Automatic Polarograph for Use with Solutions of High Resistance PAUL ARTHUR, PHILLIP A. LEWIS,' NELSON A. LLOYD,2 AND RICHARD K. VANDERKAM Deparfmenf of Chemistry, Oklahoma State University, Sfillwafer,
b A new polarograph and cell for use with solutions of high electrical resistance are described. The apparatus employs a strip-chart function plotter, the pen of which moves as a function of one potential while the chart follows another. By using the pen to trace current while the chart measures the changing potential of the microelectrode with respect to a reversible electrode through which no current is drawn, polarographic curves of current vs. effective voltage are obtained, thus eliminating iR drop. Many organic solvents not usable with conventional polarographs can b e employed in polarography with this equipment. Aqueous cells with resistances as great as 500,000 ohms have been successfully employed, and organic solvents (e.g., 1 -0ctanol) with cell resistances up to 6 megohms have given satisfactory polarograms with this apparatus.
I
polarography, polarographic cells and circuits are designed to keep resistances as low as reasonably feasible; consequently, it is both possible and customary to plot the current as a function of the voltage applied to the cell. With the polarograms so obtained, corrections for iR drop in the cell or circuit are required only when unusually large currents or resistances are encountered and when the utmost accuracy in voltage measurements is required. Modern polarographs, consequently, are unsuitable for use when solutions of high resistance are to be employed; for in such cases iR distortion is so bad the curves may not be even recognizable as polarograms. The reason for this distortion lies in the fact that of the voltage supplied and plotted by the conventional polarograph (the applied voltage, Ea),only a portion (which might be called the effective voltage, E d ) ,is used to bring about the electrolysis itself. The remainder (the iR drop) is used to overcome the resistance in the cell and circuit: N MODERN AQUEOUS
E , = E.
+ iR
1 Present address, Iowa JF'esleyan College, Mt. Pleasant, Iowa 2 Present address, Northeast Louisiana
State College, Monroe, La.
488 *
ANALYTICAL CHEMISTRY
Oklu.
The problem of high resistance can be met, therefore, either by use of an attachment which automatically introduces into the circuit enough voltage to compensate for the iR drop or by employing a polarograph which is designed to plot the current as a function of E. instead of E,. The latter approach was employed in the research reported here. Jackson and Elving (4) and Nicholson (7) have described methods of automatic iR compensation for use in situations where high resistance is encountered. Both methods, however, require a knowledge of the resistances to be encountered and a corresponding setting of the compensator before the polarogram is made, and neither will compensate properly if the resistance changes while the polarogram is being recorded. Sawyer, Pecsok, and Jensen (9), following earlier reports by Arthur et al. (1) on the use of a second reference electrode, have described the use of X - Y recorders in a manner similar to the method described in this paper. Their instrument differs, however, in that it is limited to a selected 1-volt range and to a cell resistance probably not much greater than their tested range of 22,000 ohms, whereas the instrument described here has a 3.5-volt effective range and has been shown to work excellently with cells having resistances as high as 6 megohms. More recently, Oka (8) described an iR compensator and Kelley et al. (6, 6) published a complete circuit for derivative and controlled potential polarography, both of which employ adaptations of the three-electrode cell. APPARATUS
Three parts are characteristic of this particular polarograph: a strip-chart function plotter in which the pen movement i s a function of one potential and the chart movement is a function of another; a special polarographic cell (see Figure 1) which, in addition to the usual microelectrode compartment, has two anodes, one of which, together with the microelectrode, serves for the electrolysis, while the other serves as a reference electrode against which the effective voltage of the microelectrode can be measured without iR drop; and a high-impedance cathode follower
circuit inserted in the circuit between the cell and the chart circuit of the recorder. The principle involved is best seen by studying Figures 1, 2, and 3. A conventional motor-driven rotating bridge is employed to supply a steadily increasing voltage across the microelectrode-electrolysis reference electrode (E.R.E.) branch of the cell. The pen circuit of the recorder is connected across a selected standard resistance, Sillany resulting movement of the pen, therefore, being proportional to the current flowing in the electrolysis circuit. The chart circuit, on the other hand, is connected through the high impedance cathode follower circuit (C.F.) to the microelectrode and the second (the stable) reference electrode (S.R.E.). The impedance of this cathode follom-er is above 1000 megohms, which keeps the current in this branch of the circuit so low that the potential difference between the microelectrode and the stable reference electrode (S.R.E.) is measured by the chart movement with neligible iR loss. In this way, while the pen moves back and forth across the chart as a function of the polarographic current, the chart advances and recedes as the effective voltage increases and decreases. The resulting polarogram needs no correction for any iR drop, whether this drop occurs in the circuit or in the cell (see Figure 4). Details of the circuit employed in this work are shown in Figure 3: The bridge, rotating a t 0.08 r.p.m., is supplied, by batteries, B1, the snitch SI. and a linear-taper radio potentiometer, PI. with a span voltage variable from zero to 18 volts. Runs can be started a t any preset voltage desired by Sn, Ss,PI, and Bz. Switch S4controls the polarity of the microelectrode; S, is used to keep the polarity of the electrolytic damping condenser in line with that of the microelectrode; and Slo controls the degree of damping in the current-measuring circuit. SI1 is a selector switch by which the proper standard resistance needed to measure current can be brought into the circuit. The circuit unit involving 133, PI, Pq, and Sj provides for moving the chart up or donm scale as desired, while B4,R1, P6, P6,and S9 serve the same purpose for the pen. The function plotter employed in this work was a Brown Electronik function plotter, Model No. Y 153X32(W) - X-l2O(V), with a 4.5-second pen movement having a full-scale sensitivity of 2.5 mv. and a 4 &second chart move-
.
4 A. B.
Vent inlet for connecting wire Mercury for connections D. Platinum wire to make connection with mercury pool of calomel or other reference electrode E. Thermometer F. inlet for dropping mercury electrode G. Nitrogen inlet H. Rubber tubing 1. Dropping mercury electrode K. Standord taper joints 1. Medium-porosity frits (Note1 cotton moistened with appropriate electrolyte between these) M. Gas dispersion tube N. Colomel (or other) reference electrodes C.
I-
---
-----I I
9 '
PI
- 1 0
u Figure 1.
CF:
*to6 CHART
N
Cell design
ment having a sensitivity of 3 volts for 10 inches. The chart circuit was especially designed for this research to operate on input impedances up to 0.1 megohm. Actually this last requirement was unnecessary, for it was soon found that a much higher impedance than that of the recorder circuit was needed to prevent drawing current through the stable reference electrode. The cathode follower used for the purpose was essentially that described by Gucker et al. (31, a 5692 tube being used and no reference voltage being employed. Although the strip-chart function plotter has a self-contained calibration mechanism for both the chart and the pen, introduction of the cathode follower into the chart circuit made an external chart calibration necessary. Switch Ss, together mith two Weston standard cells, S.C., serves this purpose, allowing calibration a t zero potential (short circuit), at the voltage represented by a single standard cell, or a t a potential represented by the sum of the two. OPERATIONAL PROCEDURE
For cathodic work, the operation of this apparatus is very much the same as for conventional polarographs. Daily calibration of the instrument is recommended, however, and both Sg and S7 must be closed upward for cathodic runs. For anodic work, it is necessary to close both S4 and S, downward; then upscale both the pen and the chart. During the run, the pen and chart potentials increasingly oppose the upscale potentials, yielding a polarogram that is the inverted mirror image of an identically shaped cathodic wave. Many different circuits were tested in an effort
ERE
SRE
Figure 2.
to obviate the need for using the upscaling technique in anodic work; but all failed. Seemingly, in the recorder employed, the negative of the chart and that of the pen need either t o be completely disconnected from each other (an impossibility in a polarographic circuit) or to be connected through only a small resistance. When these are connected through larger resistances, the chart movement becomes sluggish and indefinite, often with less than 50% of its usual response to a given applied potential. The circuit shown in Figure 3 was the only one of the many tested that gave good results. Many polarograms were made during the testing of this apparatus, some with low and high resistance aqueous cells and some with organic solvents. Aqueous solutions were employed so the behavior of the polarograph and the effect of resistance on curves could be studied with solutions whose polarographic behaviors have been well established. For studies of aqueous solutions, the cell in Figure 1 was modified by connecting the two reference electrodes through long small-diameter tubes, coiled to conserve space and filled with 0.1N potassium chloride solution. By employing different sizes of tubing it was possible to obtain resistances (measured from the dropping electrode to each reference electrode) from 600 to 500,000 ohms. Nonaqueous solvents studied involved all the normal alcohols from methanol through 1-octanol, 2-butanone, 3-pen-
Schematic circuit
tanone, and mixtures such as 1 : l mixtures of iso-octane and 1-butanol. Lithium chloride or lithium nitrate was the supporting electrolyte except for 3-pentanone in which magnesium perchlorate was used. Cadmium ion, elemental sulfur, b e n d , and m-nitroaniline are typical of the reducible substances which were tested in solvents of these types and although cell resistances were from 2000 ohms to 6 megohms, no indications of iR drop were seen in the polarograms obtained (Figure 5). CELL DESIGN
Certain cell designs tested earlier in the research were found, with solutions of high specific resistance, to reintroduce iR drop into the system. This was probably caused by the fact that in these cells a part of the electrolyte path between the microelectrode and the electrolysis reference electrode coincided with that between the microelectrode and the stable reference electrode. Experiments were run, therefore, eniploying 1-octanol solutions of lithium chloride containing cadmium ion, in a cell like that of Figure 1 but with long connecting tubes and with the microelectrode compartment interchanged first with the stable reference electrode, then with the electrolysis reference electrode. Various cell resistances from 500,000 ohms to 5 megohms were used. In all cases, iR distortion, equal to that calculated for the electrolyte path comVOL. 33, NO. 4, APRIL 1961
489
4 V . Lead to chart negative VI. Lead to pen positive V I / . Lead to pen negative One to three Eveready Hotshot 6-volt batteries, No. 1461, in series Two to three Burgess No. 4F, 1 '/2-volt batteries, in series B3. Two to three Burgess No. 4F, 1 '/z-volt batteries, in series One Burgess No. 4F. 1 '/*-volt battery B4. C. Capacitor, Mallory, ko w P 0 4 1,200b-mfd. electrolytlc C.F. Cathode follower PI. Industrial control, wire-wound, 4-watt, 200-ohm Pz. 550-ohm Pt. 25,000-ohm Pa. 400-ohm Pg. 100-ohm PO. 2-ohm R1. Resistor, Continental carbon, X-type, precision, 30,000-ohm Rz. Resistor, IRC, type WW4J, wire-wound, 1% accuracy, 1 000-ohm Following resistors are those shown in SI^, listed in counterclockwise order. All are IRC, type W W 4 J , wire-wound, with 1 % accuracy 5000-ohm, 2000-ohm, 1 000-ohm, 500-ohm, 200-ohm, 1 00-ohm, 50.ohm, 20-ohm, 1 0-ohm SI. SPST toggle switch SZ,Sa, Sa, S12, S13. DPDT toggle switches S4 and &. DPDT center-off toggle switches Ss, Ss, Slo. Switchcraft lever-type No. 3036L, 3-position, locking-type switches SS and Sll. Ohmite power tap switches, Model 111, eleven-contact type 61. 62.
Figure 3. I. I/. 111.
IV.
Polarographic circuit
Lead to microelectrode Lead to electrolysis electrode l e a d to stable reference electrode Lead to chart positive
mon to the two branches, occurred in the polarograms. When the cell illustrated in Figure 1 was employed, iR distortion disappeared. With solutions of very high specific resistance, however, even in this cell it was found best to locate the microelectrode approximately midway on a line between the openings leading to the reference electrodes. The nature of the reference electrodes is likewise important. This polarograph measures the potential of the microelectrode us. the stable reference electrode regardless of the source of the microelectrode potential. A dropping mercury electrode in a solution containing electrolyte constitutes itself a half cell of unstable and largely unpredictable potential. ]\'hen a polarogram is started, this half cell is connected to the electrolysis reference electrode by the bridge, the two together thus constituting a voltaic cell. The starting point of the trace, therefore, is strongly modified by the nature of the electrolysis reference electrode. Seldom does the starting point coincide with the true zero potential obtained by short-circuiting the input to the chart through the cathode follower. In aqueous studies, when both reference electrodes are saturated calomel electrodes, the difference between the starting point of the trace and true zero is smali (usually about 0.03 volt) and relatively constant. The same is true when acetone-saturated calomel electrodes of the type described by Arthur and Lyons (2) are employed for nonaqueous studies. When aqueous studies were made with a saturated calomel electrode as the stable reference electrode and a mercury-mercurous sulfate ( I N potassium sulfate) electrode 490
ANALYTICAL CHEMISTRY
as the electrolysis reference electrode, the trace always started a t significantly large positive potentials, the magnitude of the potential being strongly dependent upon cell resistance. Thus with 0.001iM cadmium ion in 0.1111 potassium chloride, at a cell resistance of 800 ohms the trace started at +0.356 volt; a t a cell resistance of 270,000 ohms, a t $0.095 volt, The current was strongly anodic a t the starting point, this current being caused by the dropping mercury electrode and the mercurymercurous sulfate electrode acting as a voltaic cell. Since the polarograph does not correct for the loss of voltage due to iR drop in this case, a calculated correction mas added to each. In this way the true starting voltage was calculated to be +0.376 and +0.375, respectively. The potential of the mercury-mercurous sulfate electrode was measured and was t 0 . 4 0 3 volt with respect to the calomel electrode employed. The fact that the difference between this value and the 0.375 volt for the starting point of the curve is of the same order of magnitude as the 0.03 volt representing the usual deviation from zero when both reference electrodes are saturated calomel electrodes gives credence to the idea that the difference represents the normal operating potential of the dropping microelectrode in 0.1V potassium chloride as a half cell us. the saturated calomel electrode. Similar results, but in the direction and of the order of magnitude to be expected, were obtained when a combination of a 0.1N calomel electrode and a saturated calomel electrode were employed, with first one, then the other as stable reference electrode. Half-wave potentials also showed
peculiarities when dissimilar reference electrodes were employed. As would be predicted, measurements of these made from the true (short circuit) zero voltage line proved to be the most useful; but with dissimilar reference electrodes, a small but definite resistance and concentration dependence was observed. Thus cadmium in 0.1M potassium chloride, in a cell with a mercury-mercurous sulfate electrolysis reference electrode and a saturated calomel stable reference electrode, gave half-wave potentials of - 0.550 volt for 0.001M cadmium a t a cell resistance of 800 ohms and -0.563 volt for the same concentration a t 270,000 ohms. A 0.0002JI solution gave -0.593 volt a t 700 ohms and -0.600 a t 320,000 ohms. When both electrodes were saturated calomel electrodes, changes from 500 to 500,000 ohms produced no measurable difference, the half-wave potential being -0.600 =t 0.003 volt both on this polarograph and on a Sargent Model XXI Polarograph employing a conventional cell CELL RESISTANCE AND OSCILLATION CHARACTERISTICS
When cells and solutions of low resistance are used with this polarograph, the polarograms are conventional in appearance (curve a, Figure 4). Since the polarograph plots current as a function of effective voltage and since the iR drop is negligible with small currents and low cell resistances, the effective voltage is equal to the applied voltage a t all times and the oscillations caused by the growth and fall of niercury droplets are, as usual, largely vertical.
I 1
- 2 036
-liOIE
EFFECTIVE VOLTAGE
Figure 4. Cadmium in 0 . 1 M aqueous potassium chloride Cell resistance each branch: A. 800ohms B. 580,000 ohms plus 1 -megohm external series resistor
With high resistances the situation is different. When the applied voltage is such that an appreciable current is flowing, as the size of the mercury droplet increases, this current also increases. In consequence, the iR drop increases and the effective voltage, which is the applied voltage less the iR drop, decreases. At maximum drop size, the effective voltage reaches a minimum; a t minimum drop size, the effective voltage is a t a maximum. This leads to the large horizontal oscillations observed in curve b, Figure 4. With the polarograph that was employed in this work, when the cell resistance is very high the vertical component of these oscillations sometimes almost disappears unless the current damping capacitor is cut out of the circuit. Undoubtedly interaction between the chart and pen circuits of the recorder used accounts for this. Owing to the large horizontal oscillations obtained, it is necessary to use the envelope of the points a t the wave front (the points of maximum drop size) and the top of the nave for meaningful measurements. JVith either low or high resistances and with either aqueous or nonaqueous solutions, very usable waves of the types shoivn are characteristic. At a range of 40,000 to 60,000 ohms or more (depending upon the nature and concentration of the solute and upon the solvent system employed) the oscillations, n-hich are changing from the vertical to the horizontal form, become erratic loops and the resulting wave becomes difficult to interpret. TTThen such intermediate resistances are encountered, however, excellent waves can be obtained by simply connecting the electrolysis reference electrode to the circuit through a 0.2-megohm or greater resistor. This added resistance causes no difficulty, for excellent polarograms are obtained even a t cell re-
sistances of 6 megohms-the highest resistance tested. A comparison of these results with some obtained when an oscillograph was substituted for the X - Y recorder indicates that the loops are probably the result of phase shifts caused by the lag in the response of the recorder; for with the oscillograph and no damping, steeply slanting, almost straight lines were obtained instead of loops. MICROELECTRODE BEHAVIOR
One difficulty encountered in this work v, as the tendency for the dropping mercury electrode to behave erratically in nonaqueous solvents such as 1-butanol or less polar solvents. Although actual maxima were not observed, the use of maximum suppressors proved effective in combating this. Thus a few milligrams of methyl red greatly improved polarograms in either 2-propanol, 1butanol, or 1-octanol. Very small amounts of diethylhesadecylmethylammonium bromide were likewise effective in 1-propanol. Such electrode behavior is not surprising considering the variables involved. Not only is the dropping mercury electrode subjected to the widely varying voltages mentioned earlier, but also the resistance a t the solution-electrode interface should change greatly during the life of a droplet. Surface tension effects must be very great and variable, and it was this concept that led to the successful employment of maximum suppressors to stabilize the electrode behavior. WAVE MEASUREMENTS
When cells of sufficiently low resistance are employed with this instrument, the polarograms resemble those obtained conventionally and half-wave potentials and average diffusion currents can be measured and emploqed in the usual ways. With cells of high resistance, it is necessary to make all measurements of half-wave potentials to the envelope through the points representing the wave front. Likewise, all diffusion current measurements must be made to the top of the wave. Whether this current measurement more nearly represents an average current or a maximum current is difficult to decide; however, experiments have shown that, with the current damped and with the resistance high enough to give the characteristic wave form shown in Figure 4, curve b, the current so measured is independent of resistance and is, within polarographically acceptable limits, a linear function of concentration. The current is significantly higher than the average current measured on a low resistance cell for the same solution, however. As is true of any polarograph, over-
I -2,036
I
EFFECTIVE
ELTAGE
Figure 5. 0 . 0 0 1 M Cadmium in 0.1M lithium nitrate in 1 -butanol Cell resistance, 0.8 megohm, each branch
damping, with the resultant failure of the recorder to follow changes in current with sufficient rapidity, shifts apparent half - wave potentials appreciably, cathodic waves, for example, shifting to more negative values. Diffusion currents decrease somewhat with overdamping; consequently, if current measurements are to be compared, they need to be made under comparable damping conditions. SUMMARY AND CONCLUSIONS
Properly used, the polarograph and cell combination described in this paper are well suited to polarographic studies and determinations in nonaqueous media. The apparatus has been tested on a wide variety of organic solutions and has also been employed in amperometric titrations of a variety of waterinsoluble substances in mixtures of iso-octane with either 2-propanol or 1-butanol. These studies will be the subject of later articles. ACKNOWLEDGMENT
The authors express their appreciation for assistance rendered in the form of a contract, No. A F 18(600)-477, from the Air Research and Development Command through the Oklahoma State University Research Foundation. LITERATURE CITED
(1) Arthur, P., Lewis, P. il., Lloyd, 1;.A., ANAL.CHEM.26, 1853 (1954). ( 2 ) Afthur, P., Lyons, H., Ibid., 24, 1422
(1932). (3) Gucker, F. T., Jr., Peterson, G. H., Ibzd., 2 5 , 1577 (1953). ( 4 ) Jackson, W.,Jr., Elving, P. J., Ibid., 28. 378 (1966). (5) Kelley; M. 'T., 1%.C., Ibid., 32, 1:
(G)_K_elley, -. . .M.- .TI,. .
RECEIVED for review March 31, 1960. Accepted January 18, 1961. VOL 33, NO. 4, APRIL 1961
491
