Gases and volatile agents

2.1 Pressure units 36 2.5 The solubility of gases in blood and

2.2 Ideal and nonideal gases 36 tissues 49

2.3 Vapour pressure 37 Summary 53

2.4 Solubility of gases in liquids 46 References 54

Gases and volatile substances are encountered in pharmacy mainly as anaesthetic gases, volatile drugs and aerosol propellants. This chapter deals with the properties of gases and vapours, including the way in which the vapour pressure above solutions varies with the composition of the solution and the temperature. The factors governing the solubility of gases in liquids are reviewed and related to the solubility of anaesthetic gases in the complex solvent systems comprising blood and tissues. Formulation issues arise from the replacement of chlorofluorocarbons (CFCs) by hydrofluorocarbons - not only because of differences in vapour pressure but also because of the changes in solvency of the propellants.

2.1 Pressure units

In practice, pressure is expressed in terms of a wide range of units. The SI unit is the pascal (Pa) where 1 Pa = 1 N m 2. Pressure should usually be converted to this unit before substitution into equations. The relationship of other commonly used pressure units to the pascal is as follows:

1 mmHg = 1 torr = 133.32 Pa 1 atm = 1.013 x 105 Pa 1 psi* = 6894.76 Pa

Standard atmospheric pressure is 760 mmHg = 760 torr = 1.013 bar = 1.013 x 105 Pa * It is common to report vapour pressure of propellants as 'pounds per square inch gauge', psig. Gauge pressure uses the actual atmospheric pressure as the zero point for measurement and hence atmospheric pressure (14.7 psi at sea level) must be added to measurements quoted in psig to obtain the absolute pressure in pounds per square inch, psia; i.e.

expel the product as a solid stream, a mist or a foam. The pressure of gas in an aerosol container of this type is readily calculated using the gas laws, as in Example 2.1.

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