Methods for preparing some commonly used gasses

2 Methods for preparing some commonly used gasses 

Methods for preparing some common gases are described below (see ref. 3 for other methods). The properties of these and other gases are listed in Appendix 2. Many of these gases are very harmful and they should only be handled in an efficient fume cupboard, taking care to prevent leaks and to scrub any excess gas. 

Prepare by addition of water to calcium carbide and dry by passing through a column of molecular sieves. Acetylene can explode spontaneously, especially when pressurized. For this reason the gas should be generated very cautiously, the generator should be cooled, and the gas line should be fitted with a bubbler so that the pressure cannot rise above 15 psi. Dispose by venting slowly in an efficient hood. 

The simplest method is to allow solid carbon dioxide to evaporate, and the gas can be dried by passage over molecular sieves. It can also be prepared by addition of dilute hydrochloric acid to calcium carbonate. Passage over molecular sieves will remove any aqueous acid in the gas stream. 

Carbon monoxide is very toxic by inhalation TLV 50ppm. Prepare by slow addition of anhydrous formic acid to concentrated sulphuric acid at 90-100°C (frothing tends to be a problem). One millilitre of formic acid generates 26.6 mmoles of gas. The CO is contaminated with small amounts of carbon dioxide and sulphur dioxide which may be removed by passage over potassium hydroxide or the commercial product Ascarite (sodium hydroxide on silica). Dispose of carbon monoxide by slow venting in an efficient hood. 

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