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AIDS FOR ANALYTICAL CHEMISTS
Solenoid Operated Switching Valve for Automatic Titrators Jorge A. Blay and Lyndle J. Betz Celanese Chemical Company Technical Center, P . 0. Box 9077, Corpus Christi, Texas 78408
DURING THE PAST few years, the increasing influence of instrumentation and the tendency to automate analytical methods have created needs for new electronic and mechanical devices adapted to laboratory requirements. During the design of automatic titrators built for specific control analyses, one of the modules needed was a n automatic switching valve to change the connections of the titrant pump from the delivery line to the titrant reservoir. The requirements for such a valve are complete inertness to corrosive liquids, minimum internal volume, no changes in internal volume when switching, no mixing cavities o r dead volume, low energy requirement for automatic operation, compactness, and low cost, Commercial valves available failed to meet one o r more of these criteria. The valve described in this paper fulfills all the conditions described above using standard components, thus decreasing to a minimum the labor involved. The valve is made of Teflon (Du Pont) and Kel-F, the internal volume is 10 pl, it has no dead volume or mixing cavities, it is compact (3 X 1.5 X 1.5 inches) and is low in cost ($18 for parts). The energy requirements were reduced substantially by using inch in diameter and reducing the Teflon septums only stroke to less than '116 inch. With the lower friction and shorter stroke, an inexpensive solenoid with spring return could be used to operate the valve.
Solenoid C o i l
Solenoid Plunger
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Valve Body
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Teflon S e p t u m L u e r Connector
CONSTRUCTION
An exploded view of the valve components is given in Figure 1 . The valve body is made from a a/4-inchslice of Chemfluor T F E extruded heavy wall tubing (Chemplast Inc.), 1.5-inch o.d., I/e-inch i.d. The plunger is a 1 inch long unplasticized Kel-F rod (Chemplast Inc.) '/%-inch diameter. The male luer adapters are obtained from Chromatronix (Catalog No. 107B8). The solenoid operator with return spring can be any solenoid with a t least 7 pounds pull at l/la-inch stroke. The one we used is a Stearn Industrial solenoid model 4-202301 ($5.70) 110 VAC coil. The Kel-F rod is turned and polished to move freely inside the Teflon body. A '/s-inch hole is drilled o n center about 3/s-inch into one end of the Kel-F rod, and a corresponding hole is drilled into the end of the solenoid plunger. The two holes are then tapped and the rod connected to the plunger with a n 8-32 N C screw. The stroke is adjusted by turning the Kel-F rod o n that screw. Three holes are drilled 120" apart and on the same plane inch from the front of the Teflon body and tapped to accept the male luer connectors. Two more holes l/&nch diameter are made o n the front across the thickness of the body to hold it o n the solenoid frame (Figure 2). The septums are cut from a 'is-inch Teflon sheet, slightly less than a ljr-inch diameter to fit under the luer connectors (Figure 1). The center hole is made with a 17-gauge needle with the tip cut flat and sharpened. Before the septums and luer connectors are installed, the stroke is adjusted so the hole to be drilled in the Kel-F rod
Figure 1. Exploded view of three-way solenoid valve
Figure 2. Finished solenoid valve
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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Lurr Connrc tor
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guide plate prevents rotation of the Kel-F rod once it is adjusted (Figure 1). The 0.030-inch holes on the Kel-F rod are made with a No. 68 drill bit introduced through the male luer connector and a ?/*-inchpiece of 17-gauge tubing used as a guide. First, with the solenoid deenergized, a hole on the top and another on the right connector are drilled until they meet at the axis of the Kel-F rod. Then the solenoid is energized and the operation repeated drilling on the top,and the left connector, Finally, the Teflon septums and luer connectors are set tight and the plunger moved manually several times so the septum surface seals perfectly against the Kel-F rod. Then the connectors are loosened until the plunger can be moved freely by the return spring.
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RESULTS AND DISCUSSION
Figure 3. Detail of the male her connector alignment with the holes on valve plunger Note that a hole is either aligned with septum hole or blocked by the speetum surfac
under the top connector is centered when the solenoid is deenergized and moves to 0.010 inch from the edge when energized (Figure 3). A sector cut at the end of the rod and a
The valves described have been in operation for almost two years with only occasional tightening of the Teflon septums. They are used with 5N solutions of perchloric acid or potassium hydroxide without problems. The liquid flows are between 0.5 and 2.5 ml per minute with a pressure drop of less than a psi. The seals have been checked up to 100 psig without leaks. If Viton septums are used insted of Teflon, the valve will hold 200 psig pressures without leaks. The three-way valve adapts itself to any automatic reagent dispenser o r reagent pump and can be easily connected for automatic refill from a reagent reservoir. This valve can also be made as a two- o r four-way valve by drilling the '/,,-inch holes distributed symmetrically around the Kel-F plunger. Several on-off or two-way valves are now being used in automated Karl Fischer water titrators to control the addition of water standard in methanol and Karl Fischer reagents. RECEIVED for review April 5 , 1971. Accepted June 29, 1971.
Direct Sample Injection into a Helium Ionization Gas Chromatograph Hugh C. Dodd Department of Environmental Medicine, The Medical College of Wisconsin, Allen-Bradley Medical Science Laboratory, 8700 West Wisconsin Aoenue, Milwaukee, Wis., 53226
OUR LABORATORY purchased a Helium Ionization Gas Chromatograph (Varian Aerograph-Helium Detector Instrument-Mode1 No. 1532) normally equipped with a sample loop injector because this instrument possesses parts per billion sensitivity to all gases and vapors-specifically, COS, CO, 02,CHa, N2, and H2 ( I ) . Only ultrapure helium carrier gas meets the requirements of the instrument. Our applications require that we be able to take air samples from any source (room air, saran bag, blood reaction vial, etc.) and immediately inject them via syringe through a septum into the gas chromatograph. In addition, to calibrate several
types of analytical instruments simultaneously, we must prepare our calibration air mixtures ( 2 ) in a saran bag. To give the analyst direct access to the sample loop system without exchanging atmospheric gases through the septum, we have designed and constructed a helium purge chamber (Figure 1 shows the chamber added to the standard instrument) consisting of two aligned septa, 1 and 2 , close enough together for a long syringe needle, 3, to inject the sample into the pre-sample loop chamber, 4. A tee, 5 , diverts part of the helium flow to purge the interseptal purge chamber, 6, constantly and then to vent to the atmosphere through tubing,
( I ) C. H. Hartmann, and K. P. Dirnick, J . Gas Chromatogr.,4,163 ( 1966).
(2) R. D. Stewart, and H. C. Dodd, Amer. Ind. H y g . Ass. J . , 25,439
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(1964).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
