Acoustic method of measuring critical properties of thermally unstable substances

An acoustic method of measuring critical temperatures and pressures of thermally unstable substances is described. In the cell with the liquid under investigation there is a wire probe, 0.02 mm in diameter, which is heated by electric-current pulses with a duration of 0.025 to 0.25 ms. At the moment of liquid boiling-up in the layer that surrounds the probe, one can observe an acoustic wave, which is registered by a ceramic piezoelectric element on a base (PbZrO3 + PbTiO3). The probe temperature at the moment of boiling-up is determined from its resistance. With an increase of the pressure in the cell with the substance under investigation the amplitude of an acoustic signal decreases. The cessation of boiling-up and disappearance of the acoustic signal show that the critical state is reached. A more precise critical pressure is found by linear extrapolation of the amplitude of the acoustic signal to zero in an amplitude-log (pressure) plot. The paper reports the experimental critical temperatures T-c and the critical pressures p(c) of reference substances: n-hexane, n-heptane, n-decane, benzene, and of thermally unstable n-alkanes: docosane and tetracosane, and also the critical curve of the system n-hexadecane-benzene. The critical constants of reference substances were measured by this acoustic method with an error that does not exceed 0.015 T-c and 0.015 p(c). (C) 1999 Elsevier Science B.V. All rights reserved.