Plank - Clausius Equation

We emphasize here that Planck [11] has already calculated the vapor pressure -temperature coexistence curve by using an equation, Eq. (8.10), which goes back to Clausius  

The equation used by Clausius contains four parameters, however, namely a, b, c, R. In order to use the concept of corresponding states, we have three conditions only, i.e., the equation at the critical point itself, the conditions that the first derivative and the second derivative of the pressure with respect to the volume must be zero. Therefore, we can deliberately chose one of the parameters a,b,c,Rto fix it. 

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