Thermochimica Acta 531 (2012) 6–11 Contents lists available at SciVerse ScienceDirect Thermochimica Acta journa l h o me page: www.elsevier.com/locate/tca Thermodynamic properties of potassium nitrate–magnesium nitrate compound [2KNO 3 ·Mg(NO 3 ) 2 ] Ramana G. Reddy ∗ , Tao Wang, Divakar Mantha Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487-0202, USA a r t i c l e i n f o Article history: Received 10 April 2011 Received in revised form 8 December 2011 Accepted 14 December 2011 Available online 24 December 2011 Keywords: DSC Mg(NO3)2–KNO3 binary system 2KNO3·Mg(NO3)2 compound Melting point Heat capacity Thermodynamic properties a b s t r a c t The Mg(NO 3 ) 2 –KNO 3 binary system phase diagram has a congruent melting compound, 2KNO 3 ·Mg(NO 3 ) 2 . The thermodynamic properties for this compound are not available in the liter- ature. In this study, the nitrate compound was synthesized and the melting point and heat capacity were determined using differential scanning calorimetry (DSC). Two endothermic peaks were observed at 404.8 K and 468.83 K corresponding to solid state transition and melting of the compound with the enthalpies of transitions as 2.71 kJ/mol and 20.73 kJ/mol, respectively. The heat capacity data as function of temperature are fit to polynomial function and thermodynamic properties like enthalpy, entropy and Gibbs energies of the compound as function of temperature are subsequently deduced. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Molten salts have been used as thermal energy storage media for solar energy applications. Nitrates are being used in the solar energy applications for their low melting point, low unit cost, high heat capacity, high thermal stability, negligible vapor pressure and high energy storage density [1]. Solar salt (NaNO 3 /KNO 3 : 60/40) is the most popular thermal energy storage medium which is cur- rently being used with the freezing point of 494.15 K [2]; another ternary system HITEC which contains NaNO 3 , KNO 3 and NaNO 2 has freezing point of 414.15 K [3]. Newer nitrate salt mixtures are being studied and projected as potential candidates for thermal energy storage (TES) and heat transfer (HT) applications. Based on these favorable features, molten salt can work directly as the energy storage medium below 773.15 K [1]. Development and synthesis of newer molten salt mixtures with freezing point lower than those currently used for thermal energy storage applications is necessary for sustained utilization of solar energy. The approach to develop lower freezing point molten salt mixtures is by the prediction of new eutectic mixtures and also by the development of new nitrate compounds. In this context, the congruently melting compound, 2KNO 3 ·Mg(NO 3 ) 2 can be a promising additive. ∗ Corresponding author. Tel.: +1 205 348 4246; fax: +1 205 348 2164. E-mail address: rreddy@eng.ua.edu (R.G. Reddy). The phase diagram of Mg(NO 3 ) 2 –KNO 3 binary system has two eutectics and one congruently melting solid phase, 2KNO 3 ·Mg(NO 3 ) 2 . The compound 2KNO 3 ·Mg(NO 3 ) 2 will be labeled as MgKN in this article. The two eutectic points appear on either side of the congruently melting solid. The melting point of the congruently melting compound, MgKN that can be read from the phase diagram is 498 K [4]. However, no experimental data are available in the literature on the accuracy of this melting point. Thermodynamic properties such as heat capacity, enthalpy, and entropy and Gibbs energy are also not available in the literature. In this paper, we determine the melting point and heat capacity of MgKN using the differential scanning calorimetry (DSC) technique to re-verify the melting point given in the phase diagram and also to deduce the thermodynamic parameters as function of temperature. 2. Experimental 2.1. Materials The MgKN compound is synthesized from magnesium nitrate hexahydrate (98%, Alfa Aesar) and potassium nitrate (ACS, 99.0% min, Alfa Aesar). Potassium nitrate is used without fur- ther purification whereas the magnesium nitrate hexahydrate is dehydrated before synthesizing the 2KNO 3 ·Mg(NO 3 ) 2 com- pound. The synthesized 2KNO 3 ·Mg(NO 3 ) 2 compound has 98% of purity. 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.12.010