Quantitative Determination of Carbon Suboxide by Gas Chromatography SIR: The presence of carbon suboxide (c302) in a reacting gas system has recently been detected by its infrared emission ( 2 ) . Published literature does not however, appear to contain any report of its detection by gas chromatography. We have succeeded in doing this, using a Beckman GC-1 chromatograph with the usual filamenttype sensing cell and a 1-cc. sample size. The original column used in this work was an 18-foot length of l/r-inch copper tubing packed with Apiezon N grease on a firebrick substrate. The CaOn retention time was too long (ea. 33 minutes) on this column, so it was shortened to 7.5 feet. The packing was prepared by dissolving the grease in benzene and then adding the substrate (30 X 60 mesh) in proportions such that the final packing would contain 23% by weight of Apiezon N and 77% by weight of the firebrick. After thorough mixing, the benzene was removed a t slightly elevated temperature and reduced pressure. The ( 2 3 0 2 was prepared by a modification of the method of Stock and Stoltzenburg (3), and was checked for purity by its mass spectrum and its infrared absorption spectrum ( 1 ) . No impurities were detectable by either method. Since ( 2 3 0 2 reacts readily with water, moisture was carefully excluded from the system. Decomposition of the c302 prior to analysis was avoided by keeping its partial pressure between 10-3 and 10-2 atm. We have found no evidence of decomposition during periods of time up to 2 hours when ( 2 3 0 2 is kept in a closed vessel a t a partial
Figure 1. Plot of concentration vs. area under C302 peak Sensing cell operated a t 250 ma.
AREA UNDER PEAK in mm?
pressure as high as 25 cm. of Hg, a t room temperature. Helium a t 15 p.s.i.g. is used as the carrier gas. With the column a t room temperature, the retention time of c302is 6 minutes. Under the esperimental conditions, the column retention time for air is 1.2 minutes, followed closely by CO, with COz appearing a t approximately 3 minutes. I n Figure 1 is shown a plot of CaO2 concentration vs. peak area, taken with the sensing cell operating a t 250 ma. A fourfold increase in the peak area has since been obtained by changing the current to 400 ma. Under the latter conditions, a peak height of 16 mm. and a halfheight width of 11 mm. are obtained when the C302concentration is 24.92 X 10-8 mole per cc. The peak is very symmetrical.
LITERATURE CITED
(1) Long, D. A., Murfin, F. S., Williams, R. L., Proc. Roy. Soc. (London) A223,
251 (1954).
( 2 ) Roblee, L. H. S., Jr., Agnew, J. T., Wark, K., Jr., Combustion and Flame 5 , 65 (1961). (3) Stock, A., Stoltzenburg, H., Ber. 50, 498 (1917).
Department of Fuel Technology THOMAS J. HIRT College of Mineral Industries HOWARD B. PALMER The Pennsylvania State University University Park, Pa. RECEIVED for review October 23, 1961. Accepted November 22, 1961. Work supported by the U. S. Atomic Energy Commission under contract AT(30-1)1710.
Rotating Disk Electrodes SIR: ‘The use of rotating disk electrodes has been developed to a high degree by the schools of Levich (9I I ) , Frumkin (S), Riddiford (2, 6, 12)) and others, but has received practically no attention from chemists in the United States. The fact that no translation of Levich’s monograph has appeared may account in part for this lack of appreciation. Disk electrodes have not been easy to fabricate, and it is felt that the full potentialities of rotating disk electrodes have not been
154
ANALYTICAL CHEMISTRY
viewed in the light of modern electroanalytical practice. We wish to emphasize the unique advantages of these electrodes for a wide variety of electrochemical studies. Simplicity in electrode fabrication can be achieved b l using carbon pastes for the disk surface. The most intriguing aspect of the rotating disk electrode (RDE) is that the convective mass transport problem was solved rather rigorously by Levich (10, I I ) via hydrodynamics and can be
verified to a high degree in euperimental applications. As far as is known, only one other convective electrode, the point probe of Jordan, Javick, and Ranz, has the similar advantage of a rigorous limiting current equation supplemented by experimental verification ( 7 ) . The latter electrode is, however, not ideally suited for routine applications. The fundamental limiting current equation for the RDE is given by Levich as: ZL~,,, = 0.62 nFACbD2/3v-%”2 (1)
where
Cb
= bulk concentration of electro-
active species in moles per liter v = kinematic viscosity of the solution in sq. cm. per second w = angular velocity of the disk given by w = 2nN with AT = revolutions per second iL,, = limiting current in milliamperes, and the remaining terms have the usual significance.
'Phis equation has been modified slightly by Gregory and Riddiford (6) and most recently by Kholpanov (8),who corrected the anomaly of Equation 1 which = 0 mhen ~3 = 0. In indicates iLlm reality, the RDE a t w = 0 approaches unshielded linear diffusion conditions and has a finite current behavior. For all applications but routine analysis, the requirements are a device for precise control of rotation speeds and suitable disk electrodes. The former can be a relatively simple motor driven stirrer with an attached tachometer generator to provide a feedback loop for a controller. Details of a suitable apparatus will be given elsewhere (6). The carbon paste electrodes developed in this laboratory function very me11 when used as rotating disks. Figure 1 illustrates two such electrodes. Details of the electrode construction and carbon paste preparation have been given (1, 13). For use as rotated disks, the Teflon plugs are mated to a glass rod which serves as the rotating spindle. These electrodes have been used up to
35
T
A B Figure 1. Typical carbon paste rotating disk electrodes A
B.
Single disk W. Electrical connection to disk G. Glass tubing spindle, 6-8 mm. diameter T. Teflon holder, typical diorneter 1 2 mm. D. Carbon paste dlsk, typical diameter 2-8 mm. Ring-disk or double disk WJ. Electrical connection to inner disk Wr. Slip ring electrical connection to outer ring (wire shielded where contact with solution is possible) T. Teflon holder, typical diameter 12 rnm. D. Carbon paste inner disk, typical diameter ca. 3 mm. R. Carbon paste outer ring, typical width 1-1.5 rnm.
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30 revolutions per second without any serious deviations from theory. Figure 2 shows how well measurements obtained with such electrodes conform to Equation 1, which predicts a linear The dependence of iLlm on "I2. potential applications of such RDE's are summarized below. Routine Analysis. For analysis via i L i m , it is unnecessary to vary rotation velocity. The usual 600 r. p. m. constant speed equipment used for rotated wires, etc., can be used. At 600 r. p. m., the limiting current for the oxidation of 1 X 10F4Mp-phenylenediamine in pH 2.4 buffer is about 5.4 times greater than the peak current obtained under stationary conditions. The important criterion for sensitivity is, however, not merely the total current, but the ratio of faradaic to residual current. With the carbon paste disk, the residual current is practically negligible for anodic reactions and almost independent of stirring rate (ca. 0.02 pa. for stationary or rotated electrodes). Determination of Diffusion Coefficients. Using arbitrary fixed rotation velocities, D values can be obtained with excellent precision and are in good agreement with values obtained by other techniques. Table I summarizes some data for 3,3'dimethoxybenzidine (0-dianisidine) and compares these with those obtained by other electrochemical methods. Applications to Electrode Mechanism Studies. The carbon paste RDE's have been used to study the mechanism of the anodic oxidation of N-methvl- and N.N-dimethvlaniline with considerable success. Kinetic parameters such as the order of the electrochemical reaction can be evaluated without difficulty. Equation 1 can be modified to account for processes controlled by both mass transfer and charge transfer (irreversible reactions). The mass transfer can be extended to high rates by increasing rotation speeds before turbulence renders Equation 1 sible to study the electron transfer kinetics of processes which are normally considered reversible. The femme fatale of RDE's in electrode mechanism studies is the double disk or ring-disk shown in Figure 1. This electrode is patterned after the design of Frumkin and others (3). Consider the over-all anodic electrode process: A +. B nle at E, (2) A+B-rC (3) C -P D nie at E2
