Introduction Enthalpy calculation is essentially needed in process simulation and design of heat-transfer equipment. Excess molar enthalpy data are useful for determination or verification of model parameters with which the enthalpy values can be calculated with confidence. Experimental H E results also provide information on the molecular interactions between constituent components. Diethyl carbonate is a solvent used in a variety of liquid extraction processes, especially in the synthesis of pharmaceuticals and agricultural chemicals. This compound is also a good gasoline additive [1] to pre- vent the crack of abnormal burning and promote the blending octane values. The present study is undertaken to measure excess enthalpies of {x 1 p-xylene+x 2 octane+(1–x 1 –x 2 ) diethyl carbonate}, {xp-xylene+(1–x) octane}, {xp-xylene+ (1–x) diethyl carbonate} and {x octane+(1–x) diethyl carbonate} at 298.15 K. Measurements were carried out using a Calvet microcalorimeter. Excess molar enthalpies of {x diethyl carbon- ate+(1–x) octane} and {x diethyl carbonate+(1–x) p-xylene} at 298.15 K were measured by García et al. [2] and Pei-Jung Lien et al. [3] respectively. We are not aware of any previous measurement of H m,123 E of the ternary mixture to which this study is directed. Experimental Chemicals Diethyl carbonate (Fluka, purity >99%), p-xylene (Fluka, purity >99%) and octane (Fluka, purity ³99.5%) were subjected to no further purification other than drying with Union Carbide 0.4 nm molecu- lar sieves to eliminate residual traces of water and de- gassed by ultrasound technique. Densities (Table 1) were measured with an Anton-Paar DMA 4500 densimeter (accuracy ±5×10 –5 g cm –3 ) at 298.15 K. Apparatus and procedure The mixtures were prepared by mass using a Mett- ler H51 balance (precision ±1×10 –5 g), ensuring a probable error in the mole fraction less than 10 –4 . The experimental excess molar enthalpies were measured using a Calvet microcalorimeter. A Philips PM2535 multimeter and a data acquisition system were linked to the microcalorimeter. Calibra- tion was performed electrically using a Setaram EJP30 stabilized current source. The apparatus and proce- 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 88 (2007) 2, 583–586 DETERMINATION OF EXCESS MOLAR ENTHALPIES FOR THE TERNARY SYSTEM p-XYLENE+OCTANE+DIETHYL CARBONATE P. V. Verdes 1 , M. M. Mato 2 , J. L. Legido 2 , E. Jiménez 3 and M. I. Paz Andrade 1* 1 Departamento de Física Aplicada, Campus Universitario Sur, Universidade de Santiago, 15782 Santiago de Compostela, Spain 2 Departamento de Física Aplicada, Facultad de Ciencias del Mar, Campus As Lagoas- Marcosende, Universidade de Vigo 36310 Vigo, Spain 3 Departamento de Física, Facultade de Ciencias, Universidade de A Coru Ôa, 15071 A CoruÔa, Spain Excess molar enthalpies, H E , for the binary mixtures {p-xylene+(1–x) octane}, {xp-xylene+(1–x) diethyl carbonate}, {x oc- tane+(1–x) diethyl carbonate} and the corresponding ternary system {x 1 p-xylene+x 2 octane+(1–x 1 –x 2 ) diethyl carbonate} have been measured by using a Calvet microcalorimeter at 298.15 K under atmospheric pressure. The experimental H E values are all pos- itive for the binary and ternary mixtures over the entire composition range. Keywords: diethyl carbonate, excess molar enthalpies, octane, p-xylene, ternary mixture Table 1 Densities of pure components at 298.15 K Compound Density/g cm –3 This work Literature p-xylene 0.8567 0.8567 [3] Octane 0.6986 0.6986 [4] Diethyl carbonate 0.9691 0.9692 [3] * Author for correspondence: fapazand@usc.es