greater distinction should be made between the results for methane (or ethane) o n 4A and on 5A zeolite. The peaks observed on 4A zeolite are primarily dead-space peaks because the retention volume to the peak maximum is independent of temperature and the contribution from external adsorption would be negligible a t this temperature. The observed tailing represents a small amount of material entering the micropores but as the rough analysis given above indicates, the diffusion Iates are too low to give a true peak. Under these conditions, it does not seem useful to use the peak mean as a measure of the retention volume because such a value reflects a combination of the two processes. I n the case of 5A zeolite, the peaks seem to be primarily normal chromatographic peaks corresponding to micropore equilibration. This is confirmed by the observed temperature dependence of the retention volume, the rough agreement with retention volumes on CaX zeolite (for ethane), and the conclusions from Figure 2. In this case the measurement of the retention volume to the peak mean in conformity to recent theory is probably of more significance than measurement t o the maximum. The retentjon volume to peak mean for ethane on 5A
zeolite shows a small decrease with increasing flow rate. The maximum plate height observed for ethane is still only 0.5 cm orL/40 which is one tenth of the value a t which we assumed the virtual disappearance of the peak. This small observed decrease in retention volume may be a preliminary indication of incomplete equilibration but the data are inconclusive. It would appear that the gas chromatographic technique for measuring intracrystalline diffusivities can be extended to overlap the more conventional techniques based on rate of adsorption such as used by Brandt and Rudloff. It would be desirable to carry out experiments to compare the two methods for the same system. H. W. HABGOOD W. R. MACDONALD
Research Council of Alberta Edmonton 7 Alberta. Canada
RECEIVED for review December 12, 1969. Accepted February 3, 1970.
AIDS FOR ANALYTICAL CHEMISTS An Automatic Gas Sampling Device David C. Weber and Edward J. Spanier Department of Chemistry, Seton Hall University, South Orange, N . J. 07079
INSTUDIES of the polymerization of benzylchloride ( I ) , a device was required for sampling a gas stream at fixed intervals over an extended period of time. While such an apparatus has been reported (2), it is relatively costly and difficult to construct. A simple and inexpensive sequential sampler is described below. Gas Sampling System. The principal portion of the apparatus, Figure 1, consists of a 12-inch length, 1-inch o.d., l/aa-inchwall, of copper pipe which serves as the gas manifold. Two rows of holes, 90" apart and inch in diameter, were drilled in the manifold starting 1 inch from the right end. The distance between successive holes in either row was 1 inch, o n centers. Thus the distance between consecutive holes in alternate rows was inch. Seats, inch in diameter, concentric with the holes were machined to accept l 1 / 2 inch lengths of 'I4 inch 0.d. heavy wall copper tubing. These tubes were silver-soldered to the manifold and served as exhaust ports for the gas stream. The manifold was rigidly held to an aluminum backplate (24 X 8 X ' 1 4 inches) by two aluminum straps. A piston, Figure 2, travels through the manifold and directs the gas stream through successive exhaust ports. The piston, inch 1'Iz inches long, was turned from solid Teflon stock, (1) F. E. Caropreso and E. J. Spanier, J. Poly. Sci. (AZ) 7, 2679 (1969). (2) W. R. Parker and N. A. Tuey, J . Air Pollution Control Assoc., 17 (6), 388 (1967). 546
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
n OIil
Figure 1.
Gas sampling device
D. Drag H. Handle M. Manifold N. R.
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under the inside diameter of the manifold. A hole inch in diameter) was drilled down the center of the piston. Two additional holes, of the same diameter, were drilled through the wall of the piston perpendicular to each other and also perpendicular to the axial hole, all meeting at the center. A tapered seat (5/a2 inch a t maximum diameter) was machined, inch deep concentric with the axial hole, and into this was fitted a piece of glass tubing (approximately 3 inches long
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Figure 2. Teflon piston
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2 and l,i, inch 0.d.) held in place by a friction fit. I n the present device two radial holes were used to eliminate any pressure drop across the manifold ; however, in most applications one would be adequate. Two semicircular grooves (’116 inch across and inch deep) were machined around the piston ‘14 inch on either side of the two radial holes. Buna-N ‘‘0”-Rings were custom made (Locktite 0-Ring Splicing Kit). Onto the solid end of the piston was fitted a n aluminum baseplate with a raised hub which connected the piston to the driving mechanism of the system. To ensure free movement’of the piston, the wall of the manifold was lubricated with a light coating of Dow Corning Stopcock grease. A piece of steel stock (13 X ‘14 X ”14 inches) was machined in a saw-tooth fashion for the middle 10 inches. This served as a ratchet to drive the piston through the manifold. The distance between teeth was inch and the depth of the perpendicular cut was 7J16 inch. A track to support the ratchet was made from a 30-inch long “L” shaped piece of aluminum. The wide portion of the “L” was 1 inches while the narrow side was ‘1. inch. A slot inch wide) was cut through the narrow portion of the “L” shaped track and two threaded studs ( 3 / 4 inch long X inch in diameter) were attached to the bottom of the cogged bar and passed through the slot. Nuts were attached to prevent the bar from being pulled away from the track but at the same time the bar was free to travel smoothly and without binding along the track. The length of the slot limited the travel of the bar to 10 inches. A handle was attached to the side of the bar to permit manual positioning. The track was then fastened to the backplate in such a manner that, if it were extended, the center of the ratchet would travel parallel to and within the walls of the manifold. The ratchet was attached to the piston by a steel strip (’/,! X 1 / 1 ~X 12 inches) one end of which was bolted to the bar and the other end to the baseplate o n the piston. In both cases the linkage was allowed to pivot freely. The holes in the piston wall were aligned with one of the exhaust ports of the minifcld and the clamps holding the manifold tightened to prevent further movement. During operation the bar was advanced by means of a lever a r m positioned above it. The lever was cut from the same stock as the ratchet and was attached to the backboard while at the same time being free to swing in a plane parallel to and inch above the backboard. The forward motion of the lever was restricted by a stop placed in such a manner that from the time the lever engages the sawtooth bar until it hits the stop the bar will have travelled 1/2 inch. A spring then returns the bar to its original position. A stop was positioned to limit backward motion to a 2-inch arc. The driving end of the lever was fitted with a pawl catch which was rigid as the lever traveled forward and engaged the bar but released and
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Figure 3. Wiring diagram for timing circuit Internal relay External relay Clutch D. Delay tube E. Solenoid of external relay M . Motor for clock A. B. C.
passed over the next sawtooth as the lever returned to its starting position. The solenoid was positioned in the backboard and its plunger was attached to the lever by means of a piece of notched stock (1 ’/? X 3/8 X ‘ 1 2 inches). Because of the force exerted by the solenoid, it was connected to the lever 2 inches from the fulcrum. It was also necessary to install a drag, a piece of spring steel pushing against a small steel ball which in turn pushed against the ratchet, to prevent the bar from being carried too far forward by the momentum imparted by the solenoid. To further control the motion of the bar, only 95 volts was applied to the solenoid. The driving solenoid itself, was reclaimed from a discarded automatic washing machine; however, a Guardian Relay, Type A C No. 18, or its equivalent, could also be used with minor modifications. Timing Mechanism. The mechanism used to energize the sampling system was constructed from a commercial electric timer, Figure 3 (Cramer Type 412, Time Delay Relay, 120 minutes). The timer contained a n adjustable clock, an electric clutch to engage the clock and a normally open internal relay which would be closed a t the end of the timing cycle. Line current was connected to the clock motor, t o the internal relay of the timer and also to the switch section of the external relay (Potter and Brumfield, spdt., Type KA, llOV 60 hz). The timer clutch was operated through the external relay which was modified slightly by bending the relay contacts slightly closed to obtain a faster response. When the external relay was opened, the current engaged the clutch and started the timing cycle. When the cycle was complete, the internal relay closed and passed current to the solenoid of the external relay causing it to close. Current then flowed to the solenoid of the driving mechanism through a delay tube (Edison Model 501, B1922, 3 second). As the external relay closed, the clutch of the timer disengaged, the clock reset itself, and the contacts of the internal relay opened. Since it was necessary to supply power to the driving solenoid ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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while the piston advanced, a delay tube was incorporated in the circuit. The tube contacts were normally closed and passed current to hold the external relay closed, thus powering the driving solenoid, despite the fact that the internal timer relay opened. After approximately one minute the delay tube opened, thus opening the external relay and deactivating the driving mechanism solenoid. Simultaneously the timer clutch would be engaged to start another timing cycle. Because of the recycling time and the time needed for the solenoid to advance the piston, 1 minute was the shortest time possible between successive samples; however, with modification this time could be shortened. Polyethylene tubing connected the piston and the reaction system. The exhaust ports were also fitted with lengths of polyethylene tubing which in turn were attached to short
lengths of glass tubing which were then immersed in 125-1111 Erlenmeyer flasks. G a s would now flow, 10-20 cc/min., from the reaction vessel through a mercury trap, (keeping water out of the reaction vessel) through the manifold and piston to exit through the exhaust ports and then be expelled into the water. The sampling device would switch the gas flow to successive exhaust ports after the set time interval and the fractions collected could be subsequently assayed.
REcEivED for review September 8, 1969. Accepted January 22, 1970. We gratefully acknowledge an Undergraduate Research Participation Grant from the National Science Foundation for DCW as well as support from The Research Corporation.
The Tubular Carbon Electrode W. D. Mason' and Carter L. Olson College of Pharmacy, The Ohio State University, Columbus, Ohio 43210
DEVELOPMENT of small tubular electrodes for use in flowing streams was first reported by Blaedel et al. ( I ) . This tubular platinum electrode was applied to the enzymatic determination of glucose and illustrated the convenience of these specially designed electrodes with very low holdup volume for use as analytical monitoring devices in flowing streams (2). Since that time several papers have appeared describing thebehavior andtheory of tubular electrodes (3-6). Oneof the immediate shortcomings of the tubular electrode was that it was constructed from platinum and had all of the problems associated with platinum surfaces. I n order to extend the utility of the tubular electrode, Osterling and Olson prepared and evaluated a mercury-coated tubular platinum electrode (7). A tubular carbon electrode has been described by Sharma and Dutt, but it is a rigid assembly similar in construction to the early tubular platinum electrodes (8). Because a mercury surface has a very limited anodic voltage range and the fact that construction of tubular platinum electrodes presents some fabrication problems, the development and evaluation of a tubular carbon electrode that is easy and rapid to construct was undertaken. EXPERIMENTAL Instrumentation. Voltammetric studies were carried out using a Heath chopper stabilized polarograph. Recordings were made o n a Varian Model GlOOO strip chart recorder. Solutions were pumped through the electrode by means of a
Present address, School of Pharmacy, University of Georgia, Athens, Ga. (1) W. J. Blaedel, Carter L. Olson and L. R. Sharma, ANAL. CHEM., 35, 2100 (1963). (2) W. J. Blaedel and Carter L. Olson, ibid., 36, 393 (1964). (3) W. J. Blaedel and L. N. Klatt, ibid., 38, 879 (1966). (4) L. N. Klatt and W. J. Blaedel, ibid., 39, 1065 (1967). ( 5 ) Zbid., 40, 512 (1968). (6) T. 0. Osterling and Carter L. Olson, ibid., 39, 1546 (1967). (7) Zbid., p. 1543. (8) L. R. Sharrna and J. Dutt, Ztzdian J. Chem., 6 , 593 (1958).
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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Harvard Apparatus Model 600-1200 peristaltic pump. By placing the pump downstream from the electrode, the solution can be pulled from the sample through a short piece of glass capillary tubing, minimizing the entrance of oxygen into the solution. Electrodes and Cell. The tubular carbon electrode ( W E ) is constructed by press-fitting a tubular piece of wax-impregnated spectroscopic-grade graphite into a specially designed holder (Figure 1). Several factors were considered in arriving at this cell design. The TCE is constructed so that it can be assembled easily and can be quickly replaced if the electrode surface becomes damaged or fouled. In addition it is important to obtain a smooth bore to and through the electrode so that a parabolic velocity profile with laminar flow is established before the solution enters the electrode which would not be disturbed by turbulence as the solution enters the electrode in order that derived equations be valid. This was accomplished conveniently by placing a solid plug in front
