The derivatives along the critical line in a binary mixture near the critical points of pure components in one-fluid theory

Analytic expressions are obtained for all the first derivatives along the critical line in a binary mixture, near the critical points of pure components, in terms of the one-fluid Van der Waals theory. The conditions for the existence of the gas-gas equilibria of the first kind are shown and discussed for the reference substance represented by the accurate NBS equation of state of argon and by the Van der Waals and Guggenheim equations of state.