Calorimetric determination of dissolution enthalpy with a novel flow-through method

A new calorimetric flow-through system for determining the enthalpies of dissolution with small amount of solids (<1 mg) was developed. The system was designed to be used as an add-on cell with a 4 ml twin heat conduction calorimeter 2277 TAM but the principle is adoptable also for other heat conduction calorimeters. The system was tested with two salts (NaCl, KCl), sucrose and different polymorphic forms of theophylline at 25 °C and 40 °C by using water as the solvent. The system gave more accurate and precise results at 25 °C. The precision was not affected by the extent of the dissolution enthalpy. The accuracy was dependent on the calibration utilized but even the normal electrical calibration gave acceptable values. The results obtained at 40 °C were also acceptable but not as good as at 25 °C due to heat leaks. The effect of heat leaks can be minimized by heating the inflowing solvent outside the calorimeter prior to entering the flow-through cell.