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Pt electrode processes in molten Na2Si2O5
CHRONOPOTENTIOMETRIC INVESTIGATION OF Pb(II)/Pt ELECTRODE PROCESSES IN MOLTEN Na,Si,O, BOZENA
STRZELBLCKA and ALEKSANDER BOGACZ
Institute of Inorganic Chemistry and Metallurgy of Rare Elements,Technical Wroclaw, Poland
University ofWroclaw,
5G370
(Receiwd 18 June 1984) Abstrac-The results are presentedon chronopotentiomctric investigations of the transport process and mechanism of the Pb(II) ion reduction on platinum electrodes in liquid Na&lOs over the temperature range from 1173 to 1423 IL The Sand product was found to be constant in the region of the second-long transition time. A slight increase in the Sand product 1-j
INTRODUCTION Chronopotentiometry seems to be a very useful method in studying the transport processes and mechanisms of the electrode reactions in liquid sodium silicates[l, 21. E. Franks and A. Mukherjee[i] investigated chronopotentiometrically diffusion of ions in liquid NazSi,05, among others, diffusion of Ph(I1) ion. In these studies a graphite crucible was the measuring cell, a macroelectrode, and a reference electrode while a tungsten wire was the indicator electrode. The authors reported the diffusion coefficient of Pb(II) ion in NazSiz05 at 1173 K and activation energy of diffusion as equal to 4.8 x lO-‘cn~s-~ and 129.7kJmol-‘, respectively. Basing on the Stokes-Einstein’s equation analysis the authors suggested diffusion of simple Pb ion in liquid Na2SiZ05. Unfortunately, they reported neither the current density ranges nor corresponding values of the Sand products. In this paper we present our results on application of chronopotentiometry in liquid silicates, analysis of variations of Sand product with the current density, and determination of diffusion coelI%cientsof Pb(I1) ion in liquid silicates. This study is related to probable crucial importance of Pb(II) ion diffusion in liquid silicates when fire-refining of metals by liquid slags[3].
14 % by weight of lead oxide. The components were homogenized at 1473 K. The overall homogenization time did not exceed 48 h. The measuring cell is shown in Fig. 1. This cell was a flat-bottomed quartz tube in which a corundum crucible with the solution to be investigated was
EXPERIMENTAL In the chronopotentiometric measurements glass and liquid sodium disilicate were used as basic electrolytes and the Pb(II) ions as a depolarizer. Sodium disilicate was prepared by melting stoichiometric mixtures of anhydrous sodium carbonate and silicon dioxide at 1473 K. Then it was crushed and mixed with
Fig. 1. The measuring cell: (1) quartz cell, (2) corundum crucible, (3) Pt/Pt, Rh thermocouple, (4) reFerence electrode, (5) indicator electrode, (6) auxiliary electrode, (7) glass dome.
865
866
BOZENA STRLELBICKAAND
placed. A distributing dome was put on the top part of the tube terminated with a flat ground joint. The dome was provided with four vertical ground joints for 3 electrodes and a thermocouple as well as with a threeway cock for supplying argon. All electrodes were made of platinum. An auxiliary electrode was a 1 x 2 cm plate welded to a wire 1 mm in diameter. The wire was placed in a tightly fitting corundum shield. An oxide electrode was used as a reference electrode. It was platinum wire placed in a fairly loose corundum with the open out-let and operated under air. The indicator electrodes were made from 1 x 1, 1 x 2, 2 x 2 mm plates 0.1 nun thick, welded to a thin wire (0.2 mm in diameter) which was welded to a thick wire (1 mm in diameter). The thicker wire was placed in a tightly fitting corundum shield. The indicator electrodes designed in this way enabled us to neglect any possible variations in their electrochemically active surface areas due to the creeping effect of fused silicate at high measuring temperatures. The surface area of each indicator electrode was determined chronopotentiometriclly in 1 M aqueous potassium nitrate solution containing thallous ions (concentration of 2.5 x IO-’ kmolme3) as a depolarizer. The electrode surface area was calculated basing on 8-10 measurements according to transformation of the Sand equation:
where I, T, n and D are current density, transition time, number of electrons involved, and diffusion coefficient of Tl(I), respectively. The surface area of the indicator electrode determined chronopotentiometricaally was usually several percent higher than that measured geometrically. The chronopotentiometric curves were recorded using the following set: --direct current was supphed from a CHP-3 chronopotentiometer which was coupled with a digital potential recorder CRP-02, -the curves of transition times longer than 1 s were recorded by means of a TZ 21 S recorder, faster runs were photographed from a PT-516 A oscilloscop-e. In experiments carried out the depolarizer concentration was the function of temperature due tochanges of a solution density. This density was calculated basing on the relationship reported by J. O’M. Bockris et al.[4].
ALEKSANDER BOGACZ RESULTS
AND
DISCUSSION
A typical chronopotentiometriccurve for deposition of lead on platinum from the PbO-Na,Si,O, solutions is shown in Fig. 2. Series of experimental curves were recorded for concentrations equal to 1,2,3 and 4 % by weight of PbO in Na2Si20s over the temperature range from I 173 to 1423 K. All these curves were very well shaped. Current density was changed from several to transition to tens of mA cm- ’ which corresponded time variations from tens to tenth parts of a second. The dependence of the Sand product us current density has been shown in Figs 3 and 4. The curves were plotted for the region of minimal current densities when characteristic inflexion appeared (Fig. 3), and for higher current densities (Fig. 4). At low current densities (Fig. 3) for all concentrations measured the rectilinearity range was shifted with the increasing temperature towards higher current densities and the Sand product decreased with the initial increase of current density. Decrease of the Sand product with the increasing current density might be due to: -sphericity of a diffusion field due to small radius of a spherical indicator electrode, -preceding chemical reaction, -additional, other than diffusion, transport of ions to the surface of the electrode (convection is the most probable). The effect of the diffusion field sphericity might be neglected since the diffusion layer thickness, calculated as JGY, was 4 x ICI-’ cm for longest T. It was, therefore, low enough when compared with 0.2 cm* of the indicator electrode surface. Decomposition of lead polysilicate complexes might be the preceding chemical reaction. The existence of such complexes may be evaluated based on similarity of ionic potentials (expressed by the charge/radius ratios) of Pb(II), Ca(II) and Fe(I1) &ions equal to 1.67, 2.02 and 1.92 urn- ‘. respectivelv. CafII) ions and Fe(H) ions are repoyted to f&m pol&ilic&d complexesc5]: We have found a strong asymmetric band with an explicit maximum between 960 and 98Ocm-’ in the ir spectrum of the PbO-NazSi20, This band _ system. _ may be assigned to skeleton iibrations of Si,O::ions[6, 71. Thus the following reaction of lead polysilicate decomposition we have assumed as the preceding reaction: PbSi,O:!’
-“I &Pb+2+Si,0:;-. kz
Fig. 2. Chronopotentiogram of lead deposition from 2.034 x 10e4 molecm-’ solution of PbO in liquid Na,SizO, at 1373 K 1-j,l = Il.072 Arm-‘, r = 3.8 s.
(2)
Pb(II)/Pt electrode processesin molten Na2Si205
867
1423 K
.
2,3.. 1373
P
‘,Q .-
K
1273 A I
‘07 ..
K
A =
1223 K
x
1173 K
‘15 .T
o-:~!~+::!:::::!::4
a
12
16
20
24
28
~-j~~-103/A~criiz
Fig. 3. Plots of the Sand product uslow currentdensityfor chronopotentiometricdepositionof lead from 2% by weight PbO solution in NazSilOs at various temperatures.
\
l&23 K
14 12
10
8
6
10
20
30
LO
50
60
70
80
90
100
lCt3. I-jcI/A.Cd
Fig. 4. Plots of the Sand product trs current density during chronopotentiometric deposition of lead from 3% by weight PbO solution in Na,Si,O, at various temperatures. The equilibrium constant (K) of this reaction was estimated based on the ratio of the value of the Sand product in its constancy region and the value of Sand product obtained from extrapolation to zero current density according to the relationship[&lOJ: 1-j,lz:‘”
=
1-jC(z”’ 1 + (l/K).
(3)
Equation (3) is valid if the error function argument [(k, + k,)“2tk] -z 2. The values of the K& + k,)‘fz parameterwere determinedas the slopes of rectilinear segments of curves: the Sand product us current density extrapolated to zero current density. The values of the equilibrium constants (K), dccomposition (k,) and formation (k,) rates constants of a chemical reaction (2) are presented in Table 1.
BOZENA STR~ELBICKA AND ALEKSANDER BOGACZ
868
Table 1. Values of equilibrium constants, decomposition and formation rate constants for the first order chemical reaction assumed preceding the reduction process of the Pb(I1) ions on platinum in Na,SizO, _~~ k, x lo2 k2 x lo2 K(k, + k,) [&
[s”‘]
1173 1223 1273 1373 1423
0.666 0.869 1.000 0.333
F_-‘I
[s-q
K
+
1.76 4.33 26.7 0.321
0.386 1.17 17.8 0.0595
4.5 3.7 1.5 5.4
0.22 0.27 0.67 0.19
These data are of tentative nature. An increase of the Sand product was found only for long 7, of about 10 s, and the number of measuring points used for extrapolation was low. They cannot provide an explicit answer if the preceding reaction or thermal convection produced the initial decrease of the Sand product. The stability constants (1/K) of the polysilicate complex of Pb(II) are equal to 0.2, and proved that this complex is unstable. Diffusion coefficients of Pb(I1) ion determined under an assumption that preceding reaction of decomposition of lead polysilicate complexes occured were of the same order like those determined in the range of the Sand product constancy (Table 2).
I-jcl~T a,05
‘A.cm
-2
Table 2. Comparison of the diffusion coefficients determined from ttie Sand product constancy region with the diffusion coefficients obtained by assuming a preceding reaction D&o, for DPb(II) for 1-j,/
rl’
4-A =O3 =
con&.
Gb(ll) x 10’ [cm2
1173 1223 1273 1373 1423
s- ‘1
2.66 3.77 2.92 10.1 8.17
h’b(U)
x lo7
[cm’s_‘] 1.92 2.22 2.92 3.95 4.92
However, the dependence between diffusion coefficients determined assuming preceding chemical reaction and temperature was not rectilinear. It was ascertained for all PbO concentrations measured. In light of the above consideration thermal convection seems to be responsible for more intensive transport of Pb(I1) ions to the electrode surface examined. For high current density (Fig. 4) the Sand product increased markedly with the increasing current density. No effects related to charging of the double electric layer and oxidation of the electrode during
.,I ’ 1423 K
1273 K 1223 K
Fig. 5. Plots of the Sand product us concentration for Lead deposition from PbO-Na,Si,O, various temperatures.
solutions
at
Pb(II)/Pt electrode proecs;scsin molten NazSizOs
anodic dissolution of the depolarizer reduced were observed. Initial segments of the experimental curves corresponded to simple Faraday processes and were almost normal to the time axis. No platinum oxide was found on the surface of indicator electrode using a scanning electron microscope. The indicator ektrode’s potential reached the value of static potential after 15min of measurements.Thus we have explained increase in the !%andproduct as the result of an increase of the electrochemically active surface of the indicator electrode. Shorter 7 resulted in better adaptation of the
869
diffusion layer boundary to irregularities of the electrode surface[ll]. The relationship of the Sand product us concentration from the region of its constancy was a straight line in&sMing the origin of the coordinate axis (Fig. 5). The slopes of these straight lines were used to determine numerical values of the diffusion coefficients. The temperature dependence of diffusion coefficients is presented in Fig. 6. The slope of the straight line In D - 1jT was used to determine the activation energy of diffusion of the Pb(I1) ions in Na2Si20, according to
kl Q I
-62
- 6,3
-6,5 -6,6 -6,1
or
6
7
8
Fig. 6. Plot of diffusion coeficients of Pb(I1) ions in Na2Si205
9
lo*.
i/K_
us temperature.
Fig. 7. Linearizationof experimentalcurves for lead deposition on platinum from solution of 3 % by weight PbO in Na,Si,O, in the E - [lg 1 - (t/r)“*] coordinates.
BOZENA STRZELBICKAANDALEKSANDER
870 the Arrhenius equation: D=D,,exp
(
-$=
>
,
where E, and D,, are activation energy of diffusion and diffusion coefficient at OK, respectively, and its logarithmic form: (52.3 f 2.7) InD =(-4440*0.111)RT * (5) The activation energy of the Pb(I1) ion diffusion in Na,Si,O, was found to be equal to 52.3 f 2.7 kJ mole.
Assuming the process of lead deposition on the platinum electrode as a reversible process with an insoluble product or the process determined by the charge transfer rate, the results of linearization of chronopotentiograms in coordinates E us lg [1 - (C/T)“*] for various current densities and the same PbO concentration in NalSitOS were done (Fig. 7). The numbers of electrons transferred calculated from the slopes of rectilinear segments of the curves varied from 1.8 to 2.3. These numbers are too high to be transference coefficients a,. Thus, the electrode process investigated could not be dependent on the charge transfer rate all the more so as the diffusion coefficients of Pb(I1) ions in Na&O, are low (lo-’ cm* S-I in order). The rectilinear segments of the curves were plotted covering about 70 y0 of experimental points on each chronopotentiometric curve analysed.
Booacz
The slopes of straight segments of curves are close to the theoretical values corresponding with the discharge of the Pb(II) ions on platinum in sodium disilicate. Thus we have defined the electrode process as a two-electron electrode reaction dependent on the mass transfer rate.
REFERENCES 1. E. Franks and A. Mukhcrjeq J. elecrroanal. ckem. inte$ic. Electrockem. 49, 456 (1974). 2. A. Bogacq B. Strzelbicka and J. KaZmierczak, Electrockim. Acta 30, 731 (1985). 3. A. Bogacz et al., Proc. Conf. ‘Zagadnienia ogniowej rafinacji miedzi od raniezyszzeh metalowych’. Vol. III, Metalurgia. Part I. Gliwice (1977). 4. J. O’M. Bockris, J. W. Tomlinson and J. L. White, Tmns. Faraday Sot. 52,299 (1956). 5. A. Bagacz, Pr. Nauk. Inst. Chem. Nieorg. Pol. W. Nr 27; Seria Studb i Materialv. Nr. 14 (1975). 6. T. Yanagae and Y. Sugkohara, T&ns. jap. Inst. Met. 11, 400 fl97OL 7. T. Yka&.e and Y. Suginohara, Tetsu To Hagam 57,192 (1971). 8. L. Gierst and A. L. Juliard. J. akvs. Ckem. 57. 701 (19531. 9. W. H. Reinmuth, Ana~yt. &kem:32, 1514 (l&O). ~ W. H. Reinmuth, Analyt. Ckem. 33, 322 (1961). :: M. Chemla, Prclc. 3rd Int. Conf on Molren S&s Chemistry, p. 25. Wroclaw-ICarpaa (1979).
STRZELBLCKA and ALEKSANDER BOGACZ
Institute of Inorganic Chemistry and Metallurgy of Rare Elements,Technical Wroclaw, Poland
University ofWroclaw,
5G370
(Receiwd 18 June 1984) Abstrac-The results are presentedon chronopotentiomctric investigations of the transport process and mechanism of the Pb(II) ion reduction on platinum electrodes in liquid Na&lOs over the temperature range from 1173 to 1423 IL The Sand product was found to be constant in the region of the second-long transition time. A slight increase in the Sand product 1-j
INTRODUCTION Chronopotentiometry seems to be a very useful method in studying the transport processes and mechanisms of the electrode reactions in liquid sodium silicates[l, 21. E. Franks and A. Mukherjee[i] investigated chronopotentiometrically diffusion of ions in liquid NazSi,05, among others, diffusion of Ph(I1) ion. In these studies a graphite crucible was the measuring cell, a macroelectrode, and a reference electrode while a tungsten wire was the indicator electrode. The authors reported the diffusion coefficient of Pb(II) ion in NazSiz05 at 1173 K and activation energy of diffusion as equal to 4.8 x lO-‘cn~s-~ and 129.7kJmol-‘, respectively. Basing on the Stokes-Einstein’s equation analysis the authors suggested diffusion of simple Pb ion in liquid Na2SiZ05. Unfortunately, they reported neither the current density ranges nor corresponding values of the Sand products. In this paper we present our results on application of chronopotentiometry in liquid silicates, analysis of variations of Sand product with the current density, and determination of diffusion coelI%cientsof Pb(I1) ion in liquid silicates. This study is related to probable crucial importance of Pb(II) ion diffusion in liquid silicates when fire-refining of metals by liquid slags[3].
14 % by weight of lead oxide. The components were homogenized at 1473 K. The overall homogenization time did not exceed 48 h. The measuring cell is shown in Fig. 1. This cell was a flat-bottomed quartz tube in which a corundum crucible with the solution to be investigated was
EXPERIMENTAL In the chronopotentiometric measurements glass and liquid sodium disilicate were used as basic electrolytes and the Pb(II) ions as a depolarizer. Sodium disilicate was prepared by melting stoichiometric mixtures of anhydrous sodium carbonate and silicon dioxide at 1473 K. Then it was crushed and mixed with
Fig. 1. The measuring cell: (1) quartz cell, (2) corundum crucible, (3) Pt/Pt, Rh thermocouple, (4) reFerence electrode, (5) indicator electrode, (6) auxiliary electrode, (7) glass dome.
865
866
BOZENA STRLELBICKAAND
placed. A distributing dome was put on the top part of the tube terminated with a flat ground joint. The dome was provided with four vertical ground joints for 3 electrodes and a thermocouple as well as with a threeway cock for supplying argon. All electrodes were made of platinum. An auxiliary electrode was a 1 x 2 cm plate welded to a wire 1 mm in diameter. The wire was placed in a tightly fitting corundum shield. An oxide electrode was used as a reference electrode. It was platinum wire placed in a fairly loose corundum with the open out-let and operated under air. The indicator electrodes were made from 1 x 1, 1 x 2, 2 x 2 mm plates 0.1 nun thick, welded to a thin wire (0.2 mm in diameter) which was welded to a thick wire (1 mm in diameter). The thicker wire was placed in a tightly fitting corundum shield. The indicator electrodes designed in this way enabled us to neglect any possible variations in their electrochemically active surface areas due to the creeping effect of fused silicate at high measuring temperatures. The surface area of each indicator electrode was determined chronopotentiometriclly in 1 M aqueous potassium nitrate solution containing thallous ions (concentration of 2.5 x IO-’ kmolme3) as a depolarizer. The electrode surface area was calculated basing on 8-10 measurements according to transformation of the Sand equation:
where I, T, n and D are current density, transition time, number of electrons involved, and diffusion coefficient of Tl(I), respectively. The surface area of the indicator electrode determined chronopotentiometricaally was usually several percent higher than that measured geometrically. The chronopotentiometric curves were recorded using the following set: --direct current was supphed from a CHP-3 chronopotentiometer which was coupled with a digital potential recorder CRP-02, -the curves of transition times longer than 1 s were recorded by means of a TZ 21 S recorder, faster runs were photographed from a PT-516 A oscilloscop-e. In experiments carried out the depolarizer concentration was the function of temperature due tochanges of a solution density. This density was calculated basing on the relationship reported by J. O’M. Bockris et al.[4].
ALEKSANDER BOGACZ RESULTS
AND
DISCUSSION
A typical chronopotentiometriccurve for deposition of lead on platinum from the PbO-Na,Si,O, solutions is shown in Fig. 2. Series of experimental curves were recorded for concentrations equal to 1,2,3 and 4 % by weight of PbO in Na2Si20s over the temperature range from I 173 to 1423 K. All these curves were very well shaped. Current density was changed from several to transition to tens of mA cm- ’ which corresponded time variations from tens to tenth parts of a second. The dependence of the Sand product us current density has been shown in Figs 3 and 4. The curves were plotted for the region of minimal current densities when characteristic inflexion appeared (Fig. 3), and for higher current densities (Fig. 4). At low current densities (Fig. 3) for all concentrations measured the rectilinearity range was shifted with the increasing temperature towards higher current densities and the Sand product decreased with the initial increase of current density. Decrease of the Sand product with the increasing current density might be due to: -sphericity of a diffusion field due to small radius of a spherical indicator electrode, -preceding chemical reaction, -additional, other than diffusion, transport of ions to the surface of the electrode (convection is the most probable). The effect of the diffusion field sphericity might be neglected since the diffusion layer thickness, calculated as JGY, was 4 x ICI-’ cm for longest T. It was, therefore, low enough when compared with 0.2 cm* of the indicator electrode surface. Decomposition of lead polysilicate complexes might be the preceding chemical reaction. The existence of such complexes may be evaluated based on similarity of ionic potentials (expressed by the charge/radius ratios) of Pb(II), Ca(II) and Fe(I1) &ions equal to 1.67, 2.02 and 1.92 urn- ‘. respectivelv. CafII) ions and Fe(H) ions are repoyted to f&m pol&ilic&d complexesc5]: We have found a strong asymmetric band with an explicit maximum between 960 and 98Ocm-’ in the ir spectrum of the PbO-NazSi20, This band _ system. _ may be assigned to skeleton iibrations of Si,O::ions[6, 71. Thus the following reaction of lead polysilicate decomposition we have assumed as the preceding reaction: PbSi,O:!’
-“I &Pb+2+Si,0:;-. kz
Fig. 2. Chronopotentiogram of lead deposition from 2.034 x 10e4 molecm-’ solution of PbO in liquid Na,SizO, at 1373 K 1-j,l = Il.072 Arm-‘, r = 3.8 s.
(2)
Pb(II)/Pt electrode processesin molten Na2Si205
867
1423 K
.
2,3.. 1373
P
‘,Q .-
K
1273 A I
‘07 ..
K
A =
1223 K
x
1173 K
‘15 .T
o-:~!~+::!:::::!::4
a
12
16
20
24
28
~-j~~-103/A~criiz
Fig. 3. Plots of the Sand product uslow currentdensityfor chronopotentiometricdepositionof lead from 2% by weight PbO solution in NazSilOs at various temperatures.
\
l&23 K
14 12
10
8
6
10
20
30
LO
50
60
70
80
90
100
lCt3. I-jcI/A.Cd
Fig. 4. Plots of the Sand product trs current density during chronopotentiometric deposition of lead from 3% by weight PbO solution in Na,Si,O, at various temperatures. The equilibrium constant (K) of this reaction was estimated based on the ratio of the value of the Sand product in its constancy region and the value of Sand product obtained from extrapolation to zero current density according to the relationship[&lOJ: 1-j,lz:‘”
=
1-jC(z”’ 1 + (l/K).
(3)
Equation (3) is valid if the error function argument [(k, + k,)“2tk] -z 2. The values of the K& + k,)‘fz parameterwere determinedas the slopes of rectilinear segments of curves: the Sand product us current density extrapolated to zero current density. The values of the equilibrium constants (K), dccomposition (k,) and formation (k,) rates constants of a chemical reaction (2) are presented in Table 1.
BOZENA STR~ELBICKA AND ALEKSANDER BOGACZ
868
Table 1. Values of equilibrium constants, decomposition and formation rate constants for the first order chemical reaction assumed preceding the reduction process of the Pb(I1) ions on platinum in Na,SizO, _~~ k, x lo2 k2 x lo2 K(k, + k,) [&
[s”‘]
1173 1223 1273 1373 1423
0.666 0.869 1.000 0.333
F_-‘I
[s-q
K
+
1.76 4.33 26.7 0.321
0.386 1.17 17.8 0.0595
4.5 3.7 1.5 5.4
0.22 0.27 0.67 0.19
These data are of tentative nature. An increase of the Sand product was found only for long 7, of about 10 s, and the number of measuring points used for extrapolation was low. They cannot provide an explicit answer if the preceding reaction or thermal convection produced the initial decrease of the Sand product. The stability constants (1/K) of the polysilicate complex of Pb(II) are equal to 0.2, and proved that this complex is unstable. Diffusion coefficients of Pb(I1) ion determined under an assumption that preceding reaction of decomposition of lead polysilicate complexes occured were of the same order like those determined in the range of the Sand product constancy (Table 2).
I-jcl~T a,05
‘A.cm
-2
Table 2. Comparison of the diffusion coefficients determined from ttie Sand product constancy region with the diffusion coefficients obtained by assuming a preceding reaction D&o, for DPb(II) for 1-j,/
rl’
4-A =O3 =
con&.
Gb(ll) x 10’ [cm2
1173 1223 1273 1373 1423
s- ‘1
2.66 3.77 2.92 10.1 8.17
h’b(U)
x lo7
[cm’s_‘] 1.92 2.22 2.92 3.95 4.92
However, the dependence between diffusion coefficients determined assuming preceding chemical reaction and temperature was not rectilinear. It was ascertained for all PbO concentrations measured. In light of the above consideration thermal convection seems to be responsible for more intensive transport of Pb(I1) ions to the electrode surface examined. For high current density (Fig. 4) the Sand product increased markedly with the increasing current density. No effects related to charging of the double electric layer and oxidation of the electrode during
.,I ’ 1423 K
1273 K 1223 K
Fig. 5. Plots of the Sand product us concentration for Lead deposition from PbO-Na,Si,O, various temperatures.
solutions
at
Pb(II)/Pt electrode proecs;scsin molten NazSizOs
anodic dissolution of the depolarizer reduced were observed. Initial segments of the experimental curves corresponded to simple Faraday processes and were almost normal to the time axis. No platinum oxide was found on the surface of indicator electrode using a scanning electron microscope. The indicator ektrode’s potential reached the value of static potential after 15min of measurements.Thus we have explained increase in the !%andproduct as the result of an increase of the electrochemically active surface of the indicator electrode. Shorter 7 resulted in better adaptation of the
869
diffusion layer boundary to irregularities of the electrode surface[ll]. The relationship of the Sand product us concentration from the region of its constancy was a straight line in&sMing the origin of the coordinate axis (Fig. 5). The slopes of these straight lines were used to determine numerical values of the diffusion coefficients. The temperature dependence of diffusion coefficients is presented in Fig. 6. The slope of the straight line In D - 1jT was used to determine the activation energy of diffusion of the Pb(I1) ions in Na2Si20, according to
kl Q I
-62
- 6,3
-6,5 -6,6 -6,1
or
6
7
8
Fig. 6. Plot of diffusion coeficients of Pb(I1) ions in Na2Si205
9
lo*.
i/K_
us temperature.
Fig. 7. Linearizationof experimentalcurves for lead deposition on platinum from solution of 3 % by weight PbO in Na,Si,O, in the E - [lg 1 - (t/r)“*] coordinates.
BOZENA STRZELBICKAANDALEKSANDER
870 the Arrhenius equation: D=D,,exp
(
-$=
>
,
where E, and D,, are activation energy of diffusion and diffusion coefficient at OK, respectively, and its logarithmic form: (52.3 f 2.7) InD =(-4440*0.111)RT * (5) The activation energy of the Pb(I1) ion diffusion in Na,Si,O, was found to be equal to 52.3 f 2.7 kJ mole.
Assuming the process of lead deposition on the platinum electrode as a reversible process with an insoluble product or the process determined by the charge transfer rate, the results of linearization of chronopotentiograms in coordinates E us lg [1 - (C/T)“*] for various current densities and the same PbO concentration in NalSitOS were done (Fig. 7). The numbers of electrons transferred calculated from the slopes of rectilinear segments of the curves varied from 1.8 to 2.3. These numbers are too high to be transference coefficients a,. Thus, the electrode process investigated could not be dependent on the charge transfer rate all the more so as the diffusion coefficients of Pb(I1) ions in Na&O, are low (lo-’ cm* S-I in order). The rectilinear segments of the curves were plotted covering about 70 y0 of experimental points on each chronopotentiometric curve analysed.
Booacz
The slopes of straight segments of curves are close to the theoretical values corresponding with the discharge of the Pb(II) ions on platinum in sodium disilicate. Thus we have defined the electrode process as a two-electron electrode reaction dependent on the mass transfer rate.
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