Heat capacity of a two-phase system, with applications to vapor corrections in calorimetry

Abstract

A formula is derived that gives the heat capacity of a system composed of solid or liquid in equilibrium with saturated vapor in terms of the specific heat of the condensed phase and certain auxiliary data. This formula is valid throughout the enti;e range from 0 to 100 percent of vapor, and at the latter extreme reduces to a well-known relation between the specific heats of saturated liquid and saturated vapor.The formula is applied to the calculation of vapor corrections in calorimetry.

Specific heats of oxygen at coexistence

“…The accurate formulation of adjustment calculations for experimental specific heat measurements on a two-phase sys te m is an exacting challenge [8,10]. It therefore may be helpful to record the following details, pointed out by Barieau. In eq (3 .9) the adjustment for vapor entering the capillary tube is not rigorous.…”

“…Hoge, several difficult adjustments are eliminated by performing experiments with relatively large and also with relatively small amounts of liquid in the calorime ter [8]. When two phases are present,…”

Specific heats Cv of fluid oxygen from the triple point to 300 K at pressures to 350 atmospheres

“…1), touching it at th e critical point. Through eq (4-c) the shape of this locus is adjusted by constant C1• The first three derivatives vanis h at the c riti cal density (we explored fj= 1,3,5). W e hope soon to report on a PVT equation of state incorporating eq (4-d) for nonanalyticity at the critical point.…”

The specific heats, Cs, and Cv, of compressed and liquefied methane

“…Then the effects 'of the latent heat of vaporization and heat absorbed by the vapor are subtracted [3,5,7] to give CfT (column 10). This type of correction is derived by Hoge [11] . The temperature increment, resulting from a con· stant power input over a time D.t, is evaluated at the middle of the heating interval by extrapolating the temperature drift rates evaluated just before heating and after an equilibrating time has elapsed (about 20 min).…”