Determination of Polynuclear Aromatic Hydrocarbons by Anodic Differential Pulse Voltammetry at the Glassy Carbon Electrode in Sulfolane and Acetonitrile as Solvents J. F. Coetzee,* G. H. Kazi, and J. C. Spurgeon Department of Chemistry, University of Pittsburgh, Pittsburgh, Pa. 15260
Anodic voltammetric characteristics of 11 polynuclear aromatic hydrocarbons, including benr[a]pyrene, have been measured at stationary and rotated platinum and glassy carbon disk electrodes in sulfolane containing 0.1 M tetrabutylammonium hexafluorophosphate. For three of the hydrocarbons, measurements were also made in acetonitrile containing sodium perchlorate. In the stationary mode, the platinurn electrode experienced severe surface contamlnatlon, and the glassy carbon electrode was much superlor. Relatlve peak potentials and peak widths of the hydrocarbons in sulfolane were slmilar to those in acetonitrile and offered no advantages to offset the disadvantage of the high viscosity of the solvent. The sensitivity of differential pulse voltammetry for the hydrocarbons studied is ca. 5 X M in sulfolane and 2 X 10-* M in acetonitrile.
This work was undertaken with two objectives. First, polynuclear aromatic hydrocarbons constitute a serious air pollution problem in many localities, and we wished to establish whether differential pulse voltammetry, which possesses high sensitivity and other potentially attractive features, would be competitive with existing methodology for the determination of these compounds. Second, sulfolane appeared to offer certain potential advantages for anodic voltammetry that merited further study. Particulate matter in urban atmospheres contains a benzene-soluble fraction which accounts for approximately 10% by weight of the particulate (1).Included in this fraction are the polynuclear aromatic hydrocarbons (PAH). The analysis of particulate samples for PAH is usually a 4-stage operation involving collection, separation of the organic fraction, isolation of the PAH, and their subsequent determination. Typically, the sample is collected with a "High-Volume" air sampler, the organic fraction is extracted with benzene in a Soxhlet apparatus, the PAH are isolated by column, paper, or thin-layer chromatography, and are then determined by ultraviolet spectrophotometry or fluorimetry. Most recently, high pressure liquid chromatography coupled with the use of fluorescence detection or of a variable wavelength micro ultraviolet detector was found to improve results (2). Other details have been summarized elsewhere (3,4). The principal objective of the present work was to establish whether differential pulse voltammetry offers any advantages over the existing methods in the final step of the analysis. We selected sulfolane for this purpose because its useful range extends t o very positive potentials and it is chemically much more inert than acetonitrile, which has been the most popular solvent for anodic voltammetry. For example, it is resistant to even the powerful oxidant nitronium ion ( 5 ) ,which raised the hope that the follow-up chemical reactions which complicate anodic oxidations in acetonitrile may be minimized in sulfolane. The properties of sulfolane have been summarized elsewhere (6). Its principal disadvantage for the present work is its high viscosity of 0.103 poise a t 30 O C , which is some 10 times t h a t 2170
of water and 30 times that of acetonitrile. However, this high viscosity would be an advantage for techniques such as quantitative chronopotentiometry in which it is desirable t o minimize natural convection so that linear diffusion conditions can be maintained over long periods of time. The anodic voltammetry of aromatic hydrocarbons has been studied in great detail in methylene chloride, nitrobenzene, and particularly acetonitrile. Apparent half-wave potentials obtained for the hydrocarbons of interest here a t platinum microelectrodes in acetonitrile as solvent are summarized in Table I. Perylene and 9,lO-diphenylanthracene are the only hydrocarbons listed that undergo reversible oxidation under conventional voltammetric conditions. It has been established by cyclic voltammetry in acetonitrile (11,13,14) and methylene chloride (15) and by rotated disk voltammetry in nitrobenzene (16) and several other solvents (17),and confirmed by E P R spectrometry (15,16), that many (perhaps all) hydrocarbons undergo an initial one-electron loss. The radicals produced, AP+, vary widely in stability for different aromatic hydrocarbons, Ar, depending on such factors as the degree of charge delocalization, the presence of electrondonating groups, and the presence of groups blocking reactive sites in Ar.+. For the majority of hydrocarbons listed in Table I, the radicals are relatively unstable and undergo ECE follow-up reactions (18) a t rates observable in fast-sweep voltammetry and a t rotated disk electrodes. The stability of Ar-+ is affected markedly by the solvent, and (where tested) follows the order nitrobenzene > methylene chloride > acetonitrile. Illuminating general discussions of the electrochemical oxidation of hydrocarbons have been given by Adams (18)and by Mann and Barnes (19). I t was not our purpose t o make a detailed study of the stabilities of hydrocarbon cation radicals in sulfolane. However, as a prelude to the pulse voltammetric work, we did make exploratory measurements a t slow rotation rates (w = 60 radians s-l) a t disk electrodes and slow sweep rates (0.02 V s-l) a t the stationary glassy carbon electrode, comparing the results with those given by the model compound ferrocene, in order to identify any gross departures form behavior in other solvents.
EXPERIMENTAL Apparatus. Electrochemical measurements were made with a Princeton Applied Research Corporation Model 170 Electrochemistry System, using a three-electrode configuration in a water-jacketed Brinkmann cell (EA-876-5) fitted with a Brinkmann top (EA-874) and thermostated with a Sargent circulating bath (S-84880). The temperature was 30.0 f 0.5 "C for sulfolane and 25.0 f 0.5 "C for acetonitrile. The reference electrode for sulfolane [AgRE(SL)] and that for acetonitrile [AgRE(AN)] consisted of a Sargent silver electrode (S-30515-C) dipping into a 0.10 M AgC104 solution in the corresponding solvent contained in an 8-mm i.d. glass tube fitted with an asbestos fiber sealed into the tip. The resistance of such electrodes varied between 0.5 and 0.7 megohm. The potentials of the AgRE(SL) and the AgRE(AN) were +0.70 and +0.34 V vs. SCE(aq), respectively. The counter electrode was 1-cm2platinum foil. Indicator electrodes were either plantinum or glassy carbon disk electrodes. The platinum
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table I. Apparent Half-Wave Potentials for the Oxidation of Polynuclear Aromatic Hydrocarbons at Platinum Electrodes" in Acetonitrile as Solvent at 25 "C Reference Supporting elecE1/2, V Ref. electrolyte trode Hydrocarbon Anthracene
Benz[a]anthracene Benzo[ghi]perylene Benzo [a]pyrene Chrysene
0.5 M NaC104
Agb
2MNaC104 0.1 M Et4NC104 0.5 M NaC104
Ag' SCE Agd Agb
2MNaC104 0.1 M Et4NC104 2MNaC104
0.84 0.84 0.88 1.09 0.91 0.92
(7) (8) (9) (IO) (11) (7)
SCE Agd SCE
1.18
(IO)
1.00 1.01
(11)
0.5 M NaC104 0.5 M NaC104
Agb Agb
0.76 1.13
(7) (7) (12)
2MNaC104 0.1 M Et4NC104 2MNaC104 0.5 M NaC104
SCE Agd SCE Ag
1.35 1.22 1.23 1.00
2MNaC104 0.1 M Et4NC104
SCE Agd
1.26 (2) ( 7 ) 1.19 (10) 0.92 (11)
2MNaC104 0.5 M NaC104
SCE Agb
0.85 0.86
1.10
Coronene Dibenz[a,h]anthracene 9,lO-Diphenylanthracene Perylene Pyrene
1.12
Triphenylene
2MNaC104 0.1 M Et4NC104 2MNaC104 0.1 M Et4NC104
SCE Agd SCE Agd
1.16 1.06 1.55 1.46
(10)
(IO) (11)
Table 11. Voltammetry of Polynuclear Aromatic Hydrocarbons at the Rotated Platinum Disk Electrode" in Sulfolane as Solvent at 30 "C Hydrocarbon
El/zb
Slopec
illCd
Anthracene Benz[a]anthracene Benzo [ghi]perylene Benzo[a]pyrene Chrysene Coronene Dibenz[a,h]anthracene 9,lO-Diphenylanthracene Perylene Pyrene Triphenylene (Ferrocene
0.53 0.63 0.47 0.42 0.81 0.73 0.68 0.51 0.31 0.54 1.00 -0.31
67 73 67 53
3.4 3.3 1.0 2.5 5.5
81
146e 50 59 60 50 65 60
1.8
2.5 0.7 1.7
f f
0.8)
" Nominal area -0.01
cm2;rotated at 600 rpm; scan rate = 2 mV s-l. V us. Ag/(O.l M AgC104 in sulfolane) external reference electrode; supporting electrolyte: 0.1 M Bu4NPF6. Refers to plot of E vs. log [i/(il- i)]; in mV. Units: pA/mM. e Electrode may have been fouled. f Poorly defined, but much higher than for other hydrocarbons.
(IO) (1)
(10) (1) (2) ( 7 ) (10) (11) (10) (11)
Rotated wire electrode, except vibrating electrode for Ref. 7. Ag/(O.l M AgC104 + 0.5 M NaC104), with Ag+ present in test solution. Same as b, but with Ag+ separated from test solution. Ag/(O.Ol M AgClOd), with Ag+ separated from test solution. electrode was constructed by sealing platinum wire into a 6-mm i.d. glass tube and grinding the end of the tube flush with the wire. A layer of Wood's metal was placed over the platinum inside the tube and mercury was used for electrical contact. The nominal area of the electrode was 0.01 cm2.The glassy carbon electrode (from Chemtrix, Inc.) was a button of glassy carbon sealed into a 6-mm i.d. glass tube, with a nominal surface area of 0.07 cm2 and a resistance of 7 ohms. Glassy carbon has a total pore volume of only 0.35% as compared to 11%for high-density graphite and is essentially impermeable to gases; it also has other favorable properties (20,21). In their excellent discussion of the applicability of solid electrodes in nonaqueous solvents, Panzer and Elving (22)pointed out substantial advantages of glassy carbon over pyrolytic graphite electrodes, such as reduced susceptibility to surface contamination. In previous studies, polishing the surface of the glassy carbon electrode to a mirror finish with 0.5-p alumina not only reduced the residual current (221, but provided a general improvement in electrode response (23).In this work, intermittent polishing of fouled electrodes provided little improvement, but merely holding the potential at -2.0 V for a minute or so removed adsorbed films and restored reproducibility of currents and peak potentials. Such simple cathodic cleansing of glassy carbon electrodes has also been observed by Zittel and Miller (211. Reagents. Our previously reported procedure for the purification of sulfolane (24), which was designed for acid-base potentiometry and cathodic voltammetry, was modified slightly to better meet the requirements of anodic voltammetry. The ion-exchange step in the previous procedure was considered unnecessary here and was omitted. In its place was introduced a step in which the solvent is twice heated with 2 g KMn04 per liter for 3 h at 90 "C and then filtered. This treatment markedly improves the ultraviolet spectrum of the solvent; further details are given elsewhere ( 3 ) .It is important to note that, quite generally, it is necessary t o tailor the purification of relatiuely
Table 111. Slow Linear-Sweep Voltammetry of Polynuclear Aromatic Hydrocarbons at the Glassy Carbon Electrode" in Sulfolane as Solvent at 30 'C Hydrocarbon Anthracene Benz[a]anthracene Benzo[ghi] perylene Benzo[a]pyrene Chrysene Coronene Dibenz[a,h]anthracene 9,lO-Diphenylanthracene Perylene Pyrene Triphenylene (Ferrocene
E P b ( E P-
(ip/CV1/2)d
0.55 0.70 0.53 0.45 0.88 0.78 0.77 0.56
47 67 70 54 84 66 57 59
78 71 14 78 73 13 69 16
0.37 0.65
57 e 62 60
15 130 110 17)
1.11
-0.28
Nominal area -0.07 cm2;only relatively slow sweep rates of 0.005 to 0.02 V s-l were used. V vs. -4g/(O.l M AgC104 in sulfolane) external reference electrode; supporting electrolyte: 0.1 M Bu4NPF6. In mV. Units: i, in FA, C in mM; V = 0.01 V s-l. e Distorted wave.
inert solvents t o their intended use. Acetonitrile was purified as described before (25). The supporting electrolyte chosen for sulfolane was tetrabutylammonium hexafluorophosphate (Bu4NPFs) since it has a more extended anodic range than that of a perchlorate, which is electroactive at very positive potentials (18).It was prepared from Bu4NI and KPF6 (Alfa Inorganics) after purification of the starting materials. The ByNI was recrystallized from ethyl acetate. The KPFs was first recrystallized from water and was then boiled in concentrated ammonium hydroxide for an hour, after which crystallization was induced by cooling. Approximately 34 g of purified Bu4NI in 100 ml of methanol was added slowly and with stirring to a hot solution of 17 g of purified KPFs in 150 ml of water. Sufficient (ca. 800 ml) of methanol was then added to dissolve the precipitate, after which the methanol was evaporated slowly, the solution cooled, and the ByNPF, crystals were collected. Finally, the product was recrystallized twice from methanol and dried in vacuo at 85 "C for 24 h. With 0.1 M Bu4NPF6, the resistance between the indicator and counter electrodes was 2300 ohms. Residual currents at the stationary glassy carbon electrode in freshly-prepared 0.1 M solutions of Bu4NPFs in sulfolane were low, reaching a value of 2 FA at +2.0 V vs. AgRE(SL) or +2.7 V vs. SCE(aq). However, after aging for more than a few days, such solu-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2171
Table IV. Differential Pulse Voltammetry of Polynuclear Aromatic Hydrocarbons at the Glassy Carbon Electrode in Sulfolane and Acetonitrile as Solventso Hydrocarbon Anthracene Benz[a]anthracene Benzo[ghi] perylene Benzo[a]pyrene Chrysene Coronene Dibenz[a,h]anthracene 9,lO-Diphenylanthracene Perylene Pyrene Triphenylene (Ferrocene
EpbJ
0.57 0.68 0.51 0.85e 0.44 0.76e 0.82 0.77 0.75 0.53 0.34 0.67'? 0.57 1.09 -0.31 +0.01e
Wll2'3d
47 90 102 90" 76 90e
83 83 78 102 100 86 e 62 106 106 103")
0.2
0.4
0.3
E,V
a Data are for sulfolane as solvent a t 30 OC, except where indicated otherwise. V vs. Ag/(O.l M AgC104 in sulfolane) external as supporting electrolyte, reference electrode with 0.1 M Bu~NPFF, except where indicated otherwise. Varies with pulse amplitude; listed values have been extrapolated to AE = 0. Width a t halfpeak height, in mV. e For acetonitrile as solvent a t 25 "C, with Ag/(O.l M AgC104 in acetonitrile) external reference electrode and ca. 0.01 M NaC104 as supporting electrolyte.
0.5
0.6
vs. A g R E ( S L )
Figure 1. Differential pulse voltammograms at the glassy carbon electrode ( A = 0.07 cm2)in sulfolane containing 0.1 M Bu4NPFeat 30 OC. Scan rate = 2 mV s-l ( A ) 10 pM benzo[a]pyrene, pulse amplitude A € = 10 mV, sample duration t = 10 ms, pulse frequency u = 1 s-', full scale = 1 pA, peak current Aip = 0.29
+
PA. (8) mixture containing 48 pM perylene 4- 60 pM benzo[a]pyrene 32 pM benzo[ghi]perylene, A € = 50 mV, t = 15 ms, u = 1 s-', full scale = 10 &A. (C) Same as for 8,but with A€ = 5 mV and full scale = 1 pA. Curves have been
offset for clarity
tions exhibited a marked increase in residual current at +2 V and beyond. Also, after solid Bu4NPFs had been allowed to age in the bottle, its freshly-prepared solutions in methanol as solvent showed increasing absorbance at 220 nm. Consequently, only freshly-prepared solutions of freshly-purified Bu4NPF6 were used as supporting electrolyte. The hydrocarbons were obtained from either Aldrich Chemicals or Eastman Chemicals. Coronene, benzo[ghi]perylene and dibenz[a,h]anthracene were recrystallized from benzene, benz[a]anthracene from ether, and benzo[a]pyrene from 2080% by volume benzene-methanol. The remaining hydrocarbons were eluted from an alumina column with pentane-ether mixtures. Caution: Some of these hydrocarbons are carcinogenic.
RESULTS AND DISCUSSION All potentials are reduction potentials; values for sulfolane are referred t o AgRE(SL), and new values for acetonitrile are referred to AgRE(AN). T h e results of exploratory measurements at the rotated platinum disk electrode in sulfolane are reported in Table I1 and should be compared with previous results obtained in acetonitrile and shown in Table I. For a reversible oxidation, t h e half-wave potential should be related to the gas-phase ionization potential (EiJ as follows (10):
Ell2 = Eip + AE,
- TASo/F - (RT/F)In (f+/fo)
+ (RT/F)In (D+/D0)lI2+ constant
vs. A g R E ( A N )
( A ) Mixture containlng 1.2 pM perylene
+ 3.2 pM benzo[a]pyrene + 1.8 pM
benzo[ghl]perylene In 0.001 M NaC104. ( A ' ) Supporting electrolyte alone. (6) Mlxture containlng 12 pM perylene 61 pM benzo[a]pyrene t 17 pM benzo[gh/]peryleneIn 0.02 M NaCl04. (6')Supporting electrolyte alone
+
(1)
where AEs is the difference in the real solvation energies of Arm+ and ArO, and f and D represent, respectively, the activity coefficients and the diffusion coefficients of the two species. Consequently, relative half-wave potentials for a series of hydrocarbons may be solvent dependent, and it was hoped t h a t sulfolane would provide better resolution than acetonitrile does. However, this is not the case. T h e apparent halfwave potentials in Table I1 correlate linearly with most of those for each individual set in Table I, and the slope of the correlation line is near unity. Consequently, the apparent half-wave potentials in sulfolane show the same correlations 2172
E,V
Figure 2. Differential pulse voltammograms at the glassy carbon electrode (A = 0.19 Cm2) in acetonitrile at 25 OC. Scan rate = 2 mV s-I, pulse amplitude = 10 mV, sample duration = 10 ms, pulse frequency = 1 s-'
with ionization potentials, with molecular orbital (HMO and especially SCF) parameters, with interaction energies of charge-transfer complexes with trinitrofluorenone, and with p-absorption band spectra observed in acetonitrile (10). T h e results of some exploratory linear-sweep measurements at the glassy carbon electrode in sulfolane are shown in Table 111.For a reversible oxidation i t 30 "C the following relationships are valid (18):
E , = El12 and
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
+ 0.029/nV
(2)
Ep/a= El12 - 0.028/n V
E,
- Ep/2= 0.057/n V
(3)
(4)
whereas for the totally irreversible case Equation 4 becomes
E, - E,/z = 0.048/ana V In previous studies in acetonitrile (11),methylene chloride (15), and nitrobenzene (16)) 9J0-diphenylanthracene and perylene have been shown to give peak currents corresponding to one-electron overall oxidations without contributions from follow-up reactions, even a t relatively slow sweep rates. Also in sulfolane, peak currents of these two hydrocarbons are similar to each other and to that of the model compound ferrocene, and correspond to a diffusion coefficient near 1 X cm2 s-1, which is reasonable. Likewise, (E, - E,/P)values for these three compounds are close to those predicted by Equation 4. Most of the other hydrocarbons (particularly pyrene and triphenylene) give substantially higher peak currents, consistent with ECE follow-up reactions occurring within the time scale of the measurements. The results of differential pulse measurements in sulfolane are listed in Table IV. Since resolution proved to be no better than that expected in acetonitrile, some measurements were subsequently made in acetonitrile, in which much higher sensitivity could be obtained. Those results are also given in Table IV. Peak widths and relative peak potentials were similar in the two solvents, and the following discussion of these two quantities will concern the more extensive data obtained in sulfolane. Differential pulse voltammetry was introduced by Barker in 1958 (26) and much of its theory was presented by Parry and Osteryoung in 1965 (27). I t is primarily an analytical technique and possesses the major asset of sensitivity down to the micromolar range (28)and below. Experimental pitfalls have been discussed (29). For a reversible oxidation, the peak potential should be related to the half-wave potential through the pulse amplitude as follows:
E, = El/2 - AE/2
(6)
Le., E, should shift in a cathodic direction as A E increases. All hydrocarbons listed in Table IV, with the exception of coronene, dibenz[a,h]anthracene and triphenylene, follow Equation 6 well. The peak half-width also depends on the pulse amplitude when the latter is large, but approaches a limiting value at small amplitudes given by (Wl/2)0= 3.52 RT/nF = 92 mV ( n = 1,T = 303)
(7)
The model compounds, 9,10-diphenylanthracene, perylene, and ferrocene, which undergo reversible one-electron oxidations, give limiting peak half-widths some 10 mV larger than that predicted by Equation 7. With the exception of anthracene and pyrene, all hydrocarbons give limiting peak halfwidths between 76 and 106 mV. At a pulse amplitude of 100 mV, peak half-widths are up to 40% larger. Peak potentials and half-widths vary only slightly with concentration. From the peak half-widths, the degree of resolution attainable for mixtures of hydrocarbons can be estimated. For small pulse amplitudes and a reversible oxidation, the peak current is given by Ai, = (n2F2/4RT)ACAJ3(D/at)1/2
(8)
where t is the duration of the pulse and other symbols have their usual meaning (27). It is to be noted that the units of Ai, and C are amperes and moles ~ m - respectively. ~ , For large pulse amplitudes, Ai, is given by the Cottrell equation (27). The ratio Ai&' should be independent of C, and this indeed was the case for 9,10-diphenylanthracene,perylene, and even anthracene. For the more irreversible cases, the ratio Ai,# decreased with increasing concentration, e.g., by a factor of 2 for an increase in the concentration of triphenylene frbm 2 X to 1 X M. For most of the hydrocarbons, analytically useful signals can be obtained a t concentrations near 5X M in sulfolane containing 0.1 M Bu4NPF6. In acetonitrile containing 10-2-10-3 M NaC104, the sensitivity is much higher and measurements can be made at 2 X M. The main reason is that the much lower viscosity of acetonitrile provides two benefits. First, the diffusion coefficient of a given hydrocarbon should be some 30 times larger in acetonitrile than in sulfolane, so that for a reversible oxidation the peak current (Equation 8) should be 5 or 6 times larger in acetonitrile. Second, the roughly 30-fold increase in conductivity allows the use of a correspondingly lower concentration of supporting electrolyte with the attendant benefit of a lower background current. Results obtained in sulfolane with a mixture of perylene, benzo[a]pyrene and benzo[ghi]perylene for which half-wave potentials differ by only 0.10 and 0.07 V, respectively, are given in Figure 1. While resolution is incomplete, such mixtures can be analyzed by constructing calibration curves of Ai,/C vs. C. The choice of the optimum pulse amplitude involves a compromise: increasing the amplitude increases the sensitivity but decreases the resolution. The higher sensitivity obtained in acetonitrile is illustrated in Figure 2. We conclude that differential pulse voltammetry a t the glassy carbon electrode in sulfolane and particularly in acetonitrile as solvent can provide useful results in the determination of polynuclear aromatic hydrocarbons individually and in the analysis of their mixtures. In acetonitrile sensitivity of the method is superior to that of ultraviolet spectrophotometry ( 4 )and in both solvents its resolving power appears to be a t least comparable. Fluorescence spectrometry offers much higher sensitivity, but in the analysis of mixtures quenching and other difficulties (4,30,31)necessitate preliminary separation, e.g., by thin-layer chromatography. Differential pulse voltammetry may require less preliminary separation and in any event should be a useful independent technique.
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(1964). (9)K. E. Friend and W. D. Ohnesorge, J. Ofg. Chem., 28, 2435 (1983). ( I O ) E. S.Pysh and N. C.Yang, J. Am. Chem. SOC.,85, 2124 (1963). (11)M. E. Peover and B. S.White, J. Electfoanal. Chem., 13, 93 (1987). (12)J. W. Loveland and G. R. Dlmeler, Anal. Chem., 33, 1195 (1961). (13)T. A. Gough and M. E. Peover, "Proceedlngs Of the Third International Polarography Congress, Southampton, 1964",Macmillan, New York, N.Y., 1966. (14)M:E. Peover, In "Electroanalytlcal Chemistry", Vol. 2,A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1967. (15)J. Phelps, K. S. V. Santhanam, and A. J. Bard, J. Am. Chem. Soc., 88, 1752 (1967). (16)L. S.Marcoux, J. M. Fritsch, and R. N. Adams, J. Am. Chem. Sac.,89, 1752 11967). (17)L. Jeftlc and R. N. Adams, J. Am. Chem. Soc., 82, 1332 (1970). (18)R. N. Adams, "Electrochemistry at Solid Electrodes", Marcel Dekker, New York, N.Y., 1969.
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