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Silver based mercury film electrode
J. Electroanal. Chem., 67 (1976) 301--314
301
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
SILVER BASED MERCURY FILM ELECTRODE PART II. THE INFLUENCE OF AGING OF ELECTRODE ON THE ELECTRODE PROCESSES OF VARIOUS METAL IONS
ZBIGNIEW STOJEK, PIOTR OSTAPCZUK and ZENON KUBLIK
Institute of Fundamental Problems of Chemistry, University of Warsaw, Pasteura 1, 02093 Warsaw (Poland) (Received 17th June 1975)
ABSTRACT The cyclic voltammetric behaviour of 8 metal ions at solid silver amalgam electrodes prepared by aging of a thin silver based mercury film electrode (SBMFE) and by deposition of silver and mercury on platinum were investigated. It was established that such electrodes behave in relation to some metals (Pb, Bi, Sn) similarly as silver electrodes i.e. the cyclic curves obtained with these electrodes at concentration 10- 3 M range show a prepeak--postpeak system corresponding to deposition and dissolution of the monolayer of deposit. On the other hand under the same conditions no prepeaks were observed for cadmium, zinc and thallium. In all cases investigated the heights of anodic stripping peaks were lower on curves obtained with aged SBMFE than on those obtained with fresh SBMFE having a mercury layer I u m thick.
INTRODUCTION
"Thin, silver based mercury film electrodes (SBMFE) transform with time into electrodes coated with a layer of solid silver amalgam. The conditions under which the transformation proceeds were given recently [1]. No precise information has been published, however, on the influence which this change can exert on cyclic and stripping voltammetric curves of different metals. The influence can be significant because on curves obtained under similar conditions b u t with a pure silver electrode usually t w o peaks occur [2], one of which reflects the formation of the monolayer of the deposit. The present work reports a more careful examination of the characteristics of the solid silver amalgam electrode formed b y "aging" of a S B M F E , In this examination the cyclic and stripping voltammetric curves are recorded for 8 metal ions using the following types of electrodes: fresh and aged SBMFE, pure silver and pure mercury electrodes as well as several silver amalgam electrodes of definite composition.
302 EXPERIMENTAL
The voltammetric curves were recorded with a Radelkis OH-102 polarograph with 3-electrode arrangement. The reference electrode, saturated calomel electrode, was separated from solution b y a salt bridge. The counter electrode consisted of a 2 cm 2 platinum foil. During a deposition period, in stripping voltammetry experiments the solution was stirred with a magnetic stirrer. As a pure mercury electrode, a HMDE of a type described b y Kemula and Kublik [3] was used. The working silver electrode with geometric area of 6.5 mm 2 was prepared by fusing a silver wire into a soft glass tubing. The silver of this electrode was also used as a support metal for fresh and aged SBMFE. By "aged electrode" we mean a SBMFE, the fluid mercury of which was transformed to solid silver amalgam. For 1 pm thick mercury deposits the ageing period was in no case shorter than 24 h. The precise manner of preparation of these electrodes was described in a previous paper [1]. The remaining working electrodes were prepared b y a deposition of Ag and Hg in different proportions on a platinum electrode from stirred solution containing 1 M NaC104, 0.3 M NaCN, 5 × 10 - 2 M Ag(CN)2 and different concentration of Hg(CN)~-. At equimolar concentrations of Ag(I) and Hg(II) in solution the deposition currents of silver and mercury were practically the same. The deposition was performed at --1.0 V under potentiostatic conditions using a Radelkis OH-404 coulometric universal-analyser. The reference and counter electrodes used at this stage were the same as described above. The deposits had thicknesses of 1 ttm and according to the phase diagram of the system Ag--Hg [4], with rising content of mercury, the following phases are formed: ~ phase - - a solid solution of mercury in silver (up to nearly 45 at.% of Hg), fi phase -- with a range of composition close to AgHg formula, 7 phase -- with a range of composition close to Ag5 Hga formula (for this phase sometimes the formula Ag3 Hg4 is given [5]). Above 60 at.% of Hg a two phase system containing crystals of Ag5 Hga (Ags Hg4) and fluid mercury was formed. All solutions were prepared with reagent grade chemical and d o u b l y distilled water. The solutions were flushed with nitrogen to remove dissolved oxygen before the curves were recorded. RESULTS
Cyclic voltammetry Typical current--voltage curves obtained with various kinds of electrodes for different concentrations of lead ions are shown in Figs. 1 and 2. The curves obtained with HMDE and SBMFE have a reversible shape in all supporting electrolytes used (0.5 M N a C 1 0 4 , 1 M NaOH, KC1 + HC1, acetate buffer pH 4.8) irrespective of the concentration of lead ions present in solution. The curves obtained with SBMFE are shifted cathodically along the potential
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Fig. 1. E f f e c t o f k i n d o f electrode on cyclic v o l t a m m e t r i c curves of' 2.5 X 10 - 5 M Pb 2+ ions in 0.5 M KCI, p H 3. ( --) S B M F E , ( . . . . . ) aged S B M F E , ( . . . . ) HMDE, ( ...... ) silver electrode. V ol t age scan rate 0 . 7 5 V rain - 1 . Fig. 2. E f f e c t o f k i n d o f e l e c t r o d e o n c y c l i c v o l t a m m e t r i c curves o f 1 × 10 - 3 M Pb 2+ ions in 0.5 M KC1, p H 3. ( ) SBMFE, (..... ) aged S B M F E , ( . . . . ) HMDE, ( ...... ) silver electrode. Voltage scan rate 0 . 7 5 V m i n - 1 .
axis compared with curves obtained at HMDE. According to de Vries and van Dalen's theory [6] of film electrodes this shift is Nernstian in nature. On the other hand the curves obtained with pure silver and aged SBMFE exhibit some peculiarities both for dilute and concentrated solutions of lead ions. The single cathodic--anodic system obtained with silver electrode for dilute solutions of Pb 2+ ions indicates more symmetry than theory predicts for uncomplicated Nernstian charge transfer. The curves obtained at aged SBMFE are very similar to curves obtained at fresh SBMFE and are shifted cathodical ly along the potential axis compared with curves obtained at pure silver elec-
304
trode. The height of the cathodic peak obtained under these conditions at silver and aged SBMFE increases with increasing concentration of Pb 2÷ ions b u t only up to occurrence of the second peak which appears at more negative potential. N o w the height of the second cathodic peak rises while the height of the first peak attains a constant value. As shown in Fig. 2 t w o cathodic and two anodic peaks appear under these conditions. The less negative cathodic peak is sometimes called a prepeak while the more positive anodic peak is a postpeak. Such prepeak--postpeak systems were observed at pure silver electrodes for some metals [2]. The appearance of this system corresponds to the deposition and dissolution of a monolayer of foreign metal onto and from a solid electrode. As is shown in Fig. 2 this effect occurs n o t only at pure silver b u t also at solid silver amalgam electrode. In the last case the prepeak--postpeak system is shifted cathodically. The differences in potentials of monolayer deposition on pure silver and on solid silver amalgam may indicate that deposited lead interacts with solid silver amalgam n o t so strongly as with pure silver. The shape and maximal height of the prepeak depend on the rate of potential scan and to a lesser degree on the kind of supporting electrolyte used. To obtain more information a b o u t the influence of surface composition of amalgam electrode on electrode processes of lead the cyclic current--voltage curves were recorded with electrodes prepared by deposition of Ag and Hg in different proportions on platinum. Selected curves obtained at such electrodes are presented in Fig. 3. It should be noted that currents obtained at a silver electrode prepared by deposition of silver on platinum are greater than those obtained at a pure silver wire electrode with the same geometric area. It is probable that in the course of deposition of silver the surface area of the electrode increases. At low content of mercury in amalgam (up to 10%) only insignificant changes of curves are observed. With further rise of mercury in the electrode (up to 25 at.%) the height and slope of the rising part of the prepeak change only slightly while the descending part of the prepeak increases considerably. It appears as if in the range of potential between prepeak and normal peak a second prepeak arose. Two prepeaks are visible quite well at an electrode with 35 at.% of Hg. These results can be interpreted as if at two kinds of surfaces two peaks arose. This conclusion is however inconsistent with the fact that, instead of being constant, the quantity of electricity needed to form both prepeaks shows an increase with increasing mercury c o n t e n t in deposit (up to 35 at.% of Hg). We suppose that this inconsistency m a y be eliminated by t h e assumption that deposits containing 20--35 at.% of Hg have increased surface area. The second interpretation takes into account the possibility of successive deposition of two monolayer layers. The first prepeak begins to diminish at 35 at.% of Hg and at 40 at.% of Hg it disappears completely. Above 40 at.% of Hg the second prepeak begins to diminish, too, and at approximately 60 at.% of Hg the prepeak---postpeak
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Fig. 3. E f f e c t o f i n c r e a s i n g a m o u n t o f m e r c u r y in silver a m a l g a m e l e c t r o d e s o n c y c l i c v o l t a m m e t r i c c u r v e s o f 1 × 10 - 3 M P b 2+ i o n s in 0.5 M KCI, p H 3. ( ) Electrodes p r e p a r e d b y d e p o s i t i o n o f A g a n d Hg in v a r i o u s p r o p o r t i o n s o n p l a t i n u m e l e c t r o d e w i t h g e o m e t r i c a r e a = 4.9 m m 2. T h e n u m b e r s a t e a c h c u r v e i n d i c a t e t h e at.% o f m e r c u r y in deposits. ( ...... ) Silver w i r e e l e c t r o d e w i t h g e o m e t r i c a r e a = 6 . 5 m m 2, ( . . . . . ) aged S B M F E w i t h g e o m e t r i c a r e a = 6 . 5 m m 2, ( . . . . ) HMDE with geometric area = 3.8 m m 2. V o l t a g e s c a n r a t e 0 . 7 5 V r a i n - 1 .
system disappears fully. The curves are n o w very similar to curves obtained with SBMFE. These results are in accordance with data obtained from the phase diagram of the system Ag--Hg. According to these data amalgam electrodes with approximately 60 at.% of Hg should contain some fluid mercury. Further increase of mercury c o n t e n t in electrodes improve the shape of curves and shifts them to more positive potential. From these facts it results that in silver amalgam electrodes containing excess of mercury over Ag5 Hgs formula the surface of electrodes is coated by a thin film of fluid mercury. With increasing amounts of mercury the thickness of the film rises and according to de Vries and van Dalen's theory [6] the whole curve shifts anodically along the potential axis. Contrary to theoretical predictions the values
306
of half peak width found are, however, lower than 38 mV predicted for this case by the theory. Half peak width rises from 20 mV observed for electrodes with 60 at.% of Hg to 25 and 27 mV obtained for electrodes with 63 and 66 at.% of Hg respectively. This discrepancy may be a result of deposition of lead in two forms. Due to the very low c o n t e n t of mercury some a m o u n t of lead may be deposited as a solid. Simultaneous anodic oxidation of lead dissolved in mercury and present on its surface as a solid may lead to diminishing of the peak width. To evaluate some quantitative data concerning the amounts of metal deposited in the course of formation of the prepeak the quantity of electricity (pC cm - 2 ) used for the formation of prepeak was determined from the voltammetric curves. These data as well as data concerning the differences of potentials of prepeak and normal peak (AE) are listed in Table 1. The results obtained from anodic postpeak were usually few per cent greater than values evaluated from corresponding cathodic prepeak. The listed values are the average values taken from the cathodic and anodic runs. Nearly the same values were obtained at different scan rates(0.25--1.5 V min -1 ) if the concentrations of Pb 2÷ ions in solution were quite high (5 X 10 - 3 M). For lower concentrations (5 X 10 - 4 M) the faster scan rates gave lower results. From u n k n o w n reasons lower values are found for KOH solutions. The experimental data given by Schmidt and Wiithrich [2] are somewhat higher (446.6 pC cm-2). The value predicted for the closest packing of lead atoms (R = 1.75 A) on 1 cm 2 area equals to 301.5 pC cm - 2 . The value found for the electrode prepared by electrolytic deposition of silver on platinum was nearly three times higher than the value predicted theoretically. Still higher values are found when some amounts of mercury were deposited simultaneously with silver. These values a m o u n t e d to 908.2, 1295.2, 2193.9 and 2577.5 pC cm - 2 respectively for 0, 11, 20 and 30 at.%
TABLE 1 The separation of prepeak and normal peak (AEp) and quantity of electricity under a prepeak (Q)
[Pb 2+ ] /
mM 1 0.5 5 1 1
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Acetate buffer pH = 4.8 0.1 M KCI + 1 X 10 - 3 M HC1 0.1 M KC1 + 1 x 10 - 3 M HCI 0.5 M NaClO 4 1 M KOH
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308 of mercury in deposit. It should be noted that at the same platinum electrode coated by a thin mercury layer peaks are observed the height of which was in accord with the geometric area of electrode. It is obvious that amalgamation of silver under these conditions leads to an increase of surface area of the electrode. For this reason it is n o t easy to determine strictly the surface composition of aged SBMFE by comparison of results obtained on both kinds of electrodes. Approximate evaluation leads to the conclusion that the surface composition of aged SBMFE ranged between 25 and 35 at.% of Hg. In all supporting electrolytes used, the formation of a multilayer of lead begins at the same potential both on silver and on aged SBMFE. The current corresponding to this process rises steeply and under certain conditions, after reversal of the direction of the scan, the anodic line cuts the cathodic branch. This all means that for formation of a multilayer of lead a crystallization overvoltage is required. Similar experiments were performed for other metal ions. Some quantitative results of these investigations are presented in Table 2. From these experiments it results that the appearance of prepeaks on curves obtained with aged SBMFE is limited to few metals only. Prepeaks were n o t observed for Cu, Zn, Cd and T1 although for the last t w o metals the formation of monolayers proceeds quite distinctly on pure silver electrode [7]. From the remaining metals tin(II) in highly concentrated, acid solution of chloride ions gave curves very similar to those obtained for lead. The general similarity to lead is also characteristic for bismuth, although in this case several discrepancies appear. As is shown in Fig. 4 the prepeak--postpeak of bismuth is well defined both on curves obtained with pure silver and with aged SBMFE. The formation of a multilayer on pure silver proceeds, however, with a slightly higher overpotential. Slight overpotential is also observed for multilayer dissolution. On aged SBMFE these overpotentials are lower. Whereas in the case of lead the prepeak--postpeak system obtained with aged electrode was shifted cathodically compared to pure silver, no shift was observed for curves obtained with aged SBMFE for bismuth. Moreover, the shape of curves obtained with electrodes prepared by deposition of Ag and Hg on platinum practically did n o t change, when the concentration of mercury was raised from 0 to 54 at.%. In this case as in the case of lead the quantity of electricity under the peak increased when the low amounts of mercury were deposited simultaneously with silver. In the concentration range 54--63 at.% of Hg the curves began to change and above 63 at.% their shape was nearly the same as the shape of curves obtained at 1 pm SBMFE. Only peak potentials of these two curves differed b y nearly 30 mV. The voltammetric behaviour of antimony is complicated. The curves obtained at SBMFE were often disturbed due to the low solubility of antimony in mercury. The curves obtained at a silver electrode are poorly-defined. Though a cathodic peak with a constant height was observed under these conditions, resolution of this peak from t w o other cathodic peaks was poor. Hence it was impossible to measure the quantity of electricity under the pre-
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Fig. 4. E f f e c t o f k i n d o f e l e c t r o d e o n c y c l i c v o i t a m m e t r i c curves o f 5 X 10 - 4 M Bi 3+ in 1 M HC1. ( . . . . . . ) Silver w i r e e l e c t r o d e w i t h g e o m e t r i c area = 6 . 5 m m 2, ( . . . . . ) aged S B M F E w i t h g e o m e t r i c area = 6 . 5 m m 2, ( . . . . ) electrode prepared by simultaneous d e p o s i t i o n o f A g a n d Hg o n P t wire w i t h g e o m e t r i c area = 4.9 m m 2 and w i t h m e r c u r y c o n t e n t = 53 at.%, ( ) as a b o v e b u t w i t h 6 3 at.% o f m e r c u r y . V o l t a g e scan rate 0 . 3 7 5 V rain - 1 .
peak. The anodic dissolution of a monolayer of antimony proceeded with a slight overpotential. The greater overpotential was observed for the dissolution of the multilayer and therefore the resolution of dissolution peaks of monolayer and multilayer was also poor. The curves obtained at aged SBMFE do not differ much from the curves obtained at the silver electrode. In this case only the multilayer dissolution peak was a little better defined. As was mentioned above, thallium, copper, cadmium and zinc did not form any distinct prepeak--postpeak system in cyclic experiments carried out with aged SBMFE. Such behaviour was established for all supporting electrolytes used, i.e., KC1 + HCI for thallium; KC1, HC1, NaC104 and acetate buffer for cadmium; NaC104, K2 SO4 and KNOa for copper; KC1 and KOH
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for zinc. Under the same conditions b u t with a pure silver electrode a very distinct prepeak--postpeak system was observed for thallium and less distinct for cadmium. At solid amalgam electrodes with definite composition the prepeak of cadmium was observed only up to 10 at.% of Hg. It was impossible to record any peak of zinc at a pure silver electrode because the useful potential range of the silver electrode was too low. The reduction of Cu(II) ions on the silver electrode gave poorly defined curves in accordance with data of Schmidt et al. [8] according to which no prepeaks are characteristic for the deposition of this metal on silver. Figure 5 illustrates the behaviour of two metals which do n o t form any prepeaks on aged SBMFE. In both cases the amounts of deposited metal were sufficient to form deposits consisting of a few monolayers. In spite of this the curves obtained with aged SBMFE resemble rather the curves characteristic for fluid mercury than for a solid electrode. The aged SBMFE influenced both electrode processes in different ways. While for cadmium the slope of the curves was diminished, when fresh SBMFE was changed for aged SBMFE, for zinc just the reverse was observed.
Anodic stripping voltammetry Typical stripping curves, obtained for lead deposited at submonolayer range, are shown in Fig. 6. Only the curve obtained at the HMDE exhibits distinct tailing. The remaining curves differ slightly only in peak width and height. For example the half peak width a m o u n t e d to 37, 45 and 43 mV respectively for curves obtained at fresh and aged SBMFE and silver wire elec-
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Fig. 6. Effect of k i n d of electrode on anodic stripping curves of 2 X 10 - 7 M Pb 2+ ions in 0. 5 M KCI, pH 3. Preelectrolysis time 4 min, preelectrolysis potential --0.7 V. Voltage scan rate 0.75 V m i n - 1 . ( ) SBMFE, ( . . . . . ) aged SBMFE, ( . . . . ) HMDE, ( . . . . . . ) silver wire electrode.
trode. The differences in height were responsible for different slopes of dependences of peak current of lead dissolution on concentration of Pb 2÷ ions in solutions. As is shown in Fig. 7 this dependence is linear for SBMFE. For aged SBMFE the linearity is maintained only when the deposit does n o t exceed the submonolayer range. Bismuth behaved similarly to lead. On the other hand the dissolution curves for tin obtained at aged SBMFE were poorly defined and irreproducible. When both Bi(III) and P b ( I I ) a r e present together in solution the height and shape of dissolution peaks depend on amounts of deposited metals. The dissolution peaks obtained after 3 min preelectrolysis for 0.5 M HC1 solution containing 1 X 10 - 7 M Bi(III) and 2 X 10 - 7 M Pb(II) had nearly the same appearance as peaks obtained in the absence of the second metal. It is evident that under these conditions both metals are deposited at the submonolayer range. In similar experiments in which a few monolayers of lead and bismuth were deposited, the anodic postpeak of bismuth disappeared. A few experiments with cadmium at fresh and aged SBMFE were performed by Gardiner and Rogers [9]. The authors stated that after 4 days of aging of 2 pm SBMFE the peak current of cadmium dissolution rose considerably with simultaneous appearance of the second anodic peak at slightly more positive potential. After 5 days of aging of the electrode the decrease in sensitivity was noted. The authors supposed that due to solid amalgam formation the deposition of cadmium could n o t be effected because of t o o low overpotential of hydrogen. These results could n o t be confirmed by us. In our stripping experiments carried o u t for cadmium at fresh and aged SBMFE
312
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only one peak was observed, also under the conditions given by Gardiner and Rogers. As is shown in Fig. 7 for both fresh and aged SBMFE the dependence of ip of cadmium dissolution on concentration of Cd(II) in solution is linear both in the submonolayer and multilayer range. The lower slope of the plot obtained for the aged electrode is a result of broadening of the peak. Half peak width was equal to 41 and 55 mV respectively for fresh and aged SBMFE. On the other hand for lead and bismuth the linearity in the multilayer range is attained for SBMFE only. DISCUSSION
From the experiments described above it follows that the transformation of SBMFE to solid silver amalgam influences the processes of deposition and dissolution of metals in so various ways that it is not possible to find any simple diagnostic criteria enabling the determination of whether the surface of SBMFE consists of solid or fluid amalgam. In cases where a prepeak appears prior to the normal peak the presence
313 of solid surface will usually be obvious. Such a situation exists, however, only when deposited amounts of metals are greater than a monolayer and when the interaction of deposited metal with electrode surface is strong enough. The majority of metals studied by us interacts distinctly with silver to form prepeaks. However, the energy of this interaction is changed when mercury is introduced to the crystal lattice of silver. For bismuth this change is comparatively small, because the position and shape of the prepeak is practically unchanged even when the c o n t e n t of mercury in solid silver amalgam rises up to 54 at.%. Such a behaviour indicates that bismuth interacts n o t only with silver b u t also with mercury present on the surface of the electrode. For lead the appearance of t w o peaks leads to similar conclusions. On the other hand the presence of mercury in solid silver amalgam exerts a negative influence on silver--cadmium and probably on silver--zinc interaction. The cadmium prepeak disappears when the proportion of Ag : Hg in the electrode attains a value of 9 : 1. As shown in Fig. 5 the deposition and dissolution of cadmium onto and from aged SBMFE proceed at an even more negative potential than on fresh SBMFE. Neither are prepeaks observed on curves obtained with aged SBMFE for zinc and thallium. When deposition is carried o u t in the submonolayer range single stripping peaks are observed both for fresh and aged SBMFE. Under these conditions the transformation of the surface of the electrode may cause a shift of curves along a potential axis as well as a decrease and widening of peaks. These shifts are n o t greater than 30 mV, and therefore cannot be used as a diagnostic criterion for changes of the surface of the electrodes. The lowering o f the peak heights leads consequently to lower slopes of the dependence ip--c. Because these changes are usually n o t striking their occurrence can be missed and then the usage of the higher calibration curve, instead of a lower one (Fig. 7), would lead to erratic results. The magnitude of errors o f this type depends on differences in slopes of the ip--c line. For example on the basis of the curves shown in Fig. 7 one can conclude that greater errors would be made for cadmium than for lead. Silver amalgam electrodes, prepared by deposition of silver and mercury on platinum, containing excess of mercury over the Ag5 Hgs formula gave curves very similar to those obtained with SBMFE. The only serious difference between these curves consisted of the shift of the first curve along a potential axis compared to the second curve. This shift was greatest when the mercury content in deposit amounted to a b o u t 65 at.% and it diminished with further increase of mercury content. Such a behaviour implies that silver amalgam electrodes with mercury content higher than 65 at.% were coated with a film of fluid mercury. We tried to evaluate the thickness of this film using an equation given by de Vries and van Dalen for the dependence of the cathodic peak potential on the film thickness. The reproducibility of evaluations obtained for electrodes containing 65 at.% of mercury was, however, poor, i.e. the values found ranged from 2.8 × 10 - 2 pm to
314
4.5 X 10 - 2 pm. With increasing a m o u n t of mercury in the deposit, the reproducibility increased and for electrodes with 85 at.% of mercury practically the same results, amounting to 0.48 pm, were f o u n d from curves recorded both for lead and bismuth. The cause of the irreproducible results obtained for electrodes with a low c o n t e n t of fluid mercury may lie in the significant increase of concentration of deposited metal in thin mercury film. For example after recording the cathodic branch from an 1 X 10 - 3 M solution of lead ions the evaluated concentration of lead in a mercury layer with a thickness of 1 pm was 2.5 X 10 - 2 M. This value is lower than the solubility of lead in mercury (0.96 mol l - i ) , b u t after further diminishing of the thickness of the film the solubility limit may be attained. It should also be n o t e d that the diminishing of half peak width of anodic peaks obtained at electrodes with a low c o n t e n t of fluid mercury may be interpreted in the same way.
REFERENCES 1 2 3 4 5 6 7 8 9
Z. Stojek and Z. Kublik, J. Electroanal. Chem., 60 (1975) 349. E. Schmidt and N. Wfithrich, J. Electroanal. Chem., 28 (1970) 349. W. Kemula and Z. Kublik, Anal. Chim. Acta, 18 (1958) 104. M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, p. 24. A. Weryha, Z. Kristallogr., 86 (1935) 335. W.T. de Vries and E. van Dalen, J. Electroanal. Chem., 14 (1967) 315. E. Schmidt, M. Christen and P. Bayeler, J. Electroanal. Chem., 42 (1973) 275; E. Schmidt and H.R. Gygax, Heir. Chim. Acta, 48 (1965) 1584. E. Schmidt, P. Beutler and W.J. Lorenz, Bet. Bunsenges., 75 (1971) 71. K.W. Gardiner and L.B. Rogers, Anal. Chem., 25 (1953) 1393.
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© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
SILVER BASED MERCURY FILM ELECTRODE PART II. THE INFLUENCE OF AGING OF ELECTRODE ON THE ELECTRODE PROCESSES OF VARIOUS METAL IONS
ZBIGNIEW STOJEK, PIOTR OSTAPCZUK and ZENON KUBLIK
Institute of Fundamental Problems of Chemistry, University of Warsaw, Pasteura 1, 02093 Warsaw (Poland) (Received 17th June 1975)
ABSTRACT The cyclic voltammetric behaviour of 8 metal ions at solid silver amalgam electrodes prepared by aging of a thin silver based mercury film electrode (SBMFE) and by deposition of silver and mercury on platinum were investigated. It was established that such electrodes behave in relation to some metals (Pb, Bi, Sn) similarly as silver electrodes i.e. the cyclic curves obtained with these electrodes at concentration 10- 3 M range show a prepeak--postpeak system corresponding to deposition and dissolution of the monolayer of deposit. On the other hand under the same conditions no prepeaks were observed for cadmium, zinc and thallium. In all cases investigated the heights of anodic stripping peaks were lower on curves obtained with aged SBMFE than on those obtained with fresh SBMFE having a mercury layer I u m thick.
INTRODUCTION
"Thin, silver based mercury film electrodes (SBMFE) transform with time into electrodes coated with a layer of solid silver amalgam. The conditions under which the transformation proceeds were given recently [1]. No precise information has been published, however, on the influence which this change can exert on cyclic and stripping voltammetric curves of different metals. The influence can be significant because on curves obtained under similar conditions b u t with a pure silver electrode usually t w o peaks occur [2], one of which reflects the formation of the monolayer of the deposit. The present work reports a more careful examination of the characteristics of the solid silver amalgam electrode formed b y "aging" of a S B M F E , In this examination the cyclic and stripping voltammetric curves are recorded for 8 metal ions using the following types of electrodes: fresh and aged SBMFE, pure silver and pure mercury electrodes as well as several silver amalgam electrodes of definite composition.
302 EXPERIMENTAL
The voltammetric curves were recorded with a Radelkis OH-102 polarograph with 3-electrode arrangement. The reference electrode, saturated calomel electrode, was separated from solution b y a salt bridge. The counter electrode consisted of a 2 cm 2 platinum foil. During a deposition period, in stripping voltammetry experiments the solution was stirred with a magnetic stirrer. As a pure mercury electrode, a HMDE of a type described b y Kemula and Kublik [3] was used. The working silver electrode with geometric area of 6.5 mm 2 was prepared by fusing a silver wire into a soft glass tubing. The silver of this electrode was also used as a support metal for fresh and aged SBMFE. By "aged electrode" we mean a SBMFE, the fluid mercury of which was transformed to solid silver amalgam. For 1 pm thick mercury deposits the ageing period was in no case shorter than 24 h. The precise manner of preparation of these electrodes was described in a previous paper [1]. The remaining working electrodes were prepared b y a deposition of Ag and Hg in different proportions on a platinum electrode from stirred solution containing 1 M NaC104, 0.3 M NaCN, 5 × 10 - 2 M Ag(CN)2 and different concentration of Hg(CN)~-. At equimolar concentrations of Ag(I) and Hg(II) in solution the deposition currents of silver and mercury were practically the same. The deposition was performed at --1.0 V under potentiostatic conditions using a Radelkis OH-404 coulometric universal-analyser. The reference and counter electrodes used at this stage were the same as described above. The deposits had thicknesses of 1 ttm and according to the phase diagram of the system Ag--Hg [4], with rising content of mercury, the following phases are formed: ~ phase - - a solid solution of mercury in silver (up to nearly 45 at.% of Hg), fi phase -- with a range of composition close to AgHg formula, 7 phase -- with a range of composition close to Ag5 Hga formula (for this phase sometimes the formula Ag3 Hg4 is given [5]). Above 60 at.% of Hg a two phase system containing crystals of Ag5 Hga (Ags Hg4) and fluid mercury was formed. All solutions were prepared with reagent grade chemical and d o u b l y distilled water. The solutions were flushed with nitrogen to remove dissolved oxygen before the curves were recorded. RESULTS
Cyclic voltammetry Typical current--voltage curves obtained with various kinds of electrodes for different concentrations of lead ions are shown in Figs. 1 and 2. The curves obtained with HMDE and SBMFE have a reversible shape in all supporting electrolytes used (0.5 M N a C 1 0 4 , 1 M NaOH, KC1 + HC1, acetate buffer pH 4.8) irrespective of the concentration of lead ions present in solution. The curves obtained with SBMFE are shifted cathodically along the potential
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Fig. 1. E f f e c t o f k i n d o f electrode on cyclic v o l t a m m e t r i c curves of' 2.5 X 10 - 5 M Pb 2+ ions in 0.5 M KCI, p H 3. ( --) S B M F E , ( . . . . . ) aged S B M F E , ( . . . . ) HMDE, ( ...... ) silver electrode. V ol t age scan rate 0 . 7 5 V rain - 1 . Fig. 2. E f f e c t o f k i n d o f e l e c t r o d e o n c y c l i c v o l t a m m e t r i c curves o f 1 × 10 - 3 M Pb 2+ ions in 0.5 M KC1, p H 3. ( ) SBMFE, (..... ) aged S B M F E , ( . . . . ) HMDE, ( ...... ) silver electrode. Voltage scan rate 0 . 7 5 V m i n - 1 .
axis compared with curves obtained at HMDE. According to de Vries and van Dalen's theory [6] of film electrodes this shift is Nernstian in nature. On the other hand the curves obtained with pure silver and aged SBMFE exhibit some peculiarities both for dilute and concentrated solutions of lead ions. The single cathodic--anodic system obtained with silver electrode for dilute solutions of Pb 2+ ions indicates more symmetry than theory predicts for uncomplicated Nernstian charge transfer. The curves obtained at aged SBMFE are very similar to curves obtained at fresh SBMFE and are shifted cathodical ly along the potential axis compared with curves obtained at pure silver elec-
304
trode. The height of the cathodic peak obtained under these conditions at silver and aged SBMFE increases with increasing concentration of Pb 2÷ ions b u t only up to occurrence of the second peak which appears at more negative potential. N o w the height of the second cathodic peak rises while the height of the first peak attains a constant value. As shown in Fig. 2 t w o cathodic and two anodic peaks appear under these conditions. The less negative cathodic peak is sometimes called a prepeak while the more positive anodic peak is a postpeak. Such prepeak--postpeak systems were observed at pure silver electrodes for some metals [2]. The appearance of this system corresponds to the deposition and dissolution of a monolayer of foreign metal onto and from a solid electrode. As is shown in Fig. 2 this effect occurs n o t only at pure silver b u t also at solid silver amalgam electrode. In the last case the prepeak--postpeak system is shifted cathodically. The differences in potentials of monolayer deposition on pure silver and on solid silver amalgam may indicate that deposited lead interacts with solid silver amalgam n o t so strongly as with pure silver. The shape and maximal height of the prepeak depend on the rate of potential scan and to a lesser degree on the kind of supporting electrolyte used. To obtain more information a b o u t the influence of surface composition of amalgam electrode on electrode processes of lead the cyclic current--voltage curves were recorded with electrodes prepared by deposition of Ag and Hg in different proportions on platinum. Selected curves obtained at such electrodes are presented in Fig. 3. It should be noted that currents obtained at a silver electrode prepared by deposition of silver on platinum are greater than those obtained at a pure silver wire electrode with the same geometric area. It is probable that in the course of deposition of silver the surface area of the electrode increases. At low content of mercury in amalgam (up to 10%) only insignificant changes of curves are observed. With further rise of mercury in the electrode (up to 25 at.%) the height and slope of the rising part of the prepeak change only slightly while the descending part of the prepeak increases considerably. It appears as if in the range of potential between prepeak and normal peak a second prepeak arose. Two prepeaks are visible quite well at an electrode with 35 at.% of Hg. These results can be interpreted as if at two kinds of surfaces two peaks arose. This conclusion is however inconsistent with the fact that, instead of being constant, the quantity of electricity needed to form both prepeaks shows an increase with increasing mercury c o n t e n t in deposit (up to 35 at.% of Hg). We suppose that this inconsistency m a y be eliminated by t h e assumption that deposits containing 20--35 at.% of Hg have increased surface area. The second interpretation takes into account the possibility of successive deposition of two monolayer layers. The first prepeak begins to diminish at 35 at.% of Hg and at 40 at.% of Hg it disappears completely. Above 40 at.% of Hg the second prepeak begins to diminish, too, and at approximately 60 at.% of Hg the prepeak---postpeak
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Fig. 3. E f f e c t o f i n c r e a s i n g a m o u n t o f m e r c u r y in silver a m a l g a m e l e c t r o d e s o n c y c l i c v o l t a m m e t r i c c u r v e s o f 1 × 10 - 3 M P b 2+ i o n s in 0.5 M KCI, p H 3. ( ) Electrodes p r e p a r e d b y d e p o s i t i o n o f A g a n d Hg in v a r i o u s p r o p o r t i o n s o n p l a t i n u m e l e c t r o d e w i t h g e o m e t r i c a r e a = 4.9 m m 2. T h e n u m b e r s a t e a c h c u r v e i n d i c a t e t h e at.% o f m e r c u r y in deposits. ( ...... ) Silver w i r e e l e c t r o d e w i t h g e o m e t r i c a r e a = 6 . 5 m m 2, ( . . . . . ) aged S B M F E w i t h g e o m e t r i c a r e a = 6 . 5 m m 2, ( . . . . ) HMDE with geometric area = 3.8 m m 2. V o l t a g e s c a n r a t e 0 . 7 5 V r a i n - 1 .
system disappears fully. The curves are n o w very similar to curves obtained with SBMFE. These results are in accordance with data obtained from the phase diagram of the system Ag--Hg. According to these data amalgam electrodes with approximately 60 at.% of Hg should contain some fluid mercury. Further increase of mercury c o n t e n t in electrodes improve the shape of curves and shifts them to more positive potential. From these facts it results that in silver amalgam electrodes containing excess of mercury over Ag5 Hgs formula the surface of electrodes is coated by a thin film of fluid mercury. With increasing amounts of mercury the thickness of the film rises and according to de Vries and van Dalen's theory [6] the whole curve shifts anodically along the potential axis. Contrary to theoretical predictions the values
306
of half peak width found are, however, lower than 38 mV predicted for this case by the theory. Half peak width rises from 20 mV observed for electrodes with 60 at.% of Hg to 25 and 27 mV obtained for electrodes with 63 and 66 at.% of Hg respectively. This discrepancy may be a result of deposition of lead in two forms. Due to the very low c o n t e n t of mercury some a m o u n t of lead may be deposited as a solid. Simultaneous anodic oxidation of lead dissolved in mercury and present on its surface as a solid may lead to diminishing of the peak width. To evaluate some quantitative data concerning the amounts of metal deposited in the course of formation of the prepeak the quantity of electricity (pC cm - 2 ) used for the formation of prepeak was determined from the voltammetric curves. These data as well as data concerning the differences of potentials of prepeak and normal peak (AE) are listed in Table 1. The results obtained from anodic postpeak were usually few per cent greater than values evaluated from corresponding cathodic prepeak. The listed values are the average values taken from the cathodic and anodic runs. Nearly the same values were obtained at different scan rates(0.25--1.5 V min -1 ) if the concentrations of Pb 2÷ ions in solution were quite high (5 X 10 - 3 M). For lower concentrations (5 X 10 - 4 M) the faster scan rates gave lower results. From u n k n o w n reasons lower values are found for KOH solutions. The experimental data given by Schmidt and Wiithrich [2] are somewhat higher (446.6 pC cm-2). The value predicted for the closest packing of lead atoms (R = 1.75 A) on 1 cm 2 area equals to 301.5 pC cm - 2 . The value found for the electrode prepared by electrolytic deposition of silver on platinum was nearly three times higher than the value predicted theoretically. Still higher values are found when some amounts of mercury were deposited simultaneously with silver. These values a m o u n t e d to 908.2, 1295.2, 2193.9 and 2577.5 pC cm - 2 respectively for 0, 11, 20 and 30 at.%
TABLE 1 The separation of prepeak and normal peak (AEp) and quantity of electricity under a prepeak (Q)
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308 of mercury in deposit. It should be noted that at the same platinum electrode coated by a thin mercury layer peaks are observed the height of which was in accord with the geometric area of electrode. It is obvious that amalgamation of silver under these conditions leads to an increase of surface area of the electrode. For this reason it is n o t easy to determine strictly the surface composition of aged SBMFE by comparison of results obtained on both kinds of electrodes. Approximate evaluation leads to the conclusion that the surface composition of aged SBMFE ranged between 25 and 35 at.% of Hg. In all supporting electrolytes used, the formation of a multilayer of lead begins at the same potential both on silver and on aged SBMFE. The current corresponding to this process rises steeply and under certain conditions, after reversal of the direction of the scan, the anodic line cuts the cathodic branch. This all means that for formation of a multilayer of lead a crystallization overvoltage is required. Similar experiments were performed for other metal ions. Some quantitative results of these investigations are presented in Table 2. From these experiments it results that the appearance of prepeaks on curves obtained with aged SBMFE is limited to few metals only. Prepeaks were n o t observed for Cu, Zn, Cd and T1 although for the last t w o metals the formation of monolayers proceeds quite distinctly on pure silver electrode [7]. From the remaining metals tin(II) in highly concentrated, acid solution of chloride ions gave curves very similar to those obtained for lead. The general similarity to lead is also characteristic for bismuth, although in this case several discrepancies appear. As is shown in Fig. 4 the prepeak--postpeak of bismuth is well defined both on curves obtained with pure silver and with aged SBMFE. The formation of a multilayer on pure silver proceeds, however, with a slightly higher overpotential. Slight overpotential is also observed for multilayer dissolution. On aged SBMFE these overpotentials are lower. Whereas in the case of lead the prepeak--postpeak system obtained with aged electrode was shifted cathodically compared to pure silver, no shift was observed for curves obtained with aged SBMFE for bismuth. Moreover, the shape of curves obtained with electrodes prepared by deposition of Ag and Hg on platinum practically did n o t change, when the concentration of mercury was raised from 0 to 54 at.%. In this case as in the case of lead the quantity of electricity under the peak increased when the low amounts of mercury were deposited simultaneously with silver. In the concentration range 54--63 at.% of Hg the curves began to change and above 63 at.% their shape was nearly the same as the shape of curves obtained at 1 pm SBMFE. Only peak potentials of these two curves differed b y nearly 30 mV. The voltammetric behaviour of antimony is complicated. The curves obtained at SBMFE were often disturbed due to the low solubility of antimony in mercury. The curves obtained at a silver electrode are poorly-defined. Though a cathodic peak with a constant height was observed under these conditions, resolution of this peak from t w o other cathodic peaks was poor. Hence it was impossible to measure the quantity of electricity under the pre-
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Fig. 4. E f f e c t o f k i n d o f e l e c t r o d e o n c y c l i c v o i t a m m e t r i c curves o f 5 X 10 - 4 M Bi 3+ in 1 M HC1. ( . . . . . . ) Silver w i r e e l e c t r o d e w i t h g e o m e t r i c area = 6 . 5 m m 2, ( . . . . . ) aged S B M F E w i t h g e o m e t r i c area = 6 . 5 m m 2, ( . . . . ) electrode prepared by simultaneous d e p o s i t i o n o f A g a n d Hg o n P t wire w i t h g e o m e t r i c area = 4.9 m m 2 and w i t h m e r c u r y c o n t e n t = 53 at.%, ( ) as a b o v e b u t w i t h 6 3 at.% o f m e r c u r y . V o l t a g e scan rate 0 . 3 7 5 V rain - 1 .
peak. The anodic dissolution of a monolayer of antimony proceeded with a slight overpotential. The greater overpotential was observed for the dissolution of the multilayer and therefore the resolution of dissolution peaks of monolayer and multilayer was also poor. The curves obtained at aged SBMFE do not differ much from the curves obtained at the silver electrode. In this case only the multilayer dissolution peak was a little better defined. As was mentioned above, thallium, copper, cadmium and zinc did not form any distinct prepeak--postpeak system in cyclic experiments carried out with aged SBMFE. Such behaviour was established for all supporting electrolytes used, i.e., KC1 + HCI for thallium; KC1, HC1, NaC104 and acetate buffer for cadmium; NaC104, K2 SO4 and KNOa for copper; KC1 and KOH
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for zinc. Under the same conditions b u t with a pure silver electrode a very distinct prepeak--postpeak system was observed for thallium and less distinct for cadmium. At solid amalgam electrodes with definite composition the prepeak of cadmium was observed only up to 10 at.% of Hg. It was impossible to record any peak of zinc at a pure silver electrode because the useful potential range of the silver electrode was too low. The reduction of Cu(II) ions on the silver electrode gave poorly defined curves in accordance with data of Schmidt et al. [8] according to which no prepeaks are characteristic for the deposition of this metal on silver. Figure 5 illustrates the behaviour of two metals which do n o t form any prepeaks on aged SBMFE. In both cases the amounts of deposited metal were sufficient to form deposits consisting of a few monolayers. In spite of this the curves obtained with aged SBMFE resemble rather the curves characteristic for fluid mercury than for a solid electrode. The aged SBMFE influenced both electrode processes in different ways. While for cadmium the slope of the curves was diminished, when fresh SBMFE was changed for aged SBMFE, for zinc just the reverse was observed.
Anodic stripping voltammetry Typical stripping curves, obtained for lead deposited at submonolayer range, are shown in Fig. 6. Only the curve obtained at the HMDE exhibits distinct tailing. The remaining curves differ slightly only in peak width and height. For example the half peak width a m o u n t e d to 37, 45 and 43 mV respectively for curves obtained at fresh and aged SBMFE and silver wire elec-
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Fig. 6. Effect of k i n d of electrode on anodic stripping curves of 2 X 10 - 7 M Pb 2+ ions in 0. 5 M KCI, pH 3. Preelectrolysis time 4 min, preelectrolysis potential --0.7 V. Voltage scan rate 0.75 V m i n - 1 . ( ) SBMFE, ( . . . . . ) aged SBMFE, ( . . . . ) HMDE, ( . . . . . . ) silver wire electrode.
trode. The differences in height were responsible for different slopes of dependences of peak current of lead dissolution on concentration of Pb 2÷ ions in solutions. As is shown in Fig. 7 this dependence is linear for SBMFE. For aged SBMFE the linearity is maintained only when the deposit does n o t exceed the submonolayer range. Bismuth behaved similarly to lead. On the other hand the dissolution curves for tin obtained at aged SBMFE were poorly defined and irreproducible. When both Bi(III) and P b ( I I ) a r e present together in solution the height and shape of dissolution peaks depend on amounts of deposited metals. The dissolution peaks obtained after 3 min preelectrolysis for 0.5 M HC1 solution containing 1 X 10 - 7 M Bi(III) and 2 X 10 - 7 M Pb(II) had nearly the same appearance as peaks obtained in the absence of the second metal. It is evident that under these conditions both metals are deposited at the submonolayer range. In similar experiments in which a few monolayers of lead and bismuth were deposited, the anodic postpeak of bismuth disappeared. A few experiments with cadmium at fresh and aged SBMFE were performed by Gardiner and Rogers [9]. The authors stated that after 4 days of aging of 2 pm SBMFE the peak current of cadmium dissolution rose considerably with simultaneous appearance of the second anodic peak at slightly more positive potential. After 5 days of aging of the electrode the decrease in sensitivity was noted. The authors supposed that due to solid amalgam formation the deposition of cadmium could n o t be effected because of t o o low overpotential of hydrogen. These results could n o t be confirmed by us. In our stripping experiments carried o u t for cadmium at fresh and aged SBMFE
312
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only one peak was observed, also under the conditions given by Gardiner and Rogers. As is shown in Fig. 7 for both fresh and aged SBMFE the dependence of ip of cadmium dissolution on concentration of Cd(II) in solution is linear both in the submonolayer and multilayer range. The lower slope of the plot obtained for the aged electrode is a result of broadening of the peak. Half peak width was equal to 41 and 55 mV respectively for fresh and aged SBMFE. On the other hand for lead and bismuth the linearity in the multilayer range is attained for SBMFE only. DISCUSSION
From the experiments described above it follows that the transformation of SBMFE to solid silver amalgam influences the processes of deposition and dissolution of metals in so various ways that it is not possible to find any simple diagnostic criteria enabling the determination of whether the surface of SBMFE consists of solid or fluid amalgam. In cases where a prepeak appears prior to the normal peak the presence
313 of solid surface will usually be obvious. Such a situation exists, however, only when deposited amounts of metals are greater than a monolayer and when the interaction of deposited metal with electrode surface is strong enough. The majority of metals studied by us interacts distinctly with silver to form prepeaks. However, the energy of this interaction is changed when mercury is introduced to the crystal lattice of silver. For bismuth this change is comparatively small, because the position and shape of the prepeak is practically unchanged even when the c o n t e n t of mercury in solid silver amalgam rises up to 54 at.%. Such a behaviour indicates that bismuth interacts n o t only with silver b u t also with mercury present on the surface of the electrode. For lead the appearance of t w o peaks leads to similar conclusions. On the other hand the presence of mercury in solid silver amalgam exerts a negative influence on silver--cadmium and probably on silver--zinc interaction. The cadmium prepeak disappears when the proportion of Ag : Hg in the electrode attains a value of 9 : 1. As shown in Fig. 5 the deposition and dissolution of cadmium onto and from aged SBMFE proceed at an even more negative potential than on fresh SBMFE. Neither are prepeaks observed on curves obtained with aged SBMFE for zinc and thallium. When deposition is carried o u t in the submonolayer range single stripping peaks are observed both for fresh and aged SBMFE. Under these conditions the transformation of the surface of the electrode may cause a shift of curves along a potential axis as well as a decrease and widening of peaks. These shifts are n o t greater than 30 mV, and therefore cannot be used as a diagnostic criterion for changes of the surface of the electrodes. The lowering o f the peak heights leads consequently to lower slopes of the dependence ip--c. Because these changes are usually n o t striking their occurrence can be missed and then the usage of the higher calibration curve, instead of a lower one (Fig. 7), would lead to erratic results. The magnitude of errors o f this type depends on differences in slopes of the ip--c line. For example on the basis of the curves shown in Fig. 7 one can conclude that greater errors would be made for cadmium than for lead. Silver amalgam electrodes, prepared by deposition of silver and mercury on platinum, containing excess of mercury over the Ag5 Hgs formula gave curves very similar to those obtained with SBMFE. The only serious difference between these curves consisted of the shift of the first curve along a potential axis compared to the second curve. This shift was greatest when the mercury content in deposit amounted to a b o u t 65 at.% and it diminished with further increase of mercury content. Such a behaviour implies that silver amalgam electrodes with mercury content higher than 65 at.% were coated with a film of fluid mercury. We tried to evaluate the thickness of this film using an equation given by de Vries and van Dalen for the dependence of the cathodic peak potential on the film thickness. The reproducibility of evaluations obtained for electrodes containing 65 at.% of mercury was, however, poor, i.e. the values found ranged from 2.8 × 10 - 2 pm to
314
4.5 X 10 - 2 pm. With increasing a m o u n t of mercury in the deposit, the reproducibility increased and for electrodes with 85 at.% of mercury practically the same results, amounting to 0.48 pm, were f o u n d from curves recorded both for lead and bismuth. The cause of the irreproducible results obtained for electrodes with a low c o n t e n t of fluid mercury may lie in the significant increase of concentration of deposited metal in thin mercury film. For example after recording the cathodic branch from an 1 X 10 - 3 M solution of lead ions the evaluated concentration of lead in a mercury layer with a thickness of 1 pm was 2.5 X 10 - 2 M. This value is lower than the solubility of lead in mercury (0.96 mol l - i ) , b u t after further diminishing of the thickness of the film the solubility limit may be attained. It should also be n o t e d that the diminishing of half peak width of anodic peaks obtained at electrodes with a low c o n t e n t of fluid mercury may be interpreted in the same way.
REFERENCES 1 2 3 4 5 6 7 8 9
Z. Stojek and Z. Kublik, J. Electroanal. Chem., 60 (1975) 349. E. Schmidt and N. Wfithrich, J. Electroanal. Chem., 28 (1970) 349. W. Kemula and Z. Kublik, Anal. Chim. Acta, 18 (1958) 104. M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, p. 24. A. Weryha, Z. Kristallogr., 86 (1935) 335. W.T. de Vries and E. van Dalen, J. Electroanal. Chem., 14 (1967) 315. E. Schmidt, M. Christen and P. Bayeler, J. Electroanal. Chem., 42 (1973) 275; E. Schmidt and H.R. Gygax, Heir. Chim. Acta, 48 (1965) 1584. E. Schmidt, P. Beutler and W.J. Lorenz, Bet. Bunsenges., 75 (1971) 71. K.W. Gardiner and L.B. Rogers, Anal. Chem., 25 (1953) 1393.