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Adsorption of the No−3 ions at the polycrystalline silver electrode-aqueous electrolyte solution interface
ADSORPTION OF THE NO, IONS AT THE POLYCRYSTALLINE SILVER ELECTRODE-AQUEOUS ELECTROLYTE SOLUTION INTERFACE MAGDALENA
Laboratory of Electrochemistry. Department {Received
and STEFAN
MKKOWSKA
5 January
of Chemistry, Warsaw Warsaw, Poland 1983; in rroisedform
MING University,ul. Pasteura I. 02-093
19 July 1983)
Abstract-Differential capacityof polycrystallinesilver electrodeaqueous LiN03 solution interfacewas measuredfor 0.01,0.03,0.04,0.1 and0.2 M concenrrations. Adsorption of the nitrate ion hasteen analyzedin termsof the Damaskin’s method [Elekrrokhiniya 12,646 (1976)]. Some adsorptionparametershave been determinedand comparedwith the resultsobtainedfor mercuryand gold in order to determinethe role of metal in the double-layerstructure.
INTRODUCTION A rapid development of the studies of solid metalelectrolyte solution interfaces has been observed in the recent years. Quantitative studies of adsorption on solid electrodes make it possible to determine the role of metal in the electrical double-layer structure. However, these studies are complicated by difficulties in preparing the metal surface. The object of our studies is the silver-electrolyte solution interface. It is difficult to measure adsorption of some ions as it is impossible to renew the metal surface during the measurement; this complication is inherent in the technique of solid electrode investigation. Therefore, the choice of electrolyte is essential in the work with solid metal electrodes. In our work we are studying the adsorption of nitrate ions on silver. Adsorption of the NO; ionson mercury was studied by Grahame and Soderberg[ l] and then by Payne who studied adsorption of nitrate ions from a constant ionic strength solution in the Hg-NH,NQs-NH4F system[Z] and from aqueous KNO, solution@]. Adsorption of nitrate ions was also studied on gold[4]. A comparison of the results obtained in this work with those obtained for mercury and gold may cast a light on the effect of metal nature on the double-layer structure. It results from the position of silver in the hydrophilicity series of metals[S] that the adsorption properties of silver and mercury should be very similar. However, the Fermi energy levels of these metals suggest that the adsorption properties of silver should be similar with those of gold rather than with those of mercury[d]. Setting this question is an essential purpose of this work.
the Parsons and Zobel method[S]. Roughness coefficient of the electrode in the studied system was 1.15;it was determined from the differential capacity curves of the silver electrode immersed in a KF solution, as detailed previously[6]. Solutions were prepared using triply distilled water of 1.6 x 10- 6 n- ’ cm- ’ specific conductance. Analytical grade lithium nitrate (POCh Gliwice) was purified by threefold crystallization from triply distilled water. The solutions were deoxygenated with purified argon before the measurements. All the measurements were done with respect to a saturated calomel electrode.
RESULTS
DISCUSSION
Differential capacity measurements of the silver electrode-aqueous LiN03 solution interfaces have been carried out for 0.01,0.02,0.03,0.06,0.1 and 0.2 M concentrations. The results are plotted in Fig. 1 and compared with those for 0.1 M KF[6]. The capacity curves of KF and LiNO, converge at - 1110 mV potential; this value has been accepted as the desorption potential of the nitrate ions from silver. The potentials of zero charge of the studied systems have been calculated by back-integration from the known potential of zero charge values of silver as presented in Fig. 2. It is seen in the plot that the potential of zero charge shifts of silver in the LiNOs solutions are: Eo.os = 12 mV, E0.06 = 35mV, E,,, = 4SmV and E,.Z = 57 mV. The analogous data for gold are[4]: EO.OZ = 200 mV. E, nlc = 230 mV. En OS= 270 mV and E,,, = 300 mV. For mercury they are: E,., = 46 mV according to Grahame and Soderberg[l], Eo.i = 35 mV in a constant ionic strength solution and E n, = 100 mV in a KNOX solution according to Pavner21. The- r&.ults have been further analyzed by the Grahame and Soderberg method[l] modified by Damaskin et nl.[9]_ This method permits to calculate the adsorption parameters from the differential capacity results with respect to the electrode potential when the surface tension value of the metal is un_.II_
EXPERIMENTAL
Y._
The differential capacity measurements were done using the potentiostatic method by means of the apparatus made in our laboratory and described in ref.[7]. Silver electrodes were prepared by electropolishing with interrupted current in the cyanide bath[6]. Real surface area of the electrode was determined by 257 Eli *9:*-H
AND
_.__
2%
MAGDALENA
MFLKOWSKAAND
STEFAN MINC
3
3 N 'E Lc * 2!5-
2'0-
1 -120
I -300
I -460
-660
I -840
N
I -Ice0
I -lxx]
mV
Fig. 1. Differential capacity curves of silver electrode in aqueous LiNO, solutions us the electrode potential t 0.1 MKF, Aa. M LiNO,, +a.03 M LiNO,, oM LiNOJ 0.06 M LiN03, x 4.1 M LiNO, and W.2
Fig. 2. Dependence of the electrode charge on the polarization potential to.1 M KF, x a.03 M LiNO,, +0.06 M LiNO,, A.1 MLiN03, W.2 M LiNO,.
Fig. 3. Dependence of the adsorbed charge q’ on the electrode charge q. A-silver electrode in aqueous LiNO, solution: AI-O.03 M, A,-O.O4 M, A,--O.l M, A,-O.02 M; &gold electrode in aqueous LiN03 solution[ll]: B,0.035 M, B,--O.05 M; C-mercury electrodes in 0.01 M aqueous KN03 solution[3].
those obtained for gold and mercury. The curve slopes of silver are higher than those of gold and mercury. Adsorption in the range of low positive charges is weaker than in the case of gold and mercury but
2
4
6
6
10
12
14
16
4*//.&c.fn-'
known. The activity coefficient values of LiN03 in water have been taken from the paper[lO]. Figure 3 presents the plot of q’ us q for several LiN03 concentrations; the results are compared with
Fig. 4. Dependences of the b”-’ potential on the adsorbed charge q’ at constant electrode charge compared with those. obtained for mercury[3]. The numbers at the curves denote the constant electrode charges.
Adsorption of NO; ions at silver electrode-aqueous electrolyte interface
stronger than in the case of mercury at high positive charges. The 4”-2 potential of the inner part of the doublelayer was determined from the relationship:
several adsorbed charge values as functions of electrode charge are rectilinear and their slopes are (84”-*/aq),,. = 0.066 pF-‘cm2 = (p.Ki)-l. As
@‘--2 = (E,=,,+b*-‘)-EEq where #2m* was determined from the Gouy and Chapman diffuse layer theory. The dependences of $M-2 on the adsorbed charge are presented in Fig. 4 for constant metal surface charges and compared with the data obtained for mercury. The @M-* = f(q’) dependences are parallel straight lines; their slope is 0.0201 cm’ pF-l_ The value is twice as high than that obtained by Payne for mercury in KN@ solutions (0.014 cm2 pcF_ ‘) and four times higher than the value of the H -(NH,NOJ + NH.+F) system amounting 0.0051 cm f PF- ‘. The slope reciprocal has the dimension of capacity and is equal to the capacity of a condenser with plates consisting of the inner and the outer Helmholtz planes. This capacity is aq’/d@-2 = $ = 49.6 yFcrn-.l for the studied system, 96.15 PF cn- * for mercury in KNO, solutions and 196.1 pF cm- * for mercury in the constant ionic strength solutions. The #M-* = f(q),, dependence is also rectilinear. The curves plotted in Fig. 5 for
>
600
-
500
-
400
-
3cQ-
E .
I-8
259
qK1_
x*-x1
--
@fK’
x2
the distance ratio has been determined for the studied system. It amounts to 0.30. Comparing this value with the data obtained for mercury in the Hg-KNO, system for which it is equal to 0.25[3] and in the Hg-constant ionic strength solution system where it varies from 0.15 to 0 one may conclude that the distance x2 increases and the interaction of ions in the diffuse part of the compact layer are stronger than in the case of mercury. A constant slope of the ti”-2 = f(q’), curves indicate that oriented dipoles do not contribute significantly to the inner part of the electric double-layer. Therefore, a constant distance ratio for various charges may suggest a small contribution of the oriented dipoles to the inner part of the double-layer; it seems to indicate a hydrophobic character of the metal rather than the hydrophilic one. In this context this paper confirms the conclusions drawn from ValetteLl that silver and gold are hydrophobic, not hydrophilic. On the other hand, the problem of hydrophilicity of silver is still controversial as it results from TrasattiC131, and more experimental data are needed to confirm or to reject the concept of hydrophilicity of silver.
REFERENCES 1.
D. C. Grahame and B. A. Soderberg, J. them. Phys. 22,
449 11954). 2. R. P&ne,‘J. phys. Chem. 69, 4113 (1965). 3. R. Payne, J. elecrrochem. SOL-. 113. 999 (1966). 4. M. Bnostowska-Smolska and S. Mint, Pal. J. Chem.54,
2391 (1980).
5. S. Trasatti, J. elecrrounal.Chem. 33, 531 (1971). 6. S. Mint and M. Milkowska, Pd. J. Chem. 55,169 (1981). 7. M. BrzosTowska-Smolska,M. Miikowska,A. Kalinowski and S. Mint, 1. [email protected]. 89, 389 (1978). 8. R. Parsons and G. R. Zobel, J. elecrroanal. Chem. 9, 333
(1965).
Fig. 5. Dependences of the $M * potential on the electrode charge for constant adsorbed charge values: o-q’ e-q’= +3Kcm-‘, x--q’= f6&cm-*. = 0/Kcm-~,
9. B. B. Damaskin, T. A. Severova and R. V. Ivanova, EIekrrokhimiya 12, 646 (1976). of Elecrrochemical Consrants, 10 R. Parsons, Handbook Butterworths, London (1959). 11 M. Brzostowska-Smolska, private communication. 12: G. Valette, J. electroanal. Chem. 139, 301 (1982). 13. S. Trasatti, J. &crrounoL Chem. 138, 449 il9S?).
Laboratory of Electrochemistry. Department {Received
and STEFAN
MKKOWSKA
5 January
of Chemistry, Warsaw Warsaw, Poland 1983; in rroisedform
MING University,ul. Pasteura I. 02-093
19 July 1983)
Abstract-Differential capacityof polycrystallinesilver electrodeaqueous LiN03 solution interfacewas measuredfor 0.01,0.03,0.04,0.1 and0.2 M concenrrations. Adsorption of the nitrate ion hasteen analyzedin termsof the Damaskin’s method [Elekrrokhiniya 12,646 (1976)]. Some adsorptionparametershave been determinedand comparedwith the resultsobtainedfor mercuryand gold in order to determinethe role of metal in the double-layerstructure.
INTRODUCTION A rapid development of the studies of solid metalelectrolyte solution interfaces has been observed in the recent years. Quantitative studies of adsorption on solid electrodes make it possible to determine the role of metal in the electrical double-layer structure. However, these studies are complicated by difficulties in preparing the metal surface. The object of our studies is the silver-electrolyte solution interface. It is difficult to measure adsorption of some ions as it is impossible to renew the metal surface during the measurement; this complication is inherent in the technique of solid electrode investigation. Therefore, the choice of electrolyte is essential in the work with solid metal electrodes. In our work we are studying the adsorption of nitrate ions on silver. Adsorption of the NO; ionson mercury was studied by Grahame and Soderberg[ l] and then by Payne who studied adsorption of nitrate ions from a constant ionic strength solution in the Hg-NH,NQs-NH4F system[Z] and from aqueous KNO, solution@]. Adsorption of nitrate ions was also studied on gold[4]. A comparison of the results obtained in this work with those obtained for mercury and gold may cast a light on the effect of metal nature on the double-layer structure. It results from the position of silver in the hydrophilicity series of metals[S] that the adsorption properties of silver and mercury should be very similar. However, the Fermi energy levels of these metals suggest that the adsorption properties of silver should be similar with those of gold rather than with those of mercury[d]. Setting this question is an essential purpose of this work.
the Parsons and Zobel method[S]. Roughness coefficient of the electrode in the studied system was 1.15;it was determined from the differential capacity curves of the silver electrode immersed in a KF solution, as detailed previously[6]. Solutions were prepared using triply distilled water of 1.6 x 10- 6 n- ’ cm- ’ specific conductance. Analytical grade lithium nitrate (POCh Gliwice) was purified by threefold crystallization from triply distilled water. The solutions were deoxygenated with purified argon before the measurements. All the measurements were done with respect to a saturated calomel electrode.
RESULTS
DISCUSSION
Differential capacity measurements of the silver electrode-aqueous LiN03 solution interfaces have been carried out for 0.01,0.02,0.03,0.06,0.1 and 0.2 M concentrations. The results are plotted in Fig. 1 and compared with those for 0.1 M KF[6]. The capacity curves of KF and LiNO, converge at - 1110 mV potential; this value has been accepted as the desorption potential of the nitrate ions from silver. The potentials of zero charge of the studied systems have been calculated by back-integration from the known potential of zero charge values of silver as presented in Fig. 2. It is seen in the plot that the potential of zero charge shifts of silver in the LiNOs solutions are: Eo.os = 12 mV, E0.06 = 35mV, E,,, = 4SmV and E,.Z = 57 mV. The analogous data for gold are[4]: EO.OZ = 200 mV. E, nlc = 230 mV. En OS= 270 mV and E,,, = 300 mV. For mercury they are: E,., = 46 mV according to Grahame and Soderberg[l], Eo.i = 35 mV in a constant ionic strength solution and E n, = 100 mV in a KNOX solution according to Pavner21. The- r&.ults have been further analyzed by the Grahame and Soderberg method[l] modified by Damaskin et nl.[9]_ This method permits to calculate the adsorption parameters from the differential capacity results with respect to the electrode potential when the surface tension value of the metal is un_.II_
EXPERIMENTAL
Y._
The differential capacity measurements were done using the potentiostatic method by means of the apparatus made in our laboratory and described in ref.[7]. Silver electrodes were prepared by electropolishing with interrupted current in the cyanide bath[6]. Real surface area of the electrode was determined by 257 Eli *9:*-H
AND
_.__
2%
MAGDALENA
MFLKOWSKAAND
STEFAN MINC
3
3 N 'E Lc * 2!5-
2'0-
1 -120
I -300
I -460
-660
I -840
N
I -Ice0
I -lxx]
mV
Fig. 1. Differential capacity curves of silver electrode in aqueous LiNO, solutions us the electrode potential t 0.1 MKF, Aa. M LiNO,, +a.03 M LiNO,, oM LiNOJ 0.06 M LiN03, x 4.1 M LiNO, and W.2
Fig. 2. Dependence of the electrode charge on the polarization potential to.1 M KF, x a.03 M LiNO,, +0.06 M LiNO,, A.1 MLiN03, W.2 M LiNO,.
Fig. 3. Dependence of the adsorbed charge q’ on the electrode charge q. A-silver electrode in aqueous LiNO, solution: AI-O.03 M, A,-O.O4 M, A,--O.l M, A,-O.02 M; &gold electrode in aqueous LiN03 solution[ll]: B,0.035 M, B,--O.05 M; C-mercury electrodes in 0.01 M aqueous KN03 solution[3].
those obtained for gold and mercury. The curve slopes of silver are higher than those of gold and mercury. Adsorption in the range of low positive charges is weaker than in the case of gold and mercury but
2
4
6
6
10
12
14
16
4*//.&c.fn-'
known. The activity coefficient values of LiN03 in water have been taken from the paper[lO]. Figure 3 presents the plot of q’ us q for several LiN03 concentrations; the results are compared with
Fig. 4. Dependences of the b”-’ potential on the adsorbed charge q’ at constant electrode charge compared with those. obtained for mercury[3]. The numbers at the curves denote the constant electrode charges.
Adsorption of NO; ions at silver electrode-aqueous electrolyte interface
stronger than in the case of mercury at high positive charges. The 4”-2 potential of the inner part of the doublelayer was determined from the relationship:
several adsorbed charge values as functions of electrode charge are rectilinear and their slopes are (84”-*/aq),,. = 0.066 pF-‘cm2 = (p.Ki)-l. As
@‘--2 = (E,=,,+b*-‘)-EEq where #2m* was determined from the Gouy and Chapman diffuse layer theory. The dependences of $M-2 on the adsorbed charge are presented in Fig. 4 for constant metal surface charges and compared with the data obtained for mercury. The @M-* = f(q’) dependences are parallel straight lines; their slope is 0.0201 cm’ pF-l_ The value is twice as high than that obtained by Payne for mercury in KN@ solutions (0.014 cm2 pcF_ ‘) and four times higher than the value of the H -(NH,NOJ + NH.+F) system amounting 0.0051 cm f PF- ‘. The slope reciprocal has the dimension of capacity and is equal to the capacity of a condenser with plates consisting of the inner and the outer Helmholtz planes. This capacity is aq’/d@-2 = $ = 49.6 yFcrn-.l for the studied system, 96.15 PF cn- * for mercury in KNO, solutions and 196.1 pF cm- * for mercury in the constant ionic strength solutions. The #M-* = f(q),, dependence is also rectilinear. The curves plotted in Fig. 5 for
>
600
-
500
-
400
-
3cQ-
E .
I-8
259
qK1_
x*-x1
--
@fK’
x2
the distance ratio has been determined for the studied system. It amounts to 0.30. Comparing this value with the data obtained for mercury in the Hg-KNO, system for which it is equal to 0.25[3] and in the Hg-constant ionic strength solution system where it varies from 0.15 to 0 one may conclude that the distance x2 increases and the interaction of ions in the diffuse part of the compact layer are stronger than in the case of mercury. A constant slope of the ti”-2 = f(q’), curves indicate that oriented dipoles do not contribute significantly to the inner part of the electric double-layer. Therefore, a constant distance ratio for various charges may suggest a small contribution of the oriented dipoles to the inner part of the double-layer; it seems to indicate a hydrophobic character of the metal rather than the hydrophilic one. In this context this paper confirms the conclusions drawn from ValetteLl that silver and gold are hydrophobic, not hydrophilic. On the other hand, the problem of hydrophilicity of silver is still controversial as it results from TrasattiC131, and more experimental data are needed to confirm or to reject the concept of hydrophilicity of silver.
REFERENCES 1.
D. C. Grahame and B. A. Soderberg, J. them. Phys. 22,
449 11954). 2. R. P&ne,‘J. phys. Chem. 69, 4113 (1965). 3. R. Payne, J. elecrrochem. SOL-. 113. 999 (1966). 4. M. Bnostowska-Smolska and S. Mint, Pal. J. Chem.54,
2391 (1980).
5. S. Trasatti, J. elecrrounal.Chem. 33, 531 (1971). 6. S. Mint and M. Milkowska, Pd. J. Chem. 55,169 (1981). 7. M. BrzosTowska-Smolska,M. Miikowska,A. Kalinowski and S. Mint, 1. [email protected]. 89, 389 (1978). 8. R. Parsons and G. R. Zobel, J. elecrroanal. Chem. 9, 333
(1965).
Fig. 5. Dependences of the $M * potential on the electrode charge for constant adsorbed charge values: o-q’ e-q’= +3Kcm-‘, x--q’= f6&cm-*. = 0/Kcm-~,
9. B. B. Damaskin, T. A. Severova and R. V. Ivanova, EIekrrokhimiya 12, 646 (1976). of Elecrrochemical Consrants, 10 R. Parsons, Handbook Butterworths, London (1959). 11 M. Brzostowska-Smolska, private communication. 12: G. Valette, J. electroanal. Chem. 139, 301 (1982). 13. S. Trasatti, J. &crrounoL Chem. 138, 449 il9S?).