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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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
In order to center the shaft in the cylinder, a piece of 2inch lumber is clamped securely to a drill press, and a hole, exactly the same diameter as the outer electrode cylinder, is drilled to a depth of about 3 cm. The bit is removed and the electrode cylinder is placed in the freshly-drilled hole. A steel shaft about 10 cm long is clamped into the chuck of the drill press. A steel shaft is preferred because it is less apt to become bent in service. Slight irregularities in the electrode shaft may lead to large eccentricities during rotations. The lead wire is then pulled out of the way, the potting resin prepared and poured into the cylinder, and the shaft is then lowered via the drill press to the desired depth. Finally, the lead wire is wrapped around the shaft and secured with electrical tape. The procedure outlined above more or less ensures centering. Organic systems possessing well-defined electrode reactions were used to prove that this type of RDE behaved according to the Levich (3) equation (ill,,, = 0.62 nFAD2j3 ~-1/%~1’2C, where v is the kinematic viscosity in stokes and w is the rotation rate in radian second-’ and all other symbols have their standard electrochemical significance). The RDE polarogram of a 1.01 X lO-3M solution of 5,10-dihydro-5,10dimethylphenazine ( 4 ) obtained at w = 126 radian seconds-’ in acetonitrile, 0.1M tetraethylammonium perchlorate (TEAP) is shown in Figure 1. The limiting current obtained from both the first and second waves for this compound are shown as a function of w * i Zin Table I. The results for the oxidation of 9.93 x 1O-aM trianisylamine ( 5 ) in acetonitrile, 0.1M TEAP, are shown in Table 11. The value of the constant ili,/w1’2C was calculated at several concentrations of trianisylamine and these results are shown in Table 111. These data were all obtained by means of standard techniques which have been previously described (2).
All of the above data are in complete agreement with theory and are representative of a great deal of data obtained by means of this electrode. N o rigorous hydrodynamic testing was carried out; however, it has been shown in this laboratory that the electrode of cylindrical design yielded the same results as those built to more rigorous hydrodynamic standards (2). Beckman electrodes of the older design are somewhat conical in shape and are probably more prone to edge effects because the area of the inert material (glass) around the disk is much less. The present commercial design has a glass shroud which is quite adequate. It has also been noted in the case of some recently acquired Beckman electrodes that the platinum electrode surface was not absolutely centered in the glass shroud. Although RDE’s constructed from these electrodes function satisfactorily for analytical purposes, they should be avoided when making careful measurements. Numerous platinum RDE’s of this type have been prepared and the results have been excellent. None of the electrodes have ever leaked or been in any way unsatisfactory. The cost of the commercial electrode is nominal and the conversion is simple and rapid. The performance of these RDE’s has been superior to any others constructed in this laboratory.
(3) V. G . Levich, “Physicochemical Hydrodynamics,” Prentice Hall, Englewood Cliffs, N. J., 1962. (4) R. F. Nelson, D. W. Leedy, E. T. Seo, and R. N. Adams, Z . Anal. Chem., 224,185 (1967). ( 5 ) E. T. Seo, R. F. Nelson, J. M. Fritsch, L. S . Marcoux, D. W. Leedy, and R. N. Adams, J . Am. Chem. Soc., 88,3498 (1966).
RECEIVED for review June 16, 1967. Accepted September 11, 1967. Work supported by the National Aeronautics and Space Administration research grant NsG-298 to the University of Kansas.
Table 111. RDE Data for Trianisylamine Oxidation (i,im/dT)
Concentration, M 4.07 x 10-E 8.14 x 10-5 2.09 x 10-4 4.17 x 10-4 6.11 x 10-4 8.14 x 10-4
il im/w
x
0.690 5 0.006 1.35 & 0.01 3.24 Z!C 0.02 6.28 i.0.04 10.0 i 0.1 13.1 5 0.1
10-”M
1.69 1.66 1.58
1.64 1.64 1.62
Channel Selector for a Single-Channel Pulse-Height Analyzer Ralph A. Johnson Shell Development Co., Emeryville, Calij. Donald C. Blair 1055 Locust Street, Livermore, Calij.
IN ACTIVATION ANALYSIS laboratories and other laboratories using nuclear methods, it is often desirable to use single-channel, pulse-height analyzers for a variety of determinations, each requiring a different analyzer channel. When a variety of determinations is carried out concurrently or sequentially within a relatively short interval, it is necessary to make a rapid and precise change in the channel setting between each run. The channel selector described here accommodates a conventional single-channel analyzer to this kind of usage by offering through one analyzer unit and a turn of a switch a choice of any one of a number of accurately pre-set channels. The advantages that it offers are better reproducibility of the window settings and greater speed and convenience in making the settings. In the laboratory without an ADC-type, multi-channel analyzer, this selector offers some of the capabilities of several
working analyzer modules at expenditures of cost and space which are but a little more than those of only one analyzer unit. In the laboratory with both an ADC-type, multi-channel analyzer and differential-discriminator, single-channel analyzers, the latter is preferred in many applications to relatively simple spectra. The single-channel analyzer offers a dead time which is much smaller and which is more easily evaluated in certain applications. The discriminator-scaler combination provides a convenient integration, accumulation, and readout of counts falling within a region of interest. The optimum adjustment of the discriminators is easily accomplished with the aid of the multi-channel analyzer in the coincidence mode. Furthermore, single-channel analyzers are well adapted for use in pulse-coincidence analysis and in sample-to-monitor comparisons in which two independent detector-amplifier-analyzer units are operated simultaneously. VOL. 39, NO. 14, DECEMBER 1967
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