Kinetics and Thermodynamics of the Formation of MnFeP 4 O 12 Banjong Boonchom* ,†,‡ and Montree Thongkam ‡ King Mongkut’s Institute of Technology Ladkrabang, Chumphon Campus, 17/1 M.6 Pha Thiew District, Chumphon, 86160, Thailand, and Department of Chemistry, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Ladkrabang, Bangkok, 10520, Thailand The kinetics and thermodynamics of dehydration and intermolecular polycondensation reactions of Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 · H 2 O and Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 were studied under nonisothermal heating by thermo- gravimetry (TG). On the basis of three calculation procedures and 24 kinetic mechanism functions, kinetic triplets (E, A, kinetic model) using the Arrhenius equation were calculated for both processes. The comparison of the results obtained with these calculation procedures show a strong dependence on the selection of the mechanism function for the process. The thermodynamic functions (∆H*, C p , ∆G*, and ∆S*) of both reactions are calculated from differential scanning calorimetry (DSC) experiments. Kinetic and thermodynamic results indicate that the polycondensation step has a lower rate and is a stronger reaction than the dehydration step and corresponds to the breaking of the strong hydrogen- bonded P-OH group, in connection with the polycondensation reaction, which confirms that the decomposition product (manganese iron cyclotetraphosphate, MnFeP 4 O 12 ) was obtained. Introduction The cyclotetraphosphates of some bivalent metals are rela- tively stable compounds, both thermally and chemically. 1-5 This phosphate group exhibits properties of color anticorrosion ability and luminescence which allow their application as special inorganic pigments. 6-9 Additionally, they are a good source for macro- (P) and micronutrient (Ca, Mg, Fe, Mn, Co, Ni) fertilizers due to their solubility in soils. They are formed by the calcination of dihydrogenphosphates of bivalent metals or by the calcination of a mixture containing phosphorus and bivalent metal components in the corresponding ratio. 9-12 A number of papers have dealt with the use of thermal analysis (TA) under quasi-isothermal, quasi-isobaric conditions to follow the dehydration reactions of some binary metal dihydrogen- phosphate hydrates. 13-15 So far, however, no report has appeared on the application of kinetic and thermodynamic methods to the investigation of the important condensation products, cyclotetraphosphates. The presence of water molecules of binary dihydrogenphosphate hydrates influences the intermolecular interactions (affecting the internal energy and enthalpy) as well as the crystalline disorder (entropy) and, hence, influences the free energy, thermodynamic activity, solubility, stability, and electrochemical and catalytic activity. 16-20 Studies on the thermodynamics, mechanisms, and kinetics of solid-state reac- tions are a challenging and difficult task with complexity resulting from a great variety of factors with diverse effects such as reconstruction of solid state crystal lattices, formation and growth of new crystallization nuclei, diffusion of gaseous reagents or reaction products, materials heat conductance, and static or dynamic character of the environment, physical state of the reagentssdispersity, layer thickness, specific area and porosity, type, amount, and distribution of the active centers on solid state surface, etc. 19-22 The results obtained from such studies can be directly applied in materials science for the preparation of various metals and alloys, cements, ceramics, glasses, enamels, glazes, polymers, and composite materials. 21-25 The aim of the present paper is to study the kinetic and thermodynamic parameters of the formation of MnFeP 4 O 12 from the decomposition of Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 · H 2 O and was followed using differential thermal analysis-thermogravimetry (TG/DTG/ DTA), differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), and Fourier transform-infrared (FT-IR) spectroscopy. This work seeks to characterize the thermal decomposition processes of Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 · H 2 O, in relation to its thermodynamic (∆H*, C p , ∆S*, ∆G*) and kinetic (E, A, mechanism and model) properties, which are discussed for the first time. Experimental Section The starting binary dihydrogenphosphate hydrate Mn 1/2 Fe 1/2 - (H 2 PO 4 ) 2 · H 2 O (white-gray powder) was prepared in our labora- tory from the Fe(c)-Mn(c)-H 3 PO 4 system in a water-acetone medium at ambient temperature. 25 The qualities of the starting reactant and decomposition product were confirmed by atomic absorption spectroscopy (AAS, Perkin-Elmer, Analyst100), FTIR spectrophotometry (a Perkin-Elmer Spectrum GX FT-IR/FT- Raman spectrometer), X-ray diffraction analysis (a D8 Advanced powder diffractometer, Bruker AXS, Karlsruhe, Germany), and scanning electron microscopy. 25 Thermal analysis measurements (thermogravimetry, TG; differential thermogravimetry, DTG; and differential thermal analysis, DTA) were carried out by a Pyris Diamond Perkin-Elmer apparatus by increasing the temperature from (323 to 673) K with calcined R-Al 2 O 3 powder as the standard reference. The experiments were performed in static air, at heating rates of (5, 10, 15, and 20) K · min -1 . The sample mass was between (6.0 and 10.0) mg and placed into an alumina crucible without pressing. Differential scanning calorimetry was carried out for samples [(5 to 10) mg] in aluminum crucibles, over the temperature * Corresponding author. Tel.: +66-7750-6422ext 4565. Fax: +66-7750- 6410. E-mail address: kbbanjon@kmitl.ac.th. † King Mongkut’s Institute of Technology Ladkrabang, Chumphon Campus. ‡ Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang. J. Chem. Eng. Data 2010, 55, 211–216 211 10.1021/je900310m CCC: $40.75  2010 American Chemical Society Published on Web 09/16/2009