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Immersion Method for the Potential of Zero Charge Determination
Immersion Method for the Potential of Zero Charge Determination. An Electrode Pretreatment Sang Hyung Kim’ Department of Chemistry, University of Pennsylvania, Phiiadeiphia. Pennsyivania 79704 Manuscript Received Juiy 9. 7973)
(Received November 6, 7972; Revsed
We describe experimental details of an attempt to improve obtaining a zero charge state of the electrode in the immersion method for the potential of zero charge (E,) determination. The attempt utilizes a preimmersion treatment in a dry space separated from the solution by a thin Teflon tape, through which the rapid immersion takes place. The treatment in the dry space consists of heat treatment with hydrogen and argon, and washing with water as well as drying with argon. The present method with dry electrodes yields more negative E, values for gold and nickel than the earlier immersion methods with wet electrodes.
Introduction The immersion method has been developed by Jakuszewski and Kozlowski2-6 to determine the potentials of zero charge (EZ)7of solid metal electrodes. This method has been studied further by Jendrasic8 to determine the surface charge of the electrodes. Matsuda, Damjanovic, and Bockrisg have recently applied a rapid immersion technique to the ionic adsorption measurements a t sohd metal-solution interfaces. The immersion technique involves rapid immersion of a clean and dry electrode into an electrolyte solution and measurement of the charging current required to form the electric double layer a t the preset electrode potential. E , of the electrode corresponds to the potential a t which no charging current flows. The interference of faradaic reactions in the charging current measurement can be minimized by immersing the electrode as rapidly as possible. At the preimmersion state the electrode should have zero charge condition. Therefore, the adsorbed gas from the preimmersion environment should not interact with the electrode or alter natural electrode-solution double layer structure? In the previous studies2-G,S.Ythe electrode was dried in an inert gas atmosphere above the test solution before immersion. The electrode dried in this way could be covered by moisture films, which might cause some local cell reactions on the surface and thus result in a departure from zero charge at the preimmersion state especially in the case of active metals. Such uncertainties of the preimmersion environment of the electrodes leave doubt in the previous immersion method^.^,^" I t is probably due to some cathodic faradaic reactions of adsorbed films that the E, values by earlier immersion methods238 are more positive than those by other methods for goldii-19 and nickelX5J0 electrodes. In this paper we report an attempt to improve obtaining zero charge state of the electrode before immersion. The electrode is prepared in a dry space separated from the test solution by a thin Teflon tape, through, which the rapid immersion is performed without any prior contact with water vapor or air. The attempts were applied for gold and nickel electrodes. Experimental Section An one-compartment cell and an electrode preparation column above the cell shvwn in Figure 1 were separated
by a thin Teflon tape. M (Almac plastics of Penna., 0.005 in. thickzl), between “0”ring glass joints ( 1 8 j 9 ) with a Viton “0”ring sitting on the upper side of the tape. The preelectrolyzed solution, S, was filled up to the tape. About 50-cm long test electrode, T, made of ground glass precision bore tubing (0.25 in. 0.d.) was prepared above the tape in inert atmosphere and then immersed through the tape. M. The rapid immersion of the test electrode was obtained with a speed of 100 200 cm/sec by the application of magnetic field from a solenoid (Model 149-1, Dormeyer Industries, 111.) on a metal bar holding the upper end of the test electrode.9 The potential of the test electrode, controlled by a potentiostat (Tacussel PIT 202X) with a platinum counter electrode, C, was measured by a Keithley electrometer with respect to a palladiumhydrogen electrode, R. The charging current a t a given preset electrode potential was recorded en a Tektronix 564 storage oscilloscope with 3B3 time base and 3A3 amplifier. A standard resistor (1 kB) was connected in the platinum counter electrode circuit. The surface charge of the electrode was obtained by graphical integration of the current-time curve by means of a planimeter (Keuffel and Esser). The nickel and gold wire electrodes (1 mm diameter, 2 cm long) were prepared in the following ways. Nickel wire22 was spot welded to a thick tungsten wire connected to a long copper wire. The tungsten wire was sealed to the Pyrex glass first, then the nickel-tungsten joint was slightly covered by a Pyrex glass ( a few mm), and finally a long ground glass precision bore tubing was connected. The nickel electrode was dipped in dilute HC1 to remove nickel oxides, and washed with conductivity water. Then the electrode was placed in the furnace, F, and heated in argon for 5 to 60 min a t 300” to remove moisture, then in hydrogen for 5 t o 15 rnin at 300” to reduce the oxides if any, then again, in argon for 20 min at 450” to diffuse the absorbed hydrogen. and finally in cold argon in the cooling compartment, U, with water-cooled condenser for 10 min.z3 Gold wirezz electrode was made by a “house-keeper” The gold electrode. initially cleaned with hot 1 M HCI, hot 2 M HzS04, and d i d l e d water, was placed in F, and was heated in argon for 20 min a t 300”, in hydrogen for 15 min a t 300”, in argon for 20 min a t 450”, and then in cold argon in L for 10 min.23 Once the electrode
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The Journalof Physicai Chemistry, Vol. 77. No. 23, 1973
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Sang Hyung Kim
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Figure 2. The measured charge ( 4 ) of the electrode vs. elecM HC104 (pH 3) and trode potential ( E vs. nHe): gold in nickel in M HCIO4 (pH 1 2 ) . The charge, Q (pC/cmz) is based on geometric area. r\r or s in
Figure 1 . Cell and electrode preparation column. was purified in the above ways, the 6-mm Teflon stopcock, G (widened to 8-mm bore), was opened and the electrode was immersed through the tape, M, into the solution. Precision bore tubing, P (0.25 in. i.d.), provided a dynamic seal. The end of the electrodes was pointed by grinding on siiicone carbide paper so that it could punch through the tape easily. After each measurement a new tape was placed by pulling the punched tape. While replacing it, argon was blown around the tape and cell in addition to the positive pressure applied inside the cell. The electrode was washed with predeaerated conductivity water in the washing compartment, W, a few times and dried in a cold argon stream in U. Then the above heating procedure was repeated so that the electrodes were prepared with identical treatments. The stopcock, G, was closed, and the water in the washing compartment, W, was removed and evacuated by using a duo-seal vacuum pump (0.1 p ) through a Dry Ice-acetone trap. The furnace, F, was made of flexible heating tape wrapped around Pyrex tubing and insulated by an asbestos sheet and insulating cloth. The temperature of the furnace was regulated by a powerstat and measured by a clomel-alumel thermocouple inserted in a housing inside furnace, H. All connections were made of Fisher solv-seal joints and Teflon stopcocks. Prepurified argon gas (5 ppm 0 2 ) was further purified by passing over two copper turning tubings, two molecular sieve columns (Linde 13X), and further molecular sieve columns cooled down to -75" by a Dry Ice-acetone mixture. Prepurified hydrogen was also further purified by The Journal of Physical Chemistry, Vol. 77, No. 23, 1973
passing it through a Surfass hydrogen purifier (Milton Roy Co., Model CH-A). Solutions were made of reagent grade chemicals and with conductivity water. All glassware was cleaned with 1:1 concentrated "03-concentrated H2S04 mixture and rinsed with conductivity water. The measurements were performed in an open laboratory (28 f 3").
Results and Discussion The charging current measurements were performed for the gold electrode in 10-3 A4 HCi04 (pH 3) and the nickel electrode in lo-* M HC104 (pH 12). The measured excess charges ( 4 ) of the electrodes were averaged from two (for Au) and two to nine (for Ni) measurements a t each potential. The ranges of error in g are up to h0.3 and f2.5 pC/cm2 for Au and Ni, respectively. The worse reproducibility for Ni may be due to its high reactivity with water vapor a t high temperature and a fast faradaic reaction such as dissolution of Ni.25 Figure 2 shows the measured g of gold and nickel electrodes as a function of electrode potential. The measured potentials a t q = 0 of gold (0.34 V) and nickel (0.03 V) may correspond to the respective EZ's in the absence of specific adsorption (all potentials are us. nHe). The Ez's are estimated to be hO.05 V. The reported values of E , for gold from the least adsorbing solutions vary from 0 to 0.4 V.71-19 The present result for gold is in good agreement with c a p a c i t a n ~ e ~ ~ J ~ and organic adsorption12J5 methods. By an immersion method, however, Jendrasics reported more positive E, than our value and the failure of the E , measurement of gold in 0.1 M HC104 due to the nonequilibrium between the solution and the electrode surface.
Immersion Method for the Potential of Zero Charge Determination
2789
The measured E , of nickel is about 0.3 V more positive than other data from c a p a c i t a n ~ e ,organic ~ ~ ~ ~ ~adsorption,l5 and frictionlj methods from, neutral and alkaline solutions, By an immersion method, Jakuszewski and Kozlowskiz however obtained +0.193 V for 0.01 M KC1 solution. Due to :he specific adsorption of the C1their value may be more positive than our result by more than 200 mV. The present method with dry electrodes gives more negative E,'s for both gold and nickel than the earlier immersion methods238 with wet electrodes. This is in good agreement with the expectation that the dry immersion method minimizes some cathodic faradaic reactions probably due to adsorbed films on the wet electrode. However, the E, values by the present method are also more positive than those by other in situ methods11.13-17.20 determining E, especially for the case of nickel. I t seems unlikely that the adsorbed argon on the electrode in the present method may alter the natural electrode-solution double layer structure. The cause of the above discrepancy between our method and other in s i t u methods may be due to differences in preparation methods of the electrode, electrode surface conditions due to different duration of contact of the electrode with solution (the latter methods employ a prolonged contact), etc. At present, the exact cause of the positive shift of our E , values from the other in situ methods11,13-17Jo IS not well understood. Nevertheless, since we are more likely to remove spurious faradaic reactions, it is desirable to work with dr3 electrodes rather than wet electrodes in :he immersion method; however, there still may be a remaining cathodic faradaic process of unknown origin. The present pretreatment of the electrode at each potential takes more than 1.5 hr. This shortcoming weakens the immersion method whose advantage is the rapidity of the measurements. Other faster pretreatment than ours. if any, should be employed. The present method with dry electrodes provides the E,'s of gold and nickel to be closer to the other methods11-20 and more negative than the earlier immersion methodsZ.8 with wet electrodes,
suggestions, and discussions during the beginning period of this work. The author also wishes to thank the members of the Electrochemistry Laboratory of University of Pennsylvania for useful discussions. This work was supported by the Office of Naval Research.
Acknowledgment. The author wishes to express his appreciation to Dr. J. O'M. Bockris for his suggestions, discussions, and support of this work. The author is indebted to Drs. Y. Matsuda and A. Damjanovic for their help,
References and Notes (1) Present address, Biomedical Engineering Center, Technological Institute, Northwestern University, Evanston, 111. 60201, (2) B. Jakuszewski and Z. Koziowski, Rocz. Chem., 36, 1873 (1962). (3) B. Jakuszewski and Z. Kozlowski, Rocz. Chem.. 38,93 (19641. (4) B. Jakuszewski and Z. Kozlowski, SOC.S o . Lodz Acta Chim.. 9,25
(1964). (5) B. Jakuszewski and Z. Kozlowski, SOC. Sci. Lo& Acta Chim.. 19, 5 (1965). (6) Z. Kozlowski and B. Jakuszewski, SOC.Sci. Lodz. Acta Chim., 1 1 , 5 (1 966). (7) R. S. Perkins and T. N. Andersen in "Modern Aspects of Eiectrochemistry," Vol. 5 , J . O'M. Bockris and 8 . E. Conway, Ed., Plenum Press, New York, N. Y., 1969. This review includes a complete reference on the potentiais of zero charge.
(8) V. Jendrasic, J. Electroanal. Chem., 22, 157 (1969). (9) Y. Matsuda. A. Damjanovic, and J. O'M. Bockris, unpubiisned work.
(IO) R. S. Perkins, R. C. Livingston, T. N. Andersen. and H . Eyring, J . Phys. Chem., 69,3329 (1965). ( 1 1 ) G . M. Schmid and N. Hackerman, d. Eiectrochem. Soc., 109, 243 (1962);110. 440 (1963). (12) M. Green and H . Dahms, J. Electrochem. SOC., 110, 466, 1075 (1 963). (13) D. D. Bode, T. N. Andersen, and H . Eyring, J . Phys. Chem., 71, 792 (1 967). (14) M, Petit and J. Clavilier, C. R. Acad. Scr., Ser. C., 265, 145 (1967). (15) J. O'M. Bockris, S. D. Argade, ana E. Gileadi. Electrochim. Acta. 14,1259 (1969). (16) I . Morcos, J. Colloid lnterface Sci., 37,410 (1971). (17) M. Shimokawa and T. Takamura, J. Elecfroanai. Chem.. 32, 314 (1971i. (18) A. Hamellin and J. Lecoeur, Collect. Czech. Chem. Commun., 36, 714 (1971). (19) J. P. Carr and N. A. Hampson, d . Electrochem. Soc.. 119, 325 (1972). (20) L. V . Voikov, A. F. Ponomarev, and B. P. Yurev, Tr. Lenmgrad Poiitekh. lnst.. 304,94 (1970):Chem. Abstr.. 73,83099v (1970). (21) The per cent absorption of water for 24 hr is nil for the Teflon fiim with thickness up to 0.125 in. ("Modern Plastics Encyclopedia," 1972-1973,p 343.)The tapes thinner than 0.005 in. could not hold the vacuum we used.
(22) 99.99% pure gold and 99.95% pufe nickel wires were obtained from A. D . McKay, inc.. New York, N . Y.
(23) S. D . Argade, Ph.D. Dissertation, University of Pennsylvania, 1968. Heating time in argon at 450" in this work is shorter than Argade's. However the same calculation as Argade's showed a negiigible amount of hyarogen (-5 X l o - * g atom/cm3) present in nickel after heat treatment. Rough calculation gave about 0.1 p C / c m 2 error in actual measurements which is within the error range of the present data. (24) B. Cahan, Ph.D. Disserration, University of Pennsylvania, 1968. (25) T. N. Andersen, J. L. Anderson, D. D. Bode, Jr., and H. Eyring, d . Res. lnst. Catal. Hokkaido Univ.. 16,449 (1968). (26) E. I. Mikhailova and Z. A. iofa, Elektrokhimiya, 6,231 (1970).
The Journal of Physcal Chemistry, Voi. 77, No. 23, 1973
