Interfering influence of silver on the voltammetric behaviour of Cd2+ and Zn2+ ions on hmde in supporting electrolytes containing nitrates

J. Electroanal. Chem., 68 (1976) 193--202 © Elsevier Sequoia S.A., L a u s a n n e - Printed in The Netherlands

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INTERFERING INFLUENCE OF SILVER ON THE VOLTAMMETRIC B E H A V I O U R O F C d 2+ A N D Z n 2+ I O N S O N H M D E I N S U P P O R T I N G ELECTROLYTES CONTAINING NITRATES

PIOTR OSTAPCZUK and ZENON KUBLIK Institute o f Fundamental Problems o f Chemistry, University o f Warsaw, 02093 Warsaw, Pasteura I (Poland) (Received 2nd August 1975)

ABSTRACT A new kind of secondarily formed peaks was found in cyclic and stripping voltammetry in neutral sulphate, perchlorate and nitrate supporting electrolytes containing some divalent cations and a substance (for example 02) , the reduction of which gives as a by-product OH-- ions. The hydroxides deposited in the vicinity of the mercury electrode, in the course of a cathodic scan, react during the anodic scan according to the reaction Hg + Me(OH)2 = Hg(OH)2 + Me 2+ + 2e forming a new, separate anodic peak. It was found that silver exerts a catalytic effect on the reduction of NO~ ions on the mercury electrode. In neutral nitrate supporting electrolyte containing Ag + ions the hydroxides of some cations (Cd 2+, Zn 2+, Mn 2+, Co 2+ and Ni 2+) were deposited during the cathodic scan or during the preelectrolysis. Afterwards, in the course of the anodic scan, a new peak, of the kind described above, was observed. The same effect was formerly interpreted, for Zn 2+ and Cd 2+, as evidence for the formation of intermetallic compounds, AgZn and AgCd.

INTRODUCTION D u r i n g t h e c o u r s e o f i n v e s t i g a t i o n o f i n t e r m e t a l l i c c o m p o u n d s o f s i l v e r in m e r c u r y w e h a v e n o t e d t h a t t h e s t r i p p i n g c u r v e s o b t a i n e d w h e n A g ÷ a n d C d 2÷ i o n s w e r e b o t h p r e s e n t in s u p p o r t i n g e l e c t r o l y t e c o n t a i n i n g n i t r a t e , h a d a n a n o m a l o u s s h a p e c o m p a r e d w i t h c u r v e s o f t h e s a m e c a t i o n s o b t a i n e d in s u p porting electrolyte containing sulphate or perchlorate. The explanation of this a n o m a l o u s b e h a v i o u r o f n i t r a t e i o n s in t h e p r e s e n c e o f A g ÷ a n d C d 2+ i o n s is the aim of the present work. EXPERIMENTAL Voltammetric curves were recorded with a Radelkis OH-102 polarograph w i t h a 3 ~ e t e c t r o d e a r r a n g e m e n t . A s a t u r a t e d c a l o m e l e l e c t r o d e w a s u s e d as a r e f e r e n c e e l e c t r o d e a n d all t h e p o t e n t i a l s a r e r e p o r t e d r e l a t i v e t o t h i s e l e c t r o d e . Solutions were deaerated with gaseous hydrogen generated electrolytically

194

and washed through a column containing acidic solutions or" vanadous sulphate over zinc amalgam. Chemically pure reagents and water twice distilled in quartz were used to prepare all solutions. As a m ercury electrode, a HMDE of a t y p e described by Kemula and Kublik [1], with surface area equal to 3.8 mm 2 was used. The silver electrode and the silver based m ercury film electrode with geometric area equal t o 6.5 'mm 2 were prepared as described in ref. 2. RESULTS

Figure 1 illustrates the influence of Ag ÷ ions on the stripping curves of cadmium. Cadmium deposited in the HMDE f r o m nitrate supporting electrolyte in the absence o f silver ions gives a well-defined, reversible stripping peak

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Fig. 1. Stripping v o l t a m m e t r i c curves of c a d m i u m after 30 s preelectrolysis at --0.7 V in stirred solution, f o l l o w e d by 30 s rest period. (1 and 5) 1 x 10 - 3 M CdSO4, 1 M KNO3; (2) as 1 + 2 × 10 - 3 M AgC104; (3) as 1 + 2 X 10 - 3 M AgCIO 4+ 1 x 10 - 2 M HC104; (4) 1 X 10 - 3 M CdSO 4 + 2 x 10--3M AgClO4, 1 M NaC104. Voltage scan rate 1.5 V min - 1 . (1--4) pure m e r c u r y electrode, (5) saturated silver amalgam electrode. ( . . . . . . ) S e c o n d cycle.

195 (curve 1). However, when the deposition of cadmium is carried out in the presence of Ag ÷ ions, the stripping, voltammetric curve changes considerably (curve 2). The reversible dissolution peak of cadmium almost disappears and in the positive range of potentials a new anodic peak arises. This new peak does not correspond to silver dissolution because, under these conditions, silver is oxidized anodically together with mercury at a more positive potential. The effect described was observed before [3] and explained as the formation in mercury of an intermetallic compound, AgCd. The effect will not appear, however, if the neutral nitrate solution becomes acidic (curve 3), if it is changed for neutral sulphate or perchlorate supporting electrolyte (curve 4) and if silver is introduced into HMDE not by simultaneous deposition with cadmium, but by the dissolution of metallic silver in the HMDE (curve 5). The effect diminishes also, when the potential of preelectrolysis becomes increasingly negative. The stripping curves obtained in the case of simultaneous cadmium and silver deposition at --1.4 V have a normal, n o t disturbed shape. From the experiments described above it follows that the formation of the intermetallic c o m p o u n d AgCd in mercury is not so evident as it was supposed in the previous paper [3]. As shown on the curve 2 cyclic polarization in the positive potential range leads to the appearance of an anodic--cathodic system with peak potentials at about +0.3 and + 0.1 V. According to ref. 3 the anodic peak of this system reflects the oxidation of the intermetallic c o m p o u n d AgCd formed in mercury. On the basis of such interpretation it is, however, not possible to explain the appearance of the cathodic peak. The products of oxidation of the supposed intermetallic compound, i.e. Ag ÷ and Cd 2÷ ions could not form any cathodic peak at this potential range. The disappearance of the peaks described after acidification of solution indicates that they are rather connected with some secondary reactions leading to the formation of O H - ions near the electrode surface. It is known, that during anodic oxidation of mercury in the presence of O H - ions an adherent film of HgO is formed on the electrode surface. This film can be stripped during the cathodic scan with the formation of a cathodic peak. To demonstrate the presence of an insoluble mercury c o m p o u n d adherent strongly to the surface of mercury electrode we used the m e t h o d given in ref. 4. Namely, after withdrawing some of mercury to the electrode container, the hanging drop covered with an insoluble c o m p o u n d did not diminish uniformly, but it t o o k a shape of a hanging bag with many wrinkles. In our case, after 30 s deposition of cadmium and silver at --0.7 V, the electrode did not give any wrinkled bag when polarized to +0.2 V, and gave a very distinct, wrinkled bag at +0.4 V. The wrinkled bag will not be observed in the same solution at +0.4 V, when the concentrating electrolysis at --0.7 V is not performed. To verify further the supposition that the peaks arising in the positive potential range are connected with O H - ions formed in an, as yet, u n k n o w n secondary reaction, we decided to carry out few experiments in solutions containing various metal ions and dissolved oxygen.

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The influence o f oxygen on voltammetric curves o f Cd e+, Zn 2+, Cu2 ÷, Pb 2÷, Ni 2+, Co 2+ and M n 2+ ions in neutral supporting electrolytes. Figure 2 illustrates the influence of oxygen dissolved in solution on the cyclic curves of Cd 2+ ions. In the presence of oxygen a hump arises on the Cd 2+ ion reduction peak (curve 3). This hump will change to the second peak, if the electrode is polarized during some time in the potential range 0 to --0.5 V. The hump or the second, more negative peak reflects the reduction of insoluble cadmium hydroxide formed at the electrode surface in the secondary reactions occurring between Cd e+ ions and the products of the reduction of oxygen, i.e. O H - ions. In spite of the fact that cadmium hydroxide could not be oxidized anodically, still in its presence a new system of peaks will arise, if the electrode is

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Fig. 2. Cyclic v o l t a m m e t r i c curves o b t a i n e d with a HMDE for 1 M KNO 3 s o l u t i o n containing: (1) dissolved air; (2) 2.5 X 1 0 - - 3 M KOH, air-free; (3) 1 x 10 3 M CdSO4, dissolved air; (4) as 3 b u t air-free; (5) 1 × 10 - 3 M CdSO 4 + 2 x 10 - 3 M AgC104, air-free. Curve 5 was r e c o r d e d a f t e r 30 s p r e e l e c t r o l y s i s at =-0.5 V in stirred s o l u t i o n f o l l o w e d b y 30 s rest period. Voltage scan rate 1.5 V m i n - 1 . ( . . . . . . ) S e c o n d cycle.

197 polarized in a cyclic way in a range of potentials 0.0 to +0.4 V (curve 3). F r o m a comparison of curve 3 with 1 and 2 it is seen t hat the c o n c e n t r a t i o n of O H - ions needed for the f o r m a t i o n of a new peak is higher than the concentratfon o f O H - ions f o r m e d during the undisturbed reduct i on of oxygen dissolved in solution (curve 1). In the presence of cadmium ions, cadmium h y d r o x i d e is f o r m e d on the electrode surface and in such a way the source o f O H - ions increases considerably. Instead of reaction 1, reaction 2 takes place. Hg + 2 O H - = HgO + H20 + 2e

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The height of a new anodic peak depends on the concent rat i on of Cd 2+ ions in solutions, but only when this concent rat i on is lower than 6 × 10 - 4 M. It depends also on the time of generation of O H - ions but after 2 min of generation o f O H - ions the height of the peak attains an a p p r o x i m a t e l y constant value. The q u an ti t y of electricity needed for the f o r m a t i o n of the peak with maximal height is equal to 2.5 mC cm -2. The anodic effect caused by the simultaneous presence of oxygen and cadmium ions in solution is n o t specific for neutral nitrate solutions. Similar effects are observed in neutral supporting electrolytes containing sulphate or perchlorate ions. In the presence of C1- ions the effect is n o t observed. As shown in Fig. 3 the secondary peaks c o n n e c t e d with a Hg + Me(OH)2 = HgO + H20 + Me 2÷ + 2e

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reaction 3, are f o r m e d in the positive potential range n o t only for Cd 2+ ions but also for Cu 2+, Pb 2+, Zn 2+ and to a lesser degree for Ni 2÷ ions. Co 2÷ and Mn 2+ ions behave in a similar way to Ni 2+ ions. These secondary peaks are better defined when instead of cyclic polarization a concentrating electrolysis is p e r f o r m e d in a potential range insufficient for a reduction of the corresponding hydroxide. In acidic solution and in the absence of dissolved air these effects do n o t appear. In a neutral solution containing dissolved air and Fe 2+ ions a secondary, anodic peak is also observed, but it appears at more negative potential than the peak f o r m e d due to reaction 3 for ot her metal ions. It is probable that this peak reflects the oxidation of Fe(OH)2 to Fe{OH)3. In neutral solutions containing dissolved air and A13+ ions no secondary effects were observed in the positive potential range. As shown in Fig. 2, curve 5 the actions of Ag ÷ ions is very similar to the actions of oxygen. In bot h cases the reduction peak of Cd 2÷ ions is disturbed and new peaks appear in the positive potential range. These results m ay be interpreted on the basis of a catalytic action of silver. In the presence of silver decomposition of a substance present in high excess must proceed and as a product o f this d eco m pos i t i on O H - ions should be formed. Because the effect described appears in nitrate solution only it is reasonable to postulate t hat the nitrate ion is catalytically decomposed. To verify this supposition we decided

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Fig. 3. Effect of air dissolved in solution on cyclic v o l t a m m e t r i c curves of various cations. 1 M K N O 3 solution containing 1 × 10 - 3 mol 1-1 of (1) Cu 2+, (2) Pb 2+, (3) Ni 2+, (4) Zn 2+. (a) Obtained in the presence of dissolved air, (b) air-free solutions. Voltage scan rate 1.5 V

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The shape of the voltammetric curves obtained with a silver electrode in a solution containing NO~ ions changed distinctly only when the c o n c e n t r a t i o n of nitrate ions exceeded the value 1 X 10 - 2 M. The potential at which the current rose to 2 pA changed systematically from --1.2 V to --0.75 V when the c o n c e n t r a t i o n of NO3 ions increased from 0.01 to 0.5 M. The effect depended on the kind of preparation of the electrode. The electrode with a fresh deposited silver surface (the deposition was carried out in a cyanide bath) gave greater currents than a silver wire electrode.

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T o d e m o n s t r a t e t h a t parallel to t h e c u r r e n t rise, O H - ions are f o r m e d at the e l e c t r o d e surface, p h e n o l p h t h a l e i n was a d d e d t o the solution. Phenolp h t h a l e i n is r e d u c e d in neutral solutions at a p p r o x i m a t e l y - - 0 . 9 V [6] b u t at the c o n c e n t r a t i o n used n o a d d i t i o n a l wave was observed in the presence o f this substance. In solutions c o n t a i n i n g s o m e a m o u n t s o f p h e n o l p h t h a l e i n the electrode was polarized at a definite p o t e n t i a l f o r 15 s. I f d u r i n g this time the s o l u t i o n in the vicinity o f the e l e c t r o d e did n o t b e c o m e pink, the polarizing action was i n t e r r u p t e d . The s o l u t i o n was stirred for a m o m e n t and w h e n it b e c a m e q u i e t again the o p e r a t i o n was r e p e a t e d at m o r e negative p o t e n t i a l . In 1 M NaC10 4 as s u p p o r t i n g e l e c t r o l y t e the c h a n g e in c o l o u r was observed

at --I.I V. With the concentration of NOT ions equal to 1 × 10 -2, 2.5 X 10 -2, 5 X 10 -2 and 1 X 10 -I M the change of the colour appeared at potentials --0.9, --0.85, --0.8 and --0.7 V respectively. Figure 4 illustrates the dependence of the shape of the reduction curves of'

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Fig. 4. Effect of silver on the reduction of NO3 ions. (1,4--8)HMDE, (2)silver electrode, (3) silver based mercury electrode. (. . . . ) 1 M NaClO4, ( ) 1 M KNO 3 containing: (1) 0, (2) 0, (3) 0, (4) 1 X 10 - 3 M C d S O 4 + 1 X 10- a M AgC104, (5) 1 x 1 0 - - 3 M CdSO 4 + 2 X 10- 5 M AgC104, (6) 3 x 10- 3 M AgC104, (7) 2 x 10--aM AgC104, (8) 1 x 10- 3 M AgC104. (4--8) recorded after 30 s preelectrolysis at --0.5 V in stirred solution followed by 30 s rest period. Voltage scan rate 1.5 V min -1.

200 NO~- ions on the material o f the electrodes (silver or mercury) and on the presence of silver in the mercury electrode. The reduction potentials of NOT ions on pure mercury (curve 1) and on pure silver (curve 2) differ considerably. Hence, when during the reduction of Ag ÷ ions the silver deposits on mercury surface, the final current rise should be displaced to a less negative potential. On the other hand the reduction of NO~- ions on a pure mercury electrode (curve 1) and on saturated silver amalgam (curve 3) begins practically at the same potentials equal approximately to --1.5 V. Such a behaviour means that silver dissolved in mercury exerts no effect on the reduction of NOT ions. Hence, when during the reduction of Ag + ions the silver crystals arise, but sink into bulk of mercury, the final current rise should be unchanged. The real curves obtained with HMDE in neutral solutions of NOT ions in the presence of Ag* ions had more complicated shapes. At low concentration of Ag ÷ ions in solution only the fluid silver amalgam is formed and consequently only the small reduction current of Ag ÷ ions is observed. With increasing concentration of Ag ÷ ions in solution the current rises at less negative potential (curve 6 and 7), but after reaching a m a x i m u m it decays to the value observed in the absence of NOT ions. At last the second rise of the current appears at a potential at which NO3 ions are reduced on m e r c u r y . Close examinations with a magnifying glass reveals that parallel to the current rise tiny, black spots appear on the electrode surface. The change in colour observed in the presence of phenolphthalein begins just around these black spots. The spots do n o t appear when the electrode is polarized at potentials more negative and more positive than the m a x i m u m of the current. Moreover, the spots formed in the potential range around --0.9 V sink into the bulk of the mercury when the potential of the electrode is made --0.3 or --1.3 V. After sinking of the spots no change in colour is observed in the presence of phenolphthalein. The sinking of the spots is an irreversible process i.e. the spots do not rise to the surface again when the potential of the electrode is changed to the potential range at which these spots were formed. The black spots appear at lower concentration of Ag ÷ ions if the electrode is covered with hydroxides of the metals investigated. The black spots may arise also in the absence of NO3 ions in solution. However, to obtain these spots in neutral solution of C10¥ ions approximately three times greater concentrations of Ag ÷ ions must be present in the solution. The chronoamperometric curves recorded in the potential range --0.7 to --1.0 V in KNO3 solution containing Ag + ions showed a characteristic rise of the current, the position of which on the time axis depended on the concentration of Ag ÷ ions in the solution. I t is evident that this rise of the current reflected the m o m e n t of the beginning of the reduction of NOT ions. From the quantity of electricity flowing to the m o m e n t of the rise of the Current, the c o n c e n t r a t i o n of silver in HMDE was evaluated~ The calculated values were, however, n o t constant (2--4.× 1 0 , 2 M). Moreover, when instead of pure mercury saturated silver amalgam was used, the: quantity of electricity flowing before the rise of the current did not drop drastically. It is evident that the

201 f o r m a t i o n of solid silver deposits on the surface of the m ercury electrode depend~ on many different factors. For example the m a x i m u m on the voltammetric curve of the r educt i on of NO T ions may arise at lower c o n c e n t r a t i o n of Ag ÷ ions in solution, when in this solution some am ount s of Cd 2÷ ions are present (Fig. 4, curves 4 and 5). The effect is n o t specific for cadmium only. It may appear also in the presence of other cations, but only when those cations form corresponding hydr oxi de s on electrode surface. The influence of Ag ÷ and NO3 ions on the voltammetric curves of other cations was observed also f or Zn 2+, Co 2+, Mn 2+, Ni 2÷ and Pb 2÷ ions. In the case of zinc the effect arose even at lower c o n c e n t r a t i o n of Zn 2+ ions than in the case of Cd 2÷ ions. For Co 2÷ and Mn 2÷ ions the effect was similar to that observed for cadmium. For lead the effect was observed only at higher c o n c e n t r a t i o n of Ag ÷ ions in solution (5 X 10 -3 M) and for Cu 2÷ ions no effects were observed. DISCUSSION The fact th at O H - ions, f o r m e d in the vicinity of the electrode as a byp r o d u c t of man y electrode reactions, may react with several cations to form insoluble h y d r o x i des was established some years ago [7,8]. In the presence of such deposits the polarographic [ 9] and voltammetric [ 5] curves were disturbed. The distortion of the voltammetric curves consisted usually in the appearance o f a new, more negative, cathodic peak and in the diminishing of the height of an anodic dissolution peak. It was shown in the experimental part of this paper t hat in the presence of such, secondarily formed, hydroxides an additional system of peaks may appear on voltammetric curves in a potential range more positive than the potential of a redox system of a t ype Me'+/Me(Hg). These new peaks were obtained easily for Cd 2+, Zn 2÷, Cu 2÷, Pb 2+, Mn 2+, Ni 2+ and Co 2+ ions in neutral sulphate, nitrate or perchlorate solutions containing dissolved air. To explain the appearance of these new peaks we assume that some of the h y d r o x i d e s deposited in the vicinity of the electrode surface may serve as a source of O H - ions for the anodic f o r m a t i o n of Hg(OH)2. When th e transfer of O H - ions f r om the deposited h y d r o x i d e to Hg 2÷ ion is possible th en the anodic dissolution of m e r c u r y is displaced to a m ore negative potential and an anodic peak, corresponding to the f o r m a t i o n of the soluble, but very little dissociated mercuric hydr oxi de, arises. Further, when in the vicinity of the electrode the concent r a t i on of Hg(OH)2 exceeds 2 X 10-4M [10,11] the HgO precipitates on the electrode surface. To verify whether such a transfer o f O H - ions is possible the values of the solubility p r o d u ct s of the hydr oxi de s investigated were compared. The ionic p r o d u c t o f mercuric hydr oxi de, usually regarded as the solubility p r o d u c t of HgO, has a value equal to 1.4 X 10 -26 while the corresponding values for other h y d r o x id es of divalent cations are greater than 10-26. The transfer of O H - ions from divalent hydroxides to Hg 2÷ ions is t herefore quite possible. The lack o f additional peaks in the case of Fe(OH)2 was c o n n e c t e d with the anodic oxidation of ferrous h y d r o x i d e to ferric hydroxide. The solubility prod-

202

ucts of Fe(OH)3 and A I ( O H ) 3 a m o u n t to 3.8 X 1 0 - 3 8 and 1.9 X 1 0 - 3 3 respectively. The concentration of O H - ions, which is in equilibrium with the Fe(OH)3 and AI(OH)3, are in the first case lower, and in the second case nearly the same as the concentration of O H - ions which are in equilibrium with Hg(OH)2. These estimations are in good agreement with our experimental results and therefore they support our interpretation of these results. The occurrence of a catalytic effect is obvious, if we take into account the fact that the overpotential of the reduction of NOT ions on mercury is by 0.9 V greater than on silver. A complex dependence of this effect on potential is u n d o u b t e d l y connected with change of interfacial tension of mercury--solution interphase. On both sides of the electrocapillary m a x i m u m the surface tension of mercury diminishes; the wetting properties of mercury become greater and therefore the deposit of silver does not sink at --0.6 V and sinks at --0.3 a n d - - 1 . 3 V. Because in neutral nitrate solutions containing sufficient amounts of Ag + ions, O H - ions are formed, nearly all what was described above about the action of O H - refers also to this case. The difference between the action of oxygen and NO~- ions in the presence of Ag ÷ consists in the different dependences of these reactions on potential. Oxygen forms O H - ions from zero applied voltage, while the catalytic reduction of nitrate proceeds in the potential range --0.6 to --1.3 V. The presence of NOT and O H - ions will not be harmful for the reduction of Cu 2÷ ions because at the potential range where these substances form O H - ions, cupric hydroxide is reduced. REFERENCES 1 W. Kemula and Z. Kublik, Anal. Chim. Acta, 18 (1958} 104. 2 Z. Stojek, P. Ostapczuk and Z. Kublik, J. Electroanal. Chem., 67 (1976) 301. 3 W. Kemula, Z. Galus and Z. Kublik, Bull. Acad. Polon. Sci., Set. Sci. Chim., 6 (1958) 611. 4 W. Kemula, Z. Kublik and J. Taraszewska in W.A. Cheronis (Ed.), Proc. Intern. Symp. on Microchem. Techniques, Interscience, New York, 1961, p. 865. 5 W. Kemula, E. Goerlich, Z. Kowalski and B. Behr, Roczn. Chem., 33 (1959) 791. 6 I.M. Kolthoff and D.J. Lehmnicke, J. Amer. Chem. Soc., 70 (1948) 1879. 7 T.A. Krjukowa and B:W. Kabanow, Zh. Fiz. Chim., 13 (1939) 1454. 8 I.M. Kolthoff and C.S. Miller, J. Amer. Chem. Soc., 63 (1941) 1013. 9 B. Behr and J. Chodkowski, Roczn. Chem., 32 (1958) 339. 10 A.B. Garret and A.E. Hirschler, J. Amer. Chem. Soc., 60 (1938) 299. 11 K. Aurivillius and O. Heidenstam, Acta Chem. Scand., 15 (1961) 1993.