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Adsorption of inorganic ions on bismuth from constant ionic strength solutions in 2-butanol
247
J. Electroanal. Chem., 353 (1993) 247-254 Elsevier Sequoia S.A., Lausanne
J E C 02573
Adsorption of inorganic ions on bismuth from constant ionic strength solutions in 2-butanol M. V~i~irtn6u Laboratory of Electrochemistry, Tartu University, EE2400 Tartu (Estonia)
(Received 16 June 1992; in revised form 4 November 1992)
Abstract
The adsorption of CI-, Br , I - and SCN ions on a drop-shaped bismuth electrode from solutions in 2-butanol has been investigated by means of differential capacity measurements. The ionic charge due to the specific adsorption was calculated using the Hurwitz-Parsons-Dutkiewicz method. The results were analysed using the virial adsorption isotherm and the classical Grahame-Parsons inner layer model. It was confirmed that the standard Gibbs energy of adsorption of ions in alcohols can be correlated with quantum-chemical indices of reactivity.
INTRODUCTION T h e n a t u r e o f t h e solvent plays an i m p o r t a n t role in a d s o r p t i o n p r o c e s s e s o n e l e c t r o d e s . Since b i s m u t h is e x t r e m e l y s t a b l e a n d ideally p o l a r i z a b l e in a w i d e p o t e n t i a l r a n g e in alcoholic solutions, we have c a r r i e d out an extensive investigation o f the a d s o r p t i o n o f i n o r g a n i c ions on b i s m u t h e l e c t r o d e s in v a r i o u s a l i p h a t i c alcohols [1]. W e have f o u n d that the a d s o r p t i o n p a r a m e t e r values a r e q u i t e similar in alcohols with d i f f e r e n t lengths o f h y d r o c a r b o n c h a i n b u t similar m o l e c u l a r structures. H o w e v e r , w h e n 2 - p r o p a n o l was u s e d as t h e solvent the p a r a m e t e r s a p p e a r e d to b e very d i f f e r e n t from those for m e t h a n o l , e t h a n o l , 1 - p r o p a n o l a n d 1-butanol [2]. T h e r e f o r e , it is o f i n t e r e s t to d e t e r m i n e how t h e a d s o r p t i o n b e haviour o f ions differs in various i s o m e r s o f b u t a n o l . I n t h e p r e s e n t p a p e r we r e p o r t studies o f t h e a d s o r p t i o n of C I - , B r - , I - a n d S C N - ions on a d r o p - s h a p e d b i s m u t h e l e c t r o d e in 2 - b u t a n o l ( s e c - B u O H ) using d i f f e r e n t i a l c a p a c i t y m e a s u r e ments. EXPERIMENTAL D r o p - s h a p e d b i s m u t h e l e c t r o d e s w e r e p r e p a r e d using a special device d e s c r i b e d e a r l i e r [3]. By m e l t i n g b i s m u t h in an a t m o s p h e r e of p u r e h y d r o g e n we can o b t a i n a 0022-0728/93/$06.00 © 1993 - Elsevier Sequoia S.A. All rights reserved
248 solid drop-shaped electrode with a very clean smooth surface. This electrode is mainly polycrystalline, but its surface contains large monocrystalline areas. High purity commercial 2-butanol with water content less than 0.1% was further purified by distillation in a vacuum. Lithium salts (LiC104, LiC1, LiBr, LiI and LiSCN) were purified by recrystallization from ethanol. T o remove oxygen, pure hydrogen was bubbled through the solutions prior to capacity measurements. The capacity was measured using an ac bridge. The capacity measurements were carried out in solutions with constant ionic strength m c M LiA + c(1 - m ) M LiC10 4
where A is a surface-active anion ( C I - , B r - , I - , S C N - ) , m is the relative concentration of the surface-active component (we used solutions with 0.01 < m < 1) and c is the total concentration (c = 0.1 M). The CIO 4 ion is very weakly specifically adsorbed on bismuth from alcohols [4] and is assumed to be surface inactive here.
RESULTS AND DISCUSSION When using the mixed-electrolyte method in solvents with low dielectric permittivity, such as alcohols, we have to solve the problem of incomplete and unequal dissociation of salts in a mixed-electrolyte system. This can cause significant errors because the constancy of ionic strength in solution is not guaranteed. If the degrees of dissociation of the salts are known, the errors in the calculated adsorption values can be estimated using the method described by Damaskin [5]. Unfortunately, very few reliable data for the dissociation constants of salts in alcohols are available. Analysis of the equations in ref. 5 shows that the errors decrease rapidly as m is reduced. For example, in the case of two salts with degrees of dissociation equal to 0.3 and 0.4 (these seem to be realistic values for lithium salts in butanols), the error is 14% for m = 1 but only 1% for m = 0.1. Therefore, we can assume that in principle the mixed-electrolyte method can be used for butanols, but the results for m > 0.2 may be inaccurate. For example, when establishing the adsorption parameters, less weight must be given to the adsorption values calculated for higher m. Similarly, quantitative theoretical conclusions are justified only on the basis of those parameters that are estimated for m ~ 0. The differential capacity versus potential curves ( C - E curves) for the system m c M LiBr + c(1 - m) M LiC10 4 are shown in Fig. 1 as an example of capacity curves in 2-butanol. The C - E curves were integrated to obtain the electrode charge o-. Following the H u r w i t z - P a r s o n s - D u t k i e w i c z method [6,7] the function A~ was calculated using the standard procedure [3] (~ = 3' + o-E where 3' is the interracial tension, and A ~ : = ~ 0 - ~ : where ~:0 is the ~: value in LiCIO 4 solution). The
249
80
0 32
I
0
0.5
i
1.0
- E/V (SCE)
Fig. 1. Differential capacity curves for a bismuth electrode in solutions 0.1m M L i B r + 0 . l ( 1 - m) M LiCIO4 in 2-butanol. The values of m are given in the figure.
specifically adsorbed charge 0.1 was calculated by differentiation of AsC-log m curves. Some of the results are presented in Fig. 2. To find the adsorption parameters, the (71 values were fitted to the simple virial isotherm I n ( - o ' , / m c ) = In/3 + 2Bo"1
(1)
where In /3 = - A G ° / R T and AG° is the adsorption standard Gibbs energy. The virial coefficient B is characte istic of the mutual repulsion of adsorbed ions. The results for the S C N - ions are plotted in Fig. 3 where it can be seen that the simple virial isotherm describes the adsorption of the S C N - ions quite well. It also fits other ions in 2-butanol and all systems investigated by us in other alcohols [1]. Since the points corresponding to m = 1 also fit the straight line, it can be assumed that the errors caused by unequal dissociation of salts are probably smaller than suggested above. Various modifications of virial isotherms using different c values have been analysed elsewhere [8,9]. It has been found that, despite the good fit, the parameters in fl and B of the simple virial isotherm (eqn. (1)) depend on the total concentration of electrolyte, i.e. they are not real constants. In contrast, in the case of the isotherm including the diffuse-layer potential term In/3 and B are real constants, but the isotherm is strictly applicable only for orl ~ 0. Substituting or -[-0"1 for or in the diffuse-layer theory equations probably does not give true
250
15 0.1
o.s
F
,
- e / g C c m -a
Fig. 2. Plot of the charge o-1 due to specifically adsorbed anions against the electrode charge o- at the b i s m u t h - 2 - b u t a n o l interface: © C I - ; • B r - ; ~ I - ; z~ S C N - . The values of m are given in the figure.
10
F 5
I
I
I
5
10
15
-%/p Ccm-2 Fig. 3. Plot of the function I n ( - o . 1 / m c ) against the charge (7"1 of specifically adsorbed S C N - ions at the b i s m u t h - 2 - b u t a n o l interface. T h e electrode charge t r / i z C cm -2 is given on each line.
251
0.2
0
>
Io u
0
-0.2
I
I
I
5
10
15
-o-lll.tC c m -z
Fig. 4. Plot of the electrode potential relative to the potential of zero charge against the charge ~rx of specifically adsorbed anions at the bismuth-2-butanol interface: o CI-; • B r - ; * I - ; zx SCN-. The electrode charge ~r//xC cm -2 is given on each line.
diffuse-layer potential values. Therefore, we have used the simple isotherm (1) and calculated the real In/3 values afterwards using the formula [8,9] In/3(real) = In/3 -F~Oo/RT
(2)
where ~00 is the diffuse-layer potential in the absence of specific adsorption calculated using diffuse-layer theory. Finding the real B values is much more complicated, and reliable calculations can be carried out only for o-~ ~ 0 using the equation [8,9] B(real) = B - (o -2 + 4A2c) -1/2
(3)
where A is a solvent-dependent constant (A = 2.64 for 2-butanol). The classical G r a h a m e - P a r s o n s inner layer model [10] was used to analyze the inner-layer structure. To obtain the inner-layer parameters, the values of the potential relative to the potential of zero charge ( E - E , = 0) were plotted against o-1 at constant charge, as shown in Fig. 4. Again, owing to the uncertainty in the ¢J0 calculations when specific adsorption occurs and possible errors in o-~ for the highest values of m, only the values of K02 (the inner-layer integral capacity in the absence of ionic adsorption) are presented here. As can be seen from Fig. 4, there is a linear dependence between E - E ~ = 0 and o"1. Extrapolation of these straight lines to o"1 = 0 gives the potential step in the double layer for o"1 = 0, and the capacity K02 is K02 = o " / ( E - Eo ~0)trl= 0
(4)
252 TABLE 1 Adsorption isotherm parameters for anion adsorption at bismuth-butanol interfaces Parameter
Ion
n-BuOH
iso-BuOH
sec-BuOH
In/3,,= 0
IBrCISCN-
9.0 5.2
5.8 3.9 5.1
9.6 5.9 5.2 6.1
I -
6.8
-
BrCISCN-
2.6
3.4 2.4 1.6
5.6 2.5 1.6 0.8
B / n m 2 ion- l
It is interesting to note that, as in all other alcohols [1], K02 in 2-butanol at given ovalues is constant for the given solvent as well and does not depend on the nature of the adsorbed ion. Therefore, we can assume that the capacity K02 is characteristic of the solvent properties in the inner layer. The main parameters characterizing the adsorption of ions in various butanols are given in Table 1. The parameters for normal butanol (n-BuOH) and isobutanol (iso-BuOH) were taken from our previous papers [11,12]. The plots of K02 v e r s u s tr for n-BuOH and sec-BuOH are presented in Fig. 5. As can be seen from Table 1 and Fig. 5, there is some difference between the parameters in primary and secondary butanols which is analogous to the corresponding propanols [2]. In primary alcohols, the capacity K02 is lower and its charge dependence is different. The lower inner-layer capacity yields higher values of B. This is in full agreement with our previous results for 1- and 2-propanol [2].
5O o It_ --t
v
20
I
I
-5 0 -o-/gCcm -z Fig. 5. Plot of inner-layer capacity K02 against electrode charge ~r at bismuth-2-butanol ( o ) and bismuth-l-butanol (o) interfaces.
253 We concluded [2] that this is caused by an increase in the inner-layer dielectric permittivity of secondary alcohols, in terms of the simple plate condenser model for the inner layer. T h e r e is also a different charge dependence of In/3 in various alcohols: for primary alcohols it is linear, but for secondary alcohols it is quadratic. At first sight, it seems that the In/3~=0 values for a given ion in various solvents are quite similar and lead to no particular conclusions. However, we have found [13] that there is a satisfactory correlation between the standard Gibbs energy of adsorption (which is related to I n / 3 ~ 0 through the relation - A G ° = R T In /3~=0), and the quantum-chemical indices of reactivity [14]. The adsorption process is often considered to be a chemical reaction involving replacing the solvent molecules on the electrode surface by adsorbing ions [15]. We calculated the indices separately for each interaction involved in the adsorption process (ion-metal, i o n solvent, solvent-metal) using the semiempirical AM1 method [16] and then carried out multilinear regression analysis. As a result of this analysis using - AG ° values for 15 systems measured in water and five different alcohols, we found the following relation with a correlation coefficient of 0.95 and a standard deviation of 1.8 kJ m o l - l : - A G ° = 101 + 0.9
am_ i -
1.9
as_ i -
1.1
(5)
am_ s
where S is the appropriate quantum chemical index, m denotes metal surface, s the solvent and i the ion. The details of the calculations are given in ref. 13. It is now of interest to establish how the results in isobutanol and sec-butanol that were not included in regression analysis are in accordance with eqn. (5). Analogously to ref. 13, we calculated the indices for ion adsorption from these butanols and used eqn. (5) to obtain the AG ° and In/3~_ 0 values. The experimental and calculated values are presented in Table 2. If we take into account the fact that the experimental In/3 values can be determined with an accuracy to 0.2-0.3 units, the coincidence is rather good. The only exception is the system C1- + iso-BuOH. It is worth noting that in other primary alcohols the In/3 values for C1- were also found to be lower than the calculated values [13]. The reason for this is not quite
TABLE 2 Experimental and calculated In/3~=0 values for anion adsorption on bismuth in various butanols Solvent
Ion
In/3(exp)
In/3(calc)
iso-BuOH
C1BrSCN-
3.9 5.8 5.1
5.0 5.7 5.5
sec-BuOH
CIBr-
5.2 5.9 9.6 6,1
5.4 6.0 9.3 5.8
I -
SCN-
254 clear. W e a s s u m e that th e I n / 3 values f o r C1- m ay be d i s t o r t e d to s o m e e x t e n t by t h e c o a d s o r p t i o n o f C 1 0 4 ions. T h e l a t t e r can be n e g l e c t e d in t h e case of m o r e strongly a d s o r b e d ions. T h e r e f o r e we c a n c o n c l u d e that t h e a p p r o a c h d e v e l o p e d in ref. 13 is g e n e r a l l y good a n d eqn. (3) is a satisfactory a p p r o x i m a t i o n for calculations of - A G ° v a l u e s for t h e a d s o r p t i o n o f i no r g an i c ions o n b i s m u t h from p r o t i c solvents. REFERENCES 1 U. Palm, M. V~i~irtn6u, M. Salve, K. Anni, E. Yuriado, M. P~irnoya and K. Lust, Trans. Tartu State University, 757 (1986) 125. 2 M. V~i~irtn6u and U. Palm, Elektrokhimiya, 17 (1981) 1567. 3 B. Damaskin, U. Palm, E. Pety~irv and M. Salve, J. Electroanal. Chem., 47 (1973) 127. 4 M. V~i~irtn6u and U. Palm, Elektrokhimiya, 13 (1977) 1211. 5 B. Damaskin, Elektrokhimiya, 12 (1976) 561. 6 H.D. Hurwitz, J. Electroanal. Chem., 10 (1965) 35. 7 E. Dutkiewicz and R. Parsons, J. Electroanal. Chem., 11 (1966) 100. 8 B. Damaskin, U. Palm and M. V~i~irtn6u, J. Electroanal. Chem., 70 (1976) 103; 9 B. Damaskin, U. Palm, E. Pety~irv and M. Salve, J. Electroanal. Chem., 51 (1974) 179. 10 D.C. Grahame and R. Parsons, J. Am. Chem. Soc, 83 (1961) 1291. 11 M. V~i~irtn6u and U. Palm, Elektrokhimija, 16 (1980) 1603. 12 P. P~irsim~igiand M. V~i~irtn6u, Elektrokhimija, 27 (1991) 420. 13 M. V~i~irtn6u and M. Karelson, Elektrokhimija, 27 (1991) 1366. 14 K. Fukui, Theory of Orientation and Stereoselection, Springer Verlag, Berlin, 1975. 15 B.E. Conway, J. Sol. Chem., 7 (1978) 721. 16 M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Soc, 107 (1985) 3902.
J. Electroanal. Chem., 353 (1993) 247-254 Elsevier Sequoia S.A., Lausanne
J E C 02573
Adsorption of inorganic ions on bismuth from constant ionic strength solutions in 2-butanol M. V~i~irtn6u Laboratory of Electrochemistry, Tartu University, EE2400 Tartu (Estonia)
(Received 16 June 1992; in revised form 4 November 1992)
Abstract
The adsorption of CI-, Br , I - and SCN ions on a drop-shaped bismuth electrode from solutions in 2-butanol has been investigated by means of differential capacity measurements. The ionic charge due to the specific adsorption was calculated using the Hurwitz-Parsons-Dutkiewicz method. The results were analysed using the virial adsorption isotherm and the classical Grahame-Parsons inner layer model. It was confirmed that the standard Gibbs energy of adsorption of ions in alcohols can be correlated with quantum-chemical indices of reactivity.
INTRODUCTION T h e n a t u r e o f t h e solvent plays an i m p o r t a n t role in a d s o r p t i o n p r o c e s s e s o n e l e c t r o d e s . Since b i s m u t h is e x t r e m e l y s t a b l e a n d ideally p o l a r i z a b l e in a w i d e p o t e n t i a l r a n g e in alcoholic solutions, we have c a r r i e d out an extensive investigation o f the a d s o r p t i o n o f i n o r g a n i c ions on b i s m u t h e l e c t r o d e s in v a r i o u s a l i p h a t i c alcohols [1]. W e have f o u n d that the a d s o r p t i o n p a r a m e t e r values a r e q u i t e similar in alcohols with d i f f e r e n t lengths o f h y d r o c a r b o n c h a i n b u t similar m o l e c u l a r structures. H o w e v e r , w h e n 2 - p r o p a n o l was u s e d as t h e solvent the p a r a m e t e r s a p p e a r e d to b e very d i f f e r e n t from those for m e t h a n o l , e t h a n o l , 1 - p r o p a n o l a n d 1-butanol [2]. T h e r e f o r e , it is o f i n t e r e s t to d e t e r m i n e how t h e a d s o r p t i o n b e haviour o f ions differs in various i s o m e r s o f b u t a n o l . I n t h e p r e s e n t p a p e r we r e p o r t studies o f t h e a d s o r p t i o n of C I - , B r - , I - a n d S C N - ions on a d r o p - s h a p e d b i s m u t h e l e c t r o d e in 2 - b u t a n o l ( s e c - B u O H ) using d i f f e r e n t i a l c a p a c i t y m e a s u r e ments. EXPERIMENTAL D r o p - s h a p e d b i s m u t h e l e c t r o d e s w e r e p r e p a r e d using a special device d e s c r i b e d e a r l i e r [3]. By m e l t i n g b i s m u t h in an a t m o s p h e r e of p u r e h y d r o g e n we can o b t a i n a 0022-0728/93/$06.00 © 1993 - Elsevier Sequoia S.A. All rights reserved
248 solid drop-shaped electrode with a very clean smooth surface. This electrode is mainly polycrystalline, but its surface contains large monocrystalline areas. High purity commercial 2-butanol with water content less than 0.1% was further purified by distillation in a vacuum. Lithium salts (LiC104, LiC1, LiBr, LiI and LiSCN) were purified by recrystallization from ethanol. T o remove oxygen, pure hydrogen was bubbled through the solutions prior to capacity measurements. The capacity was measured using an ac bridge. The capacity measurements were carried out in solutions with constant ionic strength m c M LiA + c(1 - m ) M LiC10 4
where A is a surface-active anion ( C I - , B r - , I - , S C N - ) , m is the relative concentration of the surface-active component (we used solutions with 0.01 < m < 1) and c is the total concentration (c = 0.1 M). The CIO 4 ion is very weakly specifically adsorbed on bismuth from alcohols [4] and is assumed to be surface inactive here.
RESULTS AND DISCUSSION When using the mixed-electrolyte method in solvents with low dielectric permittivity, such as alcohols, we have to solve the problem of incomplete and unequal dissociation of salts in a mixed-electrolyte system. This can cause significant errors because the constancy of ionic strength in solution is not guaranteed. If the degrees of dissociation of the salts are known, the errors in the calculated adsorption values can be estimated using the method described by Damaskin [5]. Unfortunately, very few reliable data for the dissociation constants of salts in alcohols are available. Analysis of the equations in ref. 5 shows that the errors decrease rapidly as m is reduced. For example, in the case of two salts with degrees of dissociation equal to 0.3 and 0.4 (these seem to be realistic values for lithium salts in butanols), the error is 14% for m = 1 but only 1% for m = 0.1. Therefore, we can assume that in principle the mixed-electrolyte method can be used for butanols, but the results for m > 0.2 may be inaccurate. For example, when establishing the adsorption parameters, less weight must be given to the adsorption values calculated for higher m. Similarly, quantitative theoretical conclusions are justified only on the basis of those parameters that are estimated for m ~ 0. The differential capacity versus potential curves ( C - E curves) for the system m c M LiBr + c(1 - m) M LiC10 4 are shown in Fig. 1 as an example of capacity curves in 2-butanol. The C - E curves were integrated to obtain the electrode charge o-. Following the H u r w i t z - P a r s o n s - D u t k i e w i c z method [6,7] the function A~ was calculated using the standard procedure [3] (~ = 3' + o-E where 3' is the interracial tension, and A ~ : = ~ 0 - ~ : where ~:0 is the ~: value in LiCIO 4 solution). The
249
80
0 32
I
0
0.5
i
1.0
- E/V (SCE)
Fig. 1. Differential capacity curves for a bismuth electrode in solutions 0.1m M L i B r + 0 . l ( 1 - m) M LiCIO4 in 2-butanol. The values of m are given in the figure.
specifically adsorbed charge 0.1 was calculated by differentiation of AsC-log m curves. Some of the results are presented in Fig. 2. To find the adsorption parameters, the (71 values were fitted to the simple virial isotherm I n ( - o ' , / m c ) = In/3 + 2Bo"1
(1)
where In /3 = - A G ° / R T and AG° is the adsorption standard Gibbs energy. The virial coefficient B is characte istic of the mutual repulsion of adsorbed ions. The results for the S C N - ions are plotted in Fig. 3 where it can be seen that the simple virial isotherm describes the adsorption of the S C N - ions quite well. It also fits other ions in 2-butanol and all systems investigated by us in other alcohols [1]. Since the points corresponding to m = 1 also fit the straight line, it can be assumed that the errors caused by unequal dissociation of salts are probably smaller than suggested above. Various modifications of virial isotherms using different c values have been analysed elsewhere [8,9]. It has been found that, despite the good fit, the parameters in fl and B of the simple virial isotherm (eqn. (1)) depend on the total concentration of electrolyte, i.e. they are not real constants. In contrast, in the case of the isotherm including the diffuse-layer potential term In/3 and B are real constants, but the isotherm is strictly applicable only for orl ~ 0. Substituting or -[-0"1 for or in the diffuse-layer theory equations probably does not give true
250
15 0.1
o.s
F
,
- e / g C c m -a
Fig. 2. Plot of the charge o-1 due to specifically adsorbed anions against the electrode charge o- at the b i s m u t h - 2 - b u t a n o l interface: © C I - ; • B r - ; ~ I - ; z~ S C N - . The values of m are given in the figure.
10
F 5
I
I
I
5
10
15
-%/p Ccm-2 Fig. 3. Plot of the function I n ( - o . 1 / m c ) against the charge (7"1 of specifically adsorbed S C N - ions at the b i s m u t h - 2 - b u t a n o l interface. T h e electrode charge t r / i z C cm -2 is given on each line.
251
0.2
0
>
Io u
0
-0.2
I
I
I
5
10
15
-o-lll.tC c m -z
Fig. 4. Plot of the electrode potential relative to the potential of zero charge against the charge ~rx of specifically adsorbed anions at the bismuth-2-butanol interface: o CI-; • B r - ; * I - ; zx SCN-. The electrode charge ~r//xC cm -2 is given on each line.
diffuse-layer potential values. Therefore, we have used the simple isotherm (1) and calculated the real In/3 values afterwards using the formula [8,9] In/3(real) = In/3 -F~Oo/RT
(2)
where ~00 is the diffuse-layer potential in the absence of specific adsorption calculated using diffuse-layer theory. Finding the real B values is much more complicated, and reliable calculations can be carried out only for o-~ ~ 0 using the equation [8,9] B(real) = B - (o -2 + 4A2c) -1/2
(3)
where A is a solvent-dependent constant (A = 2.64 for 2-butanol). The classical G r a h a m e - P a r s o n s inner layer model [10] was used to analyze the inner-layer structure. To obtain the inner-layer parameters, the values of the potential relative to the potential of zero charge ( E - E , = 0) were plotted against o-1 at constant charge, as shown in Fig. 4. Again, owing to the uncertainty in the ¢J0 calculations when specific adsorption occurs and possible errors in o-~ for the highest values of m, only the values of K02 (the inner-layer integral capacity in the absence of ionic adsorption) are presented here. As can be seen from Fig. 4, there is a linear dependence between E - E ~ = 0 and o"1. Extrapolation of these straight lines to o"1 = 0 gives the potential step in the double layer for o"1 = 0, and the capacity K02 is K02 = o " / ( E - Eo ~0)trl= 0
(4)
252 TABLE 1 Adsorption isotherm parameters for anion adsorption at bismuth-butanol interfaces Parameter
Ion
n-BuOH
iso-BuOH
sec-BuOH
In/3,,= 0
IBrCISCN-
9.0 5.2
5.8 3.9 5.1
9.6 5.9 5.2 6.1
I -
6.8
-
BrCISCN-
2.6
3.4 2.4 1.6
5.6 2.5 1.6 0.8
B / n m 2 ion- l
It is interesting to note that, as in all other alcohols [1], K02 in 2-butanol at given ovalues is constant for the given solvent as well and does not depend on the nature of the adsorbed ion. Therefore, we can assume that the capacity K02 is characteristic of the solvent properties in the inner layer. The main parameters characterizing the adsorption of ions in various butanols are given in Table 1. The parameters for normal butanol (n-BuOH) and isobutanol (iso-BuOH) were taken from our previous papers [11,12]. The plots of K02 v e r s u s tr for n-BuOH and sec-BuOH are presented in Fig. 5. As can be seen from Table 1 and Fig. 5, there is some difference between the parameters in primary and secondary butanols which is analogous to the corresponding propanols [2]. In primary alcohols, the capacity K02 is lower and its charge dependence is different. The lower inner-layer capacity yields higher values of B. This is in full agreement with our previous results for 1- and 2-propanol [2].
5O o It_ --t
v
20
I
I
-5 0 -o-/gCcm -z Fig. 5. Plot of inner-layer capacity K02 against electrode charge ~r at bismuth-2-butanol ( o ) and bismuth-l-butanol (o) interfaces.
253 We concluded [2] that this is caused by an increase in the inner-layer dielectric permittivity of secondary alcohols, in terms of the simple plate condenser model for the inner layer. T h e r e is also a different charge dependence of In/3 in various alcohols: for primary alcohols it is linear, but for secondary alcohols it is quadratic. At first sight, it seems that the In/3~=0 values for a given ion in various solvents are quite similar and lead to no particular conclusions. However, we have found [13] that there is a satisfactory correlation between the standard Gibbs energy of adsorption (which is related to I n / 3 ~ 0 through the relation - A G ° = R T In /3~=0), and the quantum-chemical indices of reactivity [14]. The adsorption process is often considered to be a chemical reaction involving replacing the solvent molecules on the electrode surface by adsorbing ions [15]. We calculated the indices separately for each interaction involved in the adsorption process (ion-metal, i o n solvent, solvent-metal) using the semiempirical AM1 method [16] and then carried out multilinear regression analysis. As a result of this analysis using - AG ° values for 15 systems measured in water and five different alcohols, we found the following relation with a correlation coefficient of 0.95 and a standard deviation of 1.8 kJ m o l - l : - A G ° = 101 + 0.9
am_ i -
1.9
as_ i -
1.1
(5)
am_ s
where S is the appropriate quantum chemical index, m denotes metal surface, s the solvent and i the ion. The details of the calculations are given in ref. 13. It is now of interest to establish how the results in isobutanol and sec-butanol that were not included in regression analysis are in accordance with eqn. (5). Analogously to ref. 13, we calculated the indices for ion adsorption from these butanols and used eqn. (5) to obtain the AG ° and In/3~_ 0 values. The experimental and calculated values are presented in Table 2. If we take into account the fact that the experimental In/3 values can be determined with an accuracy to 0.2-0.3 units, the coincidence is rather good. The only exception is the system C1- + iso-BuOH. It is worth noting that in other primary alcohols the In/3 values for C1- were also found to be lower than the calculated values [13]. The reason for this is not quite
TABLE 2 Experimental and calculated In/3~=0 values for anion adsorption on bismuth in various butanols Solvent
Ion
In/3(exp)
In/3(calc)
iso-BuOH
C1BrSCN-
3.9 5.8 5.1
5.0 5.7 5.5
sec-BuOH
CIBr-
5.2 5.9 9.6 6,1
5.4 6.0 9.3 5.8
I -
SCN-
254 clear. W e a s s u m e that th e I n / 3 values f o r C1- m ay be d i s t o r t e d to s o m e e x t e n t by t h e c o a d s o r p t i o n o f C 1 0 4 ions. T h e l a t t e r can be n e g l e c t e d in t h e case of m o r e strongly a d s o r b e d ions. T h e r e f o r e we c a n c o n c l u d e that t h e a p p r o a c h d e v e l o p e d in ref. 13 is g e n e r a l l y good a n d eqn. (3) is a satisfactory a p p r o x i m a t i o n for calculations of - A G ° v a l u e s for t h e a d s o r p t i o n o f i no r g an i c ions o n b i s m u t h from p r o t i c solvents. REFERENCES 1 U. Palm, M. V~i~irtn6u, M. Salve, K. Anni, E. Yuriado, M. P~irnoya and K. Lust, Trans. Tartu State University, 757 (1986) 125. 2 M. V~i~irtn6u and U. Palm, Elektrokhimiya, 17 (1981) 1567. 3 B. Damaskin, U. Palm, E. Pety~irv and M. Salve, J. Electroanal. Chem., 47 (1973) 127. 4 M. V~i~irtn6u and U. Palm, Elektrokhimiya, 13 (1977) 1211. 5 B. Damaskin, Elektrokhimiya, 12 (1976) 561. 6 H.D. Hurwitz, J. Electroanal. Chem., 10 (1965) 35. 7 E. Dutkiewicz and R. Parsons, J. Electroanal. Chem., 11 (1966) 100. 8 B. Damaskin, U. Palm and M. V~i~irtn6u, J. Electroanal. Chem., 70 (1976) 103; 9 B. Damaskin, U. Palm, E. Pety~irv and M. Salve, J. Electroanal. Chem., 51 (1974) 179. 10 D.C. Grahame and R. Parsons, J. Am. Chem. Soc, 83 (1961) 1291. 11 M. V~i~irtn6u and U. Palm, Elektrokhimija, 16 (1980) 1603. 12 P. P~irsim~igiand M. V~i~irtn6u, Elektrokhimija, 27 (1991) 420. 13 M. V~i~irtn6u and M. Karelson, Elektrokhimija, 27 (1991) 1366. 14 K. Fukui, Theory of Orientation and Stereoselection, Springer Verlag, Berlin, 1975. 15 B.E. Conway, J. Sol. Chem., 7 (1978) 721. 16 M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Soc, 107 (1985) 3902.