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Tetrathionate adsorption onto mercury
J. Electroanal. Chem., 99 (1979) 255--258
255
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands Short communication TETRATHIONATE ADSORPTION ONTO MERCURY
TETSUYA OHSAKAand ROBERT DE LEVIE Department o f Chemistry, Georgetown University, Washington, D.C. 20057 (U.S.A.)
(Received 24th April 1978; in revised form 10th October 1978)
Since the work of Krjukova [1] and Frumkin and Florianovich [2,3], anions have been used as kinetic probes for the study of double layer structure [4--6] and, also, for the testing of models of electrode kinetics which involve the rather elusive "activated complex" [7--10]. Grahame [11] showed that virtually all anions are specifically adsorbed from aqueous solutions onto mercury at positive charge densities. Therefore one must always be alert to the possibility that such adsorption might complicate the interpretation of experimental data. Below we will show an example of such a case. METHODS AND MATERIALS The d.c. and a.c. polarograms were obtained on an instrument described earlier [6,12], with the following modification. The earlier I R compensation circuit was replaced by one based on the circuit of Lamy and Herrmann [13], which has only one amplifier in the positive feedback loop. The circuit as published [ 13 ] has one inconvenience, viz. that the current sensitivity depends on the amount of I R compensation used. This was removed by the introduction of an instrumentation amplifier, which also provides postamplification, see Fig. 1. The interfacial tension data were obtained on a computer-controlled maximum bubble pressure instrument built in this laboratory by M. Krishnan. The design closely follows that published by Lawrence and Mohilner [14]. The chemicals used were J.T. Baker "Analyzed Reagent" grade, except for R
Fig. 1. The modified current-measuring circuit used. The operational amplifier (no. 1, Analog Devices 40J) is followed by an instrumentation amplifier (no. 2, Teledyne Philbrick 4253). With a fraction ~ of its output voltage fed back, the output voltage V1 of amplifier no. 1 is given by Vl = iR/(1 --c~), where i is the cell current. The output voltage V2 of the instrumentation amplifier with gain G then becomes V2 = iRG.
256
KBrO4 which was a generous gift from Dr. E.H. Appelman of Argonne National Laboratories, and Na2S406 which was obtained from Fluka and was recrystallyzed once [15]. RESULTS AND DISCUSSION
The d.c. polarogram of Na2S406 in aqueous sodium halide solutions has the typical shape of the reduction of an anion [16]. The occurrence of polarographic maxima [7] can be avoided, as usual, by restricting the measurements to sufficiently low concentrations of the electroactive species. The quadrature a.c. polarogram (Fig. 2) exhibits two interesting features: a sharp peak at about +0.2 V vs. SCE, and a pronounced lowering of the capacitance around 0 V vs. SCE. The peak near +0.2 V corresponds with an oxidation wave in the d.c. polarogram (Fig. 3), which reaches a constant height at a tetrathionate concentration of about 10 -4 M, and is most likely due to the oxidation of mercury and the formation of an insoluble film of mercury tetrathionate. However, Fig. 3 clearly established the absence of any appreciable faradaic current around 0 V, so that the low quadrature a.c. component must reflect a change in double layer capacitance, such as might result from strong specific adsorption of $40~ +. This conclusion is supported by measurements of the interfacial tension, see Fig. 4. The above observations are not restricted to dilute solutions of NaF, since 0.1 M NaC1 and 0.1 M MgSO4 solutions exhibit similar capacitance changes at potentials where no d.c. current flows. We might note here that the most likely impurities in tetrathionate, I- from its usual synthesis and S2- from its decomposition, would cause an increase in double layer capacitance rather than the observed decrease. The presence of a "double layer region" around 0 V facilitates detection of specific adsorption using capacitance measurements, see Fig. 2, at potentials where the absence of a faradaic process precludes the existence of an "activated
40
C / p F cm -2 20
0
I
*0.5
i
i
I
0
I
i
i
E / V v s SCE
,
I
i
f
J
-0.5
Fig. 2. Quadrature a.e. polarograms of 10 mM NaF containing x p M Na28406 as indicated with the curves. Measurement frequency 159 Hz.
257
150
-0.5
i/~uA
E / V v s SCE
0.5
-0.7
//o// / J
20'150
Fig. 3. D.c. polarograms of 10 mM NaF containing x pM Na2S406 as indicated with the curves.
O O g
4-2
O
alNm -I
40
s8
O
b
'
E;
' YvsSCE
'
-
'
'
Fig. 4. Interfacial tension of 10 mM NaF (open circles) and of 10 mM NaF + 15 pM Na:S406 (closed circles) at 25.0 ° C.
complex". With perbromate, no such double layer region exists, but a control measurement of the interfacial tension fails to show any measurable lowering for up to 150 pM KBrO,. ACKNOWLEDGMENTS
It is a pleasure to thank Mahadevaiyer Krishnan for making the interfacial tension measurements. This work was supported by AFOSR grant 76-3027 and NIH grant GM 22296-03.
258 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
T.A. K~ukova, Dokl. Akad, Nauk SSSR, 65 (1949) 517. A.N. Frt~mkin and G. Florlanovich, D0kl. Akad. Nauk SSSR, 80 (1951) 907. A.N. Frumkin, Progz. Polarogr., 1 (1962) 223. L. Gierst, L. Vandenberghen, E. Nicolas and A. Fraboni, J. Electrochem. Soc., 113 (1966) 1025. L. Gierst, E. Nicolas and L. Tytgat-Vandenberghen, Croat. Chem. Acta, 42 (1970) 117. R. de Levie and M. Nemes, J. Electroanal. Chem., 58 (1975) 123. M.L. Foresti and R. GuideUi, J. Electroanal. Chem., 53 (1974) 219. M.L. Foresti, D. Cozzi and R. Guideni, J. Electroanal. Chem., 53 (1974) 235. R. Guidelli and M.L. Foresti, J. Electroanal. Chem., 67 (1976) 231. I. Ze~ula, V. Rychnavsk~ and E. Alexyov~, J. Electroanal. Chem., 86 (1977) 433. D.C. Grahame, Chem. Rev., 41 (1947) 441. R. de Levie and A.A. Husovsky, J. Electroanal. Chem., 20 (1969) 181. C. Lamy and C.C. Herrmann, J. Electroanal. Chem., 59 (1975) 113. J. Lawrence and D.M. Mohiiner, J. Electrochem. Soc., 118 (1971) 1596. F. Martin and L. Metz, Z. Anorg. Chem., 127 (1923) 83. I. 7.eYula, Collect. Czech. Chem. Commun. 33 (1968) 2327.
255
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands Short communication TETRATHIONATE ADSORPTION ONTO MERCURY
TETSUYA OHSAKAand ROBERT DE LEVIE Department o f Chemistry, Georgetown University, Washington, D.C. 20057 (U.S.A.)
(Received 24th April 1978; in revised form 10th October 1978)
Since the work of Krjukova [1] and Frumkin and Florianovich [2,3], anions have been used as kinetic probes for the study of double layer structure [4--6] and, also, for the testing of models of electrode kinetics which involve the rather elusive "activated complex" [7--10]. Grahame [11] showed that virtually all anions are specifically adsorbed from aqueous solutions onto mercury at positive charge densities. Therefore one must always be alert to the possibility that such adsorption might complicate the interpretation of experimental data. Below we will show an example of such a case. METHODS AND MATERIALS The d.c. and a.c. polarograms were obtained on an instrument described earlier [6,12], with the following modification. The earlier I R compensation circuit was replaced by one based on the circuit of Lamy and Herrmann [13], which has only one amplifier in the positive feedback loop. The circuit as published [ 13 ] has one inconvenience, viz. that the current sensitivity depends on the amount of I R compensation used. This was removed by the introduction of an instrumentation amplifier, which also provides postamplification, see Fig. 1. The interfacial tension data were obtained on a computer-controlled maximum bubble pressure instrument built in this laboratory by M. Krishnan. The design closely follows that published by Lawrence and Mohilner [14]. The chemicals used were J.T. Baker "Analyzed Reagent" grade, except for R
Fig. 1. The modified current-measuring circuit used. The operational amplifier (no. 1, Analog Devices 40J) is followed by an instrumentation amplifier (no. 2, Teledyne Philbrick 4253). With a fraction ~ of its output voltage fed back, the output voltage V1 of amplifier no. 1 is given by Vl = iR/(1 --c~), where i is the cell current. The output voltage V2 of the instrumentation amplifier with gain G then becomes V2 = iRG.
256
KBrO4 which was a generous gift from Dr. E.H. Appelman of Argonne National Laboratories, and Na2S406 which was obtained from Fluka and was recrystallyzed once [15]. RESULTS AND DISCUSSION
The d.c. polarogram of Na2S406 in aqueous sodium halide solutions has the typical shape of the reduction of an anion [16]. The occurrence of polarographic maxima [7] can be avoided, as usual, by restricting the measurements to sufficiently low concentrations of the electroactive species. The quadrature a.c. polarogram (Fig. 2) exhibits two interesting features: a sharp peak at about +0.2 V vs. SCE, and a pronounced lowering of the capacitance around 0 V vs. SCE. The peak near +0.2 V corresponds with an oxidation wave in the d.c. polarogram (Fig. 3), which reaches a constant height at a tetrathionate concentration of about 10 -4 M, and is most likely due to the oxidation of mercury and the formation of an insoluble film of mercury tetrathionate. However, Fig. 3 clearly established the absence of any appreciable faradaic current around 0 V, so that the low quadrature a.c. component must reflect a change in double layer capacitance, such as might result from strong specific adsorption of $40~ +. This conclusion is supported by measurements of the interfacial tension, see Fig. 4. The above observations are not restricted to dilute solutions of NaF, since 0.1 M NaC1 and 0.1 M MgSO4 solutions exhibit similar capacitance changes at potentials where no d.c. current flows. We might note here that the most likely impurities in tetrathionate, I- from its usual synthesis and S2- from its decomposition, would cause an increase in double layer capacitance rather than the observed decrease. The presence of a "double layer region" around 0 V facilitates detection of specific adsorption using capacitance measurements, see Fig. 2, at potentials where the absence of a faradaic process precludes the existence of an "activated
40
C / p F cm -2 20
0
I
*0.5
i
i
I
0
I
i
i
E / V v s SCE
,
I
i
f
J
-0.5
Fig. 2. Quadrature a.e. polarograms of 10 mM NaF containing x p M Na28406 as indicated with the curves. Measurement frequency 159 Hz.
257
150
-0.5
i/~uA
E / V v s SCE
0.5
-0.7
//o// / J
20'150
Fig. 3. D.c. polarograms of 10 mM NaF containing x pM Na2S406 as indicated with the curves.
O O g
4-2
O
alNm -I
40
s8
O
b
'
E;
' YvsSCE
'
-
'
'
Fig. 4. Interfacial tension of 10 mM NaF (open circles) and of 10 mM NaF + 15 pM Na:S406 (closed circles) at 25.0 ° C.
complex". With perbromate, no such double layer region exists, but a control measurement of the interfacial tension fails to show any measurable lowering for up to 150 pM KBrO,. ACKNOWLEDGMENTS
It is a pleasure to thank Mahadevaiyer Krishnan for making the interfacial tension measurements. This work was supported by AFOSR grant 76-3027 and NIH grant GM 22296-03.
258 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
T.A. K~ukova, Dokl. Akad, Nauk SSSR, 65 (1949) 517. A.N. Frt~mkin and G. Florlanovich, D0kl. Akad. Nauk SSSR, 80 (1951) 907. A.N. Frumkin, Progz. Polarogr., 1 (1962) 223. L. Gierst, L. Vandenberghen, E. Nicolas and A. Fraboni, J. Electrochem. Soc., 113 (1966) 1025. L. Gierst, E. Nicolas and L. Tytgat-Vandenberghen, Croat. Chem. Acta, 42 (1970) 117. R. de Levie and M. Nemes, J. Electroanal. Chem., 58 (1975) 123. M.L. Foresti and R. GuideUi, J. Electroanal. Chem., 53 (1974) 219. M.L. Foresti, D. Cozzi and R. Guideni, J. Electroanal. Chem., 53 (1974) 235. R. Guidelli and M.L. Foresti, J. Electroanal. Chem., 67 (1976) 231. I. Ze~ula, V. Rychnavsk~ and E. Alexyov~, J. Electroanal. Chem., 86 (1977) 433. D.C. Grahame, Chem. Rev., 41 (1947) 441. R. de Levie and A.A. Husovsky, J. Electroanal. Chem., 20 (1969) 181. C. Lamy and C.C. Herrmann, J. Electroanal. Chem., 59 (1975) 113. J. Lawrence and D.M. Mohiiner, J. Electrochem. Soc., 118 (1971) 1596. F. Martin and L. Metz, Z. Anorg. Chem., 127 (1923) 83. I. 7.eYula, Collect. Czech. Chem. Commun. 33 (1968) 2327.