The Cathode Ray-Tube Polarograph Theory of Method R. H.MULLER, R. L. GARMAN, M. E. DROZ, 4 N D J. PETRAS Chemical Laboratories of Washington Square College, New York University, New York, 3.Y.
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as performance of the record, etc. It would seem natural to utilize t h e cathode ray oscillograph for this purpose, but in practice a number of difficulties arise. Oscillograph practice requires rapid recurrence of the phenomenon if a persistent stationary image is desired. It is true that transient images can be photographed, but this procedure would nullify the advantage of a continuous picture of what is going on. If we attempt to sweep through the range of potentials very rapidly in order to produce a persistent stationary pattern, the question arises whether the electrode equilibria can keep pace with the rapidly changing potentials. The authors have found a solution to this problem and the instrument based on this method yields values identical with those based upon the conventional Heyrovskp method.
polarograph as developed by Heyrovsk? and his coworkers is a n instrument well known to electrochemists and analysts. The theory and applications have been summarized in several monographs (1, 2 ) . The recording of current-potential curves of a dropping mercury electrode may be accomplished in a number of ways. Heyrovsky gives adequate reaqons for preferring the photographic method, such
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Theory Let curve OAHBC of Figure 1 represent an idealized Heyrovsk9 polarogram for a certain ion. The potential, V , corresponding to point H, is the Hulbwellenpotential and represents a characteristic identifying value, for the given ion. Now imagine a small sinusoidal alternating pot,ential of peak value, AV, applied t o series with the main potential, V . The current, I, will now vary about the mean value, H , to produce a TFave, S , of the same wave form and frequency, but with an amplit'ude n-hich depends upon the steepness of curve AHB. If the main direct current potential, V , is now shifted the sine wave, S , will become distorted at its upper or lower portion because of intersection n-ith the nonlinear portions of the curve a t B or A. If this curve, S , is continuously viewed on an oscillograph screen it will be almost perfectly sinusoidal and undistorted when and only when V has
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FIGURE 1
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FIGURE 2.
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a value corresponding to the mid-point of the current-potential curve (at H ) . Conversely, the appearance of such undistorted waves, as V is varied from zero to the maximum value of deposition potentials, will serve to detect and identify the characteristic potentials.
The Instrument The circuit is shown in Figure 2, left. Battery B supplies the potentials through regulating resistor R1 and voltage divider R1. The voltmeter, Ti, indicates the applied potential. The small alternating potential in series with the direct current potential is supplied by step-down transformer TIand voltage divider Rs. The lead to the dropping mercury cathode passes through the high-gain, low primary-impedance transformer, T,. The secondary of this transformer is connected t o the vertical deflector plates of the cathode ray oscillograph. If the oscillograph is not provided with a built-in amplifier or one of sufficient gain (3000 to 5000 X ) it must be preceded by a voltage amplifier stage. The horizontal deflector plates are driven by the usual sweep circuit and in most cases means are provided for locking in the sweep with the phenomenon under investigation (synchronizing control). The authors have eliminated the customary leveling bulb for supplying mercury to the dropping electrode in the interest of compactness and convenience of manipulation and to facilitate careful electrical shielding of the electrode assembly. Figure 2, right, shows a simple arrangement of pressure bottle PB, rubber bulb B , and two-way stopcock S with a micro bubble regulator, R . Manometer M gives a rough indication of the driving air pressure. Actually, the rate of dropping of mercury is the best criterion of satisfactory operation, and this is quickly adjusted by means of the stopcock by-pass. Since very little mercury is used, the air reservoir requires very infrequent attention. Figure 3 shows a photograph of the instrument. The oscillograph is on the right. The main case on the left contains the circuit and controls. The voltmeter indicates the critical direct current potentials. The left-hand dial controls the voltage divider, Rz. The right-hand dial, RJ,governs the magnitude of the alternating current potential. Toggle switches are provided for the battery and alternating current supply. The small copper case mounted on the right side of the instrument contains the electrode assembly and can be tightly closed by means of a copper door. The electrode connections pass directly through the wall through insulated connectors (“banana plug” type). The case is grounded during operation. Connection to the oscillograph is made through shielded cable. Reasonably careful shielding and the absence of loose, rambling wires are essential for satisfactory operation. Operation
A typical polarogram as obtained by this instrument is shown in Figure 4. I n this case the solution contained a small amount of cadmium ion ( 5 mg.), 0.002 M . At an applied potential of 0.63 volt the oscillograph pattern is as shown in a. Potentials slightly less than this value yield the distorted curve, b, whereas a t slightly higher potentials another distorted curve, c, results.
This behavior is explained in the
FIGURE3
FIGURE 4. TYPICAL POLAROGRAM
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discussion of Figure 1. On a complete analysis the operator merely increases the potential, Ti, manually from zero to the maximum and notes the potentials a t which symmetrical waves appear. The process can be repeated as often as desired. For feeble curves (traces of ion) the gain control may be stepped up in order to miss none. Under these circumstances the maxima due to large amounts of other ions will produce high deflections, beyond the edge of the screen, but the instrument is not damaged as a delicate galvanometer would be. The entire pattern disappears when a mercury droplet falls from the capillary. A new curve appears almost immediately, and the momentary interruption is not disturbing, inasmuch as the general technic requires fairly slow dropping rates. The observed potentials are very reproducible and vary by only a few millivolts-0.627 to 0.629 in the above case. Furthermore they are identical with the values obtained in the conventional way-with a voltage divider and galvanometer. I n all cases in which the new instrument was compared with the “manual” method it was absolutely necessary to correct the observed potentials for the anode potential as measured against the solution with a calomel electrode in the conventional way. Authorities (1, 2 ) agree that this is necessary to obtain the standard value for each ion as recorded in the literature. Thus for zinc ion (0.001 M)the authors observed a value of -1.110 volts, and the anode potential correction was 0.041 volt yielding -1.069 volts for the Halbwellenpotential. The accepted value is - 1.06 volts.
Discussion So far no mention has been made of the quantitative aspects of the instrument. Reference t o Figure 1will show t h a t the final deflection of the cathode ray beam a t the Halbwellenpotential depends upon the gain of the amplifier, the value of
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V, and the height of the “Heyrovsky curve” for the particular ion under observation. The gain of the amplifier may be held constant and A V adjusted to some predetermined value. Investigations are in progress for the determination of suitable values of the voltage under various conditions of operation, so that quant’itative estimation may be accomplished with the instrument. The complete instrument, including the oscillograph but not labor, costs $150. Summary Current voltage curves taken with an oscillograph using a small series alternating current potential yield patterns which are interpretable on the basis of the conventional Heyrovsky method. The polarograms are viewed continuously and require no photographing or recording. The actual identification is in terms of the same potentials used by the classical methods. The introduction of the small alternating current component does not gire rise to any complications; the observed values are the same as those obtained with direct current. The analysis can be repeated indefinitely and the results are continuously visible to the operator. Sensitivity control is not hampered by possible damage to the recording instrument.
Literature Cited (1) Heyrovsk5, J., in IT-. Bottger, “Physikalische hfethoden der chemischen dnalyse,” Leipzig, Akademische Verlag, 1935. (2) Hohn, “Chemische Analysen mit dem Polarographen,” Berlin, Julius Springer, 1937. RECEIVED March 30, 1938. Presented by the senior author a t the New York Microchemical Society November 19, 1937; t h e S o r t h Jersey Section, American Chemical Society, February 14, 1938; and t h e Metropolitan Section, Electrochemical Society, February 21, 1938.
Electrolytic Silver Wool in the Filling of Microcombustion Tubes W. MAcNEVIN Ohio State University, Columbus, Ohio
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PREPARIXG the universal combustion tube filling of Pregl for use in carbon and hydrogen analysis, it is generally recommended (2, 3) t h a t loosely wound silver wire rolls or wads be used. There are obvious mechanical difficulties in this procedure in preparing a closely packed and uniformly distributed filling, and in addition the wire does not offer the maximum surface per unit weight. Recently Elek (1) has advocated the use of rolls of silver gauze because of their greater surface. It has been found in this laboratory that a “silver wool” produced by electrolysis of metallic silver has several advantages over silver wire. Finely divided silver wool may readily be prepared electrolytically according to well-known methods for purifying silver. A description of a simple electrolytic cell is given by Richards (4). By varying the current through the cell, the crystal size of the deposit can be controlled. In this way crystals have been obtained having a diameter of 0.005 to 0.05 mm. and length of 3 to 8 mm. When removed from the electrolytic cell, the crystals are in the form of closely interwoven clusters, and are conveniently handled with pincers in filling the combustion tube. The interwoven clusters facilitate the uniform packing of the tube. Satisfactory
results are obtained without the preliminary ignitions in hydrogen and oxygen, thus saving time. In general 2 to 3 grams of the crystals are sufficient where 4 to 5 grams of wire (2) are needed. Such crystals present a much greater surface than silver wire of the same dimensions, owing to many imperfections visible under the microscope. Because of the greater surface, the silver wool often outlives the rest of the tube filling. Thus the qualities attributed to electrolytic silver wool are such as to warrant its preparation and use in microchemical laboratories in preference to the wire form.
Literature Cited (1) Elek, A,, ISD. ESG. C H E X , dnal. Ed., 10,51 (1938). (2) Niederl and Niederl, “hZicromethods of Quantitative Organic Elementary Analysis,” pp. 98, 111, Xew York, John Wiley & Sons, 1938. (3) Pregl, “Quantitative Organic Microanalysis,” 2nd ed., tr. by E. Fyleman, p. 27, Philadelphia, Pa., P. Blakiston’s Son and Co., 1930. (4) Richards, T. IT,, and Wells, R . C., J . A m . Chem. SOC.,27, 473 (1905). RECEIVED February 14, 1938.
