Differential Isothermal Microcalorimeter: Heats of Mixing of Aqueous NaCl and KCl Solutions

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Abstract
A differential microcalorimeter operating under vacuum has been designed for the study of solutions. Constant and equal heating is supplied to twin cells which are in contact with a constant temperature cold source. A temperature variation produced in the working cell is detected by a thermistor bridge; the signal from this bridge serves to control the rate of heating in the working cell such that the appearing temperature difference is cancelled. The response time of the instrument is less than one 1 min and its sensitivity is 5 μcal; heat effects larger than 10 mcal can be measured to 1% or better. The heat of mixing of aqueous solutions of KCl and NaCl at 25°C have been measured between 0.1 and 1.2 molal.