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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1899
DESIGN OF THE INSTRUMENT
The channel selector is constructed as a modification of and as an adjunct to a conventional single-channel analyzer. A number of pairs of potentiometers, corresponding to analyzer channels, are mounted in a blank module adjacent to the original analyzer module. These potentiometers set the voltage levels representing the selected boundaries for the respective optional channels. A typical arrangement, one with four additional channels, is shown in Figure 1. The potentiometers are high quality ten-turn units similar in electrical characteristics to those in the original analyzer module. The switches used for selecting the desired discriminator pair are five-pole, two-position ceramic-wafer switches. The potentiometers and switches are built into a circuit which makes it possible to make any one, but only one of the discriminator pairs controlling in the analyzer at one time. This circuit is shown in Figure 2. A brief description of the channel selector described here was given at the 1967 Annual Meeting of the American Nuclear Society, San Diego, Calif., June 1967 (I). Figure 1. Pulse height analyzer and channel selector
RECEIVED for review June 26,1967.
Accepted September 7,
1967. (1) R. A. Johnson, Trans. Am. Nucl. SOC.,10 (l), 86 (1967).
Orig:inal P u l s e -Height Module
*Analyzer
,
Multiple -Channel Selertor Module Dist,riminator Potentiometers P a r s
Channel Selector Switches r-
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1
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8
8
k 3
8
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4
6
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2
3
4
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Figure 2, Circuit diagram of channel selector module 1. The discriminators in PHA module, E, and AE,, are controlling when all selector switches are off 2. Any selector switch is controlling if it is on and all preceding switches are off 3. In above diagram, Ez and AE2 are the controlling discriminators
1900
0
ANALYTICAL CHEMISTRY
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