Use of cork and rubber stoppers

Two points must be borne in mind when selecting a cork stopper. In the first place, the cork should be e.xamined for- edom from flaws unless corks of the highest quahty are employed, they are liable to have deep holes, which render them useless. In the second jrlace, the cork should originally fit as shown in Fig. II, 3, I, a and not as in 6. 

It should then be softened by rolhng in a cork press or by wrapping it in paper and roUing under the foot. 

To bore a cork, a borer should be selected which gives a hole only very shghtly smaller than that desired. The cork borer is moistened with water or alcohol or best with glycerine it is convenient to keep a small bottle (ca. 25 ml. capacity) containing glycerine. Fig. 

Rubber stoppers are frequently employed in the laboratory in vacuum distiUation assemblies (compare Section 11,19) for distillations under atmospheric pressure bark corks are generally used. Many organic liquids and vapours dissolve new rubber stoppers slightly and cause them to swell. In practice, it is found that rubber stoppers which have been previously used on one or two occasions are not appreciably attacked by most organic solvents, owing presumably 

I VMih n little espanonce thia osn ueu Uy be soconiplished in one operation without the neceasity of atoppiiiu to see whether A atraipht hole U being out. 

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Treat the distillate with 2 drops of glacial acetic acid (to destroy the phosphorus esters present) and redistil using the same apparatus as before except that the separatory funnel is replaced by a thermometer. Collect the liquid which passes over at 50-56°. Transfer the acetyl chloride to a weighed glass-stoppered bottle (since cork and rubber stoppers are attacked) and determine the weight. The yield is 22 g. 

Choice of Stoppers. Cork and rubber stoppers are generally used in a laboratory. It is very important to choose a suitable stopper for... 

If the anhydrous (98-100%) ethylenediamine is not available, it may be prepared from the aqueous 70% product as follows One hundred grams of solid sodium hydroxide and 175ml of 70% ethylenediamine are heated in a flask with an air condenser on the steam bath overnight. (As the amine attacks corks and rubber stoppers readily, these should be covered with tin foil.) The two layers that form are separated after cooling and the upper one is heated again for several hours with 30g more of the solid alkali. When distilled, the yield of amine, boiling at 115-118°C, is almost quantitative. This method is also used to dehydrate propylenediamine b.p. 118-120° C. 

Since anhydrous ethylenediamine attacks cork and rubber stoppers, ground glass equipment must be used. Five hundred grams of NaOH and 875 ml. of commercial ethylenediamine hydrate are heated overnight on a steam bath. Two layers form the upper layer is decanted, treated with additional 150 g. of NaOH for several hours the supernatant is decanted again and distilled. B.p. 116-117°C at 760 mm. Yield almost quantitative. (Propylene-diamine can be dehydrated in the same manner.)... 

Picric acid if stored in bulk should, for safety, first be damped. Smaller quantities may be safely kept whilst dry, but should be stored in bottles having cork or rubber stoppers glass stoppers should never be used for potentially explosive substances, because on replacing the stopper some of the material may be ground between the neck of the flask and the stopper, and so caused to explode. 

An assortment of small corks for closing small bore tubing can, in addition, be very useful. If the flame is to come near to a cork or a rubber stopper it is best to wrap either in thin asbestos paper before inserting into the tube. When pyrolysis of the cork or rubber stopper seems probable it is best to use a cork and not a rubber stopper, because the pyrolysis products from a cork can be cleaned olf more easily than those from rubber. Sometimes a plug made from damp asbestos paper is adequate for closing a tube of small bore, and this plug can be heated quite strongly. 

The capillary tube is inserted (with a little glycerol as lubricant), tip first, into a narrow-bored, undamaged rubber stopper which fits tightly into the neck a of the Claisen flask. The correct position of the tip is immediately above the deepest part of the bulb of the flask. A thermometer, likewise pushed through a rubber stopper, is inserted into the neck b. If it is desired to prevent contact of the substance with rubber, Claisen flasks with constricted necks are used. The capillary tube and thermometer are held in position in these necks by means of small pieces of rubber tubing drawn over each neck and its capillary or thermometer. The proper use of cork stoppers in vacuum distillations requires much practice. 

The connection between the condenser and the flask should be made with a rubber stopper in most cases. However, it should be noted that many solvents attack rubber (benzene, ligroin, etc.), not only causing damage to the stopper, but also introducing impurities into the materials. In such cases, it is preferable to use cork stoppers which have been carefully rolled before boring. If the vapors attack both cork and rubber, the use of an apparatus with ground glass joints is necessary. 

Iodine solutions lack stability for several reasons, one being the volatility of the solute. Losses of iodine from an open vessel occur in a relatively short time even in the presence of an excess of iodide ion. In addition, iodine slowly attacks most organic materials. Consequently, cork or rubber stoppers are never used to close containers of the reagent, and precautions must be taken to protect standard solutions from contact with organic dusts and fumes. 

When it is necessary to heat with a reflux condenser substances which destroy cork and rubber, a simple device can be used which is represented in Fig. 17. A test-tube is selected which fits loosely into a long-necked, round-bottomed flask. The tube is supplied with a rubber stopper and tubes as shown in the drawing. Water is passed through the tube, which is supported in the neck of the flask by means of a clamp. 

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