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Induced adsorption of cations by −SO3 and -COOH groups anchored to the surface of a platinized platinum electrode. A radiotracer study
INDUCED ADSORPTION OF CATIONS BY -S03H AND -COOH GROUPS ANCHORED TO THE SURFACE OF A PLATINIZED PLATINUM ELECTRODE. A RADIOTRACER STUDY G. HOR~NYI and E. M. RIZMAYER
CentralResearch Institute for Chemistry ofthe Hungarian Academy of Sciences, Budapest H-l 525, Hungary (Received 4 November
1987; in revised form 1 February
1988)
Abstract-The adsorption of labelled Ca*+ Ions was studied at platinized platinum electrodes in 1 x lo-* moldm-” HCIO, supporting electrolyte in the absence and presence of organic additives containing -COOH or -SO,H groups (benzenesulfonic, maleic, glyoxylic, succinic acids etc). It was found that molecules which forms strongly chemisorbed species induce a significant adsorption. The phenomenon is explainedby an ion-exchangeprocess occurring with the -COOH and -SOaH groups anchored to the surface via the strongly chemisorbedspecies.
order to avoid interference with the phenomena caused by OH- adsorption[l].
INTRODUCTION
In previous communications[l-31 problems connected with the radiotracer study of cation adsorption induced by strongly chemisorbed anions on platinum electrodes were discussed. For instance, it was found that chemisorbed CN- ions preserve their charge in the chemisorbed state and cation-exchange processes take place at chemisorbed CN- coated Pt surfaces. Ordered ionic layers are formed when a Pt( 111) surface is immersed into aqueous KCN, KSCN, KzS, KI solutions[4-81. It has been shown that the layer of anions is not removed during extensive rinsing in aqueous solutions of simple chloride salts, but undergoes cation exchange. All these results attest that induced adsorption of cations by adsorbed anions is an almost general feature in the case of platinum electrodes. ‘%a labelled calcium ions were used for the radiotracer study of induced cation adsorption. The increase in radiation intensity coming from the surface of the electrode is a direct measure of the induced adsorption of Ca’ + ions occurring in the course of the chemisorption of an anion. A similar situation is expected in the case of strong chemisorption of organic species containing such functional groups as -COOH or -SOaH. It is known that irreversible chemisorption of unsaturated and aromatic compounds occurs at a platinum electrode[9]. Thus there is a possibility of studying the induced cation adsorption of -COOH and -SOaH groups by investigating the adsorption of Ca2 + ions labelled with 45Ca in the presence of preadsorbed organic species, for instance, maleic and benzenesulfonic acids. In accordance with the considerations outlined above, the aim of the present paper is to show that -COOH and -SOS H groups of organic species strongly chemisorbed on platinum surface induce the adsorption of labelled Ca2 + ions. It should, however, be taken into consideration that the experiments should be carried out in acid medium in
EXPERIMENTAL The adsorption of 45Ca labelled Ca2 + ions was followed in situ by means of the radiotracer method described previously[9]. The measurements were carried out in the presence of 1 x 10e2 mol dm-3 HClO, supporting electrolyte. Potentials quoted in this paper are given on rhe scale. Molar activity of the labelled species was 70 MBq mol-‘. The organic species studied were as follows: benzenesulfonic, 1,3-benzenedisulfonic, glyoxylic, maleic, acetylenedicarboxylic, tartaric, glycolic and succinic acids. RESULTS Figure 1 shows the potential dependence of the adsorption of CaZ+ ions in 1 x 10e2 moldmm3 HC104 supporting electrolyte. The I-us E curve reflects an equilibrium behaviour; no significant hysteresis can be observed changing the direction of the potential shifts. The increase of the supporting electrolyteconcentration results in a dramatic decrease in the sorption of Ca2+ ions. Thus, the adsorption of Ca2+ ions cannot be detected in 1 mol drnm3 HClO, solution at low Ca’ + ion concentrations (c < 10m3 mol dme3). It follows from Fig. 1 that no adsorption of Ca2+ ions from the supporting electrolyte can be observed above E= 300 mV, therefore, organic species were added to the solution phase at E =400 mV. At this potential neither the reduction nor the oxidation of the organic species studied take place thus the phenomena observed should be ascribed to their adsorption. Figure 2 shows some typical I- vs t curves observed at 400 mV following the addition of the organic species to the solution phase.
1161
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G. HORANYIand E. M. RIZMAYER
lb0
Fig. 1 Potential 1 X lo- Lmoldm--l
.2bo
460
3bo
E/mV
dependence of the adsorption of Ca2+ ions (~=6xlO~“moldm~~) in HClO., supporting electrolyte. The direction of the potential shifts is indicated by arrows.
5
-o-o-
OPP
.P IT
_o-o-o-
4
9
L
,
-; 400
Fig. 2. r us time curves observed following the addition ot organtc species to the solution phase. (C&Z+ = 6 x 10m4 mol dm-’ in 1 x 10m2 mol dm-” HCIO,) cOrg= 1 x 10-j mol dm-‘. (1) Succinic; (2) tartaric; (3) glyoxylicl(4) benzenesulfonic; (5) maleic acids. E = 400 mV. The results presented in Fig. 2 attest that adsorption of maleic, benzenesulfonic and glyoxylic acids induce the adsorption of Ca2+ ions. In contrast to this, no significant effect can be observed in the presence of tartaric and succinic acids. This observation is in accordance with the expectations. For instance, strong chemisorption occurs in the case of maleic acid while only loosely adsorbed species were observed for its saturated counterpart, succinic acid[lO]. The potential dependence of the adsorption of Ca2+ ions is significantly influenced by the potential dependence of the adsorption of strongly chemisorbed organic species. This is shown in Fig. 3 in the case of maleic acid. Adsorption of maleic acid increases in the potential range from 0 to 400 mV (as the reduction rate decreases) and decreases above 800 mV [9]. This tendency is reflected by curve 2. However, it may be seen from Fig. 3 that at potentials E ( 200 mV the adsorption of Ca ’ + ions in the presence of maleic acid does not attain the value observed in the absence of the organic species. This phenomenon can be ascribed, for instance, to the blocking effect of organic residues on the surface. In addition. the interaction of maleic acid
1
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Fig. 3. Potential dependence of Ca2+ adsorption (1) in the absence; (2), (3) in the presence of maleic acid (data a+ in Fig. 2). and Ca*+ ions in the solution phase may lead to a decrease of the concentration of free Ca2+ ions. It is known that generally no adsorption of organic species takes place at a preoxidized platinum surface. Curve 3 in Fig. 3 shows that above E =900 mV the adsorption of Ca’+ ions can be neglected in the case of preoxidized surface indicating that without the adsorption of maleic acid no adsorption of Ca’+ ions occurs. During the reduction of the oxide layer at 800 and 700 mV a dramatic change in the Ca2+ adsorption occurs (see count rate time curve in Fig. 4a) corresponding to the formation of free metal sites where the adsorption of maleic acid takes place. At potentials under E = 300 mV the reduction of maleic acid becomes more and more pronounced and chemisorbed molecules are eliminated from the surface. The count rate time curves obtained in this potential range are shown in Fig. 4b. The decrease of the adsorption between 300 and 100 mV can be explained by the reductive elimination of chemisorbed species. The steady state coverage with respect to these species decreases with decreasing potentials. However, in the absence of organic species, there is a sharp increase in the Ca2+ adsorption (curve 1 in Fig. 3). As a result of the two competitive processes a minimum can be observed on the r US E curve at about 100 mV.
Inducedadsorptionof cations
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Fig. 4. (a) Count-rate time curve starting from a prcoxidkd electrode(otherdataasin Fig. 2(b) Count rate time curve at low potentials (other data as in Fig. 2). The sorption rate of Ca’ + ions is governed by that of the organic species. For instance,in Fig. 5 the effect of maleic acid concentration on the adsorption rate of Ca2+ ions is shown. Being the concentration of Ca2+ ion fixed, the change in the adsorption rate of labelled species can be explained only by the change in the adsorption rate of organic molecules caused by the increase of their concentration. Similar observations were made in the case of other organic species. Figure 6 shows the r us E curves obtained for benzenesulfonic, 1.3~benzenedisulfonic and glyoxylic acids. The main characteristic features of ._I
I
200
400
600
800
E/mV
Fig. 6. Potentialdependenceof theadsorptionof Ca2+ions (6 x lo-* mol dm-‘) in the absence (1) and presence of different organic species (c = 1 x 10T3 mol dm-‘) (2) glyoxylic; (3) benzenesulfonic; (4) benzenedisulfonic acids.
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Fig. 5. The adsorption rate of Ca’ + ions at different maleic acid concentrations (1) 1 x 10e4; (2) 1 x 1Om3moldm-‘. E =4OOmV.
these curves do not differ significantly from each other but there are differences in the extent of the adsorption of Ca’+ ions. However, these differences can be considered as a normal consequence of the different molecular structure, different number and nature of functional groups etc. (We return to this problem later.) However, the curves presented in Fig. 6 differ from those presented in Fig. 3. A more pronounced difference may be observed between the cures presented in Figs 3 and 6. However,
G. HORANYI and E. M. RIZMAYER
1164
this difference can be, at least partly, explained by the difference in the adsorption behaviour of maleic acid and the other organic species studied and by the different dissociation behaviour of COOH and -SO,H groups. It was of interest to study the behaviour of adsorbed Ca2+ ions. It was easy to show by two simple experiments that the labelled Ca2+ species are very loosely adsorbed. The first experiment was the exchange of adsorbed labelled Ca’ + ions with non-labelled species added to the solution phase. A very rapid exchange can be observed as shown in Fig. 7a in the case of doubling of the total concentration. A complete displacement may be observed when the non-labelled species are added in great excess. The second observation was that the rapid increase of the H+ concentration by addition of HClO,+ results in an abrupt decrease in the Ca’+ adsorption. This is shown in Fig. 7b. (The difference between the count rates reported in Figs 7a and 7b should be ascribed to the different roughness factors of the electrodes used.) All these observations attest that no strong chemisorption occurs with Ca’ + ions. DISCUSSION In the cases where induced adsorption was observed strong chemisorption of the organic species took place. Thus the -SOaH and -COOH groups were anchored to the surface by the unsaturated, or aromatic part of the molecule under consideration. For instance, the following simplified surface structures can be given: SO&i
COOH
HOOC
I
c-c
I
Hooc\c//”
D
I I -_--__-_--
I I
--_--_--
---------
maleic acid chemisorbed via the opening of the double bond;
benzenesulfonic acid in flat orientation:
SO,H and in the presence -COOSO;
= -COO=-SO;
I
glyoxylic acid chemisorbed via the CHO group.
The strongly chemisorbed species can be eliminated by reduction and oxidation. In all cases there is a potential range where the coverage with respect to the chemisorbed molecules practically does not change. The -COOH and -S03H groups take part in the following equilibrium processes: -COOH
species results in some change in their dissociation behaviour thus we do not know the values of the dissociation constants of the -SOsH or -COOH groups attached to the surface of the electrode, but it can be assumed that there remains some parallelism with the original behaviour. On the basis of the considerations outlined above the induced adsorption of Cat + ions can be considered as an ion-exchange process. The extent of Ca’ + adsorption (at fixed supporting electrolyte concentration) should depend on the number of-COOH or -SOaH groups anchored to the surface and on the equilibrium constants of the processes involved in the ion-exchange. Evidently, owing to the exchange processes, the nature and concentration of the ions present in the solution phase exert significant influence on the adsorption of each other. In addition, it should be taken into consideration that a part of the charge of -COO and SO; groups may be compensated by the positive charge of the metal surface. This effect was observed, for instance, in the case ofchemisorbed CN- ions 131. With increasing positive potentials, ie with increasing positive charge of the platinum surface the amount of sorbed Caz+ ions should decrease even at constant surface concentration of the functional groups. The occurrence of sections between 400 and 800 mV on curves 3 and 4 in Fig. 6 can presumably be ascribed to this phenomenon. (Above 800 mV the role of oxygen adsorption comes into the foreground.) Considering the complex nature of the phenomena the experimental results presented here do not allow us
+ H’, + H+,
of metal ions: + Me’+COOMe, + Me+ = SO,Me,
where Me’ denotes a metal ion. Most of the original compounds studied are more or less strong acids (benzenesulfonic and maleic acids, for instance). Evidently, the chemisorption of the organic
to draw quantitative conclusions concerning the questions discussed. It is, however, quite clear that the induced adsorption of Ca’+ ions indicates the presence of anionic groups on the surface. A very rough estimation of the ratio of the number of COOand -COOH groups on the surface can be suggested in the case of chemisorbed maleic acid if we assume that the dissociation behaviour of the two -COOH groups involved remains unaltered after the chemisorption. As the first dissociation constant of maleic acid, K 1 is 1.5 x lo-’ the -COO-/-COOH ratio in 1 x lo-’ mol dmm3 HCIO., should be 1.5, ie about 30% of all -COOH groups anchored to the surface is in dissociated form (K2 = 5 x lo-’ thus the dissociation of the second -COOH does not play a role). The extent of Ca2’ sorption is presumably proportional to the surface concentration of -COOgroups.
Induced
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of cations
On the other hand, it follows from these considerations that the increase of hydrogen ion concentration should lead to a sharp decrease of the dissociation of -COOH groups, consequently the sorption of Ca’+ ions should decrease, as well. The phenomenon studied here offers an interesting and unique possibility for indirect radiotracer adsorption studies. In the case of complicated strongly chemisorbing species containing -SO,H or -COOH groups, the direct (very expensive) labelling of the compound can he avoided and chemisorption process can be followed by the adsorption of labelled Ca2+ ions induced by the functional groups. I
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Acknowledgement-The Hungarian Academy acknowledged.
financial support of Sciences (AKA grant)
from the is gratefully
(b)
REFERENCES
10 t/min
20
Fig. 7. (a) Effect of addition of non-labelled Ca2+ ions to the solution phase on the count rate proportional to the adsorption in the presence of maleic acid (c = 1 x lo-’ mol dm-‘). Initial concentration of labelled species: 6 x lo-& mol dm-“; final concentration 1.25 x 10m3 mol dmm3. E = 400 mV. (b) Effect of H’ concentration on the adsorption of Ca” ions in the presence of maleic acid (c = 1 x 10e3 mol drne3) concentration: at E=4OOmV. Initial HC10, 1 x 10-r moldm-3; final concentration 1 x IO-’ moldm m3.
1. G. Horanyi, J. electroanul. Chem. 36, 247 (1972). 2. G. Horanyi and E. M. Rizmayer, J. electroannl. Chem. 169. 279 (1984). 3. G. Horanyi and E. M. Rizmayer, J. electroanol. Chem. 215. 369 (1986). 4. J. H. White, M. P. Soriaga and A. T. Hubbard, J. phys. Chem. 89, 3227 (1984). 5. S. D. Rosasco, J. L. Stickney, G. N. Salaita, D. G. Frank, J. Y. Katekaru, B. C. Schardt, M. P. Soriaga, D. A. Stern and A. T. Hubbard, J. electrama/. Chem. 188,95 (1985). 6. J: L. Stickney, S. D. Rosasco, G. N. Salaita and A. T. Hubbard, Langmuir 1, 66 (1985). 7. D. G. Frank, J. Y. Katekaru, S. D. Rosasco, G. N. Salaita, B. C. Schardt, M. P. Soriaga, D. A. Stern, J. L. Stickney and A. T. Hubbard, Langmuir I, 587 (1985). 8. B. C. Schardt, J. L. Stickney, D. A. Stern, D. G. Frank, J. Y. Katekaru, S. D. Rosasco, G. N. Salaita, M. P. Soriaga and A. T. Hubbard, Inorg. Chem. 24.1419 (1985). 9. G. Horinyi, Electrochim. Acta 25, 43 (1980). 10. G. Horanyi, E. M. Rizmayer and G. Inzelt, Jsrael J. Chem. 18, 136 (1979).
CentralResearch Institute for Chemistry ofthe Hungarian Academy of Sciences, Budapest H-l 525, Hungary (Received 4 November
1987; in revised form 1 February
1988)
Abstract-The adsorption of labelled Ca*+ Ions was studied at platinized platinum electrodes in 1 x lo-* moldm-” HCIO, supporting electrolyte in the absence and presence of organic additives containing -COOH or -SO,H groups (benzenesulfonic, maleic, glyoxylic, succinic acids etc). It was found that molecules which forms strongly chemisorbed species induce a significant adsorption. The phenomenon is explainedby an ion-exchangeprocess occurring with the -COOH and -SOaH groups anchored to the surface via the strongly chemisorbedspecies.
order to avoid interference with the phenomena caused by OH- adsorption[l].
INTRODUCTION
In previous communications[l-31 problems connected with the radiotracer study of cation adsorption induced by strongly chemisorbed anions on platinum electrodes were discussed. For instance, it was found that chemisorbed CN- ions preserve their charge in the chemisorbed state and cation-exchange processes take place at chemisorbed CN- coated Pt surfaces. Ordered ionic layers are formed when a Pt( 111) surface is immersed into aqueous KCN, KSCN, KzS, KI solutions[4-81. It has been shown that the layer of anions is not removed during extensive rinsing in aqueous solutions of simple chloride salts, but undergoes cation exchange. All these results attest that induced adsorption of cations by adsorbed anions is an almost general feature in the case of platinum electrodes. ‘%a labelled calcium ions were used for the radiotracer study of induced cation adsorption. The increase in radiation intensity coming from the surface of the electrode is a direct measure of the induced adsorption of Ca’ + ions occurring in the course of the chemisorption of an anion. A similar situation is expected in the case of strong chemisorption of organic species containing such functional groups as -COOH or -SOaH. It is known that irreversible chemisorption of unsaturated and aromatic compounds occurs at a platinum electrode[9]. Thus there is a possibility of studying the induced cation adsorption of -COOH and -SOaH groups by investigating the adsorption of Ca2 + ions labelled with 45Ca in the presence of preadsorbed organic species, for instance, maleic and benzenesulfonic acids. In accordance with the considerations outlined above, the aim of the present paper is to show that -COOH and -SOS H groups of organic species strongly chemisorbed on platinum surface induce the adsorption of labelled Ca2 + ions. It should, however, be taken into consideration that the experiments should be carried out in acid medium in
EXPERIMENTAL The adsorption of 45Ca labelled Ca2 + ions was followed in situ by means of the radiotracer method described previously[9]. The measurements were carried out in the presence of 1 x 10e2 mol dm-3 HClO, supporting electrolyte. Potentials quoted in this paper are given on rhe scale. Molar activity of the labelled species was 70 MBq mol-‘. The organic species studied were as follows: benzenesulfonic, 1,3-benzenedisulfonic, glyoxylic, maleic, acetylenedicarboxylic, tartaric, glycolic and succinic acids. RESULTS Figure 1 shows the potential dependence of the adsorption of CaZ+ ions in 1 x 10e2 moldmm3 HC104 supporting electrolyte. The I-us E curve reflects an equilibrium behaviour; no significant hysteresis can be observed changing the direction of the potential shifts. The increase of the supporting electrolyteconcentration results in a dramatic decrease in the sorption of Ca2+ ions. Thus, the adsorption of Ca2+ ions cannot be detected in 1 mol drnm3 HClO, solution at low Ca’ + ion concentrations (c < 10m3 mol dme3). It follows from Fig. 1 that no adsorption of Ca2+ ions from the supporting electrolyte can be observed above E= 300 mV, therefore, organic species were added to the solution phase at E =400 mV. At this potential neither the reduction nor the oxidation of the organic species studied take place thus the phenomena observed should be ascribed to their adsorption. Figure 2 shows some typical I- vs t curves observed at 400 mV following the addition of the organic species to the solution phase.
1161
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G. HORANYIand E. M. RIZMAYER
lb0
Fig. 1 Potential 1 X lo- Lmoldm--l
.2bo
460
3bo
E/mV
dependence of the adsorption of Ca2+ ions (~=6xlO~“moldm~~) in HClO., supporting electrolyte. The direction of the potential shifts is indicated by arrows.
5
-o-o-
OPP
.P IT
_o-o-o-
4
9
L
,
-; 400
Fig. 2. r us time curves observed following the addition ot organtc species to the solution phase. (C&Z+ = 6 x 10m4 mol dm-’ in 1 x 10m2 mol dm-” HCIO,) cOrg= 1 x 10-j mol dm-‘. (1) Succinic; (2) tartaric; (3) glyoxylicl(4) benzenesulfonic; (5) maleic acids. E = 400 mV. The results presented in Fig. 2 attest that adsorption of maleic, benzenesulfonic and glyoxylic acids induce the adsorption of Ca2+ ions. In contrast to this, no significant effect can be observed in the presence of tartaric and succinic acids. This observation is in accordance with the expectations. For instance, strong chemisorption occurs in the case of maleic acid while only loosely adsorbed species were observed for its saturated counterpart, succinic acid[lO]. The potential dependence of the adsorption of Ca2+ ions is significantly influenced by the potential dependence of the adsorption of strongly chemisorbed organic species. This is shown in Fig. 3 in the case of maleic acid. Adsorption of maleic acid increases in the potential range from 0 to 400 mV (as the reduction rate decreases) and decreases above 800 mV [9]. This tendency is reflected by curve 2. However, it may be seen from Fig. 3 that at potentials E ( 200 mV the adsorption of Ca ’ + ions in the presence of maleic acid does not attain the value observed in the absence of the organic species. This phenomenon can be ascribed, for instance, to the blocking effect of organic residues on the surface. In addition. the interaction of maleic acid
1
800
I
1200
E/N
Fig. 3. Potential dependence of Ca2+ adsorption (1) in the absence; (2), (3) in the presence of maleic acid (data a+ in Fig. 2). and Ca*+ ions in the solution phase may lead to a decrease of the concentration of free Ca2+ ions. It is known that generally no adsorption of organic species takes place at a preoxidized platinum surface. Curve 3 in Fig. 3 shows that above E =900 mV the adsorption of Ca’+ ions can be neglected in the case of preoxidized surface indicating that without the adsorption of maleic acid no adsorption of Ca’+ ions occurs. During the reduction of the oxide layer at 800 and 700 mV a dramatic change in the Ca2+ adsorption occurs (see count rate time curve in Fig. 4a) corresponding to the formation of free metal sites where the adsorption of maleic acid takes place. At potentials under E = 300 mV the reduction of maleic acid becomes more and more pronounced and chemisorbed molecules are eliminated from the surface. The count rate time curves obtained in this potential range are shown in Fig. 4b. The decrease of the adsorption between 300 and 100 mV can be explained by the reductive elimination of chemisorbed species. The steady state coverage with respect to these species decreases with decreasing potentials. However, in the absence of organic species, there is a sharp increase in the Ca2+ adsorption (curve 1 in Fig. 3). As a result of the two competitive processes a minimum can be observed on the r US E curve at about 100 mV.
Inducedadsorptionof cations
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Fig. 4. (a) Count-rate time curve starting from a prcoxidkd electrode(otherdataasin Fig. 2(b) Count rate time curve at low potentials (other data as in Fig. 2). The sorption rate of Ca’ + ions is governed by that of the organic species. For instance,in Fig. 5 the effect of maleic acid concentration on the adsorption rate of Ca2+ ions is shown. Being the concentration of Ca2+ ion fixed, the change in the adsorption rate of labelled species can be explained only by the change in the adsorption rate of organic molecules caused by the increase of their concentration. Similar observations were made in the case of other organic species. Figure 6 shows the r us E curves obtained for benzenesulfonic, 1.3~benzenedisulfonic and glyoxylic acids. The main characteristic features of ._I
I
200
400
600
800
E/mV
Fig. 6. Potentialdependenceof theadsorptionof Ca2+ions (6 x lo-* mol dm-‘) in the absence (1) and presence of different organic species (c = 1 x 10T3 mol dm-‘) (2) glyoxylic; (3) benzenesulfonic; (4) benzenedisulfonic acids.
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Fig. 5. The adsorption rate of Ca’ + ions at different maleic acid concentrations (1) 1 x 10e4; (2) 1 x 1Om3moldm-‘. E =4OOmV.
these curves do not differ significantly from each other but there are differences in the extent of the adsorption of Ca’+ ions. However, these differences can be considered as a normal consequence of the different molecular structure, different number and nature of functional groups etc. (We return to this problem later.) However, the curves presented in Fig. 6 differ from those presented in Fig. 3. A more pronounced difference may be observed between the cures presented in Figs 3 and 6. However,
G. HORANYI and E. M. RIZMAYER
1164
this difference can be, at least partly, explained by the difference in the adsorption behaviour of maleic acid and the other organic species studied and by the different dissociation behaviour of COOH and -SO,H groups. It was of interest to study the behaviour of adsorbed Ca2+ ions. It was easy to show by two simple experiments that the labelled Ca2+ species are very loosely adsorbed. The first experiment was the exchange of adsorbed labelled Ca’ + ions with non-labelled species added to the solution phase. A very rapid exchange can be observed as shown in Fig. 7a in the case of doubling of the total concentration. A complete displacement may be observed when the non-labelled species are added in great excess. The second observation was that the rapid increase of the H+ concentration by addition of HClO,+ results in an abrupt decrease in the Ca’+ adsorption. This is shown in Fig. 7b. (The difference between the count rates reported in Figs 7a and 7b should be ascribed to the different roughness factors of the electrodes used.) All these observations attest that no strong chemisorption occurs with Ca’ + ions. DISCUSSION In the cases where induced adsorption was observed strong chemisorption of the organic species took place. Thus the -SOaH and -COOH groups were anchored to the surface by the unsaturated, or aromatic part of the molecule under consideration. For instance, the following simplified surface structures can be given: SO&i
COOH
HOOC
I
c-c
I
Hooc\c//”
D
I I -_--__-_--
I I
--_--_--
---------
maleic acid chemisorbed via the opening of the double bond;
benzenesulfonic acid in flat orientation:
SO,H and in the presence -COOSO;
= -COO=-SO;
I
glyoxylic acid chemisorbed via the CHO group.
The strongly chemisorbed species can be eliminated by reduction and oxidation. In all cases there is a potential range where the coverage with respect to the chemisorbed molecules practically does not change. The -COOH and -S03H groups take part in the following equilibrium processes: -COOH
species results in some change in their dissociation behaviour thus we do not know the values of the dissociation constants of the -SOsH or -COOH groups attached to the surface of the electrode, but it can be assumed that there remains some parallelism with the original behaviour. On the basis of the considerations outlined above the induced adsorption of Cat + ions can be considered as an ion-exchange process. The extent of Ca’ + adsorption (at fixed supporting electrolyte concentration) should depend on the number of-COOH or -SOaH groups anchored to the surface and on the equilibrium constants of the processes involved in the ion-exchange. Evidently, owing to the exchange processes, the nature and concentration of the ions present in the solution phase exert significant influence on the adsorption of each other. In addition, it should be taken into consideration that a part of the charge of -COO and SO; groups may be compensated by the positive charge of the metal surface. This effect was observed, for instance, in the case ofchemisorbed CN- ions 131. With increasing positive potentials, ie with increasing positive charge of the platinum surface the amount of sorbed Caz+ ions should decrease even at constant surface concentration of the functional groups. The occurrence of sections between 400 and 800 mV on curves 3 and 4 in Fig. 6 can presumably be ascribed to this phenomenon. (Above 800 mV the role of oxygen adsorption comes into the foreground.) Considering the complex nature of the phenomena the experimental results presented here do not allow us
+ H’, + H+,
of metal ions: + Me’+COOMe, + Me+ = SO,Me,
where Me’ denotes a metal ion. Most of the original compounds studied are more or less strong acids (benzenesulfonic and maleic acids, for instance). Evidently, the chemisorption of the organic
to draw quantitative conclusions concerning the questions discussed. It is, however, quite clear that the induced adsorption of Ca’+ ions indicates the presence of anionic groups on the surface. A very rough estimation of the ratio of the number of COOand -COOH groups on the surface can be suggested in the case of chemisorbed maleic acid if we assume that the dissociation behaviour of the two -COOH groups involved remains unaltered after the chemisorption. As the first dissociation constant of maleic acid, K 1 is 1.5 x lo-’ the -COO-/-COOH ratio in 1 x lo-’ mol dmm3 HCIO., should be 1.5, ie about 30% of all -COOH groups anchored to the surface is in dissociated form (K2 = 5 x lo-’ thus the dissociation of the second -COOH does not play a role). The extent of Ca2’ sorption is presumably proportional to the surface concentration of -COOgroups.
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adsorption
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of cations
On the other hand, it follows from these considerations that the increase of hydrogen ion concentration should lead to a sharp decrease of the dissociation of -COOH groups, consequently the sorption of Ca’+ ions should decrease, as well. The phenomenon studied here offers an interesting and unique possibility for indirect radiotracer adsorption studies. In the case of complicated strongly chemisorbing species containing -SO,H or -COOH groups, the direct (very expensive) labelling of the compound can he avoided and chemisorption process can be followed by the adsorption of labelled Ca2+ ions induced by the functional groups. I
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4
8
12
Vmin
Acknowledgement-The Hungarian Academy acknowledged.
financial support of Sciences (AKA grant)
from the is gratefully
(b)
REFERENCES
10 t/min
20
Fig. 7. (a) Effect of addition of non-labelled Ca2+ ions to the solution phase on the count rate proportional to the adsorption in the presence of maleic acid (c = 1 x lo-’ mol dm-‘). Initial concentration of labelled species: 6 x lo-& mol dm-“; final concentration 1.25 x 10m3 mol dmm3. E = 400 mV. (b) Effect of H’ concentration on the adsorption of Ca” ions in the presence of maleic acid (c = 1 x 10e3 mol drne3) concentration: at E=4OOmV. Initial HC10, 1 x 10-r moldm-3; final concentration 1 x IO-’ moldm m3.
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