Vapor-pressure for the pure fluids from calorimetric measurements near the critical point

Abstract

We report the values of second temperature derivatives of the vapor-pressure curve (d 2 P S/d T 2) and chemical potential (d 2 μ/d T 2) for pure fluids derived from precise two-phase isochoric heat capacity measurements over a wide temperature range including the critical region. The values of (d 2 P S/d T 2) derived from C V 2 measurements were compared with the values calculated from direct differentiation of the vapor-pressure equation from the literature. The derived (d 2 P S/d T 2) data has been used to develop a scaling-type vapor-pressure equation in the critical region. A description of the vapor-pressure calculation procedure from C V 2 measurements is given. The values of the critical pressure P C and slope of the vapor-pressure curve (d P S/d T) C at the critical point were calculated by using calorimetric measurements. The asymptotic behavior of the second temperature derivatives of the vapor-pressure curve (d 2 P S/d T 2) and chemical potential (d 2 μ/d T 2) near the critical point are discussed. The strength of the Yang–Yang anomaly, R μ , defined by Fisher as R μ = A μ /( A μ + A P ), where A μ and A P are the amplitudes of the singularity of − T(d 2 μ/d T 2)≈ A μ t − α and TV C(d 2 P S/d T 2)≈ A P t − α was estimated from derived results.