Isothermal Calorimeter and the Heat of Mixing of Iso‐Octane‐Perfluoroheptane Solutions

An isothermal calorimeter for measuring heats of mixing of binary solutions has been designed. The instrument utilizes twin calorimeters to eliminate troublesome corrections and a pair of differential thermistors for controlling the electrical circuit. There is no vapor space and the rate of mixing is controlled so that the heat absorbed is automatically compensated and the maximum temperature change is of the order of 0.001° to 0.002°C. This is accomplished by a differential bridge.The heats of mixing of perfluoroheptane and isooctane were measured and the results compared with the values calculated from vapor pressure measurements.