Thermodynamic consistency near the liquid-liquid critical point.

The thermodynamic consistency of the isobaric heat capacity per unit volume at constant composition Cp,x and the density ρ near the liquid-liquid critical point is studied in detail. To this end, Cp,x(T), ρ(T), and the slope of the critical line (dT/dp)c for five binary mixtures composed by 1-nitropropane and an alkane were analyzed. Both Cp,x(T) and ρ(T) data were measured along various quasicritical isopleths with a view to evaluate the effect of the uncertainty in the critical composition value on the corresponding critical amplitudes. By adopting the traditionally employed strategies for data treatment, consistency within 0.01 K MPa−1 (or 8%) is attained, thereby largely improving the majority of previous results. From temperature range shrinking fits and fits in which higher-order terms in the theoretical expressions for Cp,x(T) and ρ(T) are included, we conclude that discrepancies come mainly from inherent difficulties in determining the critical anomaly of ρ accurately: specifically, to get full co...