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Deposition of cobalt on mercury from potassium thiocyanate solution
E1ectroc~cnActa.1970. Vol. 15.pp.2007 to2013. PuxmmonF?ens.PrintcdhNorthemIreland
DEPOSITION OF COBALT ON MERCURY FROM POTASSIUM THIOCYANATE SOLUTION* D.
J. ASTLN
and J. A. HARRISON
Electrochemistry Research Laboratories, Department of Physical Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, England Abstract-Potentiostatic pulse and sweep curves show that cobalt deposits form thiocyanate as an amalgam and a solid phase. Mercury dissolves in thiocyanate as Hg(SCN)f-; this phenomenon is used to show that mercury and cobalt form a compound located in the surface layer. R#sum&Lesimplusions potentiostatiques et les courbes de balayage montrent que le cobalt se depose a partir du thiocyanate en un amalgame, plus une phase solide. Le mercure se dissout dans le thiocyanate en taut que Hg(SCN),g-. Ce ph6nomene est utilise pour montrcr que le mercure et le cobalt forment un compose localid sur la couche superfrcielle. Zusamme&ssnng--Potentiostatische Puls- und Anstiegskurven zeigen, dass sich Kobalt aus Thiocyanat als Amalgam und als feste Phase abscheidet. Quecksilher lost sich in Thiocyanat und bildet Hg(SCN)48-; mit dleser Erscheinung kann gezeigt werden, dass Quecksilber und Kobalt eine in der Oberflachenschicht liegende Verbindung bilden. INTRODUCTION THE BEHAVIOUR of cobalt deposited
on to mercury is of interest from two points of view; to study the electrocrystallization of cobalt metal on an ideal dislocation-free substrate, and to investigate compound formation between cobalt and mercury. The second point has been investigated almost exclusively and has been demonstrated by several authors. Hovsepian and Shain :,a using the Co(II)-pyridine complex, suggest from the observed polarographically reversible wave and irreversible sweep curves at a sitting mercury drop that Co2+(pyridine) Co(Hg) E1, CoHg, kt
+ 2 e -rt Co(Hg), CoH&,
(1) (2)
Co,H&.
The existence of the Co, dimer had been established earlier by Ficker and Meite$ on the basis of anodic stripping curves of cobalt in the presence of zinc. Kemula et al’ have compared polarographic and sweep curves for Ni(II)-pyridine and used similar arguments. Galus ef aIs analysed polarographic curves for Ni(II)-KCN and Ni(II)-pyridine in the traditional manner, assuming nickel formed an amalgam. Polarograpbic investigations have also been carried out for Ni(II)-KSCN,8 and Co(II)-KSCN.617 A 1arge number of other metals form intermetallic compounds.s The formation of a solid metallic phase instead of or parallel to amalgam formation has rarely been investigated: however, methods are well developed for the detection of solid phases. The system Ni(II)-KSCN under potentiostatic pulse shows quite clearlylo that under some conditions a solid monolayer of nickel is deposited with little amalgam formation. Solid phases of Ru, Pt from their chlorides have also been detected12 on mercury. In this case ac-impedance measurements at a * Manuscript received 22 July 1969. 11
2007
D. J. ASTLBY and J. A. HAFWSON
2008
positive potential, after polarizing cathodically, show the presence of a permanent metal layer by its effect on calomel formation or on the adsorption of Cl- on the mercury substrate. A further complication to anodic sweep measurements which has not been taken into account in these systems is the mercury dissohttion as a complex with anions present in the solution. The presence of such complexes before the formation of a passivating layer of mercury salt has now been shown for a number of systems, for examplell mercury dissolving as HgS,2- in Na,S solution. The aim of the present work is to investigate phase formation in Co(H)-KSCN. EXPERIMENTAL
TECHNIQUE
Hg was chemically’cleaned, dried and vacuum-distilled twice. The glassware was cleaned with chromic acid mixture before use. Co@JO,), (A-R. recrystallized) and KSCN (A.R. recrystallized) were made up with triply distilled water. The mercury electrode was a drop extruded from a vertical capillary from underneath by means of a syringe. The area of the drop was O-1 cm a. A helix of Pt wire symmetrically placed around the Hg drop served as counter electode. A Luggin capillary monitored the potential close to the Hg electrode. The potentials were set with respect to Ag/AgCl/ 1 M KCl; the liquid junction to the measured solution was formed at a glass frit. Potentiostat and sweep generator were by Chemical Electronics (TR70-2A, RBl). Impedance measurements were made by the method of Randles and Sommerton.” A 5-mV sine wave of frequency 1500 Hz was superimposed on the dc potential fed from a potentiostat. The impedance was measured using Lissajous figures. RESULTS
Single sweep measurements in (a) 1O-2 M Co2+ + 4 M KSCN- and (b) 10” M COG++ 1 M KSCN- show the features of Fig. 1. The first scan at 300 mV/s shows deposition and removal peaks only. The removal peak is split. On second and subsequent scans an additional cathodic peak appears. This suggests that the surface of the mercury has been changed. Two aspects of the experimental sweeps have
a
b
FIG. 1. Singleshot sweepcurvesfrom -40 mV in 1 M KSCN + lo-* M Co(NO&. a, New mercurydrop, 300 mV/s sweeprate b, secondsweepaftersittingat -60 mV for 30 s, 10 V/s sweeprate.
Deposition of cobalt on mercury from potassium thiocyanate solution
been investigated ; the nature of the cathodic peaks, and whether the cathodic film can be completely stripped. The first point has been measured by pulsing on a fresh Hg drop from a potential before Co tL+reduction of Hg dissolution, into the reduction region. The results for the same solution are shown in Fig. 2. The rising transients clearly correspond to nucleation and growth of a Co film. They lie on a common falling curve after the maximum, which indicates diffusion control. At shorter times than those shown in Fig. 2 the current falls and is probably controlled by a preceding chemical reaction. The currents at fixed times of 10 ms, 50 ms, 15 s (reproducing a polarographic experiment) plotted from Fig. 2 and data at more negative potentials gave a Tafel slope of ca 60 mV.
mA
I 0
I
I
I
80
40 ms
FIG. 2. Current/time transients on pulsingto b, -114OmV; c, -112OmV; d, -1lOOmV. a, -116OmV;
A comparison of the charge under the cathodic and total anodic peaks of Fig. 1 shows that not all the charge is recovered. In Fig. 1 the discrepancy is about 1200 &jcm2. However, measuring charges is not very accurate because at the positive “tail” the currents are low, and it is not practical to sweep more positive because of the dissolution of Hg as Hg(SCN-),z- (see below). In order to characterize the properties of mercury with and without cobalt dissolved in it, the ac impedance was measured. In the absence of Co a+, H g dissolution in SCN- shows the characteristic Randles plot. Figure 3 shows a typical plot of R,, I/COG,as a function of c&‘* in 4 M SCN(-200 mV, -190 mv). The ohmic and double layer components were negligible and Fig. 2 represents a faradaic impedance for A + ne s B given by and
R, =
G2+ Rot
(4) (5)
with
(6)
2010
D. J. Asnw
~a.
3. ude~
plot at -_200
and J. A. HrrRarso~
mv of RS, I/WCS as a function of ~-l”.
4 M =CN.
where I& is the transfer resistance, R,, C, are the series resistance and capacitance, w the frequency and the other symbols are as usual. In this case the slope of the curves in Fig. 3 is a direct measure of co (cn is large). Curves of the type Fig. 3 measured in 4, 2 and 1 M KCNS solutions at various potentials were compared with the concentrations of possible Hg(SCN)(s+‘)+ complexes predicted from known thermodynamic data. The results in Table 1 show that TABLE 1 Species HgWN.‘HgWWsH&SC%
Calculated 4MKSCN at -2OOmV 2-56 x 1O-4 M 64x10-‘M 1.6 x lo-“M
HgISCN):
2MKSCNat --14OmV 2-5 x 16’M 1.28 x lo-6 M 1.28 x 10-O M
Hg(SCNF HgW=h-HgWW,
1MKSCNat -1OOmV 4 x lo-‘M 4 x lo-“M 4~10-~M
gggy-
l3qxzimenta.l 65x10-‘M
2+3x10-‘M
2.0 x lo-* M
the Hg(SCN),*- species diffuses into the bulk. More evidence is provided by Figs. 4 and 5. In Fig. 4 the concentration co is plotted as a function of potential; it has the expected 30 mV slope, for a two-electron reversible process. Similarly Fig. 5 shows
1
1
1 -180
-220
mV
I
2012
D. J. ASTLBY and
J. A.
HARRISON
co calculated from potentiostatic pulse measurements as the slope of i/t-1’2; this graph also has a 30 mV slope. The kinetics of formation of Hg(SCN)~- have not been further investigated. At more positive potentials solid Hg2(CNS-)s can be formed. In the presence of Co2+ a fihu Co2+ Hg(SCN),2- could also form. &,, = -6.54 for this compound, and thus for a 1O-2 M Co2+ and Hg(SCN),+ = 10--4.sM, the CoHg(SCN)4 would precipitate. Both these conditions were avoided. Measurement of Randles plots, Fig. 6, after a linear sweep as in Fig. 1 showed that the concentration of Hg(SCN-)42- was much reduced. The intercept of & with the
FIG. 6. RandIes plot at -200
mV in 4 M KSCN sweep.
+ lo-’ M Co(N08),
after a cathodic
y axis is also increased showing that the kinetic rate of formation of Hg(SCN),2- is reduced. Figure 7 shows the differential capacitance C, as a function of potential, at fixed frequency. The amount of the charge lost into the crop corresponds to 6-2 x 1W M of Co in Hg. If this contributed to the faradaic impedance C’,would be increased in the presence of cobalt. The concentration is insuflicient to account for a significant lowering of the surface concentration of Hg, assuming a uniform concentration. It must be assumed therefore that the Co remaining in the Hg does not dissolve anodically and is located as a film in the surface layer. The Hg dissolves through it. CONCLUSION
The deposition of cobalt on mercury is controlled partly by nucleation and growth of the solid phase, presumably cobalt metal. Although the kinetics have not been investigated in detail it is clear that this efkct could explain the apparent polarographic reversibility observed by previous authors.
Deposition of cobalt on mercuryfrom potassiumthiocyanatesolution
With
61 -100
Co film
Without
2013
Co film
I
I
I
-300
mV
Fro.
7. CBat 1 UHZtrspotential.
Anodically, mercury dissolves as Hg(SCN),2- and this effect can be used to show that after stripping off cobalt amalgam and a solid phase of cobalt, a cobalt-mercury compound located in the surface layer remains. Ackrww&&ements-We wish to thank Dr. R. D. Armstrong and Prof H. R. Thiik for interest in this work. D. J .A. is indebted to the ScienceResearchCouncil for a maintenancegrant.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
B. K. Hovs~~ura and I. SEAIN, 3. elecfruanal. Chem. 12,397 (1966).
B. K. HOVSEPIAN and I, SHAIN,J. ekctroanai. Chem. 14, 1 (1967). H. K. FICKBR and L. Mrxms, And. CMm. Acta 26,172 (1%2). W. KBMULA, LJ. J~TIC and Z. GALUS,3. electroanal.Chem.10,387 (1965). Z. GALUSand Lr. JEFXTC, .I. electround. Chem 14,415 (1967). Adyt. Chem. 39,892 (1967). S. P. PERONB and W. F. GUTKNBCHT, D. KYRIACOU, Analyt. Chem. 37,1036 (1%5). YA. I. TURY~ and G. F. SEROVA, Russ. .T.phys. Chem. 34,480 (1960). Et BARIXND~CHT, ElectroanaZyticaZ Chemistry, Vol. 2, ed. A. J. BARD. Arnold and Deklcer. London (1967). J. A. -SON and H. R. THIRSK,Trw. Far&y Sot. 61,2742 (1965). 10. M. FLEISCHMANN, 11. R. D. ARMsTIcoNc), D. F. PORTER and H. R. TEXINIC,J. electroanoi. Gem. X4, 17 (1967). 12. R. D. Guzs, J. A. HARRIzwN and H. R. %IRSK, J. electroad. Chem. 20,47 (1969). 13. J. E. B. RAND= and K. W. SOMMBRTON, Tranr. Far&y Sot. 48,937 (1957).
DEPOSITION OF COBALT ON MERCURY FROM POTASSIUM THIOCYANATE SOLUTION* D.
J. ASTLN
and J. A. HARRISON
Electrochemistry Research Laboratories, Department of Physical Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, England Abstract-Potentiostatic pulse and sweep curves show that cobalt deposits form thiocyanate as an amalgam and a solid phase. Mercury dissolves in thiocyanate as Hg(SCN)f-; this phenomenon is used to show that mercury and cobalt form a compound located in the surface layer. R#sum&Lesimplusions potentiostatiques et les courbes de balayage montrent que le cobalt se depose a partir du thiocyanate en un amalgame, plus une phase solide. Le mercure se dissout dans le thiocyanate en taut que Hg(SCN),g-. Ce ph6nomene est utilise pour montrcr que le mercure et le cobalt forment un compose localid sur la couche superfrcielle. Zusamme&ssnng--Potentiostatische Puls- und Anstiegskurven zeigen, dass sich Kobalt aus Thiocyanat als Amalgam und als feste Phase abscheidet. Quecksilher lost sich in Thiocyanat und bildet Hg(SCN)48-; mit dleser Erscheinung kann gezeigt werden, dass Quecksilber und Kobalt eine in der Oberflachenschicht liegende Verbindung bilden. INTRODUCTION THE BEHAVIOUR of cobalt deposited
on to mercury is of interest from two points of view; to study the electrocrystallization of cobalt metal on an ideal dislocation-free substrate, and to investigate compound formation between cobalt and mercury. The second point has been investigated almost exclusively and has been demonstrated by several authors. Hovsepian and Shain :,a using the Co(II)-pyridine complex, suggest from the observed polarographically reversible wave and irreversible sweep curves at a sitting mercury drop that Co2+(pyridine) Co(Hg) E1, CoHg, kt
+ 2 e -rt Co(Hg), CoH&,
(1) (2)
Co,H&.
The existence of the Co, dimer had been established earlier by Ficker and Meite$ on the basis of anodic stripping curves of cobalt in the presence of zinc. Kemula et al’ have compared polarographic and sweep curves for Ni(II)-pyridine and used similar arguments. Galus ef aIs analysed polarographic curves for Ni(II)-KCN and Ni(II)-pyridine in the traditional manner, assuming nickel formed an amalgam. Polarograpbic investigations have also been carried out for Ni(II)-KSCN,8 and Co(II)-KSCN.617 A 1arge number of other metals form intermetallic compounds.s The formation of a solid metallic phase instead of or parallel to amalgam formation has rarely been investigated: however, methods are well developed for the detection of solid phases. The system Ni(II)-KSCN under potentiostatic pulse shows quite clearlylo that under some conditions a solid monolayer of nickel is deposited with little amalgam formation. Solid phases of Ru, Pt from their chlorides have also been detected12 on mercury. In this case ac-impedance measurements at a * Manuscript received 22 July 1969. 11
2007
D. J. ASTLBY and J. A. HAFWSON
2008
positive potential, after polarizing cathodically, show the presence of a permanent metal layer by its effect on calomel formation or on the adsorption of Cl- on the mercury substrate. A further complication to anodic sweep measurements which has not been taken into account in these systems is the mercury dissohttion as a complex with anions present in the solution. The presence of such complexes before the formation of a passivating layer of mercury salt has now been shown for a number of systems, for examplell mercury dissolving as HgS,2- in Na,S solution. The aim of the present work is to investigate phase formation in Co(H)-KSCN. EXPERIMENTAL
TECHNIQUE
Hg was chemically’cleaned, dried and vacuum-distilled twice. The glassware was cleaned with chromic acid mixture before use. Co@JO,), (A-R. recrystallized) and KSCN (A.R. recrystallized) were made up with triply distilled water. The mercury electrode was a drop extruded from a vertical capillary from underneath by means of a syringe. The area of the drop was O-1 cm a. A helix of Pt wire symmetrically placed around the Hg drop served as counter electode. A Luggin capillary monitored the potential close to the Hg electrode. The potentials were set with respect to Ag/AgCl/ 1 M KCl; the liquid junction to the measured solution was formed at a glass frit. Potentiostat and sweep generator were by Chemical Electronics (TR70-2A, RBl). Impedance measurements were made by the method of Randles and Sommerton.” A 5-mV sine wave of frequency 1500 Hz was superimposed on the dc potential fed from a potentiostat. The impedance was measured using Lissajous figures. RESULTS
Single sweep measurements in (a) 1O-2 M Co2+ + 4 M KSCN- and (b) 10” M COG++ 1 M KSCN- show the features of Fig. 1. The first scan at 300 mV/s shows deposition and removal peaks only. The removal peak is split. On second and subsequent scans an additional cathodic peak appears. This suggests that the surface of the mercury has been changed. Two aspects of the experimental sweeps have
a
b
FIG. 1. Singleshot sweepcurvesfrom -40 mV in 1 M KSCN + lo-* M Co(NO&. a, New mercurydrop, 300 mV/s sweeprate b, secondsweepaftersittingat -60 mV for 30 s, 10 V/s sweeprate.
Deposition of cobalt on mercury from potassium thiocyanate solution
been investigated ; the nature of the cathodic peaks, and whether the cathodic film can be completely stripped. The first point has been measured by pulsing on a fresh Hg drop from a potential before Co tL+reduction of Hg dissolution, into the reduction region. The results for the same solution are shown in Fig. 2. The rising transients clearly correspond to nucleation and growth of a Co film. They lie on a common falling curve after the maximum, which indicates diffusion control. At shorter times than those shown in Fig. 2 the current falls and is probably controlled by a preceding chemical reaction. The currents at fixed times of 10 ms, 50 ms, 15 s (reproducing a polarographic experiment) plotted from Fig. 2 and data at more negative potentials gave a Tafel slope of ca 60 mV.
mA
I 0
I
I
I
80
40 ms
FIG. 2. Current/time transients on pulsingto b, -114OmV; c, -112OmV; d, -1lOOmV. a, -116OmV;
A comparison of the charge under the cathodic and total anodic peaks of Fig. 1 shows that not all the charge is recovered. In Fig. 1 the discrepancy is about 1200 &jcm2. However, measuring charges is not very accurate because at the positive “tail” the currents are low, and it is not practical to sweep more positive because of the dissolution of Hg as Hg(SCN-),z- (see below). In order to characterize the properties of mercury with and without cobalt dissolved in it, the ac impedance was measured. In the absence of Co a+, H g dissolution in SCN- shows the characteristic Randles plot. Figure 3 shows a typical plot of R,, I/COG,as a function of c&‘* in 4 M SCN(-200 mV, -190 mv). The ohmic and double layer components were negligible and Fig. 2 represents a faradaic impedance for A + ne s B given by and
R, =
G2+ Rot
(4) (5)
with
(6)
2010
D. J. Asnw
~a.
3. ude~
plot at -_200
and J. A. HrrRarso~
mv of RS, I/WCS as a function of ~-l”.
4 M =CN.
where I& is the transfer resistance, R,, C, are the series resistance and capacitance, w the frequency and the other symbols are as usual. In this case the slope of the curves in Fig. 3 is a direct measure of co (cn is large). Curves of the type Fig. 3 measured in 4, 2 and 1 M KCNS solutions at various potentials were compared with the concentrations of possible Hg(SCN)(s+‘)+ complexes predicted from known thermodynamic data. The results in Table 1 show that TABLE 1 Species HgWN.‘HgWWsH&SC%
Calculated 4MKSCN at -2OOmV 2-56 x 1O-4 M 64x10-‘M 1.6 x lo-“M
HgISCN):
2MKSCNat --14OmV 2-5 x 16’M 1.28 x lo-6 M 1.28 x 10-O M
Hg(SCNF HgW=h-HgWW,
1MKSCNat -1OOmV 4 x lo-‘M 4 x lo-“M 4~10-~M
gggy-
l3qxzimenta.l 65x10-‘M
2+3x10-‘M
2.0 x lo-* M
the Hg(SCN),*- species diffuses into the bulk. More evidence is provided by Figs. 4 and 5. In Fig. 4 the concentration co is plotted as a function of potential; it has the expected 30 mV slope, for a two-electron reversible process. Similarly Fig. 5 shows
1
1
1 -180
-220
mV
I
2012
D. J. ASTLBY and
J. A.
HARRISON
co calculated from potentiostatic pulse measurements as the slope of i/t-1’2; this graph also has a 30 mV slope. The kinetics of formation of Hg(SCN)~- have not been further investigated. At more positive potentials solid Hg2(CNS-)s can be formed. In the presence of Co2+ a fihu Co2+ Hg(SCN),2- could also form. &,, = -6.54 for this compound, and thus for a 1O-2 M Co2+ and Hg(SCN),+ = 10--4.sM, the CoHg(SCN)4 would precipitate. Both these conditions were avoided. Measurement of Randles plots, Fig. 6, after a linear sweep as in Fig. 1 showed that the concentration of Hg(SCN-)42- was much reduced. The intercept of & with the
FIG. 6. RandIes plot at -200
mV in 4 M KSCN sweep.
+ lo-’ M Co(N08),
after a cathodic
y axis is also increased showing that the kinetic rate of formation of Hg(SCN),2- is reduced. Figure 7 shows the differential capacitance C, as a function of potential, at fixed frequency. The amount of the charge lost into the crop corresponds to 6-2 x 1W M of Co in Hg. If this contributed to the faradaic impedance C’,would be increased in the presence of cobalt. The concentration is insuflicient to account for a significant lowering of the surface concentration of Hg, assuming a uniform concentration. It must be assumed therefore that the Co remaining in the Hg does not dissolve anodically and is located as a film in the surface layer. The Hg dissolves through it. CONCLUSION
The deposition of cobalt on mercury is controlled partly by nucleation and growth of the solid phase, presumably cobalt metal. Although the kinetics have not been investigated in detail it is clear that this efkct could explain the apparent polarographic reversibility observed by previous authors.
Deposition of cobalt on mercuryfrom potassiumthiocyanatesolution
With
61 -100
Co film
Without
2013
Co film
I
I
I
-300
mV
Fro.
7. CBat 1 UHZtrspotential.
Anodically, mercury dissolves as Hg(SCN),2- and this effect can be used to show that after stripping off cobalt amalgam and a solid phase of cobalt, a cobalt-mercury compound located in the surface layer remains. Ackrww&&ements-We wish to thank Dr. R. D. Armstrong and Prof H. R. Thiik for interest in this work. D. J .A. is indebted to the ScienceResearchCouncil for a maintenancegrant.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
B. K. Hovs~~ura and I. SEAIN, 3. elecfruanal. Chem. 12,397 (1966).
B. K. HOVSEPIAN and I, SHAIN,J. ekctroanai. Chem. 14, 1 (1967). H. K. FICKBR and L. Mrxms, And. CMm. Acta 26,172 (1%2). W. KBMULA, LJ. J~TIC and Z. GALUS,3. electroanal.Chem.10,387 (1965). Z. GALUSand Lr. JEFXTC, .I. electround. Chem 14,415 (1967). Adyt. Chem. 39,892 (1967). S. P. PERONB and W. F. GUTKNBCHT, D. KYRIACOU, Analyt. Chem. 37,1036 (1%5). YA. I. TURY~ and G. F. SEROVA, Russ. .T.phys. Chem. 34,480 (1960). Et BARIXND~CHT, ElectroanaZyticaZ Chemistry, Vol. 2, ed. A. J. BARD. Arnold and Deklcer. London (1967). J. A. -SON and H. R. THIRSK,Trw. Far&y Sot. 61,2742 (1965). 10. M. FLEISCHMANN, 11. R. D. ARMsTIcoNc), D. F. PORTER and H. R. TEXINIC,J. electroanoi. Gem. X4, 17 (1967). 12. R. D. Guzs, J. A. HARRIzwN and H. R. %IRSK, J. electroad. Chem. 20,47 (1969). 13. J. E. B. RAND= and K. W. SOMMBRTON, Tranr. Far&y Sot. 48,937 (1957).