Water Calorimetry: The Heat Defect

Domen developed a sealed water calorimeter at NIST to measure absorbed dose to water from ionizing radiation. This calorimeter exhibited anomalous behavior using water saturated with gas mixtures of H2 and O2. Using computer simulations of the radiolysis of water, we show that the observed behavior can be explained if, in the gas mixtures, the amount-of-substance of H2 and of O2 differed significantly from 50 %. We also report the results of simulations for other dilute aqueous solutions that are used for water calorimetry-pure water, air-saturated water, and H2-saturated water. The production of H2O2 was measured for these aqueous solutions and compared to simulations. The results indicate that water saturated with a gas mixture containing an amount-of-substance of H2 of 50 % and of O2 of 50 % is suitable for water calorimetry if the water is stirred and is in contact with a gas space of similar volume. H2-saturated water does not require a gas space but O2 contamination must be guarded against. The lack of a scavenger for OH radicals in "pure" water means that, depending on the water purity, some "pure" water might require a large priming dose to remove reactive impurities. The experimental and theoretical problems associated with air-saturated water and O2-saturated water in water calorimeters are discussed.