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Voltammetry with disk electrodes and its analytical application
Electroanalytical Chemistry and Interfacial Electrochemistry, 44 (1973) 117-127
117
© ElsevierSequoia S.A., Lausanne- Printed in The Netherlands
VOLTAMMETRY WITH DISK ELECTRODES AND ITS ANALYTICAL APPLICATION VIII. THE CYCLIC VOLTAMMETRY OF COPPER(II) AT THE GLASSY CARBON ROTATING DISK ELECTRODE
M. ~TULIKOVAand F. VYDRA Analytical Laboratory, J. Heyrovsk~ Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Jilskh 16, Prague 1 (Czechoslovakia)
(Received 18th August 1972)
INTRODUCTION Glassy carbon has recently proven, in this laboratory and elsewhere 1-7, to be an advantageous material for electrodes for both voltammetric use and for' stripping analysis. So far, there is a lack of basic information on the voltammetric behaviour of various substances at this material, especially in different electrolytic media. This study was undertaken in order to characterize the behaviour of copper ions at this electrode in both acidic and basic media of varying complexing strength. EXPERIMENTAL All voltammograms were recorded on the OH-102 Polarograph (Radelkis, Hungary), using the three electrode system described previouslya. The electrode was polished using metallographic papers 2/0 and 5/0 (SIA, Switzerland) before each series of measurements. Each set of experiments was repeated several times with a freshly polished electrode surface to eliminate random errors. In all cases, the solutions were deaerated with purified nitrogen, and the voltammogram of the base electrolyte was recorded before introduction of copper(II) ions. All electrolyte solutions were prepared using p.a. chemicals with the exception of ammonia solutions where Suprapure ammonium hydroxide (Merck and Co., Darmstadt, Germany) was employed. Copper(II) solutions were prepared using "Specpure" copper(II) oxide (Johnson, Matthey, England). Doubly distilled water was used in the preparation of all solutions. All potentials were referred to the standard calomel electrode (SCE). Measurements were carried out at room temperature (24°). RESULTS The Voltammetry o f Cu 2+ in N a C l 0 4
The cyclic voltammogram of copper(II) in NaC10 4 medium (pH 4) is shown in Fig. 1. The scan from anodic to cathodic potentials is characterized by a
118 ,
M.
~TULIKOVA, F. VYDRA
!
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+0.5
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i
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i
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I
-O.S
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E/V Fig. 1. The cyclic v o l t a m m o g r a m of Cu 2+ in NaC104. p H 4; [Cu 2+] = 2 x 10 -4 M; [NaC104] = 1 M; 24 °; electrode rotation rate = 2800 r.p.m.; potential scan rate = 25 m V s - 1; a r r o w s indicate scan direction.
reversible one-electron wave with E ~ = - 0 . 1 V, which may be assigned to the reversible reduction of Cu 2+ to Cu ÷. At approx. - 1 . 0 V appears an additional wave of approximately equal height to that at - 0 . 1 V which is, however, combined with a large symmetrical peak. It was found that the area of this peak corresponds t o the area under the preceding wave. Since the wave at - 1.0 V can be attributed to the reduction of copper(I) to the metal at the electrode surface, it may be concluded that the monovalent copper produced from the copper(II) reduction at - 0 . 1 V remains adsorbed on the electrode surface, and is subsequently reduced to metallic copper at potentials more negative than about - 1.0 V. At about - 1.5 V the evolution of hydrogen commences. In the scan to anodic potentials, the reduction wave of copper ion to the metal coalesces with the wave of hydrogen evolution, and hence the peak at ,-~-1.0 V is absent. At more anodic potentials, three anodic peaks appear, with Ep= ~ 0 V, +0.16 V, and +0.23 V, hereafter referred to as peaks 1, 2, and 3, respectively. The dependence of the areas of these peaks on the initial cathodic potential was studied in order to elucidate their character. The large peak, 1, appears only when the preceding cathodic potential i,s sufficiently negative to produce reduction of copper to the metal. The dependence of the area of 1 on Ecathodic indicates that this is, in fact, a stripping peak due to the oxidation of metallic copper deposited at potentials more cathodic than - 0.5 V. Comparison with the area of the reduction peak (with ip - - 1.0 V) suggest that peak 1 corresponds to the oxidation of Cu ° to Cu +, rather than to Cu 2+, i.e. to a one-electron oxidation. It was further found that this peak becomes more anodic with deposition of copper at more cathodic potentials. The dependence of the area of peak 2 o n Ecathodic , when compared with the area under the C u 2 + ~ C u + reduction wave at these cathodic potentials, clearly indicates the connection between peak 2 and this wave. Peak 2 may then be assigned to the oxidation of the Cu + which was reduced from Cu 2 + at poteritials more negative than - 100 mV. This hypothesis is in agreement with the assignmemt of the origin of the cathodic scan peak at - 1.0 V; however, there is a discrepancy in peak size. It appears that almost all the Cu + produced at the electrode surface is adsorbed at more negative potentials, while at more positive potentials only a small amount remains adsorbed. This is, in fact, not surprising: a.c. studies have shown that, in neutral media, the zero charge potential of glassy carbon is roughly +0.1 V. Consequently, far more positively-
119
CYCLIC V O L T A M M E T R Y O F C U II AT GLASSY CARBON
charged copper will be adsorbed on a negatively charged surface than on a positively charged one. Peak 3 appears only in combination with peak 1, and, when peak 1 is sufficiently large, peaks 2 and 3 are roughly of identical size. Peak 3 will probably also correspond to the oxidation of Cu + to Cu z+ ; in this case, however, the Cu + arises as a result of the oxidation process in peak 1, which will thus, as proposed • above, correspond to the one-electron oxidation of Cu ° to Cu +. This latter hypothesis could probably be best confirmed with the help of a ring-disk electrode (cf. ref. 11). The well-defined nature of the Cu 2 + ~ C u + reduction wave and of stripping peak 1 promise useful analytical applications.
The Voltammetry of
C u 2+
in KNO 3 (pH 2)
The cyclic voltammogram of copper(II) in slightly acidic K N O a medium is shown in Fig. 2. The scan from anodic to cathodic potentials exhibits an irreversible reduction wave at - 150 mV which, according to the Levich equation 8, corresponds to two electrons (assuming D---6.9 × 10-6 cm 2 s-a)9. The subsequent anodic scan is characterized by a large stripping peak at 0 V (peak 1) and a smaller peak at + 150 mV (peak 2). It was found that peak 2 increases in size and shifts to slightly more positive potentials ( ~ + 190 mV) if the electrode is held stationary during the stripping process. It was further found that peak 2 appears with anodic stripping scan from the foot of the reduction wave ( ~ - 100 mV) and that peak 1 appears when the anodic scan is initiated from potentials more negative than about - 2 0 0 inV. Further, it was found by cutting out and weighing the area under the reduction wave and peak 1, that the charge passed during the stripping process corresponds to half the charge consumed in the reduction process i.e. peak 1 corresponds to a one-electron oxidation. Thus, by analogy with NaC104 medium, peak 1 was assigned to the oxidation of Cu ° to Cu +, and peak 2 to the oxidation of Cu + to Cu 2+. In K N O 3 medium, the Cu + is less firmly adsorbed on the electrode than in NaC10 4 (possibly due to complexing effects although competitive adsorption of electrolyte species, c.f. NO~-, may also play a certain role) and the
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Fig. 2. The cyclic voltammogram of C U 2+ in K N O 3. pH 2; [ C u 2 + ] = 2 × 10 -4 M; [KNO3]=0.5 M; conditions as in Fig. 1. ( . . . . . . ) Electrode rotated during stripping, ( - - ) electrode stationary during stripping. Fig. 3. The cyclic voltammogram of Cu a + in KC1. pH 4; [Cu 2 +] = 2 × 10 -4 M; KC1 = 1 M; conditions as in Fig. 1.
-i.O
120
M. ~TULIKOV~., F. VYDRA
peak corresponding to Cu + oxidation is thus smaller when there is greater mass transport at the electrode surface. The Vohammetry o f Cu 2 + in KCI
The voltammogram, shown in Fig. 3, is characterized by two reduction waves at + 150 (1) and - 4 7 5 (2) mV, each corresponding to one electron. These were assigned to the reduction of Cu z+ to Cu + and then to Cu °, respectively. Examination 3,1 o of the Cu 2 + --.Cu + reduction wave showed it to be nearly reversible in character (k°,-, 10-2 cm s-1). Comparison with NaC10 4 medium indicates that Cu + is probably not adsorbed strongly on the electrode surface (wave (2) exhibits no maximum). The anodic scan exhibits only one stripping peak irrespective of whether the electrode is stationary or rotated during the anodic scan. The correlation between the area of w a v e (2) and that of the stripping peak (obtained by weighing the appropriate graphical areas) indicates that the latter can be assigned to the one-electron oxidation of Cu ° to Cu +. The Cu + is again apparently not adsorbed on the electrode surface, as no further peaks are obtained. The Voltammetry o f Cu 2 + in ammoniacal media
The cyclic voltammograms of copper in N H 4 N O 3 media at various p H values are illustrated in Fig. 4. The voltammogram of the base electrolyte is without the peaks and waves illustrated and exhibits only the reduction wave of hydrogen and oxidation of hydroxyl ions ( < - 7 0 0 mV and > + 500 mV resp.). The reduction of Cu 2+ proceeds in two one-electron steps which become more distinctly separated with increasing pH. The reduction wave to monovalent copper is practically independent of pH, and has a half-wave potential of about 150 inV. This wave is accompanied by a small peak which falls either before or after the wave itself. The wave of the reduction of mono- to zero-valent copper is markedly and linearly dependent on pH, according to the equation E ~ ¢ ~ c , O = 125-83.3 p H mV
(1)
When sufficiently well defined, and when the p H is less than 9 this wave is also preceded by a small peak, about equal in size to those accompanying the abovementioned wave. This pH dependence may be effectively utilized in stripping analyses where copper is an undesirable interference. By choosing a suitable p H value, the copper reduction waves will be sufficiently separated and a pre-electrolysis potential may be chosen at which copper is not reduced to the metal. Such a procedure was utilized in Part II of this series 2, where pre-electrolysis was carried out in ammoniacal medium, p H 9.4, at - 6 0 0 mV (E~u+ _.c,O= - 660 mV, according to eqn. 1), although a slightly more anodic value should probably be chosen at higher copper concentrations or with longer pre-electrolysis times. The three cathodic peaks were designated individually, as there is no evidence to suggest that the peaks before and after the Cu2+--*Cu + reduction wave correspond to the same process, although they do not occur simultaneously. The first, (peak a), which appears only at lower p H ( .-~4) has a potential of Ep = - 100 inV. The second (peak b) was found to shift with pH, according to the equation Ep(b) = 320--75 p H mV
(2)
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Fig. 4. The cyclic voltammogram of Cu 2+ in NH4NO3. [NH4N O 3 ] = l M; [ C u Z + ] = 2 x l 0 -4 M; conditions as in Fig. 1; broken line indicates anodic scan from more positive potentials (as noted); arrows below p e a k indicate shift of Ep with increasing•y negative pre-electrolysis potentials, pH: (a) 4,2, (b) 6,3, (c) 6.8, (d) 8.0, (e) 9.0.
122
M. gTULIKOVA., F. VYDRA
It should be noted that, at pH 4.2, t h i s p e a k would lie at +5 inV. Thus peak a at - 100 mV does not conform to this equation; further there is no peak with such an Ep. Thus, either the pH dependence expressed by eqn. (2) does not hold at lower pH values, or else peak b is absent at such values. However, the explanation of peak a given below, confirms that peaks a and b have different origins. The third catl~odic peak (peak c), just before the Cu + ~ C u ° reduction wave, has a similar potential dependence as peak b: Ep(c~ = 154-75.5 pH mV
(3)
These three peaks will be discussed further below, in connection with the anodic peaks. The anodic peaks are up to four in number, although at any given pH not all may be present. These peaks will be designated l to 4 as indicated in Fig. 4. Peak 2 was found to occur with anodic scans beginning at potentials sufficiently cathodic to include the Cu 2 + ~ C u + reduction wave, or, on subsequent" scans, peak a. (Peak a occurs only after two or three cycles.) The size of peak a similarly depends on the anodic potential at which the cathodic scan is initiated, being absent when, on the previous cycle, this potential was not sufficiently anodic to include peak 2. By analogy with the previously discussed media, peak 2 was assigned to the oxidation of adsorbed Cu + to Cu 2+, and peak a to the reverse reaction. The absence of peak a at higher pH values is accompanied by gradual disappearance of peak 2. This phenomenon may be correlated to the increased complexing ability of ammonia at higher pH values although competitive adsorption of ammonia will also occur, analogously to the above described systems, resulting in desorption of the Cu + atom. The major peak, designated peak 1, was found to correspond to two electrons at pH 4.2 and to a maximum of one electron at higher pH values; by comparison with the area under the Cu + ---,Cu° reduction wave and the subsequent peak 1 area during the cyclic scan (by cutting out and weighing the appropriate areas), it was concluded that, at pH >i 6, peak 1 corresponds to the stripping oxidation of Cu ° to Cu +, a small amount of which remains adsorbed and is subsequently oxidized to Cu z+ in peak 2. At pH 4, the peak corresponds to the oxidation of Cu ° to Cu 2+, although a certain amount of Cu +, presumably a more firmly bound layer next to the electrode, remains adsorbed to be oxidized at higher potentials (cf. above, NaC104 medium, where separate peaks were also found for oxidation of Cu~ produced from oxidation and reduction processes). Peak 1 is shifted with pH, analogously to the Cu + --*Cu ° reduction wave, and its position also depends on the cath6dic potential at which the stripping scan is originated, shifting to more positive values as Ecath becomes more negative (see broken lines in Fig. 4). This latter phenomenon was found in all media and suggests that, with Cu °' deposited at more negative potentials, a more stable metallic structure is formed requiring higher energy for dissolution. When the anodic scan is initiated at more positive potentials, peak 1 exhibits a side peak (peak 3), which appears before peak 1 and which has a constant low height. When peak 1 becomes larger, peak 3 can no longer be distinguished from it. Peak 3 could well correspond to the oxidation of Cu ° to Cu + (adsorbed)which would require slightly less energy than oxidation to desorbed
CYCLIC VOLTAMMETRY OF CU II AT GLASSY CARBON
123
Cu +. This suggestion is supported by the similarity in sizes of the two peaks 2 and 3, but conclusive evidence is lacking. Another peak (peak 4) also appears at higher pH values, close to 0 V, more positive than the major peak 1, and reaches maximum height as peak 1 begins to increase in size. In analogy to previous work 11, peak 4 may correspond to the oxidation of a more firmly bound Cu ° layer next to the electrode surface (a "monolayer" or "microlayer"). The exact origin of peaks b and c is not clear, although their size is very similar to that of peaks 2, 3, and 4, as well as peak a, suggesting some form of adsorbed species. At higher pH values, the area under peak 1 is somewhat less than would correspond to one electron, this discrepancy increasing with increasing pH. It is well known 12 that, in ammoniacal media, the following reaction takes place: [Cu(NH3)4] 2+ --}-Cu ° --* 2[Cu(NHa)2] +
(4)
where the disproportionation constant, K = 2 × 10-2 1 mol-1; this reaction could indeed account for the decrease in the size of the Cu ° stripping peak at higher pH values. (Isotol~e studies could probably confirm this hypothesis.) Thus for the stripping analysis of copper, ammoniacal media at higher pH values are not to be recommended.
The Voltammetry of Copper(II) in Ethylenediamine (en) The cyclic voltammogram of Cu(II) in 1 M e n (pH 12) is shown in Fig. 5.
_......--:---*-j
a
0
-0.5
-i.o
E/v
-i.5
Fig. 5. The cyclic voltammogram of Cu 2+ in ethylenediamine, pH 12; [Cu2+]=2× 10 -4 M; [en]=l M; conditions as in Fig. 1.
The reduction is characterized by two one-electron waves of irreversible character which have, however, well defined current plateaux, suitable for analytical use. The stripping peak corresponds to two electrons, as determined by cutting out and weighing the peaks and the corresponding area under the wave at 10 different potentials between the foot of the wave up to and including the diffusioncontrolled current plateau. The stripping peak potential depends on the preelectrolysis potential, similar to the situation described for ammoniacal medium, i.e. it shifts to more positive potentials when more negative pre-electrolysis potentials are employed. The peak is sharp and useful for analytical applications. The disproportionation effect (eqn. 4), lowering the peak height in the case of ammoniacal media, is absent here, as theoretically predicted 12. Ke q ~ 10 5 for the reaction 2Cu +
124
M. STULIKOVA., F. VYDRA
C u ° + Cu 2 +. (The stripping peak was somewhat larger than would correspond to two electrons, possibly due to this reaction.) The voltammogram displayed the complete absence of small adsorption peaks, presumably due to the strongly complexing effect of en at this pH (cf. N H a at high pH), although competitive adsorption of en will probably also be important. The lack of adsorption of Cu ÷ is confirmed by the fact that, although the stripping peak corresponds to two electrons, no peak is present if the previously attained cathodic potential in a particular cycle is not sufficiently negative to result in the reduction to the metal, i.e. only Cu ÷ produced as a result of Cu ° oxidation results in a peak. Cu + resulting from Cu E+ reduction is desorbed before sufficiently anodic potentials are attained for its oxidation. The Voltammetry o f Cu z ÷ in Acetic Acid-Acetate Media
The cyclic polarogram of copper(II) in acetic acid medium at pH 2.5 (1.0 M acetic acid) is characterized by a poorly defined irreversible double wave corresponding to the step-wise reduction Cu z ÷ ~ C u + ~ C u °, accompanied by a small pre-wave (see Fig. 6). A large cathodic peak appears as the limiting current of the second wave is attained. The anodic scan is marked by either one large, or one large and one small stripping peak, depending on the cathodic potential at which the scan is initiated. The situation is very similar to that found in NaC104 except that, in this case, the large stripping peak corresponds to two electrons (as determined by cutting out, weighing, and comparing the area under the reduction wave and the stripping peak, corresponding to 10 different cathodic potential values); further, .no stripping peaks appear if the preceding cathodic potential is not sufficiently negative to reduce copper to the metal. From these dependence, and in analogy with the preceding systems, the following assignment seems probable: Cathodic w a v e s :
CH 2+ ~ CUa'~-~-CU+n__ad
Cu + ~ Cu ° Cathodic pre-wave:
Cu 2+ ~ Cth~,micro analogous to peak a, NH4NO s medium
Cathodic peak:
Cth-~-~-CLLa +, micro ~
Anodic peaks:
(1) C u ° ~ C u z+ +Cu~,micro. (2) CtL,~,micro*~Cu2 +
Cu°
analogous to cathodic peak, NaC104 medium
where ad =adsorbed, non-ad--non-adsorbed, micro refers to a more firmly bound layer next to the electrode surface, and * refers to the species produced only from reaction (1). At higher pH values (5-6), the peaks and waves, which were here correlated to adsorption phenomena, disappear, and the anodic stripping peak appears to correspond to less than two electrons, especially when more negative potentials precede the anodic scan. This effect can presumably be correlated to the greater complexing ability of acetate for Cu ÷ at higher pH values, analogously to the other systems discussed above.
CYCLIC V O L T A M M E T R Y O F C U II AT GLASSY CARBON
.
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.
.
.
.
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.
.
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125
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Fig. 6. The cyclic voltammogram of Cu 2+ in acetic acid. pH 2.6; [Cu z+] = 2 x 10 ~ M; [ C H a O O H ] = 1 M; conditions as in Fig. 1; broken line as in Fig. 4.
DISCUSSION
The character of the voltammetric curves of copper(II) on the glassy carbon r.d.e, seems to depend to a great extent on the degree to which monovalent copper, produced as a product of an electrochemical reaction, remains adsorbed on the electrode surface sufficiently long to undergo further electrochemical reaction. The formation of monolayers or ad-atoms of metallic copper, frequently noted at platinum and other metallic electrodes (e.9. ref. 11), does not appear to play a significant role here, and, with the possible exception of anodic peak 4 in NH4NO 3NH4OH medium, evidence is entirely lacking for such phenomena. The degree of adsorption of monovalent copper seems to depend to a great degree on the medium employed, and, not surprisingly, there seems to be a correlation between the complexing strength of the medium and the degree of desorption of Cu +, although competitive adsorption of other electrolyte species will also be important. An exact correlation however, is impossible due to the lack of tabulated stability constant values for monovalent copper ion (because of its instability in most aqueous solutions). An interesting phenomenon was noted in that, in many of the systems studied, the small peaks observed were of similar size. Although Cu ° ad-atom formation does not seem to occur, it is entirely possible that monovalent copper is specifically adsorbed. To further examine this idea, the sizes of these peaks were compared and, in each case, the fraction of the geometrical surface which would be covered by the number of atoms corresponding to the area under each peak was calculated (see Table 1). In these calculations, an ionic radius of 0.93 /~ was assumed for Co +12. The average surface coverage is 0.36, or about 1/3 of the geometrical area. The values are quite close, the differences presumably being attributable to the experimental error involved in measuring the peak areas (the peaks were often quite flat and drawn-out), although it is not inconceivable that the adsorption-active surface area could vary somewhat with the electrolytic medium employed (due to adsorption of other solution components, etc.). These results suggest that, in the systems studied, this ad-ion layer is somewhat more resistive than normally adsorbed Cu + to the dissolution effects of a complexing medium, even at a positively charged electrode. The waves in most cases seem to be suitable for analytical use, being linearly dependent on concentration, and the stripping peaks also on pre-electrolysis time
126
M. ,~TUL]KOVA, F. VYDRA
TABLE 1 COMPARISON OF FRACTION OF ELECTRODE SURFACE COVERED IN VARIOUS SYSTEMS, ASSUMING SECONDARY STRIPPING PEAKS CORRESPOND TO ADSORBED MONO-VALENT COPPER System and peak designation b
CH3COOH, pH 2.6, a.p. 2 KNO 3 a.p. 2, stationary electrode NH4NO 3 c.p. a c.p. b c.p. c a.p. 2 a.p. 3 NaC10 4 a.p. 1 a.p. 2
Peak area
(xlO 6)
Coulombs
Moles ( x l O -a°)
Ions ( x l O 14)
Fraction of surface" covered
18.30 14.20 14.20 13.50 14.46 11.40 13.80 20.80 14.82
1.90 1.47 1.47 1.40 1.50 1.18 1.44 2.02 1.54
1.14 0.88 0.88 0.84 0.90 0.71 0.86 1.30 0.93
0.44 0.39 0.39 0.32 0.35 0.27 0.33 0.50 0.36
Geometrical area. b a.p. =anodic peak, c.p. =cathodic peak.
a
and potential in the usual manner. F o r anodic stripping analysis, it w o u l d be preferable to choose a m e d i u m in which C u ÷ is also oxidized, preferably simultaneously in one peak with C u °, because of the resulting increase in sensitivity. ACKNOWLEDGEMENT The a u t h o r s wish to thank Dr. K. ~tulik, D e p a r t m e n t Chemistry, Charles University, for his interest and helpful remarks.
of Analytical
SUMMARY T h e cyclic v o l t a m m e t r y of c o p p e r ( I I ) at the glassy c a r b o n rotating disk electrode has been studied in a n u m b e r of media with a view to its usefulness in analytical applications. It has been found that, while C u 2 ÷ is generally reduced to Cu °, the stripping scan does not necessarily result in reoxidation to C u 2+, the reaction often ending with oxidation of C u ° to m o n o v a l e n t copper. This p h e n o m e n o n has been related to the degree to which m o n o v a l e n t copper, p r o d u c e d at the electrode surface, remains a d s o r b e d long e n o u g h to u n d e r g o further reaction, which is, in turn, strongly affected by the medium.
REFERENCES 1 F. Vydra and M. ~tulikovfi, Collect. Czech. Chem. Commun., 37 (1972) 123. 2 M. Kopanica and F. Vydra, J. Electroanal. Chem., 31 (1971) 175 3 M. ~tulikovfi and F. Vydra, J. Electroanal. Chem., 38 (1972) 349~
CYCLIC VOLTAMMETRY OF CU II AT GLASSY CARBON 4 5 6 7 8 9 10 ll 12
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F. Vydra, M. ~tulikov& and P. Pet&k, J. Electroanal. Chem., 40 (1972) 99. C. E. Plock, Anal. Chim. Acta, 53 (1971) 249. H. E. Zittel and F. J. Miller, Anal. Chem., 37 (1965) 200. H. Sunahara and T. Ishizuka, Rev. Polarogr. (Kyoto), 14 (1967) 176. V. E. Levich, Physico-Chemical Thermodynamics, State Publishing House of Physico-Mathematical Literature, Moscow, 1959. L. Meites, Handbook of Analytical Chemistry, McGraw-Hill, New York, 1963. J. Koryta,.Electrochim. Acta, 6 (1962) 67. G. W. Tindall and S. Bruckenstein, J. Electroanal. Chem., 22 (1969) 367; Anal. Chem., 40 (1968) 10~1. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Interscience, New York, 1962.
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© ElsevierSequoia S.A., Lausanne- Printed in The Netherlands
VOLTAMMETRY WITH DISK ELECTRODES AND ITS ANALYTICAL APPLICATION VIII. THE CYCLIC VOLTAMMETRY OF COPPER(II) AT THE GLASSY CARBON ROTATING DISK ELECTRODE
M. ~TULIKOVAand F. VYDRA Analytical Laboratory, J. Heyrovsk~ Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Jilskh 16, Prague 1 (Czechoslovakia)
(Received 18th August 1972)
INTRODUCTION Glassy carbon has recently proven, in this laboratory and elsewhere 1-7, to be an advantageous material for electrodes for both voltammetric use and for' stripping analysis. So far, there is a lack of basic information on the voltammetric behaviour of various substances at this material, especially in different electrolytic media. This study was undertaken in order to characterize the behaviour of copper ions at this electrode in both acidic and basic media of varying complexing strength. EXPERIMENTAL All voltammograms were recorded on the OH-102 Polarograph (Radelkis, Hungary), using the three electrode system described previouslya. The electrode was polished using metallographic papers 2/0 and 5/0 (SIA, Switzerland) before each series of measurements. Each set of experiments was repeated several times with a freshly polished electrode surface to eliminate random errors. In all cases, the solutions were deaerated with purified nitrogen, and the voltammogram of the base electrolyte was recorded before introduction of copper(II) ions. All electrolyte solutions were prepared using p.a. chemicals with the exception of ammonia solutions where Suprapure ammonium hydroxide (Merck and Co., Darmstadt, Germany) was employed. Copper(II) solutions were prepared using "Specpure" copper(II) oxide (Johnson, Matthey, England). Doubly distilled water was used in the preparation of all solutions. All potentials were referred to the standard calomel electrode (SCE). Measurements were carried out at room temperature (24°). RESULTS The Voltammetry o f Cu 2+ in N a C l 0 4
The cyclic voltammogram of copper(II) in NaC10 4 medium (pH 4) is shown in Fig. 1. The scan from anodic to cathodic potentials is characterized by a
118 ,
M.
~TULIKOVA, F. VYDRA
!
I
+0.5
0
i
I
i
I
I
-O.S
i
i
I
-I.0
E/V Fig. 1. The cyclic v o l t a m m o g r a m of Cu 2+ in NaC104. p H 4; [Cu 2+] = 2 x 10 -4 M; [NaC104] = 1 M; 24 °; electrode rotation rate = 2800 r.p.m.; potential scan rate = 25 m V s - 1; a r r o w s indicate scan direction.
reversible one-electron wave with E ~ = - 0 . 1 V, which may be assigned to the reversible reduction of Cu 2+ to Cu ÷. At approx. - 1 . 0 V appears an additional wave of approximately equal height to that at - 0 . 1 V which is, however, combined with a large symmetrical peak. It was found that the area of this peak corresponds t o the area under the preceding wave. Since the wave at - 1.0 V can be attributed to the reduction of copper(I) to the metal at the electrode surface, it may be concluded that the monovalent copper produced from the copper(II) reduction at - 0 . 1 V remains adsorbed on the electrode surface, and is subsequently reduced to metallic copper at potentials more negative than about - 1.0 V. At about - 1.5 V the evolution of hydrogen commences. In the scan to anodic potentials, the reduction wave of copper ion to the metal coalesces with the wave of hydrogen evolution, and hence the peak at ,-~-1.0 V is absent. At more anodic potentials, three anodic peaks appear, with Ep= ~ 0 V, +0.16 V, and +0.23 V, hereafter referred to as peaks 1, 2, and 3, respectively. The dependence of the areas of these peaks on the initial cathodic potential was studied in order to elucidate their character. The large peak, 1, appears only when the preceding cathodic potential i,s sufficiently negative to produce reduction of copper to the metal. The dependence of the area of 1 on Ecathodic indicates that this is, in fact, a stripping peak due to the oxidation of metallic copper deposited at potentials more cathodic than - 0.5 V. Comparison with the area of the reduction peak (with ip - - 1.0 V) suggest that peak 1 corresponds to the oxidation of Cu ° to Cu +, rather than to Cu 2+, i.e. to a one-electron oxidation. It was further found that this peak becomes more anodic with deposition of copper at more cathodic potentials. The dependence of the area of peak 2 o n Ecathodic , when compared with the area under the C u 2 + ~ C u + reduction wave at these cathodic potentials, clearly indicates the connection between peak 2 and this wave. Peak 2 may then be assigned to the oxidation of the Cu + which was reduced from Cu 2 + at poteritials more negative than - 100 mV. This hypothesis is in agreement with the assignmemt of the origin of the cathodic scan peak at - 1.0 V; however, there is a discrepancy in peak size. It appears that almost all the Cu + produced at the electrode surface is adsorbed at more negative potentials, while at more positive potentials only a small amount remains adsorbed. This is, in fact, not surprising: a.c. studies have shown that, in neutral media, the zero charge potential of glassy carbon is roughly +0.1 V. Consequently, far more positively-
119
CYCLIC V O L T A M M E T R Y O F C U II AT GLASSY CARBON
charged copper will be adsorbed on a negatively charged surface than on a positively charged one. Peak 3 appears only in combination with peak 1, and, when peak 1 is sufficiently large, peaks 2 and 3 are roughly of identical size. Peak 3 will probably also correspond to the oxidation of Cu + to Cu z+ ; in this case, however, the Cu + arises as a result of the oxidation process in peak 1, which will thus, as proposed • above, correspond to the one-electron oxidation of Cu ° to Cu +. This latter hypothesis could probably be best confirmed with the help of a ring-disk electrode (cf. ref. 11). The well-defined nature of the Cu 2 + ~ C u + reduction wave and of stripping peak 1 promise useful analytical applications.
The Voltammetry of
C u 2+
in KNO 3 (pH 2)
The cyclic voltammogram of copper(II) in slightly acidic K N O a medium is shown in Fig. 2. The scan from anodic to cathodic potentials exhibits an irreversible reduction wave at - 150 mV which, according to the Levich equation 8, corresponds to two electrons (assuming D---6.9 × 10-6 cm 2 s-a)9. The subsequent anodic scan is characterized by a large stripping peak at 0 V (peak 1) and a smaller peak at + 150 mV (peak 2). It was found that peak 2 increases in size and shifts to slightly more positive potentials ( ~ + 190 mV) if the electrode is held stationary during the stripping process. It was further found that peak 2 appears with anodic stripping scan from the foot of the reduction wave ( ~ - 100 mV) and that peak 1 appears when the anodic scan is initiated from potentials more negative than about - 2 0 0 inV. Further, it was found by cutting out and weighing the area under the reduction wave and peak 1, that the charge passed during the stripping process corresponds to half the charge consumed in the reduction process i.e. peak 1 corresponds to a one-electron oxidation. Thus, by analogy with NaC104 medium, peak 1 was assigned to the oxidation of Cu ° to Cu +, and peak 2 to the oxidation of Cu + to Cu 2+. In K N O 3 medium, the Cu + is less firmly adsorbed on the electrode than in NaC10 4 (possibly due to complexing effects although competitive adsorption of electrolyte species, c.f. NO~-, may also play a certain role) and the
I¢
Zc
L +O.S
0
E/V
-O.S -I-0.5
0
-0.5
E/v
Fig. 2. The cyclic voltammogram of C U 2+ in K N O 3. pH 2; [ C u 2 + ] = 2 × 10 -4 M; [KNO3]=0.5 M; conditions as in Fig. 1. ( . . . . . . ) Electrode rotated during stripping, ( - - ) electrode stationary during stripping. Fig. 3. The cyclic voltammogram of Cu a + in KC1. pH 4; [Cu 2 +] = 2 × 10 -4 M; KC1 = 1 M; conditions as in Fig. 1.
-i.O
120
M. ~TULIKOV~., F. VYDRA
peak corresponding to Cu + oxidation is thus smaller when there is greater mass transport at the electrode surface. The Vohammetry o f Cu 2 + in KCI
The voltammogram, shown in Fig. 3, is characterized by two reduction waves at + 150 (1) and - 4 7 5 (2) mV, each corresponding to one electron. These were assigned to the reduction of Cu z+ to Cu + and then to Cu °, respectively. Examination 3,1 o of the Cu 2 + --.Cu + reduction wave showed it to be nearly reversible in character (k°,-, 10-2 cm s-1). Comparison with NaC10 4 medium indicates that Cu + is probably not adsorbed strongly on the electrode surface (wave (2) exhibits no maximum). The anodic scan exhibits only one stripping peak irrespective of whether the electrode is stationary or rotated during the anodic scan. The correlation between the area of w a v e (2) and that of the stripping peak (obtained by weighing the appropriate graphical areas) indicates that the latter can be assigned to the one-electron oxidation of Cu ° to Cu +. The Cu + is again apparently not adsorbed on the electrode surface, as no further peaks are obtained. The Voltammetry o f Cu 2 + in ammoniacal media
The cyclic voltammograms of copper in N H 4 N O 3 media at various p H values are illustrated in Fig. 4. The voltammogram of the base electrolyte is without the peaks and waves illustrated and exhibits only the reduction wave of hydrogen and oxidation of hydroxyl ions ( < - 7 0 0 mV and > + 500 mV resp.). The reduction of Cu 2+ proceeds in two one-electron steps which become more distinctly separated with increasing pH. The reduction wave to monovalent copper is practically independent of pH, and has a half-wave potential of about 150 inV. This wave is accompanied by a small peak which falls either before or after the wave itself. The wave of the reduction of mono- to zero-valent copper is markedly and linearly dependent on pH, according to the equation E ~ ¢ ~ c , O = 125-83.3 p H mV
(1)
When sufficiently well defined, and when the p H is less than 9 this wave is also preceded by a small peak, about equal in size to those accompanying the abovementioned wave. This pH dependence may be effectively utilized in stripping analyses where copper is an undesirable interference. By choosing a suitable p H value, the copper reduction waves will be sufficiently separated and a pre-electrolysis potential may be chosen at which copper is not reduced to the metal. Such a procedure was utilized in Part II of this series 2, where pre-electrolysis was carried out in ammoniacal medium, p H 9.4, at - 6 0 0 mV (E~u+ _.c,O= - 660 mV, according to eqn. 1), although a slightly more anodic value should probably be chosen at higher copper concentrations or with longer pre-electrolysis times. The three cathodic peaks were designated individually, as there is no evidence to suggest that the peaks before and after the Cu2+--*Cu + reduction wave correspond to the same process, although they do not occur simultaneously. The first, (peak a), which appears only at lower p H ( .-~4) has a potential of Ep = - 100 inV. The second (peak b) was found to shift with pH, according to the equation Ep(b) = 320--75 p H mV
(2)
121
(a) I
c
! o
+O.S
0
-O.S
- 1.0
E//V
~
(b)
I¢
IQ
+O;S
0
la
-O.S
-I.0
E/v
" ' ~ / - "
~'0.5
0
-I.0
-O.S
(d)
i
4- 0 . 5
i
i
i
,
0
i
i
i
i
- O.S
i
i
i
i
- 1.0
E/v (e)
Ic r
IO . . . .
+OoS
0
-03
E/V
-t.O
Fig. 4. The cyclic voltammogram of Cu 2+ in NH4NO3. [NH4N O 3 ] = l M; [ C u Z + ] = 2 x l 0 -4 M; conditions as in Fig. 1; broken line indicates anodic scan from more positive potentials (as noted); arrows below p e a k indicate shift of Ep with increasing•y negative pre-electrolysis potentials, pH: (a) 4,2, (b) 6,3, (c) 6.8, (d) 8.0, (e) 9.0.
122
M. gTULIKOVA., F. VYDRA
It should be noted that, at pH 4.2, t h i s p e a k would lie at +5 inV. Thus peak a at - 100 mV does not conform to this equation; further there is no peak with such an Ep. Thus, either the pH dependence expressed by eqn. (2) does not hold at lower pH values, or else peak b is absent at such values. However, the explanation of peak a given below, confirms that peaks a and b have different origins. The third catl~odic peak (peak c), just before the Cu + ~ C u ° reduction wave, has a similar potential dependence as peak b: Ep(c~ = 154-75.5 pH mV
(3)
These three peaks will be discussed further below, in connection with the anodic peaks. The anodic peaks are up to four in number, although at any given pH not all may be present. These peaks will be designated l to 4 as indicated in Fig. 4. Peak 2 was found to occur with anodic scans beginning at potentials sufficiently cathodic to include the Cu 2 + ~ C u + reduction wave, or, on subsequent" scans, peak a. (Peak a occurs only after two or three cycles.) The size of peak a similarly depends on the anodic potential at which the cathodic scan is initiated, being absent when, on the previous cycle, this potential was not sufficiently anodic to include peak 2. By analogy with the previously discussed media, peak 2 was assigned to the oxidation of adsorbed Cu + to Cu 2+, and peak a to the reverse reaction. The absence of peak a at higher pH values is accompanied by gradual disappearance of peak 2. This phenomenon may be correlated to the increased complexing ability of ammonia at higher pH values although competitive adsorption of ammonia will also occur, analogously to the above described systems, resulting in desorption of the Cu + atom. The major peak, designated peak 1, was found to correspond to two electrons at pH 4.2 and to a maximum of one electron at higher pH values; by comparison with the area under the Cu + ---,Cu° reduction wave and the subsequent peak 1 area during the cyclic scan (by cutting out and weighing the appropriate areas), it was concluded that, at pH >i 6, peak 1 corresponds to the stripping oxidation of Cu ° to Cu +, a small amount of which remains adsorbed and is subsequently oxidized to Cu z+ in peak 2. At pH 4, the peak corresponds to the oxidation of Cu ° to Cu 2+, although a certain amount of Cu +, presumably a more firmly bound layer next to the electrode, remains adsorbed to be oxidized at higher potentials (cf. above, NaC104 medium, where separate peaks were also found for oxidation of Cu~ produced from oxidation and reduction processes). Peak 1 is shifted with pH, analogously to the Cu + --*Cu ° reduction wave, and its position also depends on the cath6dic potential at which the stripping scan is originated, shifting to more positive values as Ecath becomes more negative (see broken lines in Fig. 4). This latter phenomenon was found in all media and suggests that, with Cu °' deposited at more negative potentials, a more stable metallic structure is formed requiring higher energy for dissolution. When the anodic scan is initiated at more positive potentials, peak 1 exhibits a side peak (peak 3), which appears before peak 1 and which has a constant low height. When peak 1 becomes larger, peak 3 can no longer be distinguished from it. Peak 3 could well correspond to the oxidation of Cu ° to Cu + (adsorbed)which would require slightly less energy than oxidation to desorbed
CYCLIC VOLTAMMETRY OF CU II AT GLASSY CARBON
123
Cu +. This suggestion is supported by the similarity in sizes of the two peaks 2 and 3, but conclusive evidence is lacking. Another peak (peak 4) also appears at higher pH values, close to 0 V, more positive than the major peak 1, and reaches maximum height as peak 1 begins to increase in size. In analogy to previous work 11, peak 4 may correspond to the oxidation of a more firmly bound Cu ° layer next to the electrode surface (a "monolayer" or "microlayer"). The exact origin of peaks b and c is not clear, although their size is very similar to that of peaks 2, 3, and 4, as well as peak a, suggesting some form of adsorbed species. At higher pH values, the area under peak 1 is somewhat less than would correspond to one electron, this discrepancy increasing with increasing pH. It is well known 12 that, in ammoniacal media, the following reaction takes place: [Cu(NH3)4] 2+ --}-Cu ° --* 2[Cu(NHa)2] +
(4)
where the disproportionation constant, K = 2 × 10-2 1 mol-1; this reaction could indeed account for the decrease in the size of the Cu ° stripping peak at higher pH values. (Isotol~e studies could probably confirm this hypothesis.) Thus for the stripping analysis of copper, ammoniacal media at higher pH values are not to be recommended.
The Voltammetry of Copper(II) in Ethylenediamine (en) The cyclic voltammogram of Cu(II) in 1 M e n (pH 12) is shown in Fig. 5.
_......--:---*-j
a
0
-0.5
-i.o
E/v
-i.5
Fig. 5. The cyclic voltammogram of Cu 2+ in ethylenediamine, pH 12; [Cu2+]=2× 10 -4 M; [en]=l M; conditions as in Fig. 1.
The reduction is characterized by two one-electron waves of irreversible character which have, however, well defined current plateaux, suitable for analytical use. The stripping peak corresponds to two electrons, as determined by cutting out and weighing the peaks and the corresponding area under the wave at 10 different potentials between the foot of the wave up to and including the diffusioncontrolled current plateau. The stripping peak potential depends on the preelectrolysis potential, similar to the situation described for ammoniacal medium, i.e. it shifts to more positive potentials when more negative pre-electrolysis potentials are employed. The peak is sharp and useful for analytical applications. The disproportionation effect (eqn. 4), lowering the peak height in the case of ammoniacal media, is absent here, as theoretically predicted 12. Ke q ~ 10 5 for the reaction 2Cu +
124
M. STULIKOVA., F. VYDRA
C u ° + Cu 2 +. (The stripping peak was somewhat larger than would correspond to two electrons, possibly due to this reaction.) The voltammogram displayed the complete absence of small adsorption peaks, presumably due to the strongly complexing effect of en at this pH (cf. N H a at high pH), although competitive adsorption of en will probably also be important. The lack of adsorption of Cu ÷ is confirmed by the fact that, although the stripping peak corresponds to two electrons, no peak is present if the previously attained cathodic potential in a particular cycle is not sufficiently negative to result in the reduction to the metal, i.e. only Cu ÷ produced as a result of Cu ° oxidation results in a peak. Cu + resulting from Cu E+ reduction is desorbed before sufficiently anodic potentials are attained for its oxidation. The Voltammetry o f Cu z ÷ in Acetic Acid-Acetate Media
The cyclic polarogram of copper(II) in acetic acid medium at pH 2.5 (1.0 M acetic acid) is characterized by a poorly defined irreversible double wave corresponding to the step-wise reduction Cu z ÷ ~ C u + ~ C u °, accompanied by a small pre-wave (see Fig. 6). A large cathodic peak appears as the limiting current of the second wave is attained. The anodic scan is marked by either one large, or one large and one small stripping peak, depending on the cathodic potential at which the scan is initiated. The situation is very similar to that found in NaC104 except that, in this case, the large stripping peak corresponds to two electrons (as determined by cutting out, weighing, and comparing the area under the reduction wave and the stripping peak, corresponding to 10 different cathodic potential values); further, .no stripping peaks appear if the preceding cathodic potential is not sufficiently negative to reduce copper to the metal. From these dependence, and in analogy with the preceding systems, the following assignment seems probable: Cathodic w a v e s :
CH 2+ ~ CUa'~-~-CU+n__ad
Cu + ~ Cu ° Cathodic pre-wave:
Cu 2+ ~ Cth~,micro analogous to peak a, NH4NO s medium
Cathodic peak:
Cth-~-~-CLLa +, micro ~
Anodic peaks:
(1) C u ° ~ C u z+ +Cu~,micro. (2) CtL,~,micro*~Cu2 +
Cu°
analogous to cathodic peak, NaC104 medium
where ad =adsorbed, non-ad--non-adsorbed, micro refers to a more firmly bound layer next to the electrode surface, and * refers to the species produced only from reaction (1). At higher pH values (5-6), the peaks and waves, which were here correlated to adsorption phenomena, disappear, and the anodic stripping peak appears to correspond to less than two electrons, especially when more negative potentials precede the anodic scan. This effect can presumably be correlated to the greater complexing ability of acetate for Cu ÷ at higher pH values, analogously to the other systems discussed above.
CYCLIC V O L T A M M E T R Y O F C U II AT GLASSY CARBON
.
't-0.5
O
.
.
.
.
-0.5
.
.
E/v
125
.
-I.O
Fig. 6. The cyclic voltammogram of Cu 2+ in acetic acid. pH 2.6; [Cu z+] = 2 x 10 ~ M; [ C H a O O H ] = 1 M; conditions as in Fig. 1; broken line as in Fig. 4.
DISCUSSION
The character of the voltammetric curves of copper(II) on the glassy carbon r.d.e, seems to depend to a great extent on the degree to which monovalent copper, produced as a product of an electrochemical reaction, remains adsorbed on the electrode surface sufficiently long to undergo further electrochemical reaction. The formation of monolayers or ad-atoms of metallic copper, frequently noted at platinum and other metallic electrodes (e.9. ref. 11), does not appear to play a significant role here, and, with the possible exception of anodic peak 4 in NH4NO 3NH4OH medium, evidence is entirely lacking for such phenomena. The degree of adsorption of monovalent copper seems to depend to a great degree on the medium employed, and, not surprisingly, there seems to be a correlation between the complexing strength of the medium and the degree of desorption of Cu +, although competitive adsorption of other electrolyte species will also be important. An exact correlation however, is impossible due to the lack of tabulated stability constant values for monovalent copper ion (because of its instability in most aqueous solutions). An interesting phenomenon was noted in that, in many of the systems studied, the small peaks observed were of similar size. Although Cu ° ad-atom formation does not seem to occur, it is entirely possible that monovalent copper is specifically adsorbed. To further examine this idea, the sizes of these peaks were compared and, in each case, the fraction of the geometrical surface which would be covered by the number of atoms corresponding to the area under each peak was calculated (see Table 1). In these calculations, an ionic radius of 0.93 /~ was assumed for Co +12. The average surface coverage is 0.36, or about 1/3 of the geometrical area. The values are quite close, the differences presumably being attributable to the experimental error involved in measuring the peak areas (the peaks were often quite flat and drawn-out), although it is not inconceivable that the adsorption-active surface area could vary somewhat with the electrolytic medium employed (due to adsorption of other solution components, etc.). These results suggest that, in the systems studied, this ad-ion layer is somewhat more resistive than normally adsorbed Cu + to the dissolution effects of a complexing medium, even at a positively charged electrode. The waves in most cases seem to be suitable for analytical use, being linearly dependent on concentration, and the stripping peaks also on pre-electrolysis time
126
M. ,~TUL]KOVA, F. VYDRA
TABLE 1 COMPARISON OF FRACTION OF ELECTRODE SURFACE COVERED IN VARIOUS SYSTEMS, ASSUMING SECONDARY STRIPPING PEAKS CORRESPOND TO ADSORBED MONO-VALENT COPPER System and peak designation b
CH3COOH, pH 2.6, a.p. 2 KNO 3 a.p. 2, stationary electrode NH4NO 3 c.p. a c.p. b c.p. c a.p. 2 a.p. 3 NaC10 4 a.p. 1 a.p. 2
Peak area
(xlO 6)
Coulombs
Moles ( x l O -a°)
Ions ( x l O 14)
Fraction of surface" covered
18.30 14.20 14.20 13.50 14.46 11.40 13.80 20.80 14.82
1.90 1.47 1.47 1.40 1.50 1.18 1.44 2.02 1.54
1.14 0.88 0.88 0.84 0.90 0.71 0.86 1.30 0.93
0.44 0.39 0.39 0.32 0.35 0.27 0.33 0.50 0.36
Geometrical area. b a.p. =anodic peak, c.p. =cathodic peak.
a
and potential in the usual manner. F o r anodic stripping analysis, it w o u l d be preferable to choose a m e d i u m in which C u ÷ is also oxidized, preferably simultaneously in one peak with C u °, because of the resulting increase in sensitivity. ACKNOWLEDGEMENT The a u t h o r s wish to thank Dr. K. ~tulik, D e p a r t m e n t Chemistry, Charles University, for his interest and helpful remarks.
of Analytical
SUMMARY T h e cyclic v o l t a m m e t r y of c o p p e r ( I I ) at the glassy c a r b o n rotating disk electrode has been studied in a n u m b e r of media with a view to its usefulness in analytical applications. It has been found that, while C u 2 ÷ is generally reduced to Cu °, the stripping scan does not necessarily result in reoxidation to C u 2+, the reaction often ending with oxidation of C u ° to m o n o v a l e n t copper. This p h e n o m e n o n has been related to the degree to which m o n o v a l e n t copper, p r o d u c e d at the electrode surface, remains a d s o r b e d long e n o u g h to u n d e r g o further reaction, which is, in turn, strongly affected by the medium.
REFERENCES 1 F. Vydra and M. ~tulikovfi, Collect. Czech. Chem. Commun., 37 (1972) 123. 2 M. Kopanica and F. Vydra, J. Electroanal. Chem., 31 (1971) 175 3 M. ~tulikovfi and F. Vydra, J. Electroanal. Chem., 38 (1972) 349~
CYCLIC VOLTAMMETRY OF CU II AT GLASSY CARBON 4 5 6 7 8 9 10 ll 12
127
F. Vydra, M. ~tulikov& and P. Pet&k, J. Electroanal. Chem., 40 (1972) 99. C. E. Plock, Anal. Chim. Acta, 53 (1971) 249. H. E. Zittel and F. J. Miller, Anal. Chem., 37 (1965) 200. H. Sunahara and T. Ishizuka, Rev. Polarogr. (Kyoto), 14 (1967) 176. V. E. Levich, Physico-Chemical Thermodynamics, State Publishing House of Physico-Mathematical Literature, Moscow, 1959. L. Meites, Handbook of Analytical Chemistry, McGraw-Hill, New York, 1963. J. Koryta,.Electrochim. Acta, 6 (1962) 67. G. W. Tindall and S. Bruckenstein, J. Electroanal. Chem., 22 (1969) 367; Anal. Chem., 40 (1968) 10~1. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Interscience, New York, 1962.