Ground Glass Joints and Stopcocks

A dry, weighed polyethylene film (length 50 mm, breadth 25 mm, thickness 0.1-0.2 mm) is placed in a tube (about 70 ml capacity) fitted with a ground glass joint and stopcock. After addition of some crystals of benzophenone (as sensitizer) the sample is heated for 1 h on a water bath at 60 °C.The outside of the tube is dried, the stopcock closed, and the tube exposed to 15 min irradiation from a mercury lamp (maximum of emission in the region of A = 253-254 nm). 

The apparatus consists of a glass reaction flask fitted with a side arm and dropping funnel. The side arm is connected to two glass traps in series by means of standard ground-glass joints and stopcocks. 

The tube is placed inside another tube fitted with a ground-glass joint and stopcock at the top (Fig. 20). The stopcock is joined to a vacuum system by a rubber tube through a Dry Ice-acetone trap and a tower of sodium hydroxide pellets. It is partly opened very carefully, by adjusting the stopcock so as to give a steady but not violent ebullition. 

Reaction tubes varying in internal volume from 22 ml. to 185 ml. have been used. Presumably the upper limit in size is established by the volume of the steel bomb. The best grease to use as lubricant for the ground-glass joint and stopcock on the large transfer tube is Halocarbon stopcock grease, t... 

While it is possible to mould stoppers and cones for interchangeable ground glass joints and for hollow stopcock keys by mouth blowing, this is an exhausting and quite unnecessary task if air from a compressor is available. A simple two-way valve made in the mechanical workshop from brass or aluminium, connected up as shown in Fig. 3.3, and operated by hand or by foot-treadle, wiU do a better job with much less effort. 

Two 1-L roimd flasks with ground-glass joint and glass stopcock, dropping funnel, magnetic stirrer, syringe (20 mL), filter funnel, condenser, distillation apparatus, and ice bath. 

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. 

Owing to the corrosive action of bromine upon corks j-jg 7, l. and rubber stoppers, ground glass joints are recommended in this preparation. The apparatus, depicted in Fig. Ill, 37, 1, is particularly convenient for the preparation of bromides from alcohols. A double surface condenser is fitted into D and a round-bottomed flask is fitted on to the ground glass joint at C R is a three-way stopcock f which permits the removal of the contents of A without disconnecting the apparatus. For preparations of moderate size, A has a capacity of 60 or 100 ml. and a 250 or 500 ml. flask is attached at C. 

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