Phase Rule Criterion of Purity

Equation (13.9) is written as if the number of degrees of freedom of a system were calculated from known values of the number of phases and the number of components. In practice, an experimentalist often determines F and p from his or her observation and then calculates C, the number of components. 

The determination of the purity of a homogeneous solid from solubility measurements is an example of this application of the phase rale. The experimental procedure 

What would be the number of degrees of freedom in a system in which pure H2O was increased to a temperature sufficiently high to allow dissociation into H2 and O2  

Willard Gibbs, Trans. Conn. Acad. Sci. 3, 108-248 (1876) 3, 343-542 (1878). Reprinted in The Collected Works of J. Willard Gibbs, Vol. I, Yale University Press, New Haven, CT, 1957. 

Berger, L. Eyraud, M. Richard, and R. Riviere, Bull. Soc. Chim. France 628, 1966. 

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