Application of the Cathode-Ray Polarography to Rotating Platinum. Electrodes IRVING SHAIN' and A. L. CRITTENDEN Department o f Chemistry, University of Washington, Seattle, W a s h .
.in instrument was devised to permit the observation of current-voltage curves at rotated platinum microelectrodes in periods of time comparable to the drop times of dropping mercury electrodes. By simple modification of a commercial cathode ray oscillograph and use of a mechanically driven sweep generator, an instrument is provided which has good stability and versatility with regard to zero drift, sensitivity, and rate and range of variation of polarizing potential. The apparatus is designed primarily for application to platinum electrodes, but can easily be modified to provide synchronization of sw-eep with drop time for studies with the dropping mercury electrode.
A
€'PLICATIONS of the rotating platinum electrode to polarography have been extensive, but in many cases only poorly defined limiting current regions have been reported. A difficulty occasionally encountered is slow drifting of current Fvith time in the region of limiting current. Steady-state conditions are reached very rapidly with rotating platinum microelectrodes ( 7 ) . Rogers et al. (IC) found that a steady state is reached rapidly enough to permit automatic recording of current,-voltage curvcs with a conventional recording polarograph. The t'endency for currents to drift with time is rapid enough to influence results obtained when times of several minutes are used in recording. I n order to minimize the effects of drift, the possihilitics of more rapid recording were investigated. The rate of recording used was limited by the fact that a t high rates of change of applied potential the capacitative part of the total current bec o n m large compared to t h e electrolysis part of the current. I n this work current-voltxge rurves were measured using a linear variation of applied voltage with time i n the range of from 0.1 to 1volt per second. EXPERIMENTAL
Cathode-Ray Polarograph. For thc recording of currentvoltage curves a cathode-ray tube presentation was used. A number of instruments have been designed for this purposp (1, 3, .9, 10, 12, 15). The arrangement used was similar to that used by Snowden and Page (15) in some respects. The reference electrode of the electrolysis cell was grounded through a precision resistance decade. Currents flowing in the cell were measured by applying the voltage drop across the decade to the vertical amplifier of the oscillograph. The voltage across the cell alone was applied t o the horizontal amplifier.
A DuMont Model 304 H oscillograph was used for data presentation. This instrument is well suited to the purpose, as the amplifiers are direct coupled and the instrument has good stability and linearity. As both terminals of the electrolysis cell were a t a variable potential to grdund, it was necessary to modify the oscillograph to provide a means of plotting horizontally the difference. between the potentials of the two terminals of the cell. The oscillograph has push-pull amplifiers. The grid of tube V - l l b (manufacturer's notation) was disconnected from ground, brought external to the instrument for use as a second horizontal input, and connected to the junction of the resistor decade and the reference electrode. With the two terminals of the cell connected to the regular X axis input and the added input (called X ' ) , the oscillograph plotted the voltage difference between the cell terminals on the horizontal axis. This was strictly true only when an attenuator was provided externally for the X' input similar to the X attenuator incorporated in the oscillograph. Potentials applied to the electrolysis cell were varied linearly with time. Since the voltage developed across the series re-
'
Present address, Department of Chemistry. University of m'isconsin, XIadison, Wis.
sistor varied in a complex manner, a linear voltage sweep device could not be connected directly between ground and the rotating electrode. Various compensating circuits have been used to correct the voltage supplied to the electrolysis cell for the voltage drop in the series resistor ( 1 , 12, 15). The circuit used in this work is shown in Figure 1; R15is the series-connected decade resistor. The grid of a cathode-follower circuit (VI, four halves of two 5692 duotriodes in parallel) was connected to the reference electrode. The cathode of this circuit acted as a lowimpedance source of current, the potential of which followed closely the potential a t the reference electrode. If voltage changes a t the reference electrode were small (less than 0.05 volt ), the cathode-follower output changed in a like manner within a few millivolts. A linear sweep voltage source and an additional variable bias voltage source were connected between the cathode follower and the rotating electrode. The three voltage sources in series provide a voltage source to ground which varies in such a manner that the voltage applied to the cell alone varies in the desired way. Because sweep times required were considerably longer than those of previous investigators, a mechanically driven potentiometer was used in place of electronic sweep generators. A Helipot potentiometer having 360" of mechanical rotation and linear to 0.5% w a ~used (I&*). A small battery and a crude potentiometer supplied a variable otential across the Helipot. The Helipot was turned by a smalf synchronous motor through a gear train of variable ratio, so that the sweep time could be varied in a precisely known manner from 0.15 to 28 seconds. Although the motor operated continuously, the potentiometer was turned only when a fast-acting clutch was actuated by a relay. The clutch was so arranged that the potentiometer was automatically disengaged and stopped after completion of exactly one revolution. A bias circuit was included to determine the potential a t the start of a potential sweep. A ten-turn Helipot and battery was used for this purpose (&). A similar circuit (RZil) was included for use in calibrating the entire instrument. The input impedances of the oscillograph used were somewhat low (2.2 megohms). T o prevent loading the cell by the oscillograph, small cathode followers were included in each lead from the electrolysis cell (V3, Vs). Occasionally sensitivity of the oscillograph was increased by a factor of 10 by using preamplifiers in the leads to the oscillograph (V2, V 4 ) . The apparatus waa powered from a power supply similar to that described by Koontz and Dilatush ( 5 ) , providing 280 volts a t 150 ma. with a ripple content of about 1 mv. peak-to-peak. Voltages across all potentiometers were measured with either a voltmeter or a student potentiometer. Vertical calibration was accomplished through Sla by placing a small 60-cycle alternating current signal on the X input and calibrating voltage into the Y input. A horizontal line was obtained which was vertically dis laced by the magnitude of the calibrating voltage. For horizontafcalibration, the 60-cycle signal was placed on the Y and X ' inputs and the same signal plus the direct current calibrating voltage placed on the X input. The resulting vertical line was displaced horizontally by the magnitude of the calibrating voltage (difference between X and X' signals). This line was vertical only when the X and X' gains were balanced by use of R12 and R14. Calibrating voltages were fed through all amplifiers and cathode followers, ensuring calibration of the entire assembly. Additional verification of calibration was obtained by plotting current-voltage curves using fixed resistors in place of the elec281
ANALYTICAL CHEMISTRY
282 trolysis cell. Provision was made for observation of currenttime curves by use of &. Cell and Rotating Electrode. Cells were H cells of the type used by Lingane and Laitinen (8). The rotating electrodes were made by sealing lengths of 0.81-mm. diameter platinum wire into either glass or Lucite tubes so that the wire extended perpendicularly to the axis of rotation. Contact to an inner steel shaft was made by a pool of mercury. One electrode was sealed into a Lucite tube 10.0 mm. in diameter and extended 3.7 mm. from the tube. Two others used were sealed into 8.0-mm. glass tubes and extended 0.5 and 2.5 mm. The electrodes were rotated by a synchronous motor and gear train at 600 r.p.m. t Reagents. Reagent grade chemicals were used. Oxygen was removed from solutions by bubbling nitrogen gas through the solutions. The nitrogen was scrubbed by passing it through an acid vanadous sulfate solution (11). Sodium sulfite was used for the removal of oxygen in the work with silver ion in ammonia solution. DISCUSSIOY
Capacity Currents. It is well k n o m that currents floir-ing a t microelectrodes include both electrolysis currents and capacity
currents due to the charging of the electrical double layer. Extensive studies of the capacity of electrical double layers have been made by Grahame (2) and, recently, by Loveland and Elving (9). Robertson (IS) studied the capacity of double layers on platinum. Although the differential capacity varies with applied potential, Grahame found that the capacity of the double layer in potassium chloride a t a mercury electrode was essentially the same as that in a similar solution through which an electrolysis current was passing in the presence of small amounts of reducible materials. This applies in the region of limiting current; in the region of half-wave potentials, apparent capacities are much higher. The same effect is indicated with platinum electrodes in the cases reported here. Rates of voltage sweep used ranged from 0.1 to 1 volt per second. At these rates capacity currents were significant. At higher rates, capacity currents may become as large as electrolysis currents for dilute solutions. Limiting currents were corrected for capacitr currents by observing the variation in total
TEST
Figure 1. Bi, B7, Bs. B I , Ba, Bs.
B4, Be. Bo, Bio, Bii. C1 M i 8 Ma. MZ.
RI R 1. Ra, R7. Ra, Re. R6? Ro. Rs, Rio, Rii. Rii. Riz. Ria. Rir. Ria. Ria, Rzr. Ri7, Rig, R s .
Circuit Diagram of Cathode-Ray Polarograph
7.5-volt C batteries 45-volt B batteries 1.5-volt batteries Two 1.5-volt batteries in series 50-microfarad electrolytic condenser 0-3 voltmeters 0-100 microampere meter 3000-ohm potentiometer (compensator balance) 1200-ohm, 5-watt resistance 100,000-ohm resistances 4.7-megohm resistances 1.8-meoghm resistances 15,000-ohm resistances 3000-ohm potentiometer ( Y position) 250,000-ohm potentiometer (X'gain) 3000-ohm potentiometer ( X position) 250,000-ohm potentiometer (Xgain) 100- t o 100,000-ohm resistance 100-ohm Helipots linear t o 0.6% 200-ohm potentiometers
Ris. Rzo, Ris.
100-ohm Helipot (motor-driven) 3000-ohm potentiometers 1000-ohm resistance Rzz. 1500-ohm resistance Rza. 10,000-ohm resistance 81, A,S6, SIZ. Single-pole single-throw switches Sz Ss,S I , 811. Double-pole double-throw switches Sa: Triple-pole double-throw switch Ss. Single-pole double-throw switph so. Four-position double-gang s w t c h SlO. Four-position triple-gang switch Vi. Four halves 5692 (or 6SN7) vacuum tubes in parallel VI, Va, Vd, Va. Each one-half 6SL7 vacuum tube Unless otherwise noted, all resistances are 0.5 watt. Rir is made up of General Radio series 510 decades accurate t o 0.05 %,. Ris is a Helipot, Rlodel G, designed for continuous rotation. Rzi.
V O L U M E 2 6 , NO. 2, F E B R U A R Y 1 9 5 4
283
current with rate of voltage sweep. Plots of total current versus rate of voltage sweep gave straight lines, which, upon extrapolation to zero rate of sweep, gave the values of the limiting electrolysis currents. The extrapolation is illustrated by Figure 2. I t was also possible to eliminate the capacity current by running blanks on the indifferent electrolyte a t the same rate of voltage sweep. Both methods of obtaining limiting currents gave identical results within the experimental error limits of the procedure. Electrode Reactions Used. Six reactions were investigated. Most of these have been studied previously using various types of electrodes. Current-voltage curves are given in Figure 3, where, according to the usual convention, positive currents indicate reduction processes a t the rotating platinum electrode, and negative currents indicate oxidations.
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Figure 2. Limiting Current os. Rate o f Voltage Change for Oxidation of Iodide in Sulfuric Acid S o h tion 1. Blank 2. 0.00050M I 3. 0.0010M I -
4.
5.
ported by Laitinen and Kolthoff ( 7 ) . Rogers et al. ( 1 4 ) used reduction of silver ion in studies on the recording of currentvoltage curves a t platinum electrodes using conventional recording equipment. 'Under the conditions used here a well defined wave was obtained in solutions containing potassium nitrate and ammonia. The half-wave potential (about -0.20 volt us. S.C.E.) varied with both silver ion and ammonia concentrations. As recommended by Kolthoff and Laitinen, 0.01% gelatin was added to ensure even plating of silver on the electrode. The oxidation of hydroquinone has been studied using stationary platinum electrodes (6). I n this work 0.4 to 4.0M potassium chloride was used as the indifferent electrolyte. The currentvoltage curves obtained did not appear to depend significantly on the concentration of potassium chloride: neither were the curves affected appreciably when buffers of p H = 7 were used. The apparent half-wave potential was more negative (about -0.5 volt us. S.C'.E.) than that reported with stationary electrodes. The oxidation of ferrous ion a t the rotating platinum electrode was found to give erratic results. Distorted current-voltage curves were found in a large variety of electrolytes. Best results n ere obtained in tzcidic citrate solutions. I n the higher concentration solutions the limiting current changed erratically with change in rate of voltage sweep. I n all cases observed, better reproducibility was observed \+hen the voltage sweep was begun from a potential where only very small currents flowed. If current-voltage curves were initiated in regions of limiting current or in regions where serious decomposition of the indifferent electrolyte occurred, the limiting currents were poorly defined or depended strongly upon the length of time current was allowed to flow before beginning the voltage sweep.
0.002O.M I 0.0030M I -
Since this work was done, Kolthoff and Jordan (4)have reported studies on the voltammetry of iodine and iodide a t the rotating platinum electrode. They found that in solutions containing hydrochloric acid, iodide gave a double anodic wave. The second wave corresponding to oxidation to iodine monochloride \\as found to be rate-controlled. The authors found a similar situation to exist in the oxidation of iodide in 0.5M potassium chloride. Although the waves xere not Tvell defined, the second wave appeared to be nearly taice the height of the first. The half-wave potentials (+0.47 and f0.75 volt us. S.C.E.) agreed well with Kolthoff and Jordan's data. I n 0.1M potassium nitrate solution a single anodic wave was observed. The half-wave potential was the same as that for the first wave in potassium chloride solution. Oxidation of iodide in 5.0M sulfuric acid was found to give a single well defined limiting current region (El 2 = f0.32 volt us. S.C.E.). Laitinen and Kolthoff ( 7 ) studied the oxidation of ferrocyanide a t the rotating platinum electrode in potassium chloride solutions. I n dilute potassium chloride solutions, limiting current regions were found only after a few minutes of evolutions of oxygen a t more positive potentials. I n this laboratory, ferrocyanide in 0.1.11 sodium hydroxide solution was found to give a well defined wave, the half-wave potential of which (f0.26 volt us. S.C.E.) was in agreement with the work of Laitinen and Kolthoff. Limiting current regions were found without prior evolution of oxygen. Iodate gave a well defined cathodic wave in 5.OM sulfuric acid solution (El = +0.43 volt us. S.C.E.). I n weakly acidic solutions poorly defined waves were obtained. As iodide is oxidized a t less positive potentials, the product is considered to be iodine. The reduction of silver ion a t platinum electrodes has been re-
I
1 4.0
io75
Figure 3.
+0.50 +0.25 VOLTS V S . S.C.E.
0.0
Current-Voltage Curves
1. 0.002,M I - i n KC1 2 . 0 002.1.1 I - in KNOa 3. 0.001,M I - i n HnSOd 4. 0.002Jf F e ( C N ) a - - - 5 . 0.0002.1f IOa6. 0.001M .4g(NHs)?+ (curve shifted 1.0 volt positive) 7 . 0.001.V hydroquinone 8. 0.003Af F e + +in acidic citrate
Relation between Concentration and Limiting Current. Current-voltage curves were recorded for the reactions mentioned a t a variety of concentrations. Rates of voltage sweep were varied from 0.1 to 1 volt per second. Limiting currents and residual currents were determined by the method of Figure 2. I n all cases except the oxidation of ferrous ion, proportionality
284
ANALYTICAL CHEMISTRY
Table I.
Relation between Current and Concentration Concn. Range,
PFecision,
0.0002 t o 0 , 0 0 5 0.0005to0.004
1 3 f5
Voltage Sweep, Volts U8. S.C.E. 0.00 t o + 1 . 0 0 $0.25 t o + 1 . 2 5
0.0005 t o 0 . 0 0 4
5 3
$0.25 to +1.25
0.0005 t o 0 , 0 0 5 0.0005 t o 0.005 0.0001 t o 0 . 0 0 1 0.0003 t o 0.004 0.0002 t o 0 . 0 0 2
f 4 1 3 +3 f8 f 4
0.00 to 0.00 t o $0.75 to 0.00 t o 0.00 t o
M
I - in KNOI I - in KCl (1st wave) I - in KC1 (2nd wave) I- in Has04 Fe(CS)s--- IO3 Ag(NHs)z* Hydroquinone
%
+1.w +1.00 -0.25 -1.00 +l.OO
between limiting current and concentration was found over the ranges indicated in Table I. In the case of ferrous ion, fair proportionality was observed over the range 0.0001 to 0.003M but large errors were observed a t higher concentrations. C0;YCLUSION
Currentrvoltage curves a t rotating platinum electrodes can be recorded a t rapid rates of potential variation. I n many cases limiting current conditions are established rapidly. Corrections for large capacitative currents may be applied easily if the currents are not large compared to the electrolysis currents. Pro-
portionality between limiting current so measured and concentration is generally observed. LITERATURE CITED
Delahay, P., and Stiehl, G. L., J . Phys. and Colloid Chem.. 55, 570 (1951). Grahame, D. C., J . Am. Chent. Soc., 63, 1207 (1941). Heyrovskg, J., and Foretj, F., 2. physik. Chem., 193, 77 (1943). Kolthoff, I. M., and Jordan, J., J . A m . Chem. SOC.,7 5 , 1571 (1953). Koontz, P., and Dilatush, E., Electronics, 20, No. 7 . 119 (1947). Laitinen, H. A , , and Kolthoff, I. hI., J . P h y s . Chem., 45, 1061 (1941). Ibid.. 0 . 1079. Lingane, J. J., and Laitinen, H. A , , IXD.ENG.CHEM.,ANAL. ED., 11, 504 (1939). Loveland, J. W., and Elving, P. J., J . Phus. Chem., 56, 250 (1962). Matheson, L. A, and Nichols, N , Trans. Electrochem. Soc., 73, 193 (1938). Meites, L., and Meites, T., AXAL.CHEW, 20, 984 (1948). Handles, J. E. B., Trans. Faraday Soc., 44, 322 (1948). Robertson, W. D., J . Electrochem. Soc., 100, 194 (1953). Rogers, L. B., RIiller, H. H., Goodrich, R . B., and Stehney, A. F., A N A L . C H E M . , 21, 777 (1949). Snowden, F. C., and Page, H. T., Ibid., 22, 969 (1950). RECEIVED for review September 17, 1952. Accepted October 19, 1953. Abstracted in p a r t from t h e thesis presented by Irving Shain to the Graduate School of the University of Washington in partial fulfillment of the requirements for the degree of doctor of philosophy. August 1952.
Polarographic Studies with Gold, Graphite, and Platinum Electrodes SAMUEL S. LORD, JR.', and L. B. ROGERS Department o f Chemistry and Laboratory o f Nuclear Science, Massachusetts lnstitute o f Technology, Cambridge 39, Mars.
Up to the present, almost all polarographic studies with solid electrodes have employed platinum. It seemed desirable to examine the use of other materials as many reactions are known to be affected by the electrode material. Polarographic waves obtained with a gold electrode were almost indistinguishable from those obtained with platinum. However, those obtained using a graphite electrode were sometimes less clearly
N
OBLE metal electrodes having high oxygen overvoltages
can be used for studying polarographic reactions which take place a t potentials where mercury oxidizes and is useless. Although platinum has been employed for most studies in this range of potential, it should be possible t o extend the range by using electrodes of gold or lead dioxide, which are known t o have higher oxygen overvoltages than platinum. Graphite electrodes were investigated because they offered a rapid means of obtaining a fresh surface. This feature is of particular interest xith organic reactions, which of ten produced films of high electrical and chemical resistance which had to be removed, usually with difficulty, before starting the next polarogram. This difficulty has been encountered by others (9). It was deemed advisable t o make a critical comparison of the polarographic behaviors of gold, graphite, platinum and, t o a limited extent, lead dioxide electrodes using a number of organic and inorganic reactions. Electrode pretreatment, electrode rotation, type of electrode reaction, and direction of polarization are known to affect the value of the half-wave potential and the reproducibility of both the half-wave potential and the diffusion current, so they were systematically examined in more detail than 1 Present address, Jackson Laboratory, E. I. du P o n t de Nemoura Q: Co.. Ino., P e n n s Grove, N . 3.
defined, especially when the electrode was rotated. On the other hand, a stationary graphite electrode offered advantages over both gold and platinum for studying organic oxidations where inert, nonconducting films were often formed. In the cases examined, the formal potentials obtained polarographically for several organic compounds agreed well with the critical oxidation potentials obtained chemically.
previously. The present knowledge about these factors is summarized below. Early studies have been reported which employed a polarograph to determine the effect of electrode material upon the decomposition potentials of solutions of acid and of different metallic cations (23, 36). More recently, one of the first papers dealing with the automatic recording of polarographic data obtained with solid electrodes showed that the half-wave potentials for the reduction of silver varied by as much as 0.2 volt when electrodes of six different metals were employed. However, no comparable studies were reported for an ion-ion reaction. Except for a recent publication based upon the present study ( 6 ) , the use of graphite has not been reported, although it has been used elsewhere in polarographic studies or their equivalent on a limited scale (8,24,85). In their pioneering work, Laitinen and Kolthoff ( 1 4 )found that very poor waves resulted from the oxidation of ferrocyanide a t a rotating platinum electrode unless the electrode had been pretreated by evolving oxygen on it. Muller ( 1 8 ) used a system where the solution was allowed to flow past the electrode and found a larger wave for the reduction of ferric ion in 0.1N hydrochloric acid if the electrodes were preanodized rather than precathodized. Anodizing also produced a more positive half-wave potential. If no particular pretreatment was used, the reproducibility of the diffusion current was poor. ?Iftilleralso reported that
