V O L U M E 2 6 , NO. 4, A P R I L 1 9 5 4
789
In order to eliminate cross contamination of samples completely, work was initiated on a manifold having no glass stopcocks. Apparatus of this kind has now been in satisfactory use for almost 2 years in manifolds as well as other sample-handling systems. APPARATUS
Valves and Teflon Packing Glands. A small needle toggle valve supplied by the Hoke Valve Corp. (Catalog S o . 462) was selected to replace stopcocks. Initially, these valves were equipped with Hycar rubber seats, which were replaced with Teflon. A cork borer was used to cut a disk from a Teflon sheet 0.18-inch thick, and this disk was then pressed into the stem by use of a vise. A small rubber O-ring seal was used in the valve assembly, but numerous tests showed that this introduced no hydrocarbon cross contamination. In use, the valve was found to be consistently vacuum-tight, with the exception of an occasional leaky valve sent.
To substitute valves for stopcocks it was necessary to find a convenient method for making glass-to-metal connections Several things were tried, including wax seals, iridium-soldered joints, and glass attached to Kovar, which in turn was soldered to the valve. Each of these techniques is workable, but the Teflon packing gland diagrammed in Figure 1 was more convenient. Connections are made as shown, in which 2-mm. capillary tubing is snugly fitted inside the reamed-out valve nipple. The connection is made vacuum-tight by gentle tightening of the packing nut with a wrench. Manifold and Packing Gland Applications. The valve unit qhown in Figure 2 consists of the valve and glass connectors having joints which are ground flat to enable easy assembly of the svstem. The joints are sealed with phthalic anhydride cement after valves have been tightened in position on a Transite board. To allow freedom for positioning the valve, the brass rod holding the valve should be smaller in diameter than the hole in the TranFite through which it fits. Teflon packing glands are also used to attach sample tubes t o the manifold, as shown for a liquid pentane container in Figure 3. I t is important to cap volatile material such as this during storage
TEFLON GASKETS,
c
HOKE VALVE
i-
GLASS TUBING
$" HEX PACKING
Figure 1. Packing Gland
NUT
T -MANIFOLD
Figure 3. Liquid Sample Tube Attachment and Cap
PACKING NUT
'7.8 m.m. 0. D. -LlOUlD
SAMPLE TUBE
prior to mass spectrometer analysis and for this purpose a Teflon seated cap, diagrammed also in Figure 3, is used. Pentane was stored at room temperature for 2 months without measurable loss using the seal shown. In addition, the apparatus was found suitable for sample shipment via air express. Packing glands used as described here are tightened by means of G-inch wrenches. OPERATION
Experience has shown that the apparatus is of rugged construction. The equipment built after the experimental model. has been in use almost a year without breakage. Occasionally, leakage through valve seats occurs, but this can usually be remedied by warming the valve in its closed position and rotating the valve stem to reseal the Teflon seat. Dirt or even mercury on the valve seat can also cause leaks, so that visual inspection of the seat may sometimes locate leakage trouble. There appears to be no need to clean the apparatus, although no dirt or oil should be allowed to remain in the apparatus, as either material absorbs hydrocarbon gases. Mass spectrometer sample inlet systems having no glass stopcocks have been in operation for many years without being cleaned and the same sort of service is anticipated for these manifolds. A valve manifold which had been in continuous use for 6 months showed no measurable absorption of 2-butene. By contrast, a stopcock manifold which had been in use for only a month absorbed 8 ml. of the gas in 5 minutes. PRESEKTED before the American Petroleum Institute, New York, N. Y., May 1953.
SILVER SOLDER'
4"
BRASS ROD
-
7 I
It'
Apparatus for Stirring a Reaction Mixture through a VaporTight Seal. J. Russell Skidmore, Dow Chemical International, Ltd., Midland, Mich. ERY often the chemist wishes to carry out a reaction aided
V by agitation or stirring in a totally enclosed or vapor-tight
SAE THREAD
Figure 2. Valve Assembly
system. This usually requires a mechanical seal of some kind. In some instances, a magnetic stirrer is adequate, and no seal is required. Liquid seals--e.g., mercury-filled seals-are not entirely satie factory, especially for vacuum or pressure systems. Cylindrical seals with graphite or Teflon sleeve bearings have been used with
ANALYTICAL CHEMISTRY
790 some suceem, and a satisfactory Ped1 can he made from a groundglass syringe. A far less satisfactory seal is prepared with a spherical glass joint, one part of which is stationary and the other part rotates about the axis of rotation of the stirrer t o which i t is attached.
The stirrer has become so useful that it is beine installed far many renct.ions not requiring vapor-tight or e'
Improved Dip Coater. A. G. Roberts and R. S. Piaer, Organii Plastics Section, Kational Bureau of Standards, Washington 25, D. C.
A X I S OF ROTATION
desirable during the course of extensive laboraI toryfrequently test. programs to conserve limited supply of the mat?TIS
B
G'TAToR S H A F T COUNTER B A L A N C E DRIVE ARM UBBER TUBE STANDARD SPHERICA GROUND J O I N T
ROUND B O T T O
FLASK
AD improvement in such a seal can be ohtained by the use of a center of rotation-the center of the mherieal joint instead of. or in addition to, an axis of rotation. The diagram illustrates sehemstically the stirring mechanism which utilizes a spherical glass joint t o effect a vapor-tight seal. ,The stirring rod in action describes a cone in the reactor about the center of rotittian. The shsK should be left free to rotate about its anis to ensure symmetrical wearing of the seal. The zone of influence in the reactor is large, depending upon the length of the driving arm or its height above the seal. Becmse of the large zone of influence, the rate of rotation usually required is slow; 50 to 300 r.p.m. is satisfactory for most conditions. The stirring rad does not require paddles, blades, or othcr appendages, though these may be added as desired. When the viscosity of the reactants is ION, better mixing will be obtained hy indenting the side walls of the flask. These units have been test operated for long periods of time under vacuum of 0.05 mm. of mercurj and under pressure (with n spring load on the seal) of 18 inches of water. No spring load is required for satisfactory operation a t atmospherio pressure. The seal should he lubricated with suitable lubricant, depending upon the nature of the reaction under observation. The eketeh shows a rubber sleeve to permit adjustment or replacement of the stirring rod, but if desired, the rod may be welded to the sed for an nll-glass system. The counterbalanced drive arm Shown may easily be prepared from a piece of heavy iron wire such as '/,-inch uncoated welding rod. It could be made as well of a disk of metal, or plastic, or m y of various substances which might he available. I t s dimensions are not critical; it should be adaptahtable to a chuck or coupling attachment for a driving mechanism and should provide an opening or hole through which the agitator shaft extends to maintain i t in a fixed position relative to the center of rotation. A t least two types of motors are available to drive such stirrers; a stirrer motor with a cone drive mechanism, and a variable speed motar with a gear-reduced Phaft. Other systems may he set up as needed, depending upon the amount of work to he done hy the stirrer.
rials under investigation. Even if unlimited quantities are available. it is usually desirable to carry out a given series of tests v i t h x minimum of material in order to reduce the effect of contsinerto-container variations. The conservation problem may hecom? acute in cases where the dip-coating technique is employed with coating solutions which are activated just prior t o use and which have a short pot life that necessitates their being discarded after each f e n hours of use. The day-to-day need for freshly mixed materials may consume an enormous amount of material, from the laboratory point of vie!,-, In these circumstances, the use of the minimum quantity of material per coating hatch is an obvious necessity. Containers of the smallest dimensions that u-ill satiafactorily accommodatc the specimens to be coated are easily constructed; however, the persistent tendency of flat specimens to rotate or swing against the sides of a narrow container during the dipcoating operation constitutes a vexing problem. This problem ha3 been met simply and effectively in this laboratory by eonstruction of a special specimen holder for use with the conventional type of laboratory dip coater. Undesirahle motion of the test panel being dipped can he prevented, for example, by replacing the specimen-holding pluml, .bob. ot the familiar rimer-I'ayne .. ._ .._ . dip coater IYayne, H.Y., Ino. ENR.CHEM., ANAL.ED., 15, 48 (1943)l with 8. rigid brass strip sliding vertically through the grooves of a fixed brass mounting attuehed by a rod and conventional clamp t o a ring stand. The rod is soldered into the b x k of the slide holder. A pin is soldered near the bott,om of the brass strip a t an angle of about 45" from the vertical, which causes the specimen to slide aminst the flat face of thehrass strip and remain firmly in d a c e thrauehout the d i p h a t i n g operation. The diameter of the mounting hole in the test specimen must he sufficiently larger than that of the mounting pin to permit the panel t.o slide freely down the pin and onme to rest in a vertical position. The minimum diameter, D,of the mounting hole is given bytherelationD = 1.4 P T, where P i s pin diameter and T t,he teet panel thickness.
+
The improved specimen holder is shown. The dip-ooater attnchmentiseaeily constructed and will considerably increase the utility of the conventional dip coat,er
