FARADAIC IMPEDANCE AT PLATINUM MICROELECTRODES
3363
Faradaic Impedence at Platinum Microelectrodes. Behavior of the Iodide- Iodine Oxidation-Reduction Couple
by A. B. Thomas*and R. J. Brodd U n i o n Carbide Corporation, Consumer Products Division, Research Laboratory, P a r m a SO, Ohio (Received M a y 86,1964)
Using carefully purified reagents and controlled surface preparation, it was possible to obtain accurate and reproducible impedances a t platinum electrodes for the iodine-iodide oxidation-reduction couple. Proper electrode construction was found to be very important. It was possible to prepare and activate single crystal platinum electrodes of known surface area and reproducible faradaic impedance. The diffusion coefficients of iodine and triiodide deduced from the faradaic impedance were shown to be in agreement with measurements using rotating disk electrodes. It was shown that the electrode process follows the path proposed by Vetter with a of about 0.6. In low concentration iodide solution, a process parallel to the main reaction scheme was found that was tentatively ascribed to an adsorption of iodide ions on the electrode surface.
Introduction Several studies of the electrode kinetic behavior of the iodine-iodide oxidation-reduction couple at platinum electrodes have been made. Those using faradaic Impedance methods are the work of Vetter2&and Llopis, et aLZb A study using the rotating disk electrode has been reported by Newson and Riddiford,a who supported the rate-determining step proposed by Vettes.2a The differences between the various investigations may be ascribed as the difficulty of establishing a characterizable surface state on the solid electrode surface. A method for the preparation of a reproducible, characterizable platinum surface is described which is comparable with a mercury surface for electrokinetic investigations. Experimental Details Impedance measurements were made with the aseries resistance-capacitance bridge with the experimental cell composing one arm of the bridge. The bridge could be balanced at all frequencies to 1% or better when standard resistors and capacitors were substituted for the experimental cell. Impedances of the test electrodes were measured at 2000, 800, 500, 400, 350, 300, 250, 200, and 150 C.P.S. and were reproducible to &5%.
Distilled water was redistilled from alkaline potassium permanganate solutions. The middle fraction was redistilled and stored in seasoned Pyrex flasks. Reagent grade potassium sulfate was heated in a casserole over a Meeker burner and recrystallized twice from the triple-distilled water. The product was vacuum dried over phosphorus pentoxide. Reagent grade iodine was freshly sublimed from a mixture of potassium iodide and barium oxide. Reagent grade potassium iodide was recrystallized twice from triple-distilled water and vacuum dried over phosphorus pentoxide. Double-distilled 70% perchloric acid was mixed with 10 to 20% concentrated nitric acid and distilled at atmospheric pressure. The middle fraction was retained and chlorine was removed by pumping with a water aspirator and/or bubbling dry purified nitrogen through for several hours. The final product gave no test for chloride ion. (1) Deceased.
(2) (a) K. J. Vetter, Z . physik. Chem. (Leipeig), 199, 285 (1952); (b) J. Llopis and F. Colom, Anales Fis. Quim., 52A, 233 (1956); J. Llopis, J . Fernandes-Biarge, and M . Perez-Fernandes, ibid., 55A, 93 (1959) ; J. Llopis, J . Fernandes-Biarge, and M .Perez-Fernandes, Electrochim. Acta, 1 , 130 (1959). (3) J. D . Newson and A. C . Riddiford, J . Electrochem. Soc., 108, 699 (1961) ; D . P . Gregory and A. C. Riddiford, J. Chem. Soc., 3756 (1956).
Volume 68, Number 11
November, 196.4
Tank nitrogen was purified by bubbling through Oxorbent solution, coriceritrated sulfuric acid, and either drying towers of phosphorus pentoxide and magnesium perchlorate or triple-distilled wat,er prior to use. Solutions were prepared as rirrded by weighirig rcagents. Nostock solutioris were uscd. Pure platilium wire and shcct (Anirricari Platinum Works) were used t,o prepare rlcctrodes. Vour typrs of electrodes, spherical, wire loop, plain sheet, and flat ground, wcrc used i n t,his study. Counter rlrctrodes were 1-cui. diamrtrr disks of shrrt platinuiii plsced 1 em. beneath t,he microrlcctrodes. The proper sealing of plat,inuiii t,o glass was critical i n obtaining reproducible electrode iiiipedaiiees. Pyrex aiid cobalt glass seals were iiot, drpriidahle hut were occasionally satisfactory for wirc of lrss than 0.2-nirii. dianirtrr. Uniformly depeiidablr svals were ohtained wil h Irad glass and soft glass when the wire diameter was less than 0.5 mni. Wire loop arid sheet, electrodes wrre prrpared by sealing appropriate platiiiuin pieces int,o soft glass tubing using lead glass. Flat, polished electrodes prrpared by grinding sealed wires carefully to a filial polish with 0.1-p alumina were not rrliahlr. Spherical electrodes were produced by sealiiig appropriate lengths of plat,iiiuiii wirr irito soft, glass tubing wit,h lead glass and carefully fusirig thr plat inrun in a gas-oxygen flame. Nat,ural cooling in air yirldrd mirror-bright spheres, varyiiig lrss t,haii 5% i n diameter. Electrodes prepared i n this way were 0.01.i to 0.1 cm. i n diatnet,er or 0.0010 to 0.04 cm.* in area. X-Ray diffract,ion patteriis revealed that such rlrctrodes were single crystals wheu the diameter w m less than 0.05 cm., aiid were composed of orily two t,o fivr grains if larger in diameter. A conveiiicrit procedure for revealing grain bouridaries and othrr crystal characteristics was t,o etch with a 50-C.P.S.alternatiiig current at a current, density of 20 to 50 ma./crii.z in a solution of 0.5 M NaCI-0.5 W HCI for 15 to GO min.‘ An etched sphere 0.1 CUI. in diameter is shown in Fig. 1. Notice the grain boundary and diamond figure revealing one cryst.al face. Two different methods were used to prepare the electrodes for the measurements. In the first method, the test electrodes were allowed to starid in the t,est solution overnight @.e., 16 hr. or longer) wit.h low amplitude (10 hv.) 15 to 20 C.P.S. field applied between the electrodes. This correspouds to Vetter’s pretreatment of his electrodes. The other activatiori method consisted of prrparing an oxide filni on the electrode by anodic polarization at 1.0 v. positive to a saturatrd calomel elrctrode and thrn reducirig the oxide film The J m , m o l
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