From the form of the Clausius-Clapeyron equation given by equation (2.6), the slope of a plot of log P against 1/T is

F 2.303RT

AHvap =-2.727 x 103 x 2.303 x 8.314 = -52.2 x 103 J mol-1

The molar enthalpy of vaporisation of metamfetamine is 52.2 kJ mol-1.

be compared. So that the same equation of state will apply to each gas, it is necessary to use 'reduced' thermodynamic variables. The reduced physiological temperature Tr may be obtained by dividing the physiological temperature for mice (310 K) by the critical temperature of each gas, or, as an approximation, by the boiling point of the gas. Equation (2.6) now becomes

6 2.3O3RTr

+ constant

Figure 2.7 shows the logarithm of the partial pressure of each anaesthetic gas required to produce a given level of anaesthesia plotted against the reciprocal of its reduced physiological temperature. Apart from the fluorinated compounds (which possess unique solubility properties), a close adherence to equation (2.7) is noted.

Gaseous anaesthesia in mice is an equilibrium process between the anaesthetic gas and the phase in which the gas exerts its effect (the biophase). As such, it should be amenable to treatment by the Clausius-Clapeyron equation. Modification of this equation is required when the distribution of a series of gases is to

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