Octanol as a nonaqueous phase

Octanol is often used as the nonaqueous phase in experiments to measure the partition coefficient of dmgs. Its polarity means that water is solubilised to some extent in the 

The correlation of lipophilicity and biological activity usually involves equations of the type 

According to equation (3.49) the chemical potentials in two phases at equilibrium are equal. Thus, from equation (3.52), 

If the standard states are identical, consequently the activities will be equal in the two phases. The activity of a substance in the biophase at equilibrium is thus identical to the readily determined value in an external phase. 

For narcotic agents applied as a vapour, the standard state can be taken to be the saturated vapour, and thus activity a =pjp where is the partial pressure of the vapour and p is the saturated vapour pressure at the same temperature. When the narcotic agent was applied in solution and was also of limited solubility, activity was equated with the ratio SJSq where S, is the molar concentration of the narcotic solution and Sq its limiting solubility the ratio SJSq therefore represents proportional saturation. This is in contrast to the normal procedure of taking the standard state as an infinitely dilute solution. 

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