Standard states for bomb calorimetry

Abstract

An examination of the thermodynamics of the conditions existing in bomb calorimetry shows that the heat of combustion per unit mass of substance burned is a function of the mass of sample used, of the initial oxygen pressure, of the amount of water placed in the bomb, and of the volume of the bomb. In order to eliminate the effects of these at present unstandardized variables and to obtain a more generally useful thermal quantity which characterizes the pure chemical reaction for stated conditions, it is suggested that every bomb-calori-metric determination be first corrected (where such correction is significant) so as to give the value of A£/r, the change of ''intrinsic" energy for the pure isother-mal reaction under the pressure condition of 1 normal atmosphere for both re-actants and products. From this value the more generally useful quantity, Q v , the heat of the pure reaction at a constant pressure of 1 atmosphere is readily calculable. An equa-tion for calculating the correction is given and illustrated by examples. The magnitude of the correction varies from a few hundredths of 1 per cent up to several tenths of 1 per cent according to the nature of the substance burned and the conditions prevailing in the bomb during the combustion. It is further recommended that, in approving, for the purpose of standardizing a calorimeter, a particular value for. the heat of combustion (in the bomb) of a standardizing substance, such as benzoic acid, the value approved be accom-panied by specification of the oxygen concentration and of the ratios to the bomb volume of (1) the mass of the sample and (2) the mass of water, together with appropriate tolerances. An equation is given for correcting to any desired standard temperature the heat measured in the bomb calorimeter.