We have used a fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter (d.s.c.) to measure heat capacities of NaCl(aq) at T = (283.15 to 393.15)K at the pressure 0.3(5) MPa. Consecutive scans at a rate of 16.7 mK . s(-1) gave results that were reproducible to +/-(1 to 2) mu W for filled cells of nominal volume 0.9 cm(3). The difference between volumic heat capacities of liquids in the working cell for two separate experiments is proportional to the difference between the calorimetric outputs from temperature scans run at the same scan rate and pressure. In a "baseline" experiment, scans are made when both cells contain water. In the second experiment, the reference cell contains water and the sample cell contains the solution of interest. The calibration constant was determined over the temperature range of the experiments from results of measurements on a solution of known volumic heat capacity, 1 mol . kg(-1) NaCl(aq). More dilute solutions of NaCl(aq) were then treated as "unknowns" to test the calorimeter's ability to measure volumic heat capacities. Apparent molar heat capacities C-p,C-phi calculated from the results of multiple temperature scans both upward and downward, and also from replicate experiments, have root-mean-square deviations Delta(1) less than (0.9, 1.8, 3.2, 3.6, 34, and 68) J . K-1 . mol(-1), respectively, for m = (0.3, 0.1, 0.03, 0.01, 0.003, and 0.001) mol . kg(-1) NaCl(aq) at T = (283.15 to 393.15)K and at p = 0.3(5) MPa. Deviations Delta(2) of mean values of C-p,C-phi of these six solutions from those calculated at T = (288.15 to 393.15)K and p = 0.3(5) MPa using Archer's equations (J. Phys. Chem. Ref. Data 1992, 21, 793-829) ranged from {(-0.4 to 0.4), (-0.6 to 0.3), (0.2 to 1.1), (-1.3 to 1.7), (-8.2 to -0.3), and (-5.4 to 3.4)} J . K-1 . mol(-1), respectively. The results indicate that this calorimeter is capable of yielding values of volumic heat capacities for the most dilute solutions with a relative precision of 10(-5) to 5 . 10(-5) between T = (283.15 to 393.15)K at p = 0.3(5) MPa. (C) 1997 Academic Press Limited.
