The heat of mixing of hydrogen peroxide and water and the heat of vaporization of the mixtures were measured over a wide concentration range a t O0 C. with a Bunsen ice calorin~eter and a t 26.V with a diphenyl ether calorimeter. The heat capacities of the solutions were determined between these two temperatr~res. Similar measurements were carried out on the corresponding deuterium compounds. The heat of decompositio~l of hydrogen peroxide catalyzed by colloidal platinum was also measured a t 2G.D0 as a function of concentration. Correlatioil of all the results leads to the following recommended values for the therrnochemical properties of the pure peroxides in the liquid state a t 25' C. 1320.n,o,

.qBSTR-kCT The need for a Bunsen-type isothermal calorimeter operating as close as possible t o the standard thermochemical reference temperature, 2j°C., was fulfilled by means of diphenyl ether (m.p. 26.n0C.) as the working substance. The instrument was calibrated electrically by comparison with the ice calorimeter arid the constant was found to be 19.01+0.02 cal. per gm. of mercury. From this and other lcnown properties of diphenyl ether the density of the solid a t the melting point was estimated a t 1.188 gm. per mI. 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 a n amount equivalent to the heat t o be measured. The calorimeter was operated in a large water-thermostat kept constant to within O.OOI°C. With highly purified diphenyl ether there was no noticeable temperature drift during the measurements. Quantities of heat up to GOO 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.