Adiabatic and Isoperibol Calorimeters

Adiabatic and Isoperibol Calorimeters.—Most calorimeters used in combustion and reaction calorimetry undergo a change of temperature when reaction takes place. If the calorimeter is surrounded by a jacket, the temperature of which is controlled to be the same as that of the calorimeter, no heat-exchange occurs between the siuroundings and the calorimeter, which is then described as adiabatic. However, if the temperature of the environment is maintained constant (in a type of calorimeter conveniently described as isoperibol and sometimes, incorrectly, as isothermal) some heat-exchange occurs between the calorimeter and its surroundings, but may be accurately determined by analysis of the temperature-time curves before and after reaction takes place, provided the reaction is of short duration (say not exceeding 15 min). With slower processes, isoperibol calorimeters are less useful, and the adiabatic principle is easier to effect and yields more accurate results. 

In isoperibol and adiabatic calorimeters the quantity actually measured is the temperature change of a calorimeter. Although a detailed discussion of thermometry would be inappropriate, it is useful to survey briefly the main methods used in reaction calorimetry. 

Very few modem calorimeters employ mercury-in-glass thermometers. The limit of accuracy of the most accurate instrument of this type, the Beckmann thermometer, is about 0.001 K it is easily broken, and subject to errors caused by exposed stem, pressure, sticking of the mercury column, and drift in calibration. 

An important development in recent years has been the advent of quartz crystal thermometry. When a quartz crystal is cut at a certain orientation to the axes of the crystal lattice the temperature dependence of the resonant frequency is large and nearly linear, and the Hewlett Packard Company has 

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