid electron-transfer rates are obtained on dry graphites, it was considered that perhaps the anodic pretreatment simply rejected organic molecules from the surface and these were replaced with water, i.e. the surface after pretreatment acted more like a dry graphite surface immersed in aqueous media. If so, enhancement should also be seen with increasing negative potential pretreatments, since these also could cause desorption of organic surface layers and replacement by water. To check this possibility, DOPAC in 1 M HC104 was studied at GP-38/Nujol pastes. Each new surface was pretreated for 1 min at potentials from - 0 . 1 0 to - 0 . 8 0 V vs. SCE. Only very slight decreases in AEp were seen with ferrocyanide in 1 M KC1 using pretreatment potentials up to - 0.8 V. Thus, it can be concluded that mere potential-dependent displacement of organic surface layers by the aqueous medium is not the major contribution to the marked changes in electron-transfer rates (activation) seen with anodic pretreatment. Only in the potential range > + 1.2 V does the pretreatment cause major changes in k s. Obviously, in this potential region, background electrolysis is occurring with evolution of oxygen, etc.

98

from solutions of most supporting electrolytes. Hence, it is reasonable that the important contribution to activation is caused by oxidative changes in the graphite surface. As discussed later, this electrochemical oxidation presumably produces a more hydrophilic surface state which can promote organic layer displacement by water. This argument is borne out by studies of the chemical oxidation of graphites mentioned previously.

Influence of controlled charge at fixed potentials Here, RDE studies were used to decide whether the activation was due primarily to the magnitude of the pretreatment potential or the amount of anodic charge (i x t) passed at the potential. It was reasoned that the fraction resulting in surface state oxidation might be related to the degree of activation and hence the change in k s•

-22 ~ -3.0 z 0 k) w-35

x" +

1///0+ /+

x /

(D - 4 . O

S -4.5 0

0:2

I 04

0:6

0:8

i 1.0

1:2

4

1:2

x.

7F -2.C z 8 -2.5

x x

/ /,

/

/

Xx~/,~

. . . . x

~

/ / w / /I../. I +/~ -3.0 1+ ,+/~0 + (D £ / A -35 0 0'.2 0'4

/" ~

~

/+/+ ~/

B O16

CHARGE DENSITY

0'.8 / Ccm -2

Fig. 4. (A) The change in rate constant of DOPAC (5.0 x 10 4 M ) oxidation in 0.1 M phosphate buffer, pH 7.4, as a function of charge applied at various potentials per unit area. ( ~ ) Activation at + 1.5 V; ( + ) at + 1.55 V; ( X ) at + 1.6 V; (O) at + 1.65 V, all vs. SCE. (B) The change in rate constant of Fe(CN)6 3 (10 -3 M ) oxidation in 0.5 M Na2SO 4 as a function of charge applied. ( ~ ) Activation at + 1.4 V; ( + ) at + 1.45 V; ( x ) at 1.5 V, (O) at + 1.55 V, all vs. SCE. See text for further explanation.

99

With an RDE operating at constant 30 rps, approximately fixed current could be passed at each of the values of pretreatment potential for periods between 15 s and 15-20 min. Each experiment was begun with a fresh surface. The charge consumed by the oxidation of ferrocyanide or DOPAC could be calculated from the limiting current of an RDE potential scan taken immediately after the activation period. (The latter also provided the data for k s measurements.) The total charge passed, less that used for ferrocyanide or DOPAC oxidation, represents charge which was consumed by electrolysis of background components and surface state oxidation. Solutions of 5.0 x 10 -4 M DOPAC in 0.1 M phosphate buffer and 1.0 x 10 -3 M ferrocyanide in 0.5 M Na2SO 4 were examined. The latter supporting electrolyte was used to avoid chloride ion oxidation which could abnormally interact with the graphite. The RDE was packed with GP-38/hexadecane paste (2:1) for these studies. The dependence of log k s vs. corrected charge for DOPAC oxidation is seen in Fig. 4A. It follows from these results that for this buffer medium and redox system the electrode activation is essentially independent of the pretreatment potential over the range + 1.50 to + 1.65 V vs. SCE and is primarily dependent on the total charge passed. The plateau level in Fig. 4A shows that k s is independent of charge beyond about 0.5 C cm -2. The corresponding data for ferrocyanide are shown in Fig. 4B. Here the interpretation is not so clear. Again the value of k s levels off with charge greater than about 0.5 C cm -~, but this limit also appears to have some potential dependence.

Experimental verification of hydrocarbon loss from pastes during pretreatment An attempt was made to provide direct chemical evidence that hydrocarbons were lost from the electrode surfaces during anodic pretreatments. For this purpose a hexadecane paste (2:1) electrode (0.345 cm 2 area) was inverted and 100 ffl of phosphate buffer, pH 7.4, was placed over the electrode. Reference and auxiliary electrodes were positioned in the drop and potentials were applied for periods of up to 3 rain. At the end of this period the drop was pipetted off into a small conical tube, extracted with 500 ~tl of hexane containing tetradecane and octadecane as internal standards, and the hexane extract separated off into a sealed vial. The TABLE 4 G C - M S analysis of pretreated electrode extracts Conditions"

Hexadecane f o u n d / n g

No applied potential Persulfate-oxidized b + 1.0 V + 1.6 V

17 20 125 570

a See test for details of electrolysis conditions. b Hexadecane paste prepared from pre-oxidized GP-38 as described earlier; no applied potential.

100 pipette tip was carefully rinsed with the hexane solution as part of the extraction. This sample was submitted to gas chromatography-mass spectrometry analysis for determination of hexadecane. Blank determinations were carried out under identical conditions for 3 min with no applied potential. The results of four sets of such analyses are given in Table 4. Although the quantitative aspects of the G C - M S analysis are good, as shown by measurements of standards, we attach no particular significance to the absolute amounts of hexadecane found in Table 4. However, the trend is highly significant. There is a clear release of hexadecane from the electrode surface with the anodic pretreatment at + 1.0 V and a further increase at the more significant activation potential of + 1.6 V. Although chemically oxidized graphite appeared to be somewhat activated in terms of k s measurements, the hexadecane release (with no applied potential) cannot be distinguished from the control. Nevertheless, we believe that these data confirm the suggestion made, namely, that anodic pretreatment makes the graphite surface more hydrophilic and removes some of the adherent organic layers. DISCUSSION AND SUMMARY The present studies provide a new, more quantitative framework for understanding factors which affect the electron-transfer characteristics of graphite paste electrodes. First, it is clear that "dry'graphite, although its high residual currents may render it undesirable for voltammetric work, gives very rapid electron-transfer rates with k~ values approaching those obtained on platinum. Mixing the graphite with any pasting liquid lowers k~ and, in a series of linear chain hydrocarbons, k s decreases with increasing chain length. Either chemical or electrochemical oxidative pretreatments (activation) of these pastes causes the properties to approach the " d r y " situation, i.e. k s increases. Such behavior could be explained by either (1) the oxidative formation of functional groups on the graphite surface which participate directly in the electrode reaction (electrocatalysis) a n d / o r (2) the formation of surface groups which modify the interracial properties of the electrode but do not participate directly in the electrode reaction. The possibility of forming catalytic quinoid surface states by electrochemical and chemical pretreatment on carbons and graphites has often been suggested. Evans and Kuwana have indicated from surface spectroscopy and electrochemistry that rf plasma oxygen treatment of pyrolytic graphite may yield quinoid functionalities [9]. With graphite paste activation pretreatments we find little evidence for such surface states. Following pretreatment of a hexadecane paste electrode at + 1.65 V for several minutes, an immediate cathodic potential sweep shows a small, totally irreversible peak at about + 0.65 V. This peak has no corresponding anodic counterpart and is itself completely missing on the second or any subsequent sweeps. Thus, it shows few of the attributes of a bound quinone functionality, but rather appears to be an easily removed adsorbed component. In addition, a prolonged cathodic treatment at - 1.5 V following activation at + 1.65 V does not change the state of the electrode (i.e. k~ values for ferrocyanide or D O P A C are still at the elevated level

101 attained by the anodic treatment alone). Thus, there appear to be few, if any, electro-reducible surface groups produced in the activation which are responsible for improving the electron-transfer rates in these situations. The second explanation, on the other hand, is well represented by the experimental findings in this study. As mentioned earlier, of all compositions examined, a " d r y " graphite surface gives the highest attainable k s values. The introduction of any hydrocarbon pasting liquid, which is presumably adsorbed onto the graphite and acts as an inhibitor to electron transfer, decreases k s, often by several orders of magnitude. A freshly prepared paste electrode loses some of this inhibition as soon as it is immersed in an aqueous solution owing to the slight solubility of the hydrocarbon. This dissolution is not necessarily trivial since the solubility, especially of the smaller hydrocarbons, may be considerable (e.g. hexane solubility in 0.1 M aqueous salt solutions is ca. 10 -4 M). It follows, of course, that the longer the hydrocarbon chain, the less will be such adventitious removal of inhibition. However, this solubility factor is of limited effectiveness since even in the case of appreciable solubility at least one monolayer probably remains on the graphite. With the longer hydrocarbons the solubility effect is practically inoperative. Indeed, hydrocarbons larger than dodecane exhibit such low solubility that their increasing hydrophobicity can be used to explain the trend of decreasing k s with increasing chain length. However, in addition to passive dissolution, activation by high anodic potential pretreatment can be quite effective in removing the organic inhibitory layer since oxygen-containing surface states are formed. These render the surface much more hydrophilic and aid in removal of the hydrocarbon and its replacement by aqueous layers. The electron-transfer characteristics move toward those exhibited by "dry" graphite immersed in aqueous solutions. The process probably never completely removes all hydrocarbon, especially that retained in micro holes and cracks in the graphite. However, since the current density in such small pockets is fairly low, these remaining hydrocarbon molecules represent a minimal inhibition to the net electron-transfer rate at the overall surface. The above hypothesis is strongly supported by the G C - M S data showing release of surface hydrocarbons into the aqueous media with anodic pretreatment. It is interesting to note that untreated graphite paste electrodes have extremely low capacitance currents, but the activation pretreatment markedly raises the capacitance--however, again only toward a limiting value of the "dry" electrode in aqueous media. Although also consistent with simple surface area increases, these capacitance current changes can be interpreted as replacement of organic surface layer with aqueous media via generation of hydrophilic oxygen functionalities. In general, it seems that this activation process and its effect on k s is not so much dependent on potential as it is on the total amount of oxygenated surface states formed, i.e. on the total anodic charge passed once a sufficient potential has been reached to provide the surface oxidation. This point is equivocal and may vary with the specific redox system and supporting electrolyte, as is evident here with the somewhat potential-dependent activation of ferrocyanide oxidations.

102 ACKNOWLEDGMENTS T h e support of this work by the N a t i o n a l Science F o u n d a t i o n ( N e u r o b i o l o g y Division) via G r a n t BNS-7914226 a n d T h e N a t i o n a l Institutes of H e a l t h via G r a n t N S 16364 is gratefully acknowledged. T h e authors w o u l d like to a c k n o w l e d g e the s u p p o r t p r o v i d e d by the C e n t e r for Biomedical Research, T h e U n i v e r s i t y of Kansas. REFERENCES 1 2 3 4 5 6 7 8

P.T. Kissinger, C. Refshauge, R. Dreiling and R.N. Adams, Anal. Lett., 6 (1973) 465. P.T. Kissinger, C.S. Bruntlett and R.E. Shoup, Life Sci., 28 (1981) 455. J.C. Conti, E. Strope, R.N. Adams and C.A. Marsden, Life Sci., 23 (1978) 2705. A.W. Sternson, R. McCreery, B. Feinberg and R.N. Adams, J. Electroanal. Chem., 46 (1973) 313. R.S. Nicholson, Anal. Chem., 37 (1965) 1351. Z. Galus, Fundamentals of Electrochemical Analysis, Ellis Horwood, Chichester, Sussex, 1976, p. 249. J.S. Mattson and H.B. Mark, Activated Carbon, Marcel Dekker, New York, 1971. J.P. Randin in A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, Vol. 7, Marcel Dekker, New York, 1976, p. 22. 9 J.F. Evans and T. Kuwana, Anal. Chem., 49 (1977) 1632.