of the other two arms is unimportant provided the recorder trace is brought on scale. Resistance is then added to the arm (rl or r ~with ) the lower value until, on reversing their position, the recorder trace remains at the same setting. Wire-wound resistors, preferably of the 1 precision type, are recommended for all fixed resistors except those used to adjust the values of the individual units in rda and r4b. These values can be adjusted by shunting precision wire-wound resistors with ordinary carbon resistors of considerably higher value. For the 10,000-ohm data of Table I, the r4a resistors were adjusted by shunting 75-ohm wire-wound resistors with carbon resistors having values of 2000-6200 ohms; it was possible in this way to match the given values within 0.1 ohm. The r4bresistors were made by shunting 7.5-ohm wire-wound resistors with 360-ohm carbon resistors; matching was
within 0.05 ohm for each unit, and within 0.1 ohm for the total combined resistance. Several titrators of this general design have been built and have proved convenient and satisfactorily accurate for general temperature measurement as well as for thermometric titration. The entire circuit, including the mercury batteries, can be housed in a standard 4- X 7- X 12-inch aluminum box. Construction time is perhaps eight to 16 hours, depending on facilities available, experience, and degree of effort in fitting the resistors to the correct values. Circuit data can easily be calculated for other thermistor, recorder, and range combinations. There is no inherent reason why range should be limited to 12 O C, but extension of range would probably involve more complicated circuitry. RECEIVED for review July 3, 1967. Accepted August 31, 1967.
A Small Volume Flow Cell for Simultaneous Polarographic, Coulometric, and Spectrophotometric Measurements Jiri Janata and Harry €3. Mark, Jr. Department of Chemistry, The Unicersity of Michigan, Ann Arbor, Mich. 48104
FLOW CELLS used for electrometric measurements have often been used in industrial control processes and long duration automatically controlled laboratory experiments ( I ) . Also, cells in which the electrolysis solution is set in motion by a stream of an inert gas, such as nitrogen, have been employed in certain polarographic analyses (2); and flow systems for the determination of the electron spin resonance (ESR) spectra of electrogenerated free radicals have proved to be very valuable in electrode reaction mechanism studies and free radical investigations (3). We have employed these flow system principles for the construction of a small volume flow cell in which polarographic, coulometric, and spectrophotometric measurements can be made simultaneously. This flow cell proved to be very helpful in studying the electrode reactions of the aromatic hydrocarbons, corannulene (4) and 4,5-methylenephenanthrene (5). DESCRIPTION OF FLOW CELL CONSTRUCTION This device consists of two main parts: An all glass flow cell (Figure 1, projection in direction of light path) and a light tight metal mounting case (Figure 2) which fits into the sample cell compartment of a Cary Model 14 UV-VIS Spectrophotometer. (If no optical measurements are necessary for a particular coulometric study, the glass flow cell can be used without the metal frame.) The flow system was designed to have as small as possible solution volume (approximately 10 ml) so that it is possible to conserve sample and solution. The electrolysis solution is set in motion in the flow cell by a stream of an inert gas (usually purified nitrogen) which enters the cell through the inlet, L. (1) Z . P. Zagorski, in “Progress in Polarography,” Vol. 2, P. Zuman,
Ed., Wiley, New York, 1962.
(2) P.Zurnan and 0. Manousek, J. Heyrovsky Polarographic In-
stitute, Prague, private communication, 1967.
(3) R. Dehl and G. K. Fraenkel, J. Chem. Phys., 39, 1793 (1963). (4) J. Janata, J. Gendell, C.-Y. Ling, W. Barth, L. Backes, H. B. Mark, Jr., and R. G. Lawton, J. Am. Chem. Soc., 89, 3056
(1967). ( 5 ) J. Janata, J. Gendell, R. G. Lawton, and H. B. Mark, Jr., University of Michigan, Ann Arbor, Mich., unpublished data,
1967. 1896
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ANALYTICAL CHEMISTRY
The bubbles of gas rising in the spiral tube carry along the solution, which results in a uniform counterclockwise flow (provided a uniform gas flow is maintained). The gas bubbles then separate from the solution in the cell head (the gas escapes from the cell through outlet, M) and the solution then flows into the lower part of the system toward the working electrode. The dropping mercury electrode (DME), indicated by symbol A , an auxiliary gas inlet, B, and the salt bridge from the external reference electrode, C , are introduced into the cell head through the Teflon cell cap, D . A flexible polyurethane strip, E, is wrapped around the neck of the cell head to hold it in the center of the split Teflon ring, F, which is then slid sideways into the top of the metal case. A slotted metal piece, N , is then slid into the metal case top. This arrangement gave a flexible yet firm support for the glass body of the flow cell. Although this flow cell has been used only for polarographic (DME) studies, a platinum wire micro electrode, G, is sealed permanently into the glass wall of the top compartment (or section) for possible electro-oxidation studies. The lower part of the flow cell is compartmented. The Pt auxiliary electrode, I , is contained in one of the compartments which isolates the electrolysis solution under study from mixing with the solution of the auxiliary compartment. The auxiliary electrode compartment has two stopcocks which allow for separate draining and filling of this compartment. A sintered glass disk, W,separates the auxiliary electrode and flow section of the cell, but, of course, it allows electrical contact between the two compartments. The exhaustive electrolyses take place at the mercury pool electrode, J . The electrolysis solution flows across the mercury pool electrode surface, into the quartz optical flow cell, K (Precision Cells Inc.), and, then, up to the upper cell head for recirculation. It was found convenient (to prevent accidental dropping of the optical cell) to cement the ground glass inlet and outlet joints of the optical cell with a small amount of epoxy resin. Experimentally it was found that the time interval between initiation of production of colored product at the mercury pool working electrode and its subsequent arrival at the optical cell was about 0.5 to 1.0 sec using a nitrogen glass flow of 150 to 30 ml/min, respectively. The rate of flow of the nitrogen gas pump can be regulated by the stopcock, 0.
I
Figure 1. Diagram of flow cells A. B.
C.
D. E.
F. G. N. I. J.
K. L. M. N. 0.
Dropping mercury electrode (indicator electrode) Nitrogen inlet Salt bridge from reference electrode Teflon cell cap Polyethylene sheet Teflon holder Pt indicator electrode Sintered glass disk Pt auxiliary electrode Hg pool working electrode Quartz spectrophotometer cell Nitrogen gas circulating pump inlet Nitrogen gas outlet Slotted top insert Nitrogen gas pump flow regulator stopcock
As mentioned previously, the volume of the electrolysis solution in the flow cell (not counting the volume of solution in the auxiliary electrode compartment) is about 10 ml. Figure 2 shows how the flow cell is mounted onto the metal case which fits into the Cary Model 14 Spectrophotometer. Vacuum rubber tubing connects the flow cell with the gas inlet and outlet hose connections on the metal case. The electrical connections for the Hg pool, auxiliary, and Pt wire electrodes are made through the top of the metal cases through standard banana jacks. A stopcock in the mercury pool working electrode section is used to drain off the excess collected mercury flowing from the DME during exhaustive electrolysis. The best way of cleaning the flow cell was to attach an aspirator (through a trap) to the bottom stopcock of the work-
Figure 2. tometer
Details of mounting case for Cary 14 Spectropho-
ing electrode section and to flush the whole system repetitively with chromic acid cleaning solution and distilled water. The auxiliary electrode compartment is cleaned the same way by attaching the aspirator to the bottom stopcock of this compartment. DISCUSSION The polarograms obtained under the flow conditions were reproducible; the limiting currents were proportional to concentration and had normal shapes. If, however, one desires to obtain diffusion limited currents (under quiescent conditions), it is only necessary to stop the gas pump flow. In this case, a three-way stopcock in the external gas line stops the flow of gas to the pump, inlet L, but then directs the gas through inlet, B, maintaining a nitrogen atmosphere over the solution in the cell head. In the coulometric studies, the relation between the total charge, Q , passed and the limiting current of the electroactive species at any time, t, is given by the modified form of Faraday’s law :
( - n k c o Q)
id = ido 1
___.
where ido is the limiting current at the beginning of the electrolysis ( t = 0), Y is the volume of the electrolysis solution, Co is the initial concentration of the electroactives species, F is Faraday’s constant, and n is the number of electrons involved VOL. 39, NO. 14, DECEMBER 1967
0
1897
in the electrode process. A plot of the instantaneous limiting current as a function of total charge, Q,using an X-Y recorder yields a straight line plot (provided no change in the reaction path occurs during the electrolysis) having a slope that is inversely proportional to the number of electrons, n, evolved in the reaction (4). Furthermore, if C" is known and as the overall Q can be calculated from either spectrophotometric or electrometric measurement, the exact degree of completion of the electrolysis can be determined. As stated before, although this particular cell is specifically designed for cathode reaction studies, it is possible to place a
platinum gauze electrode in the upper section of the cell to study anodic reactions. In that case, of course, the transport time is much longer. One could, of course, build a cell with a Pt-gauze electrode in the place of the mercury pool, J, if necessary. It is obvious that both the electrochemical and the electromechanical elements of this device can be modified as the problem requires. RECEIVED for review August 2, 1967. Accepted September 6, 1967. Research supported in part by grants GP-6425 and GP-6420 from the National Science Foundation.
New Practical Construction of Platinum Rotated Disk Electrodes Lynn S. Mareoux and Ralph N. Adams Department of Chemistry, Unicersity of Kansas, Lawrence, Kan. 66044
THE ROTATED DISK electrode (RDE) is unique in solid electrode methodology. The voltammetric currents obtained at such surfaces have been derived rigorously from hydrodynamic theory. Actual RDE's adhere to these rigorous equations often to better than 1%. Carbon surface RDE's are easily constructed and give reliable results ( I , 2). However, there is a great need for an inexpensive, easily constructed platinum RDE. Platinum can be force-fitted into Teflon or sealed in glass and ground to form a planar disk surface. Both techniques are difficult and the latter case requires an expert glassblower. Frequently several attempts must be made to produce a usable RDE. Platinum can be sealed satisfactorily in various potting resins and machined but such electrodes are, in general, unusable in nonaqueous media. An extremely useful platinum RDE can be constructed from a commercial platinum electrode. The results with this RDE are both accurate and reproducible and it is usable in all media. The RDE is constructed from a Beckman platinum button electrode (No. 39272) in the following manner. The electrode is carefully sawed with a carborundum wheel approximately 5 crn from the end containing the electrode surface. Care is taken not to saw the connecting wire within the electrode shell. This wire is then clipped in such a way as to allow maximum length. In the case of the older commercial electrodes, the black potting material can be removed with acetone. The newer electrodes do not contain this material. The inside of the electrode cylinder is then coated with one of any of the readily available, commercial, quick-drying cements. The cement-coated cylinder is then shaken with fine sand until the inside is coated with an abrasive layer. This step is made necessary by the fact that the potting resin (Quikmount, obtained from Fulton Metallurgical Products Gorp.) later used to set the electrode shaft into the glass cylinder, was found to shrink slightly with age and, in so doing, to pull away from the cylinder. The sand provides an abrasive surface so that when shrinkage occurs, the shaft and resin cannot turn within the cylinder. Certainly there are potting materials that do not behave in this manner and these can be used to advantage. The present method has been found to be both satisfactory and expedient. (1) H. S. Swofford and R. C. Carman 111, ANAL. CHEM.,38,
966 (1966). (2) K. B. Prater and R. N. Adams, Ibid., p. 153.
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ANALYTICAL CHEMISTRY
I""
t
/ 0 25
0
0 50 i
a . SCE
1.m
j '
Figure 1. RDE polarogram of 5,10-dihydro-5,10-dimethylphenazine in acetonitrile
Table I. Limiting Currents ( i 1 3 for Both Oxidation Waves Qf 1.01 X 10-3M 5,1O-Dihydro-5,10-dimethylphenazine hi, N A sec1'2 rotations all2 radl/2 N seconds First wave Second wave
40
19.9 19.7 19.8 19.7 19.8 19.7 19.6
50
19.6
5
10 15 20 25 30
38.8 39.2 39.4 39.4 39.2 39.0 39.2 39.1
Table 11. Limiting Currents for Oxidation of 9.93 X 10-4M Trianisylamine rotations illm N A sec1!2 -_ Nu1/2 radl/Z second 5 10 15 20 25 30
40 50
15.7 15.7 15.6 15.5 15 6 15.6 15.5 15.5
