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Electrochemical reduction of silver thiosulphate and silver thiocyanate complexes
SHORT COMMUNICATION ELECTROCHEMICAL REDUCTION OF SILVER THIOSULPHATE. AND SILVER THIOCYANATE COMPLEXES A. l
H.
HUBIN,*
Vrije Universiteit
Brussel,
*Agfa
J. VEREECKEN*
TERRYN,*
Department
of Metallurgy,
Gevaert N.V., Septestraat
(Received
4 Sepwmber
and R. DE Pleiniaan
27, 2510 Mortsel,
1984; in revised
form 12
INTRODUCTION
EXPERIMENTAL Measurements were carried out in a thermostated (20& 1°C) electrochemical cell, with a platinum counter electrode and a KCl-saturated caiomel electrode as reference. The working electrode was a silver disk (99.99%) with a radius of 1 mm. Before each experiment, the electrode was polished with diamond paste (Bodson 7 and 0.25 H). The solution was deaerated by nitrogen bubbling. Stationary and non-stationary polarization curves were recorded using a potentiostat (Amel SSl), a generator (Amel S67), a millivoltmeter (TacusselMVN-79) and an oscilloscope (Nicolet Instrument Corporation-model 206). Stationary measurements were performed on a rotating disk electrode (Tacussel--Controvit).
C .WSIO,I:-
(Ml 0.1 0.05 0.01 0.1 0.1
2, 1050 Brussel, Belgium
March
1. Reduction
thiosulphate
k*R= k,(K,,[S,O:-]
parameters
for the reduction
cAg+
C s,o:-
E/nhe
4 x 10-15 4 x lo-‘5 4 x lo-‘5
of silver
AND DISCUSSION
complexes
From the stationary polarization curves, the following conclusions could be drawn[4]. Charge transfer at low overpotentials (-z 150 mV) and diffusion at high overpotentials are rate determining. The decomplexation of Ag(S20&~ does not influence the overall reaction rate. The assumption that the fret siIver ions are electroactive leads to the calculation of a rate constant of 10’ ms-*, which is highly improbable for a rate determining step. Consequently, the complexes are the electroactive species. The transfer coefficients a, the exchange current densities & and the rate constants kp, calculated from the Tafel pkots[4] (Fig. I) are given m Table 1. As the rate constant depends on the free thiosulphate concentration, the implication is that the reduction of Ag(S,O,):does not proceed uia a simple chargetransfer mechanism. In order to complete the mechanism, a nonstationary technique had to be used. The cyclic voltammograms (Fig. 2) show a prewave. This indicates that an adsorption of product occurs[5]. Although $0 : - is electro-inactive, its adsorption can cause a decrease in free energy for the reduction of complex, which explains the separation between the peaksC61. Assuming a Langmuir adsorption isotherm, the rate constant k:, independent of the concentration, is given WCi’]:
1. Kinetic
(Ml
Belgium
1985)
RESULTS
Silver diffusion transfer development is a photographic process in which complexed silver ions are reduced to metallic silver to form an image[l]. Only a few studies on physical development have been performed by means of electrochemical techniques[2]. The kinetic data, available for electrode processes involving silver complexes of possible interest in photographic systems, have been mainly obtained by measurements at the equilibrirum potential and on a mercury electrode[3]. In this work, the mechanism and the kinetic parameters of the reduction of silver thiosulphate and thiocyanate complexes are reported.
Table
KEYZER+
of Ag&O,):-
(W
W)
@OX
aR
1.15 0.81 0.36 0.40 0.8 1
- 0.048 -0.045 - 0.046 f0.014 - 0.023
0.60 0.63 0.63 0.53 0.58
0.52 0.57 0.56 0.53 0.60
1399
f I).
JO (Am-‘) 14 9 4 10 12
k”R (m s-l) 7 x lo-’ 9 x lo-’ 19 x IO_’ 13 x 10-7 8x10-’
1400
A. HIJMN, H. TERRYN, J. VEREECKEN AND R. DE KEYZER mV
A Ug i NHE
100
0
-100
3b -200
2b
lb
L
1
0
L
3
2
_I in A/m2
5
Fig. 1. Tafel lines for the polarization curves (sweep rate = 8 x 10e4 V s-‘) ofthe Ag/Ag (S,O&- systems. I-3: [Ag+] = 4 x lo-l5 M and ~&&O&-J = 0.1, 0.05 and 0.01 M, respectively. 4, 5: [Ag(S,O&-] =O.lM and [Ag+]=4xlO’ and 8~10-‘~ M, respectively. (a) Anodic reactions; (b) cathodic reactions.
mV
Ug/
NHE
average value of kz is 3.5 x lob5 ms-’ (K,, is the adsorption constant and is determined to be 3.5 dm3 mol- I).
The
2. Reduction
of silver thiocyanate
complexes
A similar series of experiments as for the thiosulphate complexes was performed. The same steps are rate determining in the different potential ranges, but the rate constant is 100x greater. An adsorption of SCN- doesn’t occur.
-100
- 200
CONCLUSIONS The following proposed:
M _ . mA
0.3
.’ 0.2
mechanism
and rate equation
(1 -Inn, -,Ag’+mL”Ag L,
.
AgLg-““)+e-
0.1
$Ag+mL”L”- G+(L”_)&
Fig. 2. Typical of Ag&O,)$-
cyclic voltammogram ([Ag&O,):-]
= 0.1
of the reductmn M; sweep rate
1 1 1 -_= J cr, +J,
= 0.01 vs-1).
Table
L”-
s,o:SCN
Diffusion coeflicien t, D (m*s-‘) 0.45x10-9 0.75 x 10-9
Transfer
2
coeflicient, a
Rate constant, k, (ms-‘)
0.58 0.74
3.5 x 10-S 2.5 x 10-S
Adsorption constant, K,, (dm3 molt ‘) 3.5 0
are
Electrochemical J is the current
speed
REFERENCES [Ag Lg -““‘]
of the electrode,
~ _ k,[AgLg-““)]exp[-((1 ILK,,[L”
Ug is the galvanic
1401
of silver compounds
density,
J d = 6 .2 FD2/3y1/2
u is the rotation
reduction
-u)FU~/RT]
-1 + I
1. T. H. James, The Theory a/ the Photographic
2. 3.
potential.
Table 2 gives the values obtained phate and thiocyanate ions.
for the thiosul-
4. 5.
Acknowledgements-The authors gratefully acknowledge the financial support provided by Agfa Gevaert N.V. and I.W.O.N.L. (Institute for Encouragement of Scientific Research in Industry and Agriculture].
6. 7.
Process (4th AI). Macmillan, New York (1977). W. Jaenickeand H. Kobayashi, Pkotogr. Sci. Engng 25,152 (1981). J. Koryta, Advances in EIectrochemisrry and Elecrrochemical Engineering, Vol. 6, Electrochemical Kinetics of Meral Complexes (Edited by P. D&hay and C. W. Tobias). Interscience, New York (1967). A. J. Bard and L. R. Faulkner, Electrochemical Methods. J. Wiley, New York (1980). D. D. Macdonald, Transient Techniques in Electrochemistry. Plenum Press, New York (1977). M. Sluyters-Rehbach and J. H. Sluyters, J. elecrroanal. Ckem. 65, 831 (1975). F. Yeager and A. J. Salkind, Techniques of Electrochemistry. Wiley Interscience, New York (1972).
H.
HUBIN,*
Vrije Universiteit
Brussel,
*Agfa
J. VEREECKEN*
TERRYN,*
Department
of Metallurgy,
Gevaert N.V., Septestraat
(Received
4 Sepwmber
and R. DE Pleiniaan
27, 2510 Mortsel,
1984; in revised
form 12
INTRODUCTION
EXPERIMENTAL Measurements were carried out in a thermostated (20& 1°C) electrochemical cell, with a platinum counter electrode and a KCl-saturated caiomel electrode as reference. The working electrode was a silver disk (99.99%) with a radius of 1 mm. Before each experiment, the electrode was polished with diamond paste (Bodson 7 and 0.25 H). The solution was deaerated by nitrogen bubbling. Stationary and non-stationary polarization curves were recorded using a potentiostat (Amel SSl), a generator (Amel S67), a millivoltmeter (TacusselMVN-79) and an oscilloscope (Nicolet Instrument Corporation-model 206). Stationary measurements were performed on a rotating disk electrode (Tacussel--Controvit).
C .WSIO,I:-
(Ml 0.1 0.05 0.01 0.1 0.1
2, 1050 Brussel, Belgium
March
1. Reduction
thiosulphate
k*R= k,(K,,[S,O:-]
parameters
for the reduction
cAg+
C s,o:-
E/nhe
4 x 10-15 4 x lo-‘5 4 x lo-‘5
of silver
AND DISCUSSION
complexes
From the stationary polarization curves, the following conclusions could be drawn[4]. Charge transfer at low overpotentials (-z 150 mV) and diffusion at high overpotentials are rate determining. The decomplexation of Ag(S20&~ does not influence the overall reaction rate. The assumption that the fret siIver ions are electroactive leads to the calculation of a rate constant of 10’ ms-*, which is highly improbable for a rate determining step. Consequently, the complexes are the electroactive species. The transfer coefficients a, the exchange current densities & and the rate constants kp, calculated from the Tafel pkots[4] (Fig. I) are given m Table 1. As the rate constant depends on the free thiosulphate concentration, the implication is that the reduction of Ag(S,O,):does not proceed uia a simple chargetransfer mechanism. In order to complete the mechanism, a nonstationary technique had to be used. The cyclic voltammograms (Fig. 2) show a prewave. This indicates that an adsorption of product occurs[5]. Although $0 : - is electro-inactive, its adsorption can cause a decrease in free energy for the reduction of complex, which explains the separation between the peaksC61. Assuming a Langmuir adsorption isotherm, the rate constant k:, independent of the concentration, is given WCi’]:
1. Kinetic
(Ml
Belgium
1985)
RESULTS
Silver diffusion transfer development is a photographic process in which complexed silver ions are reduced to metallic silver to form an image[l]. Only a few studies on physical development have been performed by means of electrochemical techniques[2]. The kinetic data, available for electrode processes involving silver complexes of possible interest in photographic systems, have been mainly obtained by measurements at the equilibrirum potential and on a mercury electrode[3]. In this work, the mechanism and the kinetic parameters of the reduction of silver thiosulphate and thiocyanate complexes are reported.
Table
KEYZER+
of Ag&O,):-
(W
W)
@OX
aR
1.15 0.81 0.36 0.40 0.8 1
- 0.048 -0.045 - 0.046 f0.014 - 0.023
0.60 0.63 0.63 0.53 0.58
0.52 0.57 0.56 0.53 0.60
1399
f I).
JO (Am-‘) 14 9 4 10 12
k”R (m s-l) 7 x lo-’ 9 x lo-’ 19 x IO_’ 13 x 10-7 8x10-’
1400
A. HIJMN, H. TERRYN, J. VEREECKEN AND R. DE KEYZER mV
A Ug i NHE
100
0
-100
3b -200
2b
lb
L
1
0
L
3
2
_I in A/m2
5
Fig. 1. Tafel lines for the polarization curves (sweep rate = 8 x 10e4 V s-‘) ofthe Ag/Ag (S,O&- systems. I-3: [Ag+] = 4 x lo-l5 M and ~&&O&-J = 0.1, 0.05 and 0.01 M, respectively. 4, 5: [Ag(S,O&-] =O.lM and [Ag+]=4xlO’ and 8~10-‘~ M, respectively. (a) Anodic reactions; (b) cathodic reactions.
mV
Ug/
NHE
average value of kz is 3.5 x lob5 ms-’ (K,, is the adsorption constant and is determined to be 3.5 dm3 mol- I).
The
2. Reduction
of silver thiocyanate
complexes
A similar series of experiments as for the thiosulphate complexes was performed. The same steps are rate determining in the different potential ranges, but the rate constant is 100x greater. An adsorption of SCN- doesn’t occur.
-100
- 200
CONCLUSIONS The following proposed:
M _ . mA
0.3
.’ 0.2
mechanism
and rate equation
(1 -Inn, -,Ag’+mL”Ag L,
.
AgLg-““)+e-
0.1
$Ag+mL”L”- G+(L”_)&
Fig. 2. Typical of Ag&O,)$-
cyclic voltammogram ([Ag&O,):-]
= 0.1
of the reductmn M; sweep rate
1 1 1 -_= J cr, +J,
= 0.01 vs-1).
Table
L”-
s,o:SCN
Diffusion coeflicien t, D (m*s-‘) 0.45x10-9 0.75 x 10-9
Transfer
2
coeflicient, a
Rate constant, k, (ms-‘)
0.58 0.74
3.5 x 10-S 2.5 x 10-S
Adsorption constant, K,, (dm3 molt ‘) 3.5 0
are
Electrochemical J is the current
speed
REFERENCES [Ag Lg -““‘]
of the electrode,
~ _ k,[AgLg-““)]exp[-((1 ILK,,[L”
Ug is the galvanic
1401
of silver compounds
density,
J d = 6 .2 FD2/3y1/2
u is the rotation
reduction
-u)FU~/RT]
-1 + I
1. T. H. James, The Theory a/ the Photographic
2. 3.
potential.
Table 2 gives the values obtained phate and thiocyanate ions.
for the thiosul-
4. 5.
Acknowledgements-The authors gratefully acknowledge the financial support provided by Agfa Gevaert N.V. and I.W.O.N.L. (Institute for Encouragement of Scientific Research in Industry and Agriculture].
6. 7.
Process (4th AI). Macmillan, New York (1977). W. Jaenickeand H. Kobayashi, Pkotogr. Sci. Engng 25,152 (1981). J. Koryta, Advances in EIectrochemisrry and Elecrrochemical Engineering, Vol. 6, Electrochemical Kinetics of Meral Complexes (Edited by P. D&hay and C. W. Tobias). Interscience, New York (1967). A. J. Bard and L. R. Faulkner, Electrochemical Methods. J. Wiley, New York (1980). D. D. Macdonald, Transient Techniques in Electrochemistry. Plenum Press, New York (1977). M. Sluyters-Rehbach and J. H. Sluyters, J. elecrroanal. Ckem. 65, 831 (1975). F. Yeager and A. J. Salkind, Techniques of Electrochemistry. Wiley Interscience, New York (1972).