Rotated and Stationary Platinum Wire Electrodes Residual Current-Voltage Curves and Dissolution Patterns in Supporting Electrolytes I. M. KOLTHOFF and NOBUYUKI TANAKA’ School of Chemistry, University o f Minnesota, Minneapolis 14, M i n n .
Polarograms observed at solid wire electrodes in the presence of nonoxidizing or reducing supporting electrolytes often exhibit small cathodic or anodic currents which may interfere with the evaluation of cathodic and anodic diffusion currents. In supporting electrolytes free of added oxidizing or reducing agents a small anodic wave is recorded over the entire pH range when current-voltage curves are measured with automatic registration from negative to positive potentials at stationary and rotated platinum wire electrodes. When the curves are recorded, starting at the evolution potential of oxygen to negative potentials, a small cathodic dissolution pattern is observed starting at a potential which approximately corresponds to the equilibrium value of the platinum-platinous hydroxide electrode. The limiting current of the anodic wave decreases rapidly to zero at a given potential indicating formation of a film at the surface of the electrode. A similar film is formed by placing the electrode in solutions of strongly oxidizing agents; it can be removed by treating the electrode with various reducing agents. Anomalies in automatically recorded residual currents at the rotated platinum wire electrode may impair its analytical applicability as a voltammetric indicator electrode. Simple procedures are given for eliminating these anomalies and for correcting them in current-voltage curves of electroreducible and electrooxidizable substances.
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EVERAL authors ( I S , 23) have observed that polarograms obtained a t solid wire electrodes in supporting electrolytes, which are free of Oxidizing or reducing constituents, often exhibit small cathodic and/or anodic currents in certain regions between the cathodic hydrogen and the anodic oxygen waves. Also, anodic and cathodic current-voltages curves a t these electrodes often exhibit irregularities and are different in shape when run from negative to positive potentials or in a reverse way (19). Similar observations have been made by most workers in this laboratory who automatically record current-voltage (c-v) curves a t the rotated platinum wire electrode. On the other hand, in measuring current-voltage curves with the manual apparatus irregular and unexpected waves were rarely observed. Generally a small anodic prewave to the oxygen wave is noticed in supporting electrolytes free of oxidizable or reducible constituents. The starting potential of this prewave varies approximately 60 mv. per unit pH change (see Table I). The appearance of the prewave undoubtedly is closely related to statements in the literature that anodic polarization of a platinum electrode gives rise to the formation of a film of oxygen ( I , 6-7, 9, 20-22, 24, 26) or platinous oxide (hydroxide) ( 4 , 8, 11, 12) on the electrode. Investigations described in the present study tend to indicate that the film formed on the preoxygen wave is due to platinous oxide. LTpon cathodic polarization of an electrode covered with such a film a cathodic dissolution pattern is observed. The characteristics of the film are described and a simple procedure is given for the removal of the film. The film formation accounts for the differences in apparent diffusion currents observed with 1 On
leave of absence from University of Tokyo, Japan.
automatic and manual recording and also for abnormnlitieq 111 current-voltage curves referred to above. Upon cathodic polarization with evolution of hydrogen, an anodic dissolution pattern is observed which is due to soibed hydrogen on the electrode. Although not further discussed here, this pattern has a larger area when a palladium electrode is used, palladium being a good solvent for hydrogen. AB shown, qorbed hydrogen can be easily removed by anodic polarization at the proper potential. When the anodically formed film of platirious oxide and sorbed hydrogen are removed from the electrode, only normal residual currents are observed a t the platinum electrode in a given range of potentials. EXPERIMENTAL
Apparatus. A recording polarograph, Sargent Model XII, was used for the measurement of current-voltage and currenttime curves. When desired, a manual instrument ( 1 6 )was wed for current-voltage curves and potential measurements. All PSperiments were carried out in a thermostat of 25.0’ rt 0.1” C. With the recording polarograph the voltage was varied a t a rate of 4.4 mv. per second, unless otherwise stated. Potentials are referred to the saturated calomel electrode (SCE).
Table I. Potentials us. SCE of Cathodic Dissolution Patterns in Various Supporting Electrolytes Supporting Electrolyte
pH
0.1MHClOi Acetate buffer Borax buffer 0.1.11 NaOH
1 5 9 13
Starting Potential +0.67
Potential of P t P t ( 0 H ) Z (13) +0.67 +0.43
+ O . 17 -0.03
+0.19
+0.39
-0.04
A platinum wire electrode was sealed into a glass tube filled with mercury. The electrode was 0.7 mm. in diameter and 4.5 mm. in length, its geometric surface area being approximately 0.1 sq. cm. The electrode was rotated with a synchronous motor a t a rate of 600 r.p.m. It gave a diffusion current of 415 er millimole per liter for the oxidation of thallous ion to oxide in deaerated 0.1M sodium hydroxide solution (IS). When being kept stationary, the electrode gave a steady state diffusion curi-ent of 14.8 pa. per millimole per liter in the same solution. A platinum foil electrode was also used of approximately 2.1 sq. cm. geometric area. Reagents. 811 reagents used were C.P. grade or better. Pretreatment of Electrode. Generally, the electrode was stored in 10M nitric acid. Before use, i t was thoroughly washed with distilled water and short-circuited against the saturated calomel electrode in deaerated 0.1M perchloric acid, until the current was nearly zero. This electrode is called a “clean electrode.” The electrode was also prepolarized cathodically or anodically a t given potentials and current-voltage curves measured after polarizatioh. Nitrogen from a Linde nitrogen tank (99.99%) was used aithout further purification for oxygen. It was found that washing the nitrogen through a column containing a sulfuric acid solution of vanadous sulfate and zinc amalgam (18) caused contamination of the gas with hydrogen, which in turn was sorbed by the platinum electrode as was evident from the appearance of an anodic dissolution pattern (see Figure 3).
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RESULTS
Behavior of Clean Electrode in Various Supporting Electrolytes. Current-voltage curves were obtained with the clean electrode in Supporting electrolytes of p H between 1 and 13 in the absence of 0x2-gen by measuring from negative to positive
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V O L U M E 2 6 , NO. 4, A P R I L 1 9 5 4
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air-saturated solutions: the dissolution patterns had the same starting potentials as those obtained in the absence of air, the dissolution pattern later overlapping with the reduction wave of oxygen. No anodic prewave was observed in 0.1M perchloric acid from +0.6 volt to more positive potentials (backward) after preB polarization of the electrode a t +1.2 volts. Behavior of Cathodically Prep o l a r i z e d Electrode. The behavior of the cathodically prep o l a r i z e d e l e c t r o d e was also investigated. After the electrode was prepolarized in oxygen-free 0.1M perchloric acid a t -2.0 volts, where hydrogen evolved vigorously a t the surface of the ilo’Ma 1 I I I I electrode, polarograms were re, *IO T O 8 0 -0 8 - 1 0 corded in the same solution from VOLTS VS. S. C. E. negative to positive potentials. Figure 1. Current-Voltage Curves Measured from Negative to Positive Potentials -4 large anodic current was obwith “Clean” Electrode i n Oxygen-Free Solutions served following the hydrogen A . In 0.1M perchloric acid C. I n sodium borate (pH 9) deposition wave (Figure 3). B. In acetate buffer (pH 5 ) D . In 0.1M sodium hydroxide solution After a cathodic prepolarization a t -2.0 volts, the electrode potentials (“backward”). Some polarograms are reproduced was washed with air-free distilled water. The polarograms recorded backward starting from -0.3 volt showed a small anodic in Figure 1. Prior to the oxygen evolution wave, a small anodic prewave was dissolution pattern. This was eliminated after placing the electrode for 10 minutes in air-saturated 0.1M perchloric acid, observed, the starting potential of which depended on the pH of dilute acid ferric sulfate, ceric sulfate, or bromine water (Figure the solution. Polarograms obtained with the clean electrode by 3). The dissolution patterns were attributed to the anodic measuring in the direction from positive to negative potentials (“forward”) did not exhibit an abnormality, provided the elecdissolution of sorbed hydrogen. h brief prepolarization a t a trode was not polarized a t a more positive potential than corresponds to the beginning of the appearance of the prewave. The same behavior v a s observed when the clenn electrode was kept for 30 hours in air- or oxygen-saturated solutions, indicating that no film was formed. Behavior of Anodically Prepolarized Electrode in Various Supporting Electrolytes. I n Figure 2 are shown polarograms measured from positive to negative potentials (forward) in various oxygen-free supporting elect r o l y t e s . T h e polarograms were started a t a potential where oxygen w a s e v o l v e d . All currenbvoltage curves exhibited cathodic dissolution i patterns. Their starting potentials t l / I were estimated as the point of intersection of the residual current line and the extrapolated line of the beginning part of the cathodic dissolution pattern (see curve A , Figure 2). Table I gives these approximate , (1.0 *OS 0 -0.5 .1.0 values. For comparison the potenVOLTS VS. S. C. E. tial of the reversible platinum-platiFigure 2. Current-Voltage Curves Measured from Positive to Negative nousoxide system as given by Latimer Potentials in Oxygen-Free Solutions ( 1 4 ) is tabulated. A . From 1.5 volts in 0.1M perchloric acid C. From 1.1 volts in sodium borate (pH 9) Polarograms were also obtained in €3. From 1.35 volt. i n acetate buffer (pH 5) D . From 0.7 volt in 0.1M sodium hydroxide solution
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small cathodic current was not sufficient to yield a hydrogen dissolution pattern (Figure 1). When nitrogen, washed through vanadous sulfate in dilute sulfuric acid over zinc amalgam, was passed over the electrode (18) in the supporting electrolyte a hydrogen dissolution pattern was found as illustrated in Figure 4. Even a short anodic polarization a t +1.5 volts did not remove all the sorbed hydrogen, while the latter did not prevent the formation of the platinous oxide film (curve B, Figure 4). On the other hand, the anodically formed film prevented the anodic dissolution of hydrogen as the cathodic dissolution pattern started a t the same potential as in the absence of sorbed hydrogen. Only after dissolution of the oxide film was the anodic dissolution of sorbed hydrogen observed (rurve B, Figure 4).
'11' 1
+
ia
ro a
*I 0
0
V O L T S VS. S. C. E.
I '
iI
I
Figure 4. Current-Voltage Curves Measured in Oxygen-Free 0.1M Perchloric Acid A. B.
Negative to positive potentials Positive to negative potentials Nitrogen washed through vanadous solution over zinc amalgam was passed through
Experiments carried out with the platinum foil electrode with an area about 20 times greater than that of the wire electrode yielded comparable results. For example, after ' 10 minutes polarization a t 1.5 volts the dissolution pattern corresponded to 9.2 X 10-4 coulomb per square centimeter as compared to 14 with the wire electrode. Considering the uncertainty in the exact values of the surface areas of both electrodes, the agreement is t I S + 10 to 5 0 V O L T S vs. s. c. E. considered satisfactory. Formation of Film with Oxidizing Agents. The clean elecFigure 3. Current-Voltage Curves Measured from trode was placed into solutions of several oxidizing agents, Negative to Positive Potentials in Oxygen-Free 0.lM Perchloric Acid washed thoroughly with distilled water, and the cathodic dissolution pattern obtained in oxygen-free 0.1M perchloric acid by A . Electrode prepolarized at -2.0 volts for 10 minutes B. Same electrode after washing with air-free distilled water, measuring from +0.7 volt to negative potentials. The dissoluatarting at -0.3volt
