Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

The use of a good chemical calibration or test reaction in Isothermal Titration Calorimetry is crucial for getting reliable enthalpy values that can be compared across different laboratories. Indeed most titration calorimeters are used to measure both equilibrium constants and molar enthalpies of reaction. But a necessary prerequisite for such measurements is to first perform the enthalpy measurement accurately and precisely. The values of the equilibrium constant(s) are then calculated by regression from an appropriate model. As such, we found it timely to extensively test a previously proposed test reaction, the dilution of propan-1-ol into water, using two calorimeters of different design (heat conduction and power compensation calorimeters) and sensitivity. Experiments were performed at 298.15 K for the previously suggested 10% mass fraction propan-1-ol solution, as well as for the lower concentrations of 5% and 2% mass fractions. Due to our capacity to use insertion heaters with one of the used calorimeters, which allows for very accurate calibration constants to be obtained, we also determined a value for the enthalpy of dilution of 10% mass fraction solution at 308.15 K, previously not available, and closer to the temperatures commonly used in titration experiments involving biological samples. The observed change in the enthalpy of dilution was found to decrease in absolute value, but to an extent that did not justify the determination of new values for the less concentrated solutions. The values obtained with the two calorimeters are in excellent agreement, as well as with the values from the literature for the 10% mass fraction solution at 298.15 K. This reaction is thus again proposed as an excellent test reaction and the detailed conditions of their use depending on instrument sensitivity are suggested. In summary, the values for the enthalpies of dilution to infinite dilution DdilH1m at 298.15 K are (1.540 +- 0.021) kJ mol 1, (0.604 +- 0.020) kJ mol 1, and (0.186 +- 0.011) kJ mol 1 for the 10%, 5%, and 2% mass fraction solutions, respectively, and at 308.15 K (1.486 +- 0.017) kJ mol 1 for the 10% mass fraction solution.