Vapor-liquid-solid phase transitions in aqueous sodium sulfate and sodium carbonate from heat capacity measurements near the first critical end point. 1. Sodium sulfate

Heat capacities at constant volume and composition are reported for aqueous sodium sulfate for three compositions, 1 mass %, 5 mass %, and 10 mass %, respectively, and along 19 isochores, from 250 kg .m(-3) to 1073 kg .m(-3) in the temperature range from 350 K to 670 a. In the range from 630 K to 65 0 K, many of these isochores display two features in the heat capacity as a function of the temperature: one peak or step when solid salt first precip itates and, on further heating, a lambda -shaped peak when the vapor- or li quid-phase disappears. In Part 2 of this paper, these features will be inte rpreted.