Activity coefficients in states of uniform composition

We shall still confine our attention to binary, two-phase systems for which, if the composition of the system is uniform, equations (18.28) and (18.29) reduce to  

First we consider a fixed pressure p . Then, assuming the heats of transfer to be independent of temperature in the range considered, we have 

In these equations T is the azeotropic temperature TJ and are the temperatures at which the two pure substances exist in equilibrium in the two phases at the given pressure. 

If the system consists of a liquid phase and a vapour phase, then we may, to a first approximation, assume the vapour to be an ideal gas mixture for which y and y 2 are equal to unity furthermore TJ and Tl are then the boiling points of the two pure substances at the pressure concerned. 

As an example let us consider a mixture of ethanol and carbon tetrachloride which forms an azeotropic mixture.f At a pressure of one atm. the azeotropic temperature is 65 08 °C. 

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