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Specific influence of electrolysis potential on the Zn-peak in chronovoltamperometry on the HMDE
ELECTROANALYTICALCHEMISTRYAND INTERFACIALELECTROCHEMISTRY Elsevier Sequoia S.A., Lausanne Printedin The Netherlands
23
S P E C I F I C I N F L U E N C E OF E L E C T R O L Y S I S P O T E N T I A L O N T H E Z n - P E A K IN C H R O N O V O L T A M P E R O M E T R Y O N T H E H M D E
ZBIGNIEW P. ZAGORSKI* AND ZENON LUKASZEWSKI hlstitute of Fundamental Chemical Problems, Technical University of Poznah (Poland)
(Received 30th September 1971)
INTRODUCTION In anodic chronovoltammetry on the hanging mercury drop electrode (HMDE), the obvious influence of the potential of the preliminary electrolysis (Eel) on the height of the peak (hpeak), has been widely investigated. The influence of Eel on the potential of the dissolution peak (Ev) has a less obvious explanation ; this effect was observed by Kemula and coworkers 1'2 in the presence of surface active compounds. This paper deals with a case in which the influence of Eel on Ep occurs without the addition of compounds oriented at the interface between mercury and solution, but is due to adsorption effects in the electrode reactions themselves. EXPERIMENTAL The investigation was carried out with the mercury drop hanging on a silver tip, because the classical H M D E cannot be used in strongly alkaline solutions. This type of electrode was found to be appropriate 3'4 in the determination of zinc. The drop hanging on a Pt contact is far less convenient for preparation 5 and is inferior in reproducibility. Although intermetallic compounds between zinc and silver have been detected in voltammetry on the H M D E (hanging mercury drop electrode) 6'7, at very low concentrations their effect is insignificant s and in our investigation they did not change the course of the phenomena involved. Immediately before each experiment, the mercury drop was deposited on the Ag (99.99 + °,'~i)tip (0.2 mm in diameter) embedded in polythene, by electrolysis in a saturated acidic mercurous nitrate solution (60 mA during 20 s), rinsed with water and dried. The freshly prepared electrode was inserted into a vessel, typical for the hanging drop technique. The solution investigated was swept with purified nitrogen, mixed with a magnetic stirrer and thermostatted at 20°C. The counter electrode was an SCE of large surface area and sufficient capacity 9. A third electrode was used continuously for the measurement of the potential of the H M D E . Potentials were measured with a millivoltmeter of high input resistance during potential scanning. * Permanent address: Institute of Nuclear Research, Dorodna 16, Warsaw 91, Poland (to which all correspondence should be addressed). J. Electroanal. Chem., 36 (1972)
24
Z. P. ZAGORSKI, Z. -EUKASZEWSKI
All curves were recorded with a LP-60 polarograph (Laboratorni Pfistroje, Czechoslovakia), usually with a scanning rate of 400 mV m i n - 1. Every 10 or 20 mV the pen was lifted for a few seconds to mark the true potential of the electrode measured independently. This procedure prevented erroneous results in cases where there are comparatively high intensities of current. This happens in analytical applications of the method developed 1°, but not in the present investigation, owing to the low concentrations of zinc ions involved. All reagents were purified, especially the concentrated N a O H and K O H solutions; the latter by electrolysis with a mercury cathode. A typical experiment was arranged as follows: the de-aerated solution was subjected to a preliminary electrolysis during uninterrupted mixing at a potential (Eel) adjusted to the desired value in the range - 1.60 to - 2 . 0 5 V (SCE), under a constant stream of nitrogen. After 9 min the potential was shifted to - 1.69 V (in the case of 5 M hydroxides, to - 1.70 V), both nitrogen and mixer were switched off, and after 1 rain the scanning was carried out from - 1.60 to +0.20 V. F r o m the resulting diagram the height of the peak (h), the width of peak (a) and the potential of the peak (Ep), were determined. RESULTS The measurements in N a O H solutions (0.l 8.7 M) and K O H solutions (0.1 9.0 M) at a concentration of zinc ions of 4 x 10-7 M have shown that both the height of the Zn peak (h) and the potential of the peak (Ep) depend on the potential of the preliminary zinc deposition. Figure 1 shows the frequently encountered, characteristic S-shaped dependence of hpeak on the potential of deposition. It is more distinct at low K OH concentrations ; at CKoH > 5 M it is already unrecognizable. The shapes of the h curves for Zn deposited from N a O H solutions are similar. Figure 2a illustrates the effect of peak-shift caused by the changed potential of deposition. This Figure shows also that the width of the peak is substantially widened with a lowering of the potential of electrolysis. The shift of the peak seems to
¢,
h/men) a~M KOI4
0.SMKOH
agt4~ p
4.~M KOH
3.0M
5,Ot~KOtt/
8.OMKOH
~X 60 6O 0
2'O
- 7
-Lg
-L7
-4.9
-4,7
-
-~.7
-L9
-4.7
-1.9
-~,7
-t.9
-~.7
-1,9 £et
CE)
Fig. 1. Influenceof electrolysispotential Eelon the peak height h in KOH solns. Zn ions concn.4 x 10- 7 M ; duration of electrolysis9 min; h = 100 divisions=0.535 x 10-6 A. Y. Electroanal. Chem., 36 (1972)
CHRONOVOLTAMPEROMETRYOF Zn AT H M D E
25
Q
-•2-0.4-0.6
-0.0-LO q.2 -1.4-l.O
A
~
-0.2-0.~ -0.6 ~.O -1,0-t.2 -1.4 -L6
direction of ~oltoge
scan
. ~3 . 0,9-0.7 . .
0.9 ~. -,/.3 -1.5-L7
C
ElY
Fig. 2. Peaks of Zn stripping with and without dependence of Ep on key (A) Peaks of Zn dissolution after electrolysis of 1.5 M KOH containing 4x 10 7M Zn ions at (a) -1.95, (b) -1.80, (c) -1.65 V/SCE. (B) Solution as (A) but with 0.0025°,,ogelatine. (C) Peaks of Zn dissolution after electrolysisof 5.0 M KOH, 4 x 10-v M Zn, without gelatine, after electrolysis at (a) - 1.95, (b) - 1.80, (c) - 1.75, (d) - 1.70 V. be an unusual phenomenon. Different conditions may, however, be created in this system, when each chosen deposition potential is encountered by the constant potential of the Zn dissolution peak. These a r e : a d d i t i o n of gelatine (Fig. 2B) or increase of K O H concentration above 5 M (Fig. 2C). V o l t a m m o g r a m s "B" and "C", obtained with the same technique, as in case "A", imply that the observed shift of the peak is not an instrumental error. Figure 3 summarises the changes of height, shift of the peak potential and the half-height width as a function of electrolysis potential for 0.5 M K O H solution. Changes of the width and the shift of peak potential occur only at hydroxide concentrations not exceeding 5 M. At higher concentrations (Fig. 4) the change of electrolysis potential results only in a slight change of h, but not of Ep. It was interesting tO compare these effects in solutions without surface active compounds to those in which gelatine is present. Keeping the same Zn concentration (4 x 10 v M), and a constant concentration of gelatine (2.5 x 10-3 O/o),only the K O H concentration was changed (0.1, 0.5 and 1.5 M). Peaks belonging to the upper plateau (on the E v vs. Eel curve) are lower in the presence of gelatine. In this case, the peak potentials E v do not depend on Eel and have values close to those obtained at a low electrolysis potential and in the absence of gelatine. In order to discover at which stage the action of gelatine is manifested, the electrolysis in 1.5 M K O H with the usual concentration of zinc was carried out without gelatine, but immediately before J. Electroanal. Chem., 36 (1972)
26
Z. P. ZAG6RSKI, Z. ,EUKASZEWSKI
s/.,v
/
200
(c)
t6o ~20
'
EJv -~.~o
~
E~/v
-
(b) - 1.30
_
E,~/v
h/mm ~oo
80
(a)
60
,~0
20
} -I.#0
Fig. 3. Dependence of(a) peak height, (b) peak potential, (c) half-height width on the potential of preliminary electrolysis in 0.5 M KOH. Concn. etc., as in Fig. 1.
Ep/V
£4 1.3
O.tHKOH
OOHKOH
O.gMKOH
i,DM KOH
~oMKOH
~O NKOH
~OMKOH
j...j,, jr f . / -
~,~ i
t.8
e.o
t.e
~.o
/~'2~o
i
4,8
J
1
2,0
,
.
i
L8
i
2,0
,
,
L8
¢
i
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'
I
"i
~.8
I
2.0 Eez/V
Fig. 4. Dependence of peak potential Ep on electrolysis potential at various hydroxide concns. Cohen. etc., as in Fig. 1.
stripping, the standard portion of gelatine was added. A distinct shift in the potential of the peak (Fig. 5), equal to that in the case when gelatine was added before the electrolysis, shows clearly that this surface active compound is influencing the Zn dissolution process rather than the deposition. Results which do not contribute to the understanding of the system were omitted, e. 9. the influence of the scan-rate was investigated but the relative changes of the peak heights and potentials were low and did not change the predicted sequence of events. J. Eleetroanal. Chem., 36 (1972)
CHRONOVOLTAMPEROMETRY OF Z n AT H M D E
27
\ -a2-a~-ae-as-t.o-4.2-t~
E/q,~.sce)
~'~dimtioaof ¢0#090 scan
Fig. 5. Influence of gelatine added after preliminary electrolysis. (a) Stripping without gelatine, (b) stripping after addition of 0.0025°,0 gelatine. 1.5 M KOH, other concns, etc., as in Fig. 1.
Other experiments which are worthy of mention are the following. Carbonates up to 0.1 M and chlorides up to 10 3 M do not change the picture. The presence of oxygen or hydrogen peroxide (up to 3 x 10-4%) only made the background worse. The use of a platinum contact instead of the silver one again worsens the background and lowers the sensitivity. Increase of concentration of Zn ions was studied in the analytical part ~° of the work. The relation is satisfactory for analytical applications and does not indicate that there is any change in the mechanism from that for the standard concentration of 4 x 10-7 M used throughout this part of the paper. For the determination of traces of Zn in KOH or Na OH the highest concentrations of alkalies are advisable: the adsorption phenomena no longer occur and the slightly lower sensitivity, due to the increased viscosity, is unimportant. DISCUSSION AND ADDITIONAL EXPERIMENTS
The Ep vs. Eel and h vs. Eel curves reported are different from the typical curves of anodic voltammetry (cf Neeb 1~). The unusual behaviour of Zn ions in alkaline solutions at different E~l cannot be explained, for example, by the differing amounts of codeposited Na or K or by formation of intermetallic compounds. The similarity of the phenomenon to that observed in the presence of gelatine suggests the decisive role played by adsorption. The working hypothesis adopted during the work is that the effect consists in the desorption or destruction of a substance covering the surface of the electrode at lower potentials. Because the values of E~r at which the inflection of h and of Ep o c c u r s suggest the highest increment of the stripped Zn, the absorbed substance may be identified as J. Electroanal. Chem., 36 (1972)
28
Z.P.
ZAGORSKI, Z. -EUKASZEWSKI
a Zn compound. Also, compounds without Zn in this simple and pure system are unlikely. According to Landsberg et al. 12, in the reduction of Zn ions in alkaline solutions on the mercury electrode, Zn(OHJz is formed as an intermediate compound. To explain the responsibility of this compound for the S-shape of the E p E e ~ curves, the desorption of Zn(OH)z between - 1.8 and - 1.9 V (SCE) has to be assumed. To confirm the hypothesis, 1.5 M KOH, containing 4 x 10- v M Zn z + was electrolysed for 9 min at - 1.95 V, at which Zn(OH)2 would not be formed, and then additionally at - 1.65 V for 3 min. The peak thus obtained (b in Fig. 6) is shifted in the anodic direction in comparison to the original peak obtained without the additional electrolysis. On the contrary, beginning with 9 rain of electrolysis at a low potential ( - 1.75 V) and additionally for only 5 s at - 1.95 V the peak is shifted negatively (b in Fig. 7). An additional electrolysis at"high" potential for 15 s shows further progress in the cleaning of the surface, accompanied by an increase in the peak. Sixty seconds of additional electrolysis indicates the full decomposition of Zn(OH)> In the presence of gelatine, the effect of E p shifted as the function of Eel disappears, because the destruction of the zinc hydroxide layer is not sufficient to unclog the surface of the electrode, the latter being protected by the adsorbed protein. Also it is not known whether the Zn(OH)2 layer does exist in the presence of gelatine. It is difficult to make precise calculations of the thickness of the zinc hydroxide layer, mainly because of the unknown orientation of this compound on the mercury
\
-o~ -o.4 -0.6 -o~ -Lo -~2 - ~ ~ireetion of uoltage scan
/ -02 -a~ -o~ -o~ -to -~.2 -4J -~di~ction off volto~ scan
e/vt~n.scE)
Fig. 6. Lowering and shift of Zn peak due to additional electrolysis at lower potential. (a) 9 rain at - 1.95 V only, (b) 9 min at - 1.95 V and 3 rain at - 1.65 V/SCE. Fig. 7. Cleaning of electrode surface due to a short increase of electrolysis potential (a) 9 min at - 1.75 V, (b) 9 rain at - 1.75 V + 5 s at - 1.95 V, (c) 9 rain at - 1.75 V + 15 s at - 1.95 V, (d) 9 rain at - 1.75 V + 60 s at - 1.95 V.
J. Electroanal. Chern., 36 (1972)
CHRONOVOLTAMPEROMETRYOF Zn AT H M D E
29
surface. The electrical measurements a n d the electrode surface are k n o w n precisely enough. The a m o u n t s of electrical charge used for cleaning the surface a n d the m o s t probable assumption of the surface area occupied by the a d s o r b e d molecule, indicates the occurrence of Zn(OH)2 as a m o n o m o l e c u l a r layer. ACKNOWLEDGEMENT The authors thank Professor W i k t o r K e m u l a of the Polish A c a d e m y of Sciences for fruitful discussions. SUMMARY 1. A d s o r p t i o n on the H g hanging d r o p electrode, established during the preliminary electrolysis, influences the Ep and a of the a n o d i c p e a k during the subsequent stripping process. 2. In the particular case of zinc ions in alkaline solutions, at a low electrolysis potential (Ee0, Zn(OH)2 is formed as an intermediate a n d is a d s o r b e d at the electrode interface. At higher potentials this layer disappears. These potentials are ca. - 1.92 V (SCE) in 0.1-1 N K O H or - 1.82 V (SCE) in 1.5-3 N K O H . The influence of this layer in the stripping process is similar to that of other a d s o r b e d layers, e.g. of gelatine. 3. The effect described is of i m p o r t a n c e in the selection of o p t i m u m conditions in the technique of anodic stripping voltammetry. 4. A l t h o u g h observed in the case of zinc ions, the effect described m a y also occur in other systems. Therefore a study of the influence of Eel on Ep and a is always advisable and should not be limited to the influence on h.
REFEREN CES 1 2 3 4
W. KEMULA,Z. KUBLIKAND S. GLODOWSKI,J. Electroanal. Chem., 1 (1959/1960) 9. W. KEMULAAND S. GLODOWSKI,Rocz. Chem., 36 (1962) 1203. E. N. VINOGRADOVA,L. N. VASILEVAAND K. IOBST,Zavod. Lab.,27 (1961) 525. E. N. VINOGRADOVAANDA. I. KAMENEV,Tr. Kom. po Anal. Khim., Akad. Nauk SSSR, lnst. Geokhim. i Anal. Khhn., 15 (1965) 175. 5 A. A. KAI'LIN,B. F. NAZAROV,M. S. ZAKHAROVANDV. S. ZHIKINA,IZV. Tomsk. Politekh. Inst., 128 (1964} 31. 6 W. KEMULA,Z. GALUSANDZ. KUBLIK,Bull. Acad. Pol. Sci., Ser. Sci., Chim., Geol. Geogr., 6 (1958) 661. 7 W. KEMULA,Z. GALUSANDZ. KUBUK,Bull. Acad. Pol. Sci., Set. Sci., Chim. Geol. Geogr., 7 (1959) 613, 723. 8 F. VON STURM AND M. RESSEL, Z. Anal. Chem., 186 (1962) 63. 9 Z. P. ZAG6RSKI,Proe. 2nd Polarogr. Congr., Vol. 3, Pergamon, Oxford, 1960, p. 1132. 10 Z. LUKASZEWSKIAND Z, P. ZAG6RSKI,Chem. Anal. Warsaw, 15 (1970) 1213. 11 R. NEEm Inverse Polarographie und Voltammetrie, Akademie-Verlag, Berlin, 1969; Claemie Verlag, Weinheim, 1969. 12 R. LANDSBERG,V. STOCKMaNNANDW. GEISSLER,Z. Phys. Chem. Leipzig, 217 (1961) 368. J. Eleetroanal. Chem., 36 (1972)
23
S P E C I F I C I N F L U E N C E OF E L E C T R O L Y S I S P O T E N T I A L O N T H E Z n - P E A K IN C H R O N O V O L T A M P E R O M E T R Y O N T H E H M D E
ZBIGNIEW P. ZAGORSKI* AND ZENON LUKASZEWSKI hlstitute of Fundamental Chemical Problems, Technical University of Poznah (Poland)
(Received 30th September 1971)
INTRODUCTION In anodic chronovoltammetry on the hanging mercury drop electrode (HMDE), the obvious influence of the potential of the preliminary electrolysis (Eel) on the height of the peak (hpeak), has been widely investigated. The influence of Eel on the potential of the dissolution peak (Ev) has a less obvious explanation ; this effect was observed by Kemula and coworkers 1'2 in the presence of surface active compounds. This paper deals with a case in which the influence of Eel on Ep occurs without the addition of compounds oriented at the interface between mercury and solution, but is due to adsorption effects in the electrode reactions themselves. EXPERIMENTAL The investigation was carried out with the mercury drop hanging on a silver tip, because the classical H M D E cannot be used in strongly alkaline solutions. This type of electrode was found to be appropriate 3'4 in the determination of zinc. The drop hanging on a Pt contact is far less convenient for preparation 5 and is inferior in reproducibility. Although intermetallic compounds between zinc and silver have been detected in voltammetry on the H M D E (hanging mercury drop electrode) 6'7, at very low concentrations their effect is insignificant s and in our investigation they did not change the course of the phenomena involved. Immediately before each experiment, the mercury drop was deposited on the Ag (99.99 + °,'~i)tip (0.2 mm in diameter) embedded in polythene, by electrolysis in a saturated acidic mercurous nitrate solution (60 mA during 20 s), rinsed with water and dried. The freshly prepared electrode was inserted into a vessel, typical for the hanging drop technique. The solution investigated was swept with purified nitrogen, mixed with a magnetic stirrer and thermostatted at 20°C. The counter electrode was an SCE of large surface area and sufficient capacity 9. A third electrode was used continuously for the measurement of the potential of the H M D E . Potentials were measured with a millivoltmeter of high input resistance during potential scanning. * Permanent address: Institute of Nuclear Research, Dorodna 16, Warsaw 91, Poland (to which all correspondence should be addressed). J. Electroanal. Chem., 36 (1972)
24
Z. P. ZAGORSKI, Z. -EUKASZEWSKI
All curves were recorded with a LP-60 polarograph (Laboratorni Pfistroje, Czechoslovakia), usually with a scanning rate of 400 mV m i n - 1. Every 10 or 20 mV the pen was lifted for a few seconds to mark the true potential of the electrode measured independently. This procedure prevented erroneous results in cases where there are comparatively high intensities of current. This happens in analytical applications of the method developed 1°, but not in the present investigation, owing to the low concentrations of zinc ions involved. All reagents were purified, especially the concentrated N a O H and K O H solutions; the latter by electrolysis with a mercury cathode. A typical experiment was arranged as follows: the de-aerated solution was subjected to a preliminary electrolysis during uninterrupted mixing at a potential (Eel) adjusted to the desired value in the range - 1.60 to - 2 . 0 5 V (SCE), under a constant stream of nitrogen. After 9 min the potential was shifted to - 1.69 V (in the case of 5 M hydroxides, to - 1.70 V), both nitrogen and mixer were switched off, and after 1 rain the scanning was carried out from - 1.60 to +0.20 V. F r o m the resulting diagram the height of the peak (h), the width of peak (a) and the potential of the peak (Ep), were determined. RESULTS The measurements in N a O H solutions (0.l 8.7 M) and K O H solutions (0.1 9.0 M) at a concentration of zinc ions of 4 x 10-7 M have shown that both the height of the Zn peak (h) and the potential of the peak (Ep) depend on the potential of the preliminary zinc deposition. Figure 1 shows the frequently encountered, characteristic S-shaped dependence of hpeak on the potential of deposition. It is more distinct at low K OH concentrations ; at CKoH > 5 M it is already unrecognizable. The shapes of the h curves for Zn deposited from N a O H solutions are similar. Figure 2a illustrates the effect of peak-shift caused by the changed potential of deposition. This Figure shows also that the width of the peak is substantially widened with a lowering of the potential of electrolysis. The shift of the peak seems to
¢,
h/men) a~M KOI4
0.SMKOH
agt4~ p
4.~M KOH
3.0M
5,Ot~KOtt/
8.OMKOH
~X 60 6O 0
2'O
- 7
-Lg
-L7
-4.9
-4,7
-
-~.7
-L9
-4.7
-1.9
-~,7
-t.9
-~.7
-1,9 £et
CE)
Fig. 1. Influenceof electrolysispotential Eelon the peak height h in KOH solns. Zn ions concn.4 x 10- 7 M ; duration of electrolysis9 min; h = 100 divisions=0.535 x 10-6 A. Y. Electroanal. Chem., 36 (1972)
CHRONOVOLTAMPEROMETRYOF Zn AT H M D E
25
Q
-•2-0.4-0.6
-0.0-LO q.2 -1.4-l.O
A
~
-0.2-0.~ -0.6 ~.O -1,0-t.2 -1.4 -L6
direction of ~oltoge
scan
. ~3 . 0,9-0.7 . .
0.9 ~. -,/.3 -1.5-L7
C
ElY
Fig. 2. Peaks of Zn stripping with and without dependence of Ep on key (A) Peaks of Zn dissolution after electrolysis of 1.5 M KOH containing 4x 10 7M Zn ions at (a) -1.95, (b) -1.80, (c) -1.65 V/SCE. (B) Solution as (A) but with 0.0025°,,ogelatine. (C) Peaks of Zn dissolution after electrolysisof 5.0 M KOH, 4 x 10-v M Zn, without gelatine, after electrolysis at (a) - 1.95, (b) - 1.80, (c) - 1.75, (d) - 1.70 V. be an unusual phenomenon. Different conditions may, however, be created in this system, when each chosen deposition potential is encountered by the constant potential of the Zn dissolution peak. These a r e : a d d i t i o n of gelatine (Fig. 2B) or increase of K O H concentration above 5 M (Fig. 2C). V o l t a m m o g r a m s "B" and "C", obtained with the same technique, as in case "A", imply that the observed shift of the peak is not an instrumental error. Figure 3 summarises the changes of height, shift of the peak potential and the half-height width as a function of electrolysis potential for 0.5 M K O H solution. Changes of the width and the shift of peak potential occur only at hydroxide concentrations not exceeding 5 M. At higher concentrations (Fig. 4) the change of electrolysis potential results only in a slight change of h, but not of Ep. It was interesting tO compare these effects in solutions without surface active compounds to those in which gelatine is present. Keeping the same Zn concentration (4 x 10 v M), and a constant concentration of gelatine (2.5 x 10-3 O/o),only the K O H concentration was changed (0.1, 0.5 and 1.5 M). Peaks belonging to the upper plateau (on the E v vs. Eel curve) are lower in the presence of gelatine. In this case, the peak potentials E v do not depend on Eel and have values close to those obtained at a low electrolysis potential and in the absence of gelatine. In order to discover at which stage the action of gelatine is manifested, the electrolysis in 1.5 M K O H with the usual concentration of zinc was carried out without gelatine, but immediately before J. Electroanal. Chem., 36 (1972)
26
Z. P. ZAG6RSKI, Z. ,EUKASZEWSKI
s/.,v
/
200
(c)
t6o ~20
'
EJv -~.~o
~
E~/v
-
(b) - 1.30
_
E,~/v
h/mm ~oo
80
(a)
60
,~0
20
} -I.#0
Fig. 3. Dependence of(a) peak height, (b) peak potential, (c) half-height width on the potential of preliminary electrolysis in 0.5 M KOH. Concn. etc., as in Fig. 1.
Ep/V
£4 1.3
O.tHKOH
OOHKOH
O.gMKOH
i,DM KOH
~oMKOH
~O NKOH
~OMKOH
j...j,, jr f . / -
~,~ i
t.8
e.o
t.e
~.o
/~'2~o
i
4,8
J
1
2,0
,
.
i
L8
i
2,0
,
,
L8
¢
i
2.0
'
I
"i
~.8
I
2.0 Eez/V
Fig. 4. Dependence of peak potential Ep on electrolysis potential at various hydroxide concns. Cohen. etc., as in Fig. 1.
stripping, the standard portion of gelatine was added. A distinct shift in the potential of the peak (Fig. 5), equal to that in the case when gelatine was added before the electrolysis, shows clearly that this surface active compound is influencing the Zn dissolution process rather than the deposition. Results which do not contribute to the understanding of the system were omitted, e. 9. the influence of the scan-rate was investigated but the relative changes of the peak heights and potentials were low and did not change the predicted sequence of events. J. Eleetroanal. Chem., 36 (1972)
CHRONOVOLTAMPEROMETRY OF Z n AT H M D E
27
\ -a2-a~-ae-as-t.o-4.2-t~
E/q,~.sce)
~'~dimtioaof ¢0#090 scan
Fig. 5. Influence of gelatine added after preliminary electrolysis. (a) Stripping without gelatine, (b) stripping after addition of 0.0025°,0 gelatine. 1.5 M KOH, other concns, etc., as in Fig. 1.
Other experiments which are worthy of mention are the following. Carbonates up to 0.1 M and chlorides up to 10 3 M do not change the picture. The presence of oxygen or hydrogen peroxide (up to 3 x 10-4%) only made the background worse. The use of a platinum contact instead of the silver one again worsens the background and lowers the sensitivity. Increase of concentration of Zn ions was studied in the analytical part ~° of the work. The relation is satisfactory for analytical applications and does not indicate that there is any change in the mechanism from that for the standard concentration of 4 x 10-7 M used throughout this part of the paper. For the determination of traces of Zn in KOH or Na OH the highest concentrations of alkalies are advisable: the adsorption phenomena no longer occur and the slightly lower sensitivity, due to the increased viscosity, is unimportant. DISCUSSION AND ADDITIONAL EXPERIMENTS
The Ep vs. Eel and h vs. Eel curves reported are different from the typical curves of anodic voltammetry (cf Neeb 1~). The unusual behaviour of Zn ions in alkaline solutions at different E~l cannot be explained, for example, by the differing amounts of codeposited Na or K or by formation of intermetallic compounds. The similarity of the phenomenon to that observed in the presence of gelatine suggests the decisive role played by adsorption. The working hypothesis adopted during the work is that the effect consists in the desorption or destruction of a substance covering the surface of the electrode at lower potentials. Because the values of E~r at which the inflection of h and of Ep o c c u r s suggest the highest increment of the stripped Zn, the absorbed substance may be identified as J. Electroanal. Chem., 36 (1972)
28
Z.P.
ZAGORSKI, Z. -EUKASZEWSKI
a Zn compound. Also, compounds without Zn in this simple and pure system are unlikely. According to Landsberg et al. 12, in the reduction of Zn ions in alkaline solutions on the mercury electrode, Zn(OHJz is formed as an intermediate compound. To explain the responsibility of this compound for the S-shape of the E p E e ~ curves, the desorption of Zn(OH)z between - 1.8 and - 1.9 V (SCE) has to be assumed. To confirm the hypothesis, 1.5 M KOH, containing 4 x 10- v M Zn z + was electrolysed for 9 min at - 1.95 V, at which Zn(OH)2 would not be formed, and then additionally at - 1.65 V for 3 min. The peak thus obtained (b in Fig. 6) is shifted in the anodic direction in comparison to the original peak obtained without the additional electrolysis. On the contrary, beginning with 9 rain of electrolysis at a low potential ( - 1.75 V) and additionally for only 5 s at - 1.95 V the peak is shifted negatively (b in Fig. 7). An additional electrolysis at"high" potential for 15 s shows further progress in the cleaning of the surface, accompanied by an increase in the peak. Sixty seconds of additional electrolysis indicates the full decomposition of Zn(OH)> In the presence of gelatine, the effect of E p shifted as the function of Eel disappears, because the destruction of the zinc hydroxide layer is not sufficient to unclog the surface of the electrode, the latter being protected by the adsorbed protein. Also it is not known whether the Zn(OH)2 layer does exist in the presence of gelatine. It is difficult to make precise calculations of the thickness of the zinc hydroxide layer, mainly because of the unknown orientation of this compound on the mercury
\
-o~ -o.4 -0.6 -o~ -Lo -~2 - ~ ~ireetion of uoltage scan
/ -02 -a~ -o~ -o~ -to -~.2 -4J -~di~ction off volto~ scan
e/vt~n.scE)
Fig. 6. Lowering and shift of Zn peak due to additional electrolysis at lower potential. (a) 9 rain at - 1.95 V only, (b) 9 min at - 1.95 V and 3 rain at - 1.65 V/SCE. Fig. 7. Cleaning of electrode surface due to a short increase of electrolysis potential (a) 9 min at - 1.75 V, (b) 9 rain at - 1.75 V + 5 s at - 1.95 V, (c) 9 rain at - 1.75 V + 15 s at - 1.95 V, (d) 9 rain at - 1.75 V + 60 s at - 1.95 V.
J. Electroanal. Chern., 36 (1972)
CHRONOVOLTAMPEROMETRYOF Zn AT H M D E
29
surface. The electrical measurements a n d the electrode surface are k n o w n precisely enough. The a m o u n t s of electrical charge used for cleaning the surface a n d the m o s t probable assumption of the surface area occupied by the a d s o r b e d molecule, indicates the occurrence of Zn(OH)2 as a m o n o m o l e c u l a r layer. ACKNOWLEDGEMENT The authors thank Professor W i k t o r K e m u l a of the Polish A c a d e m y of Sciences for fruitful discussions. SUMMARY 1. A d s o r p t i o n on the H g hanging d r o p electrode, established during the preliminary electrolysis, influences the Ep and a of the a n o d i c p e a k during the subsequent stripping process. 2. In the particular case of zinc ions in alkaline solutions, at a low electrolysis potential (Ee0, Zn(OH)2 is formed as an intermediate a n d is a d s o r b e d at the electrode interface. At higher potentials this layer disappears. These potentials are ca. - 1.92 V (SCE) in 0.1-1 N K O H or - 1.82 V (SCE) in 1.5-3 N K O H . The influence of this layer in the stripping process is similar to that of other a d s o r b e d layers, e.g. of gelatine. 3. The effect described is of i m p o r t a n c e in the selection of o p t i m u m conditions in the technique of anodic stripping voltammetry. 4. A l t h o u g h observed in the case of zinc ions, the effect described m a y also occur in other systems. Therefore a study of the influence of Eel on Ep and a is always advisable and should not be limited to the influence on h.
REFEREN CES 1 2 3 4
W. KEMULA,Z. KUBLIKAND S. GLODOWSKI,J. Electroanal. Chem., 1 (1959/1960) 9. W. KEMULAAND S. GLODOWSKI,Rocz. Chem., 36 (1962) 1203. E. N. VINOGRADOVA,L. N. VASILEVAAND K. IOBST,Zavod. Lab.,27 (1961) 525. E. N. VINOGRADOVAANDA. I. KAMENEV,Tr. Kom. po Anal. Khim., Akad. Nauk SSSR, lnst. Geokhim. i Anal. Khhn., 15 (1965) 175. 5 A. A. KAI'LIN,B. F. NAZAROV,M. S. ZAKHAROVANDV. S. ZHIKINA,IZV. Tomsk. Politekh. Inst., 128 (1964} 31. 6 W. KEMULA,Z. GALUSANDZ. KUBLIK,Bull. Acad. Pol. Sci., Ser. Sci., Chim., Geol. Geogr., 6 (1958) 661. 7 W. KEMULA,Z. GALUSANDZ. KUBUK,Bull. Acad. Pol. Sci., Set. Sci., Chim. Geol. Geogr., 7 (1959) 613, 723. 8 F. VON STURM AND M. RESSEL, Z. Anal. Chem., 186 (1962) 63. 9 Z. P. ZAG6RSKI,Proe. 2nd Polarogr. Congr., Vol. 3, Pergamon, Oxford, 1960, p. 1132. 10 Z. LUKASZEWSKIAND Z, P. ZAG6RSKI,Chem. Anal. Warsaw, 15 (1970) 1213. 11 R. NEEm Inverse Polarographie und Voltammetrie, Akademie-Verlag, Berlin, 1969; Claemie Verlag, Weinheim, 1969. 12 R. LANDSBERG,V. STOCKMaNNANDW. GEISSLER,Z. Phys. Chem. Leipzig, 217 (1961) 368. J. Eleetroanal. Chem., 36 (1972)