Coexistence curve and rectilinear diameter in the critical liquid system carbon disulphide+nitromethane

Abstract

The phase boundary of the binary liquid system CS₂+CH₃NO₂ is studied over nearly six decades in reduced temperature 3×10^-6 < ε =(T_C-T)/T_C < 2×10^-1 and over the composition range 8-98 mole % of CS₂. The critical parameters are T_C=335·13₂K and x_C=57·34 mole % of CS₂. A single critical exponent β = 0·315± 0.004 fits the observations over the entire range with no indication of β increasing to the classical value of ½ far away from T_C. The diameter of the coexistence curve shows a rectilinear behaviour only far away from T_C. Near T_C, the deviation ΔX from the rectilinear law seems to fit a curve of the form Δ X=ƒε^⅞ exp (-gε^h), the derivative of which has a singularity like that of specific heat. An ambiguity in the analysis of the data in terms of mole fractions and volume fractions is pointed out. It is also suggested that the curvature of the diameter may be much weaker in a liquid-gas system and hence might have escaped detection.