Phase-Change Calorimeter for Measuring Relative Enthalpy in the Temperature Range 273.15 to 1200 K

Abstract

The general principles of heat-capacity or enthalpy calorimetry by the method of mixtures have been presented elsewhere together with a copious bibliography.⁽¹⁾ To briefly recapitulate, this experimental approach is well suited to highly accurate and precise measurement of the heat capacity of nonreacting, solid or liquid systems in any condition of subdivision (including massive, crystallites, and powders). It is also of great use in measuring heats of phase changes and their associated temperatures. Usually, the basic datum obtainable through this method is the total enthalpy change of the system under study upon rapid transfer from an initial equilibrium state in a high-temperature furnace to a second equilibrium state in the calorimeter. Thus the chief sources of error are limited to those in temperature measurement and attainment of equilibrium in the furnace, in accurately accounting for heat losses during a sample transfer, in measurement of the heat transferred from the sample to the calorimeter, and in attainment of a reproducible thermodynamic state of the sample in the calorimeter. The equipment required is a mix of custom-fabricated parts, well within the capability of any competent instrument shop, and commercially-obtainable heating, temperature-measurement, and temperature-control components. This chapter presents a specific design and operating criteria for a phase-change calorimetric system using purified water as the working substance and capable of operating at a precision level of 0.01%. and an accuracy level of 0.1%. Except for the measurement, control, and recording of temperature, the system is manual in operation.