Asbestos stoppers

The corks are badly attacked by the nitric acid and must be renewed in each run. An asbestos stopper, prepared as described in Section III, 161, can be used repeatedly. 

An all-glass apparatus is advisable. If it is not available, one-holed asbestos stoppers may be made by soaking strips of asbestos in water, wrapping them around pieces of glass tubing of suitable size until the desired diameter has been reached, and then allowing them to dry at 110°. 

Put the mixture into a copper retort. Close the latter with an asbestos stopper and connect it to a cooler. 

Put 10 g of sodium nitrate into retort 1 and pour in 10 ml of a 96% sulphuric acid solution. Close tubulature 2 with an asbestos stopper. Fill vessel 3 for cooling receiver 4 with water and ice. Carefully heat the retort. What occurs How can you explain the appearance of brown vapour in the retort When 4-6 ml of nitric acid are collected in receiver 4, stop heating the retort. Keep the prepared nitric acid for the following experiment. 

Ah open tube ends on a piece of glassware, except the one used for blowing, should be securely stoppered with cork or asbestos stoppers or rubber plugs. Small leaks must be avoided, as they upset blowing techniques and excessive loss of air can be harmful to the chest and diaphragm muscles. 

A generous supply of cork and asbestos stoppers is essential one or two of each size should be fitted with a glass tube handle, about 20 cm long and 8 mm in diameter. 

The test-tube is fitted with an asbestos stopper into which a 5 mm bore glass tube has been inserted, and marked about 4 cm down from the open end (Fig. 6.33). 

For each pair cut a 35 cm length of 23 mm tube and slightly flare each end so that an asbestos stopper with an 8 mm tube handle will fit neatly and securely. Now pull good points in the centre and cut them open. Each one should now be like Fig. 8.12a. 

When large-diameter tube is worked in the lathe it is held, of necessity, in a horizontal position. The air inside is heated by convection and soon becomes hot enough to melt rubber stoppers and to char cork stoppers. Heat radiated from hot constrictions or test-tube ends has the same effect. Rubber stoppers should be avoided in lathe work. If the tube is of large diameter or short length, and a stopper must be used, then the inside surface of the cork should be protected from the heat by an asbestos or metal foil disc. Asbestos stoppers would be ideal, but they are not available in sizes much above 50 mm. 

Large stopcock barrels and ground glass joints are more easily strained than small ones and it is advisable that they be protected from excessive heat by wrapping them in woven asbestos tape. The key should always be removed from a stopcock during joining operations. The ends of the barrel can be plugged with asbestos stoppers. 

Accurately bored asbestos stoppers, carrying a suitable length of 8 mm tubing, make good holders. Discarded ground-joints can be used they should have a suitable tube attached. 

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