AIDS FOR ANALYTICAL CHEMISTS Simple Apparatus for Performing Reactions and Filtration Operations in the Absence of Air R. Donald Spencer Carnegie-Mellon University, Meilon Institute, Pittsburgh, Pa. 15213
THEAPPARATUS illustrated below represents the culmination of our efforts to find a convenient method to react various forms of silica with organolithium reagents. Since we were concerned with quantitative analysis of the lithiated silicas by titration with methanolic HC1, water and reactive gases such as oxygen or carbon dioxide could not be tolerated either in the initial treatment with n-butyllithium or methyllithium, or in the subsequent washing and drying of lithiated products. Conventional apparatus was not satisfactory so vacuum system techniques were tried. The apparatus illustrated below was quite adequate and convenient for the procedure involving the following series of steps : Physisorbed water was completely removed by drying at 80 O C under vacuum (to Torr). Rigorously dried and degassed n-hexane or ether was vacuum transferred into the flask. Excess n-butyllithium in n-hexane or methyllithium in ether was added. The suspension was stirred magnetically with a glass-covered magnet under an inert gas blanket (argon) for the reaction period. The solvent was filtered and all excess organolithium reagent was removed by repeatedly washing with anhydrous, degassed solvent (n-hexane or ether). The lithiated silica was vacuum dried, taking care to continue excluding air. Operation of the Apparatus (See Figure 1). ADAPTERA. Since a hydrocarbon vacuum grease (Apiezon N) was used to make the vacuum seals at the 24/40 standard taper joints, the lower joint arrangement was reversed to prevent grease from being dissolved in the hexane and contaminating the silica sample. This is also the reason for the U-bend, which prevented grease from the vacuum system joint from reaching the reaction flask. The coarse frit in the adapter reduced chances that finely powdered silica samples would be carried into the vacuum system during the drying operations. REACTION FLASK B1. Flask B1 was the prototype reaction flask and was adequate for most of the silica samples studied. The side necks were set into the flask at 60" from the vertical, so that simple rotation of the standard taper joint would bring the right-hand neck down into the vertical filtering position. The glass frit (medium porosity) was satisfactory most of the time, but was badly clogged by the extremely fine powders of some samples. This necessitated the changes in design represented by B2 and B3. The right-hand neck was designed to accept a rubber serum-bottle septum (one-hole, 6-mm diameter, 8 mm long, neoprene or silicone, very lightly lubricated with mineral oil) since glass vacuum stopcocks were sources of grease contamination. [The septum cup can be made from 8-mm tubing. If the outer diameter is kept in the range 8.0 to 8.4 mm, the resulting side neck will accept 8-mm or 6/le-inch plastic connectors (polypropylene, Teflon, etc.) available from several suppliers. Most of these are quite satisfactory for ordinary vacuum work and greatly increase the versatility of this design.] 882
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SEALAND VALVEC. A Burrell Connector No. 270-2 was modified by cutting off the short tube at the hole and silver soldering this to the stub of a 15-gauge stainless steel needle cut off near the hub. When C is inserted into the one-hole rubber septum, a good vacuum-tight seal results as long as the hole in the side of C is within or to the right of the septum. When the reaction flask is under slight argon pressure, C may be inserted all the way to the hub, which exposes the hole and opens the valve. There is no danger of intrusion of laboratory air into the reaction vessel as long as the outward flow of argon is maintained. A her-lok syringe can be attached to C, and a reagent injected into the flask while the valve is in the open position. The liquid is pulled through the frit by reducing the pressure slightly in the main part of the reaction flask (Bl). Reactions are carried out with C inserted just far enough for the hole to be sealed by the septum. The female syringe joint may be capped with a male syringe needle plug. Filtration is accomplished by rotating the flask so the filtering arm is in the vertical position. With practice, the slurry can be transferred efficiently into the filter arm during this operation. In doing this it is helpful to manipulate the inner (stirring) magnet with a powerful magnet held against the outside of the flask. Since filtration is carried out with a small positive argon pressure on the inside of the reaction flask, the septum with C attached can be slipped out whenever argon is passing through, without danger of intrusion by laboratory air. To make this contamination even less likely, however, and to speed up filtration, a one-hole rubber stopper may be slipped over the end of B1 (as illustrated for B2) and a suction flask attached. The side arm is joined to a source of vacuum and one of inert gas (dry nitrogen or argon) by means of a Y adapter. The gas inlet is kept closed while enough vacuum is applied to cause the solvent in the filtrate to boil, at which point the vacuum tube is also pinched off. When the filtration is complete, the inert-gas arm is opened and the portion of B1 below the frit is brought to atmospheric pressure before removing the suction flask. This was necessary because a low pressure of argon such as can be tolerated by our vacuum system is not great enough to penetrate a solvent-wet frit from the reaction flask side of the assembly. After each wash-and-filter cycle, the septum and C are replaced immediately, lubricating sparingly with mineral oil, if necessary. A solid stopper may be used instead if no further washing is necessary, and the only operation remaining is the vacuum drying of the sample. In all vacuum operations with C in place, care must be taken not to insert the perforation so far into the septum that leakage can occur. For easy removal, we have not inserted septa their full length into the cups, but here again leakage can occur if they are not inserted far enough. REACTION FLASKS B2 AND B3. Very finely-powdered silicas were difficult to handle with Flask B1 because the glass frit
Rubber S t o p p e r
Figure 1. Apparatus described in text was easily clogged. Filtration times were excessively long, even when the internal argon pressure was supplemented by external suction. Flasks B2 and B3 are designed to permit filtration through membrane filters. These are fast-filtering even in small pore sizes and are available from several sources in a wide range of compositions and pore sizes. (These sources include: Selas Flotronics, Spring House, Pa. ; Millipore Corp., Bedford, Mass.; and Gelman Instrument Co., Ann Arbor, Mich.) If they become clogged, it is a simple matter to replace them (while inert gas flows through the apparatus) without exposing the reaction mixture to the air. The filter arm of B2 is simply a Schleicher and Schuell micro filter apparatus (Arthur H. Thomas, Philadelphia, Pa., Cat. No. 5136-G.) that has been modified in two ways. The stem has been altered as described for Flask B1 to permit use of a rubber septum. The rubber stopper shown in the drawing permits suction filtration during the washing steps. The second modification is the vacuum seal which must be used whenever the reaction flask is evacuated, if intrusion of outside air is to be avoided. The S . and S . funnel does not contain O-ring seals and is not designed to be vacuum tight.
The vacuum seal (B2b), provided with a 65/40 0-ring joint, corrects this deficiency, but at the price of greatly increased weight and bulk. Provision is made for two septa, although the inner one would be needed only when contamination by traces of outside air must be avoided at all costs. (This permits the outer chamber to become a safety vacuum lock.) Flask B3 has a filter arm consisting of a Fischer and Porter “Solv-Seal” glass joint such as is used as the lower end of a liquid chromatography column. As with B2, a membrane filter may be used with this joint, and this has the advantage that the two O-rings provide a vacuum-tight seal. B3 is therefore much lighter and more compact than B2 and is less likely to place undue strain on the fragile glass vacuum system. However, if it is necessary to change a filter membrane during a run, the considerable effort required to separate and rejoin the components of this rather tight joint is very likely to result in breakage of the vacuum system, unless great care is exercised. RECEIVED for review October 7, 1971. Accepted October 29,1971.
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