Permanent Recording System for Single-Sweep CathodeRay Polarographs F. C. SNOWDEN’ AND H. T. PAGE General Aniline 6% Film Corp., Easton, P a .
All recording with single-sweep cathode-ray polarographs thus far reported in the literature has been done photographically This has presented definite technical and operational difficulties. The present system was devised to circumvent these difficulties and to enhance the value of this type of cathode-ray polarograph as a research tool. The recording system consists of suitable voltage and power amplifiers, which are used to operate a commercially available high speed recording galvanometer. Voltage clamps
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Pi SEVERAL investigations of oscillographic or cathode-ray
polarography (1-4, 6, 8) reported during the past 5 years, the recording was done by photographic methods. This is perhaps the only practicable method for the multisweep-type instrument (1-4). It seemed, however, that for single-sweep cathode-ray polarographs (6, 8) other more suitable systems should be available, [Neither the cathode-ray polarograph (8) nor the recording system described in the present paper is commercially available. ] The main argument against using photographic systems for recording cathode-ray polarograms is that they are too timeconsuming. From the standpoint of its use as an analytical tool, the chief advantage of the cathode-ray polarograph over one of conventional design is the speed Fyith which results can be obtained. However, when it is required that results be more accurate than those that can be read directly from the cathode-ray tube screen during the interval between sweeps, much, if not all, of this advantage is lost when a photographic recording system is used. This loss accrues from the time consumed in the developing and printing processes. If the sole use of this instrument were in analytical applications, it might be argued with some validity that this objection could be overcome by using a modern Polaroid camera, whereby finished prints are available in approximately a minute’s time. Even this, however, would not be so satisfactory as an instantaneous recording method. Moreover, a second, and even more important field in which the cathode-ray polarograph is expected to play an increasing role is that of reaction kinetics studies. In many rapid reactions (8) it is desired to record a polarogram every 4 or 5 seconds. This precludes the use of a camera of the type described. The cost of photographic equipment, materials, and processing is not inconsiderable, and the economic side of the question must also be taken into account. Finally, in many respects the photographic method is cumbersome, and it is often difficult to obtain a sufficient number of polarogram recordings n-hen a normal drop rate of 4 to 5 seconds is used, particularly when cameras using film packs are employed, It is virtually impossible to tear out the strip of paper, taking care not to move the camera, and be in position to take another picture in less than 10 seconds. The authors found that with this arrangement the recording of every third polarogram was the fastest practical rate. Thus, for very fast reaction times of 40 seconds or less-e.g., the photodecomposition of diazonium salts-the photographic recording method was entirely unsatisfactory from an operational standpoint.
* Present address, Leeds & Northrup Co., 4901 Stenton Ave., Philadelphia, Pa.
are provided at the grids of the power-amplifying stage to provide the necessary protection to the galvanometer. The fast response and 10-cm.-persecond chart speed of this instrument provide excellent “on-the-spot” polarograms for each drop formed at the dropping mercury electrode. The inclusion of this recorder greatly improves the usefulness of a single-sweep cathode-ray polarograph as an analytical tool and as a means of investigating the mechanisms and kinetics of rapid reactions.
Because these objections to photographic methods of recording are of sufficient magnitude to decrease the usefulness of the single sweep cathode-ray polarograph described by Snowden and Page ( 8 ) ,the authors decided to try to develop an alternative recording method. As the meep time used in the instrument described is variable up to 0.75 second, it was felt that a fast responding, high speed, recording galvanometer, with appropriate amplifiers and other circuitry, might provide the solution. Several recorders of this type are available commercially, among them those manufactured by the Brush Development Co. and the Photron Instrument Co., both of Cleveland, Ohio, and the Sanborn Co., Cambridge, Mass. The maximum chart speeds available with these recorders are 12.5 em. per second for the first two and 10 cm. per second for the latter.
4 PUSH-FULL FROM CRT VOLTAGE +AMPLIFIER
PUSH-PULL POWER AMPLIFIER
HIGH SPEED
- RECORD I N G GALVANOMETER
These maximum available chart speeds present the only limiting parameter of possible significance. In order to obtain accurate measurements of half-wave potentials, the abscissa of the polarogram should be a t least 5 cm. long, providing a minimum voltage scale of 0.04 volt per mm. for a 2-volt sweep. This signifies that the sweep time must be no less than 0.5 second, and preferably the maximum of 0.75 second, if chart speeds of 10 to 12.5 cm. per second are to be used. The difficulty which arises under the circumstances outlined above is brought about by the fact that during any voltage sweep of finite length across a dropping mercury electrode, there will be a corresponding increase in drop size during the sweep time. The diffusion current will understandably increase with this larger drop size-according to the current-time relationship given by the Randles-Sevbik equation (6-7’). The distortion introduced into the cathode-ray polarograni by this increacing diffusion current 1152
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component is of no real significance in kinetics studies. In such studies the initial and final concentrations of the controlling reactants and products are generally known, and only relative concentrations a t intermediate steps are required. Thus, if the sweep time is held constant throughout a particular study, the amount of distortion introduced into the polarograms will also remain constant and relative concentrations may be obtained at any intermediate point desired. It matters not whether the distortion component amounts to 1 or 10% of the total, so long as it remains constant for the duration of a single reaction study.
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Figure 2.
As an example, assume that an analyst wishes to determine only the zinc in a sample that also contains some copper. This, of course, can be rather easily accomplished by setting the starting potential for the sweep somewhat above the half-wave potential for the cuprous-copper break, but below that for the zinc break. Here is a situation in which there is a component of current due to an ion (Cu+) which is undergoing reduction throughout the entire period of voltage sweep application. This component a t time t from the start of drop growth is given by the IlkoviE e uation for instantaneous diffusion current. Fortunately this exect is also of second order for drop times of greater than 2.1 seconds with 0.5-second sweep times, and will thus obviously cause no interference when drop times in the suggested 7- to 9-second range are employed. The reason this IlkoviE effect falls off so rapidly with respect to drop times is the presence of the tl/e factor in the current-time relationship given by the IlkoviE equation.
Schematic Diagram of Recording System Ci, Cz, Ca, Cc. 1-pfd 450-w.v.d.c.
L . 3000-ohm recorder galvanometer coil R I , Rz. 470,000 o h m s Ra. 1 m e g o h m R4. 2-megohm potential (gain control) Ra, K i . 6800 o h m s RE,Rs. 470,000 o h m s Rot R I Q . 1 m e g o h m RII. 68,000 o h m s RIZ,Rls. 47,000 o h m s Rig. 370 ohms RI,. 1 m e g o h m Rls. 220,000 o h m s Ti. 6 SL 7 Ti, T L , 6H6 or 6.415 Ts. 6 A S 7
In accurate analytical work, however, the distortion factor becomes of importance when it approaches and/or exceeds the normal inherent accuracy of the polarographic method; this latter is given as &2% by Kolthoff and Lingane (6). I t is of interest, then, to calculate the minimum drop time with which a sweep of 0.5-second duration may be utilized without introducing a significant amount of distortion. For this calculation we may use Equa:ions 1 and 2. Equation 1 is a simplified version of the Randles-SevEik equation obtained by coalescing all the nominally constant factors into a single value, K . Equation 2 is the derivative of Equation 1. ir =
Kt213
Substituting this expression in Equation 1, the minimum permissible drop time is found to be 8.3 seconds, rounded off to 9 seconds for convenience and to allow for a margin of error in Equation 3. This is not an excessively long drop time for singlesweep cathode-ray polarography, wherein each drop yields a complete polarogram. Indeed, it has been suggested by Randles (6) that a minimum drop time of 7 seconds be used in cathoderay polarography regardless of the rapidity of voltage sweep adopted. This suggestion stems from the fact that the charging current becomes a second-order effect when the drop time exceeds 7 seconds. This is due to the much slower rate of increase in drop area toward the end of relatively longer-lived drops, the point at which the voltage sweep is normally applied. In the event that greater separation of ions in the polarogram is required than that given by the 0.5-second sweep considered above, 0.75-second sweeps may be satisfactorily used with drop times of 12.5 seconds. Occasionally situations arise wherein exceptional factors must be considered and evaluated.
(1)
In these equations it is the average current in microamperes for the duration of the applied voltage sweep, and t is the drop time. If *2% is taken as the espected accuracy of the method, then an increase in diffusion current no greater than 0.04 it can be tolerated during the time of voltage smeep. In Equation 2, for purposes of calculation, Ai, can be substituted for dit and At for dt, r e d t i n g in (3)
If now, in Equation 3 0.04 it is substituted for A i r ond 0.5 second for At, an expression for if is obtained in terms of t and K.
Of the commercially available, high speed recorders described, the authors decided to use the one manufactured by Sanborn, because it is comparable in all respects to the others available, and, in addition, it has the advantage of using rectilinear chart paper. The chart speed is 10 em. per second. 4 block diagram of the circuit built is shoan in Figure 1, less the supply voltage which is obtained from a standard regulated source. The recording galvanometer has a deflection coil of 3000 ohms and requires a current of 50 ma. for full scale deflection, which entails a voltage swing of 150 volts. Thus the galvanometer cannot be attached directly to the vertical deflection plates of the cathode-ray polarograph for two reasons. First of all, the maximum voltage swing at the deflection plates is only about 70 volts, and secondly, the power available there is but a small fraction of that required. This necessitated that the recording galvanometer be preceded by two stages of amplification, one voltage and one power stage, as shown in Figure 1. The voltage clamps are for the protection of the galvanometer coil and the rerording stylus arm. Their presence prevents any application of excessive voltage across the galvanometer coil, precluding the possibility of overloading or burning out this sensitive component. It also prevents the stylus arm from banging against the stops a t the extremities of the vertical scale. Figure 2 is a complete schematic of the circuit used. The voltage am lification is obtained with the 6SL7 stage and the power ampligation with the 6AS7 stage. The two 6H6 diodes act as the voltage clamps or limiters. Both amplifiers are operated in push-pull, to provide increased stability and noise elimination. The 3000-ohm galvanometer coil of the recording instrument is provided with a center tap to facilitate its use in such push-pull amplifiers. This coil is shown in the schematic in the cathode circuit of the power amplifier. The center tap is returned to ground through a 370-ohm resistor. The normal position of the recording stylus is mid-way bet m e n the top and bottom of the chart paper. This indicates
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ANALYTICAL CHEMISTRY
that in order to obtain the greatest sensitivity from the instrument the grid biases of the two halves of the 6AS7 must be 80 adjusted that the position of this stylus is offset to a position near the bottom edge of the recorder paper. This is accomplished by returning one of the grids through a 1megohm limiting resistor to a suitable positive potential obtained from a divider across the positive supply. The other grid is biased in the negative sense in a similar manner from the negative supply. These biases were initially supplied from potentiometers across the respective voltage sources and were adjusted for a suitable zero input position of the stylus. The potentiometers were then replaced by the proper fixed resistors. In the circuit shown, the ground to grid potentials of the two halves of the 6.4S7 are -61 and $56 volts, respectively. These values will undoubtedly vary somewhat from tube to tube, and it might be a wise precaution to replace the 68,000- and 220,000-ohm resistors shown in Figure 2 to screwdriver adjusted potentiometers.
cathode-ray tube trace. This results in better quantitative data for both analytical and kinetics studies. I t is of interest to note at the upper right of Figure 4 how the stylus trace levels off after the clamped voltage value is reached.
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Figure 5. Series of Superimposed Polarograms Illustrating Kinetics of a Diazo-Coupler Reaction Obtained with new recording system
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Figure 3.
Photographed Cathode Ray Polarogram of Metallic Ion Mixture
Figure 5 shows a tracing of a superimposed series of five polarograms made during a diazo-coupler reaction. These polarograms are broken in the vicinity of multiple intersections to avoid confusion. The first polarogram in the series has no azo dye break apparent and was taken at zero time-Le., before any coupler was added to the cell. The other four polarograms were obtained, respectively, 15, 30, 60, and 120 seconds after the addition of coupler to the cell. That the reaction is complete in 120 seconds is evidenced by the disappearance of the peaks originally caused by the diazo present. This reaction is similar to one previously reported (8) and is illustrated here for comparative purposes. The significant advantage of the recording galvanometer over the photographic method is realized best in such studies of kinetics. The polarograms for each drop are now available for any reasonable drop time down to 3 or 4 seconds. Thus a reasonable body of data may be obtained for very rapid reactions which go to completion in 20 to 30 seconds. ACKNOWLEDGMENT
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Figure 4.
Polarogram Similar to Figure 3
Obtained with preeent recording system
Figures 3, 4, and 5 illustrate the results obtained from this recorder. Figure 3 is a normal photographed cathode-ray polarogram, and Figure 4 depicts a tracing of the same polarogram as drawn on the permanent recorder described here. Comparison shows that the response of the recording galvanometer is more than ample. Furthermore, the’vertical resolution of the polarogram of Figure 4 is an improvement over that of Figure 3, resulting from the increased definition of the stylus line over that of the
The authors wish to thank L. T. Hallett and F. W, Mitchell for their encouragement and suggestions during the course of this work. LITERATURE CITED
(1) Bieber and Triimpler, Helv. Chim. Acta, 30, 971 (1947). (2) Delahay, P., J. P h y s . Colloid Chem., 53, 1279 (1949). (3) Ibid., 54,402 (1950). (4) Delahay, P., and Stiehl, G. L., Ibid., 55, 570 (1951). ( 5 ) Kolthoff and Lingane, “Polarography,” New York, Interscience Publishers, 1941. (6) 9ndle8, J. E. B., Trans. Faraday Soc., 44, 327 (1948). (7) Sevzik, A., Collection Czechoslov. Chem. Communs., 13, 349 (1948). (8) Snowden. F. C., and Page, H. T., AXAL.CHEM.,22,969 (1950).
RECEIVED for review December 29, 1951. Accepted Bpril 24, 1952.
