Standard State and Thermodynamic Activity

The algebraic formulation of the relationship among chemical potential p,, standard state chemical potential and thermodynamic activity a, is 

It must be emphasized that the choice of a particular standard state of reference has no influence on the result of equilibrium calculations and is only a matter of convenience. It is generally convenient to adopt as standard state for solid components the condition of pure component in the pure phase at the P and T of interest or, alternatively, the condition of pure component in the pure phase at P = 1 bar and T = 298.15 K.  

1 Standard State of Pure Component in Pure Phase at P and T of Interest 

We can identify this reference condition by means of the notation 

The relationship between the thermodynamic activity and molar fraction of a given component i in mixture a is described, over the whole compositional range, by 

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