Langmuir 1993,9, 1962-1964
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Current Spike Polarography. A New Technique for Films and Surfaces R. Heyrovskd The J. Heyrovskf Institute of Physical Chemistry and Electrochemistry, Dolejikova 3,182 23 Prague 8,Czech Republic Received October 13,1992.I n Final Form: May 28,1993 In this method, mercury drops grow in airlgas from a capillary tube and contact, just before they fall, the solution film or surface enclosed by a small silver wire ring electrode. The resulting spectra of current spikes have ‘unusual” current-voltage (i-W characteristics: They reveal, in aqueous solutions, the fast contact charge transfer with 02,the streaming maximum of the second kind but not the fmt kind, the absence of “iR drop” and of diffusion controlled processes. The short-circuit current spike polarogram for the mercury drop/mercury pool surface has also been recorded! Also, measured easily are the x-potentiale. Introduction The present research describes a new method of investigation of the fast electrical processes at interfaces formed by a metal with solution films and surfaces. The high reproducibility of results with a dropping mercury electrode (DME)and the change of the surface area (A) of the mercury drop with time (t) enable the exact measurement of charge densities ( q ) at these interfaces as currents (q dA/dt). The main “unusual” results of fundamental nature are presented here to demonstrate the new potentialities of the DME for the study of interfacial phenomena at films and surfaces. Other results like the concentration dependences of the current spikes and X-potentials, the surface/bulk transitions, technical improvements for quantitative analytical applications, etc., are reserved for a different paper.
Experimental Section
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Eleotrodesetup. A small silver wire ring electrodeof diameter
0.46 cm (used earlier for wet-and-measurepolarography*$2 was fmed to a DMES as shown in Figure 1 (inset). The plene of the film or surface (Figure 1, insets f and s, respectively) enclosed by the ring was maintained at a constant distance (ensures reproducibilityof theresults)of about the diameter of the mercury drop. Since the mercury drops grow first in air/gas prior to contacting momentarily (xO.1 8 ) the filmlsurface, the direct current-voltage (i-V) recordings are spectraof “currentspikes”. For the current spike polarogram (CSP),the drop time, surface area A, and dAldt of the drop, at the time of formation of the interface, pertain to the values in airlgas. Capillaries of fast as well as slow mercury flow rates could be used. The results reported here pertain to a flow rate in air of 21 mg/s and drop times of 3 s in air and 0.86 s in water. Films. The films (of volume 0.01 mL) were formed in the ring (due to surface tension) by dipping the latter in water or the solutionunder study. Theywere strong enoughto let the mercury drops fall through without breaking, and a fast scan rate (50 mV/s) minimized the depletion of the f i i . In this way the same f i b could be used again for testing the reproducibility of the
results. Surfaces. For the study of the surface of a liquid, the level of the latter was maintained at the plane of the ring by means of a small inlet/outlettube (seeFigure 1,inset),which also served as a gas inlet. In this case, unlike for f i b , the CSP could be recorded at slower voltage scans as well. Use of other reference electrodes dipping in the solution, instead of the silver ring (1) HeyrowkA, R. J. Electrochem. SOC.1992,139, L50. (2) HeyrowkA, R.; Heyrove&, M. Book of Abstacts; 4th European Conference on Eledroaaalysis, May/June 1992; European Society of Electrdnalytical Chemistry: The Netherlana p 124. (3) Novotng, L. Roceedings of the J. Heyrovskg Memorial Congress, Prague; 1980, Vol. 11, p 129. Czech. Patent PV-9 612, 1979; PV-9 611, 1979, Prague.
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Figure 1. Inset: The DME/Ag ring cell for f, film, 8, surface, and b, bulk. Comparisonof ‘current spike”end bulk polarograma of bidistilled water (in air): a, surface; b, film,c, bulk. vb = -0.06 v, vd -0.16v, and v,- v b = x = -0.10(ko.01)v. (Note that the current oscillations on curve c are minimum for i = 0.) After 15 min of deoxygenation with Nz: a’, b’, surface (or film); c‘, bulk. Scan rate waa 50 mV/s.
electrode, did not change the i-V characteristics of the CSP. Also, no ‘iR drop” effects‘ were observed for these transitory
interfaces (this was checked by a three-electrode setup using a mercury pool as the third electrode). (Note: One of the reviewere thinks a current interrupter method would be more suited for this purpose.)
Results and Discussion The i-V Characteristics. The results obtained for the interfaces formed by the DME with pure water and (4) Heyrow&, J.; Kuta, J. Principles of Polarography: Publishing House of the Czechoslovak Academy of Sciences: Prague, 1966.
0743-7463/93/2409-1962$04.00/00 1993 American Chemical Society
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Langmuir, Vol. 9, No. 8, 1993 1963
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Figure 2. Comparison of “current spike”and bulk polarograms of 0.1 M aqueous solution of KC1 (in air): a, b&, b, f i i c, Surface. v b -0.12v,V,J = -0.33 v,and x -0.21(Ao.01)v. Scan rate was 50 mV/s.
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00 -1 0 V Figure 4. Comparison of “current spike”and bulk polarograms of 10-9 M T1+in 0.1 M H2S04(aqueoussolution in air): a, bulk, b, surface. v b = -0.15 V, V, = -0.45 V, and x = -0.30 (f0.02) V. (The film polarogram is similar to the one in Figure 3b, with Vf = -0.33 V). Scan rate was 50 mV/s.
aqueous solutions (in air) of KC1, HzS04, and T1+ in HzSO4 are shown in Figures 1-4, respectively (other details are given in the figure captions). It can be seen from Figures 1-3 that the surface and film CSPs are alike but that they differ from the corresponding polarograms for the bulk. Deoxygenation of the solutions diminishes, in every case, the current spikes to a minimum as shown, e.g., in Figure la’,b’ for water. (For comparison with the latter, the corresponding bulk polarogram is shown in Figure lc’.) This indicates that the spike current is mainly due to the fast contact charge transfer interaction with dissolved oxygen. The likely process is the 02,Or electron transfer equilibrium6 in the adsorbed state, as on semi-
conductor surfaces.6 The redox potential of this couple is -0.16 V (NHE) in water (see ref 14 in ref 5). Where the spike current i is positive, the adsorbed 02 (as a lone pair donor) is in excess, and where i is negative, the reduced is in excess. form, 02-, Streaming Maximum of the First Kind.‘ This can be seen on the polarogram for the reduction of oxygen in KC1 in Figure 2a for the bulk. It is attributed4 to the excess transport of the depolarizer (dissolved 0 2 ) by streaming from the bulk to the “neck” of the drop. This maximum does not appear in the CSP in Figure 2b for the film since the bulk is absent for any such streaming of 02 to occur. It does not occur in the CSP in Figure 2c for the surface either since the direction of streaming“ of 0 2 at the “tip” of the mercury drop is away from the interface and toward the bulk. Absence of Diffusion-Controlled Currents. The reductions governed by diffusion to/from the bulk of K+ at -2.15 V in Figure 2a and of H+at -1.85 V in Figures 3a and 4a do not show up in the corresponding CSPs in Figures 2b,c, 3b,c, and 4b. This is probablybecause during the short life of the interface formed at the tip of the drop, the diffusion current at a growing mercury drop is negligible. Surface Charge Transfer Currents. It follows therefore that the spike current indicates the initial fast charge transfer proceses occurring during the charging of the drop.4 Reckoned as surface charge transfer current, the spike current = q dAldt, and it is directly proportional to the surface charge density q at the applied voltage V, since dA/dt is the same for all the spikes. StreamingMaximum of the Se~ondKind.~ Turning now to the case of T1+ in H2S04, the maxima on the bulk (Figure 4a) and surface (Figure 4b) polarograms appearing in the region of the reduction of T1+ions conform with the streaming maximum of the second kind.4 In these cases, the depolarizer (here T1+ ions) streams from the bulk
(6) Heyrovek9,M.; VaviiEka, s. J. Electroanal. Chem. 1992,332,309, and refs 10-14 cited therein.
Oxford, 1972; Chapters 1 and 2.
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(6) Spamaay, M.J. The Electrical Double Layer, Pergamon Press:
1964 Langmuir, Vol. 9, No. 8, 1993 ihA
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Figure 5. (a) 'Short circuit current spike" polarogram of the DME/mercurypool electrodeinterface: V,= 0 V. (b-d)'Current spike" polarograms of the surface of a thin layer (about 0.5 cm) of water over the mercury pool electrode: b, saturated with 02 (V, = -0.13 V);c, in air (21% 02)(V,= -0.25 V);d, saturated with NE(V, = -0.55 V). Scan rate was 20 mV/s. toward the interface at the "tip" of the mercury drop (in a direction opposite to that in the case of the 0 2 maximum). Hence, the maximum appearing in Figure 4a for the bulk also shows up to the same extent in the CSP in Figure 4b for the surface. At applied voltages increasinglymore negative than that corresponding to the maximum of the second kind, the excess transport by streaming of the depolarizer is supposed to gradually de~rease.~ This is confirmed by the gradual diminution of the current spikes in Figure 4b for the surface to the same level as that for the supporting electrolyte (cf. HzS04 in Figure 312). On the other hand,
in the bulk polarogram (Figure 4a), the current diminishes to the level of diffusion currents due to the reductions of T1+ and H+. (As pointed out in the cases of K+ and H+, these diffusion currents do not appear on the CSP.) On deoxygenation, the surface current spikes of Figure 4b decrease to a minimum (as in Figure la'b' for water). This implies that the presence of 02 influences the streaming maximum of T1+ in the CSP in Figure 4b. The film CSP for T1+ is similar to Figure 3b for the supporting electrolyte (HzSO4): no maximum occurs since the bulk is absent (as in the case of the maximum of the first kind). The x-Potential? The spike currents of the film and surface polarograms vanish at applied voltages Vf and V,, respectively, and the bulk polarograms cross the zero current line at vb. These are marked in the figures. For pure water (Figures la,b) and aqueous solutions of KC1 (Figures2b,c), Vf = V, (fO.O1 VI,whereas for 0.1 M HzS04 (Figures 3b,c) V, is more negative by 0.15 V than Vf (probably due to a higher concentration of 02 in the film than in the surface). The deoxygenation of water and the aqueous solutions was found to shift V,, Vf, and v b by about -0.5 V (e.g., see Figure la',b', c' for water). In general, both V, and Vf are more negative than v b , the applied voltage at which the bulk polarograms cross the zero current line. The difference V, - v b = x,is a measure of the difference in the electrical properties of the surface and bulk.6 "Short-Circuit"CSP. This was obtained (see Figure 5a) for the DME/mercury pool interface (V, = 0) by carefullyraising the level of a mercury pool electrode until it was close enough to the DME to produce a little current spike (in nA) at some small applied voltage V. In these experiments, the proximity of the mercury drop to the mercury pool reduces the drop time but does not influence
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On addition of a small amount of water over the mercury pool, after lowering the latter, the (water) surface CSPs shown in Figure 5b-d were obtained in the presence of 02, air, and Nz, respectively. The replacement of NZby 02 (cf. Figure 5b,d) shifts V, by about 0.5 V, as pointed out earlier. This fact is being worked out quantitatively for monitoring dissolved 02 in films and surfaces. All other results and detailed discussion will be published in a longer paper paper.
Acknowledgment. The author is thankful to the reviewersfor their comments which led to the improvement and revision of the manuscript.
