1023-1935/04/4010- © 2004
åÄIä “Nauka
/Interperiodica”1079
Russian Journal of Electrochemistry, Vol. 40, No. 10, 2004, pp. 1079–1083. Translated from Elektrokhimiya, Vol. 40, No. 10, 2004, pp. 1254–1258.Original Russian Text Copyright © 2004 by Bek, Shuraeva, Ovchinnikova.
INTRODUCTION
Lead adatoms are known to exert strong catalyticinfluence on a number of electrochemical processes [1],including electrodeposition of metals, for example,gold [1–3]. It is also known that admixtures of leadcompounds exert influence on the electrodeposition ofsilver as well. However, in the last case it is studiedweakly and the results of the studies are contradictory.For example, whereas in [4–6] the authors report aboutacceleration of the silver electrodeposition during theadsorption of lead, the authors of [7] discovered decel-eration of this process. In the present work a goal is setto produce systematic data on the influence exerted byadmixture of lead ions on the kinetics of the process ofelectrodeposition of silver from cyanide electrolytes.
EXPERIMENTAL
The investigations were performed with use made ofsolution, the content of components in which,expressed in units of M, amounted to the following val-ues: AgNO
3
, 0.05; KOH, 0.3; KCN, 1; hydroxy com-pounds of lead,
Ò
1
, from
5
×
10
–6
M to
2
×
10
–4
M.
An experimental value of the equilibrium potentialof silver in a solution of the composition given in theforegoing is close to –0.5 V. In a 0.3 M solution of analkali, lead ions are present chiefly in the form of anion
. The equilibrium potential of the reactionHPbO2–
+ 2e + H
2
O = Pb +
3
OH
–
, according to [8],may be calculated with the aid of the equation
E
eq
=
−
0.54 +
, where
Ò
2
is the concentration of
hydroxide ions. As lead ions form no strong complexeswith cyanide ions, at
Ò
1
≅
10
–5
M and
Ò
2
= 0.3 M, thevalue of
Ö
eq
is close to –0.65 V, i.e. it is more negativethan the equilibrium potential of silver by approxi-mately 0.15 V.
The investigations were performed at a temperatureof
23°ë
, which was maintained with the aid of a ther-mostat. The surface area of the renewable electrode wasequal to
5
×
10
–3
cm
2
. The potentials were measured rel-ative to a saturated calomel electrode and then recalcu-lated into the scale of a normal hydrogen electrode, inwhich they are presented in the present work. Solutionwas blown with purified hydrogen. The current con-nected with the reduction of residues of dissolved oxy-gen did not exceed 15
µ
A cm
–2
[9].
In order to have a chance to regulate the time of con-tact of the electrode with the solution and determine itto a certain degree of reliability, in the present work, asin [3], used was the technique of renewal of the surfaceof the electrode directly in the solution by cutting off athin surface layer of metal, which was described in[10]. In conjunction with a computerized instrument formeasuring dependences
i
–
E
–
t
[11], this techniquemakes it possible to automatically realize the renewalof a surface and to measure polarization curves with
HPbO2–
RT2F-------
c1
c23
----,ln
Silver Electrodeposition from Cyanide Solutions: Effect of Lead Ions
R. Yu. Bek
z
, L. I. Shuraeva, and S. N. Ovchinnikova
Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences, ul. Kutateladze 18, Novosibirsk, 630128 Russia
Received January 28, 2004
Abstract
—Effect of lead hydroxy compounds on the process of electrodeposition of silver from cyanide elec-trolytes is studied on an electrode whose surface is renewed in solution by cutting off a thin layer of metal. Thispermitted to perform the study on both the freshly renewed electrode and at controlled values of the time of theelectrode contact with solution
∆
t
. Shown is that on the freshly renewed electrode (
∆
t
<
1 s) the presence in thesolution of lead ions in concentrations
Ò
1
on the order of
10
–5
M leads to the process depolarization only in theinitial portion of a polarization curve. With
Ò
1
increased to
10
–4
M the effect of depolarization extends on theentire polarization curve. Keeping the electrode in solution after the renewal of the metal surface magnifiesdepolarization, and the greater the concentration
c
1
, the shorter the time period
∆
t
required to achieve the sameeffect. These regularities are attributed to catalytic influence of lead adatoms, whose surface concentrationdepends on
Ò
1
and
∆
t
,
as well as on the intensity of their incorporation in the silver deposit.
Key words
: electrode, electrodeposition, cyanide ion, adatom, lead, silver
SHORTCOMMUNICATIONS
z
Corresponding author, e-mail: [email protected]
1080
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 40
No. 10
2004
BEK
et al
.
allowance made for the time period
∆
t
that passedbetween the renewal of the surface and the beginning ofthe scan of potential. In the course of this “delay” theelectrode existed at steady-state values of potential, i.e.the external current through it did not flow. If the quan-tity
∆
t
≤
1
s, that such measurements are called in whatfollows “performed on the freshly renewed electrode.”In other cases the magnitude of
∆
t
is specified.
RESULTS AND DISCUSSION
In Fig. 1 (curve
1
) presented is a polarization curvefor the process of electrodeposition of silver in theabsence of lead ions. The polarization curve wasrecorded on the freshly renewed surface of an electrodeat a potential scan rate of 5 mV s
–1
, which in the case ofdomination of chemical polarization (see below)ensured a regime, which was almost stationary withrespect to diffusion. Its appearance does not depend inan essential manner on whether the electrode wasrenewed immediately before the beginning of measure-ments or after the renewal the electrode was present inthe solution under investigation for several minutes.Similar in appearance polarization curves for the pro-cess of electrodeposition of silver from cyanide electro-lytes had already been reported in [12–14].
Should one make use of the dissection of such apolarization curve into a number of “branches,” whichis the technique suggested in [14], then one would haveseen that in the first branch there takes place a morerapid elevation of current with increasing polarizationthan in the second. That is why “branch II” was condi-tionally labeled in [14] a “plateau.” Then, again, thespeed of the increase in the current with increasingpolarization increases (“branch III”) and, finally, at
Ö
< –0.9
V there reveals itself still another plateau(“branch IV”). The last, we have no doubts about that,reflects the attainment of the limiting diffusion currentby complex cyanide ions of silver. This is testified to bya linear dependence of its height on the square root ofthe rate of the electrode rotation [14], a low effectivevalue of an activation energy (on the order of4 kcal mol
–1
) and its independence of polarization inthis region of potentials [12, 14]. The same is also sug-gested by good agreement of the height of this plateau(approximately 3 mA cm
–2
, see Fig. 1, curve
1
) with itscalculation with the aid of first Fick’s law, specifically,
i
=
FDc
/
δ
= 3.3 mA cm
–2
, should the magnitude of adiffusion coefficient, in accordance with [15], be takento equal
6.8
×
10
–6
cm
2
s
–1
and the effective value ofthe thickness of a diffusion layer be taken to equal10
−
2
cm [16].
The regularities of a cathodic process until reachinga limiting current are defined mainly by the process ofa slow discharge of complex cyanide ions, which iscomplicated by the adsorption of cyanide ions [12, 13,17, 18]. On a chemical nature of the process hamperingin this range of potentials had already been reported in[14]. This conclusion agrees with calculations of thevalue of purely diffusion polarization (see curve
2
inFig. 1), which were performed with the aid of a com-puter by means of a procedure that had been describedin [19]. In so doing, used were values of logarithms ofcomplete constants of formation of cyanide ions of sil-
ver
β
2
–
β
4
(for Ag
, 20
; for Ag , 20.3; for
Ag , 20.8
1
borrowed from [21]). The diffusioncoefficients for these ions were assumed to be equal to
7
×
10
–6
cm
2
s
–1
(see in the foregoing) and for the cya-nide ions,
1.6
×
10
–5
cm
2
s
–1
[22]. The effective valueof the thickness of a diffusion layer, as in the foregoing,we assumed to be equal to
10
–2
cm. A comparison ofcalculated curve
2
with experimental curve
1
(seeFig. 1) testifies to the fact that the calculated values ofa diffusion polarization are much smaller than experi-mental values of the overall overvoltage.
On the nature of processes that are responsible forthe emergence of the “inflection” in a polarizationcurve, which is observed where its first branch passesinto a second one, different judgements were put forthin literature. In [23], we explained the inflection byvariations in the adsorbability of cyanide ions and bytheir influence on the effective value of a transfer coef-ficient. In [14], however, these processes were related
1
It should be noted that the value, calculated with the aid of theseconstants, of an equilibrium potential for the solution underinvestigation (–0.45 V) happened to be more positive than theexperimental value (on the order of –0.5 V). It seems that somevalues of the constants are underrated. For example, values ofconstants
β
2
and
β
3
given in [20] are larger, specifically,
β
2
= 21.1and
β
3 = 22. A calculation with these constants yields Eeq =−0.54 V. However, such variations in the values of constants can-not exert noticeable influence on the values of calculated values ofdiffusion polarization at a large excess of “free” ions of a cyanide.
CN( )2– CN( )3
2–
CN( )43–
3
0.70
0.90.5
2
1 II
III
IV 12
I
–E, V
i, mA cm–2
Fig. 1. (1) Polarization curve and (2) calculated values ofthe diffusion polarization during the process of electrodepo-sition of silver from a cyanide electrolyte. Composition:Ag(NO3), 0.05 M, KCN, 1 M, and KOH, 0.3 M. Tempera-
ture 23°ë, rate of potentials scan 5 mV s–1. See text for theexplanation of other designations.
RUSSIAN JOURNAL OF ELECTROCHEMISTRY Vol. 40 No. 10 2004
SILVER ELECTRODEPOSITION FROM CYANIDE SOLUTIONS 1081
to the passivation of the surface of silver by a film ofdifficultly soluble compounds of the typeMex(CN)y(OH)z, which presumably occurs in the regionof this inflection and at more negative potentials. With-out going into analysis of the validity of one explana-tion for this phenomenon or another, we will restrictourselves to putting it on the record that in both casesthe emergence of the “inflection” is connected with thechange in the state of the electrode surface with anincrease in the negative values of potential.
And now we will pass to a description of the influ-ence on the progress of a polarization curve exerted bythe presence in solution of microscopically small quan-tities of hydroxy compounds of lead. Some experi-ments showed that additives of lead compounds on thelevel of 10–5 M and smaller than that had not lead to per-ceptible variations in the progress of a polarizationcurve recorded in the regime that was described in theforegoing. At a concentration of these compounds Ò1 inthe vicinity of 3 × 10–5 M there is observed a marginaleffect of acceleration of the process of electrodeposi-tion of silver in the vicinity of the first “branch” of apolarization curve, which leads to the emergence of atiny maximum in the vicinity of the first inflection fol-lowing a further increase in Ò1 to 5 × 10–5 M (see curve 3plotted in Fig. 2). The degree of acceleration of the pro-cess in a second and a third “branches” in this case isnot great. With increasing ∆Ö it decreases and disap-pears altogether in the vicinity of a limiting current. Astill further increase in the concentration of lead leadsto an increase in the height of the maximum and to theexpansion of the region of decay of the current after itto more negative values of potential. The increase in theheight of the maximum with increasing concentration oflead bears a nonequilibrium character and at Ò1 > 10–4 Mdecays.
At a relatively large value of concentration of com-pounds of lead (Ò1 ≥ 10–4 M), the region of potentialswhere there is observed an acceleration of electrodepo-sition of silver in their presence expands practicallyover the entire polarization curve and the height of themaximum starts to exceed the steady-state value of alimiting current. The latter may be explained by assum-ing that in conditions of a sharp depolarization of a pro-cess in an initial portion of a curve the rate of the scanof potential that is equal to 5 mV s–1 fails to ensure asteady-state character of diffusion. In favor of the valid-ity of such an assumption we could state the fact that atÒ1 = 2 × 10–4 M with decreasing the scan rate, for exam-ple, to 2 mV s–1, the maximum turns flatter, decreasesin height, and approaches the value of the limiting dif-fusion current of electrodeposition of silver.
And now we will pass to a description of the influ-ence exerted by the length ∆t of the exposure of an elec-trode in a solution, which passes since the instant of itsrenewal and up to the beginning of the scan of potential.It was noticed that its influence manifests itself even atminimum (out of those investigated) values of concen-
tration of lead ions Ò1 = 5 × 10–6 M, provided the quan-tity ∆t is greater than or equal to 30 s. As one can inferfrom Fig. 3 (curve 2), in this case too, the maximumaccelerating influence of the presence in solution ofhydroxy compounds of lead is discovered in the firstbranch of a polarization curve. With increasing ∆t itincreases, and at ∆t = 160 s and higher in the vicinity ofthe first branch of a polarization curve and in the vicin-ity of an inflection, which is where the first branch of apolarization curve converts into the second branch ofthe same polarization curve, there appears a maximumof current. Thus, an exposure of an electrode in a solu-tion before commencing measurements influences ini-tial portions of a polarization curve to a first approxima-tion in the same manner as does an increase in the con-centration of lead. However, while in the case of anincrease in the concentration of lead its influence isobserved even at more negative values of potential, theinfluence of the exposure of electrode ∆t at Ò = const islimited to an initial portion of a polarization curve only.
A comparison of the influence exerted by values of∆t at different values of the concentration of lead showsthat in an initial portion of a polarization curve anapproximately identical effect is reached in the casewhere the magnitude of ∆t is inversely proportional tothe concentration of lead. Besides, with increasing con-centration of this particular component there isobserved an expansion of the region of its influenceover an polarization curve (compare curves 5 and 6depicted in Fig. 2).
0.8
0.50
0.7 0.9
1.6
2.4
3.2
i, mA cm–2
–E, V
1
2
3
4
5
6
Fig. 2. Effect of the concentration of lead ions on the elec-trode polarization during the process of electrodeposition ofsilver from cyanide electrolytes. The concentration ofhydroxy compounds of lead is as follows: (1) 0, (2) 25,(3) 50, (4) 70, (5) 100, and (6) 200 times 10–6 M. The com-position of the solution by other components and the condi-tions are the same as in Fig. 1.
1082
RUSSIAN JOURNAL OF ELECTROCHEMISTRY Vol. 40 No. 10 2004
BEK et al.
Of interest was to compare the regularities that weredescribed in the foregoing with the influence exerted byan additive of lead ions on the process of electrodepo-sition of gold from cyanide electrolytes. As we hadshown in [3], adding to gold-containing solutions assmall as 10–6 M of lead compounds causes discernibledepolarization of the cathodic process, which expandsover the entire region of potentials of electrodepositionof gold, with the potential of the commencement of alimiting diffusion current shifting in the positive direc-tion. With the concentration of lead increasing to2 × 10–5 M, the degree of the depolarization increases.In [3] we had explained these regularities by assumingnot all ions of lead that manage to reach the surface ofgold are included into the growing deposit, but thatsome of them, having undergone reduction, remain ona surface in the form of adsorbed atoms and exert a cat-alytic influence on the process. The authors of [24]experimentally showed that the concentration of leadon the surface of gold much exceeds the value thatcould have been expected on the basis of the ratiobetween the rates of deposition of atoms of gold andlead. One could have assumed that a similar phenome-non takes place in the course of joint deposition of leadand silver as well. However, judging from the fact thatin the last case the effect of depolarization reveals itselfat larger values of the concentration of lead com-pounds, one could make the assumption that either leadpossesses smaller catalytic activity with respect to thereaction of electrodeposition of silver as compared withgold or it more actively is included into the growing
deposit. The last assumption seems to be more prefera-ble. In its favor testifies the fact that, as opposed to gold,the effect of depolarization in the case of the process ofelectrodeposition of silver is localized predominantlyin an initial portion of a polarization curve, where val-ues of the current density are not great and that is whythe conditions for the inclusion of adsorbed atoms intothe growing deposit are still not favorable (see below).Of the same speak experiments with an exposure of theelectrode after the renewal of its surface in a solutionbefore beginning a potential scan at a current that isequal to zero. In this case lead may accumulate on thesurface of silver in the form of adsorbed atoms (theequilibrium potential of lead in this solution amounts toapproximately –0.65 V, i.e. it is more negative than theequilibrium potential of silver, which is equal toapproximately –0.5 V), but, in view of the absence ofelectrodeposition, it cannot be included into silver atany noticeable rate.2 As we have already mentioned inthe foregoing (see Fig. 3), accumulation of catalyticallyactive atoms prior to the beginning of a potential scanleads to a sharp acceleration of process at initial instantsof electrolysis. However, with the silver deposition rateincreasing, the rate at which lead is included into thedeposit increases as well. As follows from [3, 27], thisrate, v, is proportional to the product of the electrodecoverage by the adsorbed atoms that are beingincluded, θ, and the current density of electrodepositionof metal, i.e. v = biθ, where b is a coefficient of inclu-sion. At certain values of current density, a largeramount of accumulated adsorbed atoms of lead isincluded into the deposit; the silver electrodepositionrate drastically drops and again assumes values that aretypical for a process that occurs in conditions that aresteady-state for a given composition of solution.
In the case of a curve recorded on a freshly renewedelectrode, the emergence of a maximum in an initialportion of a polarization curve may be explained as fol-lows. Immediately after switching on the potentialscan, values of current are small, so that the tendency tothe “capture” of lead adatoms undergoing adsorptionby the growing deposit of silver is not great, whereasthe rate of supply of lead to a freshly renewed electrodeis maximum.3 In this case there is observed on the elec-trode growth of the surface concentration of adsorbedatoms of lead and, consequently, an increase in the cat-alytic activity of the surface. However, as the elec-trodeposition current density increases, the rate of thecapture of lead ions by the growing deposit increases in
2 The diffusion of adsorbed atoms into the bulk of the substrate is aslow process, which requires for a noticeable change in the sur-face concentration of the adsorbate at the very least a few tens ofminutes [25, 26].
3 It is defined, more likely than not, by the rate of diffusion of theseions towards the electrode, which immediately after the renewalof the electrode has an elevated value because of the agitation ofthe near-electrode layer of solution in the course of the renewal,which becomes “quiescent,” and a few seconds later the diffusionacquires a steady-state character [16].
0.8
0.50
0.7 0.9
1.6
2.4
3.2i, mA cm–2
–E, V
1
23
4
5
6
Fig. 3. Effect of the duration ∆t of exposure of the electrodein solution before beginning a scan of potential on the polar-ization of the electrode during the process of electrodeposi-tion of silver from cyanide electrolytes. Values of ∆t are asfollows: (1) 0, (2) 40, (3) 80, (4) 160, (5) 360, and (6) 60 s.The concentration of compounds of lead is as follows:(1−5) 5 and (6) 27 times 10–6 M. The composition of thesolution by other components and the conditions are thesame as in Fig. 1.
RUSSIAN JOURNAL OF ELECTROCHEMISTRY Vol. 40 No. 10 2004
SILVER ELECTRODEPOSITION FROM CYANIDE SOLUTIONS 1083
accordance with the equation v = biθ which we pre-sented in the foregoing, whereas the rate of supply ofions of this metal to the surface has an approximatelyconstant value whose maximum value is defined by thesteady-state rate of their diffusion.
CONCLUSIONS
(1) Studied is the effect of hydroxy compounds oflead on the process of electrodeposition of silver fromcyanide electrolytes. At concentrations on the order of10–5 M, lead accelerates the process mainly at smallvalues of overvoltage and with its acceleration expandsover the entire polarization curve.
(2) Shown is a polarization curve for the elec-trodeposition of silver in the presence of lead ions isseverely affected by the duration of exposure of theelectrode in solution prior to the beginning of a poten-tial scan. With its increase, the depolarization in an ini-tial portion of a polarization curve increases. However,with increasing ∆E, the effect of the increase in thedepolarization decreases even up to a complete disap-pearance at large values of ∆E.
(3) Following an increase in the concentration oflead, in order to achieve the same effect of depolariza-tion, there is required a shorter time of the electrodeexposure in solution before the beginning of potentialscan.
(4) A comparative analysis of the obtained materialwith the results of a study of the effect of lead on theelectrodeposition of gold allows us to presume that theregularities mentioned in the foregoing could beexplained by assuming that in the case of silver thecoefficient of inclusion of lead adatoms into the grow-ing deposit of metal has a substantially larger valuethan in the case of the gold electrodeposition.
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