A 27 °C. ISOTHERMAL CALORIMETER

Abstract

The need for a Bunsen-type isothermal calorimeter operating as close as possible to the standard thermochemical reference temperature, 25 °C., was fulfilled by means of diphenyl ether (m.p. 26.9 °C.) as the working substance. The instrument was calibrated electrically by comparison with the ice calorimeter and the constant was found to be 19.01 ± 0.02 cal. per gm. of mercury. From this and other known properties of diphenyl ether the density of the solid at the melting point was estimated at 1.188 gm. per ml. Check determinations based on the heat of vaporization of water showed that for such measurements the mantle of solid ether must first be melted inside by an amount equivalent to the heat to be measured. The calorimeter was operated in a large water-thermostat kept constant to within 0.001 °C. With highly purified diphenyl ether there was no noticeable temperature drift during the measurements. Quantities of heat up to 600 cal. could be measured with a reproducibility of the order of 0.2%. The new calorimeter is simpler to operate than the ice calorimeter and its sensitivity is more than three times as great.