the totality of methods for determining the heat effects (quantities of heat) accompanying various physical, chemical, and biological processes. Calorimetric methods are used to determine heat capacities of materials; heats of phase transitions (melting, boiling, and other transitions); heat effects of magnetization, electrification, solution, sorption, chemical reactions (for example, combustion), and metabolic processes in living organisms; and in a number of cases, the energy of electromagnetic radiation and that of nuclear processes.
The apparatus used in calorimetric measurements are called calorimeters. Their design is determined by the conditions of measurement (primarily by the temperature interval) and by the required accuracy. Calorimeters for use above 400°K (arbitrary limit) are called high-temperature calorimeters, whereas those for use in the temperature range corresponding to liquid nitrogen, hydrogen, and helium are called low-temperature calorimeters.
The results of calorimetric measurements are widely used in heat engineering, metallurgy, and chemical technology. They are used to calculate the quantities of heat required to heat, melt, or vaporize materials in various engineering processes and to calculate the time limits of chemical reactions and the conditions for carrying out these reactions. Thus, the temperature and pressure range in which synthetic diamonds are obtainable from graphite were determined by calculations based to a large extent on calorimetric determinations of heat capacities and heats of combustion of these materials. Calorimetric measurements make it possible to determine the regions of stability of various minerals and to elucidate the conditions for their simultaneous presence in rocks. Low-temperature calorimetric data are being widely used in studies of mechanical, magnetic, and electrical effects in solids and liquids at low temperatures as well as in calculations of thermodynamic functions (for example, the entropy of substances).
B. A. SOKOLOV
In biology, calorimeters are used for determining the heat effects accompanying the processes of life. Two types of chemical processes are continuously occurring in organisms: endothermic processes (with heat absorption) and exothermic processes (with heat evolution), the latter type predominating. Calorimetry has shown, for example, that a coliform bacterium evolves 4 × 10-9 joules (J) (10-9 cal) of heat per hour; a mouse evolves 420 J (100 cal); and a human evolves 2 × 105 J, or ∽ 5 × 104 cal [specific heat evolution presents a completely different picture: 1, 050 J/(g-hr), 21 J/(g-hr), and 4 J/(g-hr), respectively]. The organisms are usually placed into a calorimeter for measurement of their heat production. When direct calorimetry is difficult, indirect methods are used. Indirect determination of the heat production by an organism may be performed, for example, on the basis of the intensity of its gas metabolism. In this case, the quantities of oxygen (O2) absorbed by the organism per unit time and the quantities of carbon dioxide (CO2) liberated per unit time are measured. The ratio of these quantities (respiratory coefficient) yields the quantity of O2 expended separately for the oxidation of proteins, fats, and carbohydrates. The heat content of these reactions is known, which makes it possible to calculate the total heat production of the organism.
V. A. BERNSHTEIN