Voltammetric studies on carbon paste electrodes

J Electroanal Chem, 176 (1984) 169-182

169

Elsevier Sequoia S A , Lausanne - Printed m The Netherlands

VOLTAMMETRIC STUDIES ON CARBON PASTE ELECTRODES THE INFLUENCE OF PASTE COMPOSITION ON ELECTRODE CAPACITY AND KINETICS

CSABA URBANICZKY and KENT LUNDSTROM

Department of Analytical Chemtst~, Unwersay of Uppsala, P 0 Box 531, S-751 21 Uppsala (Sweden) (Received 7th March 1984, in revised form 6th April 1984)

ABSTRACT The relative importance of the carbon and the pasting hqutd on the properties of the carbon paste electrode was studied using cyclic potential sweep voltammetry. The effect on electrode capacity and kinetics for the systems hexacyanoferrate (I1) and hydroqumone m phosphate buffer of pH 6 8 was compared. Different carbon powders were used and it ~s shown that the carbon and ItS treatment have great influence on the electrode properties Hydrogen reduction of the carbon at 1000°C prior to paste preparation resulted in a decrease of the capacity from, for example, 290 to 29 mF m -2 The reaction rate of the hexacyanoferrate (II) system increased by this procedure while that of the hydroqumone system decreased Thermal oxidation of the previously reduced carbon resulted in paste properties between the unreduced and the reduced carbon paste Different pasting hqmds were compared and the composition was varied Their influence on the capacity and the reaction rates were stu&ed. INTRODUCTION

Carbon paste electrodes (CPE's) for voltammetry consist of a carbon powder combined with a pasting liquid. The CPE is characterized by a decreased residual current and a better reproducibility of currents compared to the pure carbon material and thus improved detection limits and reproducibility of analysis is obtained [1]. The properties of the CPE depend on the specific components employed, the manner of preparation and maintenance. Different types of carbon powder were screened by Olson and Adams [2] and a multicrystalline graphite (Acheson 38) was found to be preferable. Rice et al. [3] compared two carbon powders, GP-38M and UCP-1-M. Without a matrix, both carbon powders were found to gwe high reaction rates for the oxidation of hexacyanoferrate (II) and 3,4-dlhydroxyphenylacetic acid. Further, oxidative pretreatment of the carbon using chemical oxidants prior to paste preparation increased these rates. Other carbon powders have also been used for CPE's (e.g. refs. 4 and 5), as well as for other carbon composite electrodes (e.g. ref. 6). Olson and Adams [2] showed that the peak currents from the oxidation of hexacyanoferrate (II) and 3,Y-dimethoxybenzidine increased as the ratio of carbon 0022-0728/84/$03.00

© 1984 Elsevier Sequoia S.A.

170

to bromonaphthalene or Nujol increased. Similar findings were reported later by Neeb et al. [4] in their study with sihcone oil, bromonaphthalene and Nujol as pasting hquids. Lindqulst concluded in his investigation [5] that for an optimal performance different pasting liquids have to be used, depending on the reactions studied. Rice et al. studied the influence of the shorter carbon-cham length of aliphatic hydrocarbons as pasting liquids on the rate of the electrode reactions [3]. A shorter chain length diminishes the reaction rates the least. Hexadecane was found to be a better pasting liquid than Nujol. The main shortcomings of the CPE are the solubility of the pasting hquid in organic solvents and the fragile surface. Therefore, several carbon composite electrodes, based on carbon and a solid matrix, have been evaluated. Due to its insulating efficiency, the presence of a solid matrix decreases electrode reaction rates more than a pasting liquid does, unless an active carbon surface is exposed [7]. Carbon composite electrodes with ceresin wax [8,9], Kel-F [7,10], polyethylene [11,12] or polypropylene [13] have been developed. Various other types of msulator matrices have been used as well, e.g. [14-16]. The aim of this study was to characterize the CPE further with respect to the carbon powder and the pasting liquid. Combinations of five different types of carbon powders, as received as well as treated tn hydrogen atmosphere at 1000 o C, and three pasting hquids, hexadecane (HD) [3], silicone fluid MS 200 and fluorochemical inert liquid (FI1) [5], were compared. The effects on capacity and potential limits were studied using cyclic potential sweep voltammetry in phosphate buffer of pH 6.8. The influence on the electrochenucal oxadation rates of hexacyanoferrate (II) and hydroquinone were compared. While the results to be reported concern the CPE specifically, the results of the comparison of different carbon qualitxes has significance also in carbon composite electrode methodology. EXPERIMENTAL

Apparatus As before [17], the same computenzed voltammetric instrumentation [18,19] was used. The linear potential sweep is, in fact, a staircase ramp with 1 mV resolution and the voltammograms were smoothed. The filter time constant [20] was chosen so that the time taken for two steps in the potential ramp was greater than or equalled one time constant. The voltammograms were recorded on a Hewlett Packard 9044A X - Y recorder.

Cell and electrodes The cell was a 20 ml Metrohm measuring cell. All potentials are quoted vs. the reference electrode used, a SCE and a salt bridge filled with the same solution as the cell. A Pt wire served as the auxiliary electrode. The jacketed cell was thermostated at 25 + 0.5 o C with a water bath.

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The pastes. The carbons used were Merck, regular and No. 4206, B.D.H. synthetic graphite, Rlngsdorff-Werke RW-A and Ultra Carbon UCP-1-M. In one of the pastes, a-aluminium oxide, 1 #m (B.D.H.), was added. The carbons were reduced in the tube furnace set-up described previously [21]. About 10 g of carbon powder was placed in a quartz tube. The tube was heated to 1000 °C for at least 12 h. Hydrogen was always passed through the tube, even during the cooling-down period. Oxidation of previously reduced carbon was carried out m the same furnace by passing air by means of a pump through the heated tube. The following compounds were used as pasting liquids: hexadecane (purum, Fluka); silicone fluid MS 200, 5 cSt (Kebo Grave, Sweden); fluorochemical inert liquid, FC-43 (B.D.H.). Other compounds briefly used as pasting liquids or insulator matrices were from different suppliers and used as received. All paste compositions are expressed as the weight/weight ratio of carbon/pasting liquid. The pastes were prepared by directly mixing the carbon and the pasting liquid. The electrode holder described by Lindquist was used [22] and the electrode area was 28 mm 2. The electrodes with insulator matrices were made by dissolving the insulator in hexane and adding the carbon powder. The hexane was evaporated during mixing. Then the carbon/wax mixture was gently melted and packed into the preheated electrode holder. Reagents All reagents were of p.a. grade, except for sulphunc acid, wluch was of B.D.H. "Aristar" grade. Distilled water was purified further with a Millipore Milh-Q filtration system. The solutions, 0.05 M sulphuric acid and potassium phosphate buffer of pH 6.8, were prepared the same day as they were used. The total concentration of the anion was 0.2 M in the buffer. Stock solutions of 100 m M hexacyanoferrate (II) and hydroquinone were prepared weekly. The concentration of hexacyanoferrate (II) and hydroquinone in the cell was ca. 5.0 × ] 0 - 4 M.

Electrochemical measurements As previously [17], the capacity, C, was calculated as:

C= I / A v

(1)

where I is the current, A is the electrode area and v is the sweep rate. Unless stated otherwise, the capacity was calculated from the mean current at + 250 mV from a steady-state cyclic potential sweep voltammogram obtained with a sweep rate of 100 mV s-a in the range from - 2 5 0 to + 750 mV. The tabulated positive potential limits were found by initiating cyclic potential sweeps at a sweep rate of 100 mV s -1 from +750 mV towards more positive potentials. The electrode was allowed to reach steady-state behavlour before the potential where the current amounted to 50 mA m-2 was ascertained.

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The voltammograms in the figures and the tabulated values are from measurements made in the phosphate buffer. When the faradaic processes were studied, a new electrode surface was prepared before each scan. The reversal scan was started after a delay time of 3 s at the reversal potential. The method of Nicholson [23] was used to calculate the rate constant, k o, from the voltammograms for the hexacyanoferrate (II) system. The transfer coefficient was assumed to be 0.5 and diffusion coefficients in 0.1 M KC1 were used [1]. Five sweep rates, in the range 10 to 200 mV s-1, were used in the calculations of k o except when the peak separation was too large or too small. The k o interval obtained is reported. The potential difference between the current peaks for the hydroquinone system was large, thus N~cholson's method could not be used for calculation of k o. Hence the peak separations obtained with a sweep rate of 100 mV s-1 are reported instead. If it is assumed that the reproduciblhty of the residual current is proportional to the absolute residual current level, then the quotient Ip/Irc (where the residual current from the first scan, It, is measured under the peak current, Ip) mdicates how usable a paste is for analytical purposes. Therefore it was determined. A higher value of the quotient means that the detection limit is lower. The sweep rate was 100 mV s-~ in these experiments. RESULTS

Comparison of the carbon powders The pasting hquid was hexadecane in these experiments to facilitate comparison with the results of Rice et al. [3]. Pastes made from the different carbon powders have very varying capacxties (Table 1); the RW-A carbon paste has the lowest capacity and the two Merck carbon pastes have capacities that are an order of magnitude higher (Fig. 1). The reaction rates are also very different (Table 1, Figs. 2 and 3). Pastes with higher capacities have faster kinetics. TABLE 1 C o m p a r i s o n o f d i f f e r e n t c a r b o n p o w d e r s as h e x a d e c a n e p a s t e s ( 2 / 1 ) . E x p e r i m e n t a l c o n d m o n s e x p l a i n e d in t h e text Carbon

C~ mF m -2

Regular 4206 B.D H RW-A UCP-1-M

290 300 54 14 20

H e x a c y a n o f e r r a t e (II)

Hydroqulnone

Positive h r m t / V

k°/

(Ip/IrC)/

AEp/

(Ip/Irc) /

~ln s -1

mM -1

mV

mM

18-22 26-34 < 4 << 4 << 4

66 81 250 370 390

135 135 300 520 430

180 140 1000 1900 1700

1 1 1 1 1 1

0 0 1 1 1

are

173

o/

11pA

b]

~lOOnA

-2'oo

/

280 ' 400 Potent)a[IrnVvs.SCE

f ' 600

Fig 1 Steady-state cychc potential sweep voltammograms of hexadecane pastes made of (a) Merck regular and (b) Rlngsdorff-Werke RW-A carbon powder The medium was phosphate buffer of pH 6 8 Carbon/hexadecane paste composmon 2/1 and sweep rate 100 mV s - 1, ( + ) 0 V and 0 A

30 1

20

10

t0 -113

-2( [ -200

I 0

~ 2 0

I 400

I 600

Potentml/mV vs.SCE

Fig. 2. Cychc potential sweep voltammograms of 5.0 × 10-4 M hexacyanoferrate (II) with a sweep rate of 100 mV s -1 and carbon paste electrodes (1) Merck regular and (2) Rangsdorff-Werke RW-A carbon/hexadecane (2/1) paste.

174

1

100

50

30

f/

-5C

-10C I

-200

~

2~o

~b0

600

Potent~l/mV vs.SCE

Fig. 3 Cyclic potential sweep voltammograms of 5 0 × 10 -4 M hydroqumone with a sweep rate of 100 mV s - I and carbon paste electrodes (1) Merck regular and (2) Rangsdorff-Werke RW-A carbon-hexadecane (2/1) paste

<1 c

®0 -1 -2

I

I

-200

0

I

I

200 400 Potential/mY vs. SCE

I

I

I

600

800

tO00

Fig. 4 Steady-state cychc potential sweep voltammograms with different sweep amplitudes obtained with Merck regular carbon/hexadecane (2/1) paste. Sweep rate 100 mV s-1

175

Although the reaction rates for pastes with lower capacities are smaller, the Ip/IrC quotients (Table 1) are more favourable. This is advantageous for analytical use, especially in electrochemical detectors. The capacity of the CPE was sweep-amplitude-dependent (Fig. 4) m the same way as, for example, the pyrolyuc carbon film electrode [17]. Similarly, only minor differences in capacity were found between the phosphate and sulphuric acid media. For all carbon powders, the positive potential limit is about the same (Table 1). When the negative potential range was investigated, it was found that if a certain potential limit was passed then the background current increased for each additional scan. Thus, for trace analysis, the practical negative potentml limit in phosphate buffer is ca. - 300 mV for all pastes and in sulphuric acid, the limit is ca. - 500 mV. Deaeratmn did not affect the negative potential limit. In all cases, the reaction rates were faster in sulphurlc acid than in phosphate buffer. The pH dependence of the hexacyanoferrate (II) oxidation at the CPE [24] as well as at the pyrolytic carbon electrode [25] has been reported. The oxidation process of hydroquinone is much faster than the reduction in sulphurlc acid, while the current peaks are more symmetrical m phosphate buffer. The relative order of the rate constants for the systems at the different pastes was independent of the pH.

Heat treatment of the carbons In order to study the influence of carbon surface oxygen functional groups on the paste properties, batches of carbon powders were thermally reduced and oxidized. Reductton. First, two batches of carbon were heated at 1000°C overnight. Nitrogen was passed through one of these, while hydrogen was passed through the other. Pastes made from these carbon powders, compared with untreated carbon, had lower residual currents and hydrogen heat-treated carbon paste had the lowest. Thus, hydrogen reduction was used in the subsequent work. It is seen form Tables 1 and 2 that the capacity dimmishes most in those cases where it is high prior to the reduction. For instance, at + 250 mV, the capacity is lowered from 290 to 29 mF m-2. This is of the same order as that reported for the TABLE 2 Comparison of different hydrogen-reduced carbon powders as hexadecane pastes ( 2 / 1 ) Experimental condltmns are explained m the text Carbon

Regular 4206 B D H. RW-A UCP-1-M

C/

Hexacyanoferrate (II)

Hydroqumone

mF m -2

kO/

(Ip/Irc)/

AEp/

(Ip/lrc)/

/Lm s - l

m M -a

mV

m M -1

6 6 - 88 100-120 2 2 - 30 < 4 1 4 - 17

730 620 1100 610 410

320 300 470 450 450

810 720 1900 3400 1300

29 32 21 12 21

Positive h m a t / V

12 1.3 13 12 1.1

176

pyrolytic carbon film electrode [17] but tt is sttll higher than that for untreated RW-A and UCP-1-M (Table 1). Figure 5 shows the sweep-amplitude dependence for the reduced carbon powder whde Fig. 6 shows the sweep-rate dependence for the capacity.

40C

: 20C

8

-200 b -2 0

I 0

I I 200 /.00 Potenhol/mV vs.SCE

i 600

I 800

I ! 1000

Fig 5 Steady-state cyclic potential sweep voltammograms with different sweep amphtudes obtained with hydrogen-reduced Merck regular carbon/hexadecane ( 2 / l ) paste. Sweep rate 100 mV s -1

l

150

2

10C

tL E

3 4

g so

i

I

-200

0

I

!

200 400 Potenttat/mY vs.SCE

i

i

600

800

Fig. 6 Capacity at different sweep rates from a cyclic potential sweep voltammogram. A steady-state voltammogram from - 2 0 0 to + 800 mV was used in the calculations with sweep rates (1) 20, (2) 40, (3) 100 and (4) 200 mV s - 1. The paste was hydrogen-reduced Merck regular carbon/hexadecane (2/1) paste

177

Due to the hydrogen reduction, the reaction rates for hexacyanoferrate (II) increased in all cases, as seen from voltammogram 1 m Fig. 2 and voltammogram 3 in Fig. 7. The same is true for the Ip/Irc quotient. The reaction rate for the hydroqulnone system decreased except m the case of the RW-A and UCP-1-M carbon pastes, which, however, were very slow even prior to the reduction. Despite this, the lp/Irc quotient increased since the residual current decreased due to reduction of the carbon. The voltammograms could also be reproduced with a paste made of 6-month-old hydrogen-reduced carbon as well as with a 6-month-old paste. In the following work, the Merck regular carbon powder was used. OxMatton. Two batches of previously reduced carbon were oxidized at 500 and 600°C, respectively, for 12 h. The results summarized in Table 3 show that the capacity increases when carbon is oxidized. Also, the reaction rates of hexacyanoferrate diminished to the value prior to the reduction, while the rate of hydroqulnone only slightly increased. The carbon treated at 600 o C was less influenced than the carbon oxidized at 500 o C.

Selection of pasting hqmd Type of pasting hqutd. FIL, MS 200 and HD pastes were screened. A carbon/pasting liquid ratio of 2/1 was chosen for the MS 200 and HD pastes, while

1 2

/.0

20 <

® 0

-20

-/,0 I

-2oo

I

o

I

2o0

~6o

6bo

Potential/mY vs.SCE

Fig. 7 Cychc potential sweep voltammograms of 5.0 × 10 -4 M hexacyanoferrate (]]) with a sweep rate of 100 mV s- ] and carbon paste electrodes The pastes consisted of reduced Merck regular carbon and (1) FIL, (2) MS 200, respectively; (3) HD as the pasting hqmd

178

the FIL paste was 1/1 (otherwise the paste was too dry). The results are summarized in Table 4 and some voltammograms are shown in Fig. 7. The FIL paste impairs the reaction rates the least but the capacity is high and the potential range is small. Hence, the FIL paste Is preferred only for applications where the reaction rate has to be high. The HD paste has the lowest capacity and the highest Ip/Irc quotient. Although its reaction rates are the slowest of these three pasting liquids, the HD paste has the best all-round properties (see below). More volatile liquids like MS 200 (1 cSt) and octane were screened in a pre-test but they had no major advantages. Further, they were less convenient to use since they had to be prepared just prior to the experiments. Relatwe composmon. In a subsequent test, the ratio carbon/pasting liquid was varied. A carbon/MS 200 paste of ratio 3 / 2 is unusable in practical analysis since layers of carbon paste fell off when the electrode was removed from the solution after a scan. The same is true for a paste with a carbon/FIL ratio of 1/2. Thus only results obtained with varying HD paste compositions are presented. From Table 5 it is seen that with increased dryness, the capacity and the reaction rates increase. These results are in accordance with those reported by Rice et al. [3] and Neeb et al. [4]. The Ip/Irc quotients are highest about a composition of 2/1.

TABLE 3

Comparison of reduced and OXl&zed carbons as hexadecane pastes (2/1) The carbon powder was Merck regular. The other experimental con&t]ons are explained m the text

C/

Treatment

Hexacyanoferrate(II)

mF rn -2

Untreated Reduced at1000 ° C Oxl&zed at500 ° C Oxxdlzed at 600 ° C

290 29 270 220

Hydroqumone

k o~

(Ip/lrc)/

AEp/

(Ip/lrC)/

/~ms -1

mM

mV

mM -]

18-22 66-88 18-20 17-25

66 730 82 72

135 320 265 290

180 810 200 110

]

Posmtwe h m l t / V

10 12 12 11

TABLE 4

Comparison of carbon pastes with different pasting hqutds The carbon/hqmd composition was 2 / 1 for the hexadecane and MS 200 pastes and 1/1 for the FIL paste. The carbon powder was hydrogen-reduced Merck regular Experimental condlnons are explained m the text Pasting liquid

Hexadecane MS 200

FIL

C/ mF m - z

29 71 250

Hexacyanoferrate(II)

Hydroqumone

k o~

(Ip/Irc)/

AEp/

(]p/Irc)/

tLms -1

mM -]

mV

m M -a

66-88 ~ 230 ~ 230

730 300 82

320 250 150

810 460 140

Positxve h m l t / V

12 09 07

179

Alumma catalysts It has been shown that alumina can catalyse the electrochemical oxidation of hydroquinone at a glassy carbon electrode [26]. Therefore, a paste consisting of

TABLE 5 Comparison of &fferent ¢arbon/hexadecane paste composmons The carbon powder was hydrogen-reduced Merck regular The other experimental conditions are described in the text Ratio

3/1 5/2 2/1 3/2 1/1

C/

Hexacyanoferrate(II)

Hydroqumone

mF m -2

k°/

(Ip/IrC)/

AEp/

(lp/lrC)/

g i n s -1

m M -1

mV

mM -I

150-230 90-160 66- 88 28- 34 10- 12

230 310 730 650 490

280 315 320 380 510

620 780 810 1200 1100

48 30 29 18 11

30

Posmve limit/ V

12 12 12 12 12

/1

20

/

10

/

¢

~o

-10

-20

I -200

I 0

~ I 2 0 400 Potentla[/mV vs.SCE

I 600

Fig 8 Cychc potential sweep voltammograms of 5 0 × 10 -4 M hexacyanoferrate (II) with a sweep rate of 100 mV s -1 and carbon paste electrodes The pastes consisted of (1) reduced UCP-1-M/A1203/HD (3/1/2), reduced UCP-1-M/HD (2/1) and (3) UCP-1-M/HD (2/1).

180

reduced UCP-1-M carbon/AL203/HD (3/1/2) was prepared. The capacaty mcreased from 21 to 39 mF m-2. The reaction rate for the hydroqumone system was not influenced while the rate for the hexacyanoferrate (II) system increased to 46/~m s -1 (Fig. 8).

Insulatmg matrix Several waxes have been screened as insulators for wax-ampregnated graphate electrodes [27]. Castor and ceresin waxes had the lowest residual currents relative to the faradalc currents. Both waxes were tested by preparing carbon/wax electrodes of composmon 10/7 and they had a wade potential range: for example, the hmits for the castor wax electrode were + 1.6 V and - 1 . 0 V in deaerated phosphate buffer. Further, the capacity was very low, < 10 mF m -2, provided there was no leakage between the carbon/wax and the electrode holder. Unfortunately, the kinetics were greatly ampaired for the test systems, makmg the electrodes unusable for most practical work. Nevertheless, for azide and aodide determinations, carbon/wax electrodes were very suitable. Lindqulst has shown that it is possible to determine several other compounds wath this electrode [9]. Other polymers such as Carbowax 20 M, silicone MS-4, silicone oil QF-1 and KEL-F grease were tested as insulating matrices. None of them had any advantages, neither in terms of lower capacity nor faster kinetics, compared wath a HD paste. However, the latter two can have advantages in orgamc solvents. DISCUSSION

The difference between the carbon powders, with respect to the capacity as well as the reaction rates, can be due to either a different level of oxygen functlonalitaes a n d / o r impurities. The latter can function either as inhabitors or accelerators of the electrode processes. Heat treatment with hydrogen is an effective way to remove both types of Interference from the carbon powder. The major fact explaining the low capacity of the pastes made of hydrogen-reduced carbon powder is that the number of oxygen-containing functional groups on the carbon surface is decreased. The present results and those presented for the pyrolytic carbon film electrode [17] verify that this is a substrate-mdependent reaction at carbons. Reduction of the carbon increases the oxidaUon rate of hexacyanoferrate (II) whale the oxidation rate of hydroquinone decreases. This implies, of course, that the rate-determining step is different in these two cases. These results also show that the electrode kinetics at carbon electrodes with low capacity can be fast since the reaction rate for hexacyanoferrate (II) increased when the capacity decreased. By re-oxidation of the carbon, the number of oxygen functionalities is increased. The original behavlour is almost restored. The conclusaon is that oxygen-containing funcUonal groups on the carbon surface impair the reaction rate for hexacyanofer-

181

rate (II) but mcrease somewhat the reaction rate for hydroqulnone. Usually, wet chemical agents were used to oxidize the carbon, e.g. [3], thereby probably leaving traces of oxidation agents, which can catalyse the electrode processes and hence increase the reaction rates. In order to try to increase the reaction rate of the hydroquinone system, an alumina-containing paste was prepared. However, only the reaction rate of the hexacyanoferrate (II) system increased. All attempts to increase the reaction rate of the hydroquinone system failed, except the case when hydrogen-reduced carbon was re-oxidized. CONCUSIONS

The important conclusion of this work 1s that the carbon, and its treatment, has great impact on the properties of the CPE. Further, the hydrogen-reduced carbon powder is stable with time. The choice of the pasting liquid is less important than the choice of the carbon. With hexadecane as pasting hquld, a convenient all-round paste with a low capacity and a wide potential range is obtained. The results for hexacyanoferrate (II) show that the reaction rates at a carbon electrode with a low capacity can be fast for certain types of electrode processes. In determinations at trace levels with voltammetric techniques, the very low capacity obtained by the hydrogen reduction of the carbon is advantageous since thereby the residual current is low. The hydrogen reduction of the carbon powder used for CPE's is a step towards development of a method for obtaining a standard state for carbon, for electrochemical measurements. Such a state is a prerequisite to obtain consistent results by different research groups. The results of carbon treatment are not restricted to CPE's but are valid for all kinds of carbon composite electrodes. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

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