Stoppers, rubber

Rubber stoppers are also efficient, simple, temporary system closures. Properly drilled, stoppers can support thermometers or funnels. Because new stoppers have no surface cavities or cracks, there is no risk of leaks as with corks. Like corks, stoppers are used to keep things in (or out) of container. Table 1.7 includes a list of rubber stopper sizes. 

Stoppers, as opposed to corks, can react with a number of organic chemicals used in the laboratory. Likewise, a concern when using stoppers is that the closure not adversely affect the contained sample. For this reason, cork stoppers are often preferred when containing organic solvents. If you do not have any corks, you may use a rubber stopper that has been enclosed with an aluminum foil cover. This technique has a few limitations The covered rubber stopper will not grip into the seal (there is no friction to hold it in), and the crevices of the foil provide many potential small leaks. 

Although rubber stoppers are normally intended for use with water-based solutions, do not use the aluminum foil technique with an acid or strong base solution because it will destroy the foil. 

If you look in the average laboratory supply catalog, you may see listings for amber (or natural rubber), white, black, green, red, and/or clear stoppers. Ignoring the stoppers with premade single or double holes, there is not much difference between the first three-colored stoppers. The green and red/clear do provide some special characteristics. 

Amber or Natural Rubber. These colored rubber stoppers are, simply, 

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When a more delicate fractional vacuum-distillation is required, the flask and column shown in Fig. ii(b), p. 26, may be used, the side-arm of the column being fitted directly into receiver C (Fig. 14). A rubber stopper must then be used to fit the flask on to the fractionating column, and it should also carry a capillary tube leading to the bottom of the flask, to provide the usual fine stream of bubbles to prevent bumping. 

Never use unprotected bark corks for an apparatus ih which a carefully dried liquid is to be distilled, etc., as these corks always contain appreciable quantities of water, which is exuded when the cork comes into contact with a hot liquid. Rubber stoppers should therefore be used in these circumstances. 

When boring rubber stoppers, lubricate the borer well, either with aqueous glycerine or with vaseline. Then clean well and dry before using. 

The absolute ethanol obtained in this way should have d., o 794, It is very hygroscopic, and the bottle in which it is stored should preferably have a well fitting ground-glass stopper alternatively, the bottle can be closed with a tightly fitting rubber stopper but not with a cork, as corks contain appreciable quantities of water. 

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. 

For this reduction use preferably a i litre round-bottomed flask having 3 necks (Fig. 23(G), p. 46), the two necks at the flanks being straight (to avoid the obstruction, during the addition of sodium, which a curved neck might cause). Fit the central neck with a stirrer, one of the side necks with a reflux water-condenser, and the other with a glass or rubber stopper. 

Assemble in a fume-cupboard the apparatus shown in Fig. 67(A). Place 15 g. of 3,5-dinitrobenzoic acid and 17 g. of phosphorus pentachloride in the flask C, and heat the mixture in an oil-bath for hours. Then reverse the condenser as shown in Fig. 67(B), but replace the calcium chloride tube by a tube leading to a water-pump, the neck of the reaction-flask C being closed with a rubber stopper. Now distil off the phosphorus oxychloride under reduced pressure by heating the flask C in an oil-bath initially at 25-30, increasing this temperature ultimately to 110°. Then cool the flask, when the crude 3,5-dinitro-benzoyl chloride will solidify to a brown crystalline mass. Yield, 16 g., i.e,y almost theoretical. Recrystallise from caibon tetrachloride. The chloride is obtained as colourless crystals, m.p. 66-68°, Yield, 13 g Further recrystallisation of small quantities can be performed using petrol (b.p. 40-60°). The chloride is stable almost indefinitely if kept in a calcium chloride desiccator. 

Fit a 750 ml, bolt-head flask (also by a rubber stopper) to a reflux water-condenser closed at the top by a calcium chloride tube ensure that flask and condenser are quite dr). Place 150 ml. of the dried ethyl acetate in the flask and add 15 g. of sodium. The sodium for this purpose should preferably be added in the form of wire directly from a sodium press (Fig. 55, p. 82) alternatively the sodium may be added as thin slices, but in this case each slice should be quickly pressed between drying-paper before being added to the acetate to remove the wet film which may have formed during the weighing and cutting of the metal. 

Meanwhile, the organic compound can be prepared for analysis whilst the sealed end C (Fig. 72) of the Carius tube has been cooling dow n. For this purpose, thoroughly clean and dry a small tube, which is about 6 cm. long and 8-10 mm. w ide. Weigh it carefully, supporting it on the balance pan either by means of a small stand of aluminium foil, or by a short section of a perforated rubber stopper (Fig. 73 (A) and (B) respectively) alternatively the tube may be placed in a small beaker on the balance pan, or suspended above the pan by a small hooked wire girdle. 

Fit two similar 250 ml. conical flasks, A and B, with reflux water-condensers (using ground-glass joints or rubber stoppers) and connect the condensers in series as before over two water-baths. Prepare a mixture of 2 volumes of acetic anhydride and i volume of glacial acetic acid,... 

In (d) the side arm outlet extends a short distance into the long neck of the flask, thus preventing any vapour which has been in contact with cork or rubber stoppers from condensing and flowing down the side arm. 

It is usually employed for those liquids which attack cork or rubber stoppers. 

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... 

A criticism that is sometimes levelled at distillation under diminished pressure when rubber stoppers are used is that contact of the hot vapour with the rubber frequently contaminates the distillate. In the author s... 

If a bolt-head fiask is used and a rubber stopper is permissible, there is ample room for the insertion of a capillary tube and the fractionating column in the wide neck. 

A flask is provided with a long side neck (Fig. II, 25, 1) through which a capillary is inserted and attached externally by a short length of rubber pressure tubing. With this type of connexion there is essentially no contact of hot vapours with the rubber and little or no contamination results. The neck of the flask is preferably fitted with a standard ground joint if attack of the organic vapours on the rubber stopper is likely to occur. 

The technique of the filtration of hot solutions has already been described in Section 11,28. The filtration of cold solutions will now be considered this operation is usually carried out when it is desired to separate a crystalline solid from the mother liquor in which it is suspended. When substantial quantities of a solid are to be handled, a Buchner funnel of convenient size is employed. The ordinary Buchner fimnel (Fig. 11,1, 7, a) consists of a cylindrical porcelain funnel carrying a fixed, flat, perforated porcelain plate. It is fitted by means of a rubber stopper or a good cork into the neck of a thick-walled filtering flask (also termed filter flask, Buchner flask or suction flask) (Fig. 11,1, 7, c), which is connected by means of thick-walled rubber tubing (rubber pressure tubing) to a similar flask or safety bottle, and the latter is attached by rubber pressure tubing to a filter pump the safety bottle or trap is essential since a sudden fall in water pressure may result in the water sucking back. The use of suction renders rapid filtration possihle... 

II, 36, 1 is almost self explanatory two ground glass joints are used, but these may be replaced by rubber stoppers, if desired. The crude substance is placed in the flask A. Stopcocks 1 and 2 are closed, and the apparatus is exhausted through tap 3 the indifferent gas is then allowed to enter the apparatus to atmospheric pressure. The evacuation and filling with inert gas are repeated several times. The solvent is added through the tap funnel B. 

Sublimation under reduced pressure. The so-called vacuum sublimation may be carried out in the apparatus of Fig. II, 45, 4. The cold finger is fitted into the larger tube by means of a rubber stopper... 

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