A variety of free, web-based calculators are available for solution of the Peng-Robinson equation of state. One example is an algorithm developed by Justin Lu at Carnegie Melon University (last modified 12/04/00), which uses Newton’s method to solve the cubic form of the equation for V_m(P,T).

http://www.andrew.cmu.edu/user/jlu/06-221/Peng-Robinson.htm

Another example is a demonstration calculator provided by Quest Consultants Inc., which includes provision for mixed gas compositions of the user’s specification.

http://www.questconsult.com/~tam/thermot.html