The standard molar enthalpy of formation of LnPO 4 (s) (Ln 5 La, Nd, Sm) by solution calorimetry Deepak Rawat • Smruti Dash Indian Special Chapter Ó Akade ´miai Kiado ´, Budapest, Hungary 2012 Abstract The standard molar enthalpy of formation of LaPO 4 (s), NdPO 4 (s), and SmPO 4 (s) has been determined using an isoperibol solution calorimeter. The solution calorimeter vessel was held at 298.15 K. The precipitation reaction between aqueous solution of rare-earth chloride (LnCl 3 (aq.)) and ammoniacal solution of ammonium dihydrogen phosphate (NH 4 H 2 PO 4 (aq.)) was studied. The temperature of the calorimeter vessel was measured before, during, and after the reaction. The enthalpy change due to precipitation of LaPO 4 (s), NdPO 4 (s), or SmPO 4 (s) from required solutions was measured at 298.15 K. Using these values and other auxiliary data from the literature, ther- mochemical reaction scheme were devised to calculate the standard enthalpy of formation of each phosphate com- pound i.e., LaPO 4 (s), NdPO 4 (s), and SmPO 4 (s). The cal- culated values for LaPO 4 (s), NdPO 4 (s), and SmPO 4 (s) at 298.15 K were found to be -1947.5 ± 3.2, -1938.3 ± 3.6, and -1942.9 ± 3.4 kJ mol -1 , respectively. Keywords Standard molar enthalpy of formation  Isoperibol solution calorimeter  Monazite  Thermochemical cycles  Rare-earth phosphates Introduction Safe storage of radioactive waste is a major challenge for the nuclear industry. At present, nuclear waste are immobilized in glass matrix. The main weakness of a glassy waste form is that fundamentally glass is a meta-stable material which tends to re-crystallize, although this process is very slow. Crystalline waste forms (ceramics) are expected to be several orders of magnitude more durable than glasses [1]. Miner- alogy is a good basis for designing ceramics as it provides reliable data for the long-term behavior of ceramic matrix in natural environment. Natural radioactive minerals are the only source of information for long-term accumulation of radiation damage in crystals. The zircon, zirconolite, hol- landite, synroc, brannerite, apatite, and monazite are the potential minerals for storage of nuclear waste. The present study is focused on monazite which is the natural light rare- earth orthophosphate (AXO 4 , P2 1 /n), where A site is occu- pied by large cations, nine fold coordinated, such as trivalent rare-earth elements (RE) or other cations similar in size (Ca 2? ,U 4? , Th 4? ) while the X-site contains small, tetrahe- drally coordinated cations such as P 5? . The REO 9 coordi- nation polyhedra can be explained by a combination of a pentagon and a tetrahedron unit. The REO 9 unit shares its two opposite edges with PO 4 unit to form an infinite alter- nating chain along c-axis [2, 3]. In addition to RE 3? , both trivalent (Am 3? , Cm 3? , Pu 3? ) and tetravalent (Np 4? ,U 4? , Th 4? ) actinides can be incor- porated in monazite by substitution with RE 3? for trivalent ions and by coupled substitution An 4? ? Ca 2? = 2RE 3? (An = actinide) for tetravalent ions [4]. The evaluation of the long-term durability and reactivity of rare-earth materials require accurate determination of their thermodynamic properties. Thermodynamic properties should be measured in different methods to select the true value. The solution calorimetry offers an effective methodology for the deter- mination of enthalpies of formation. In this study, enthalpy of formation of LaPO 4 (s), NdPO 4 (s), and SmPO 4 (s) have been measured using semi-adiabatic solution calorimeter. D. Rawat  S. Dash (&) Product Development Division, Bhabha Atomic Research Centre, Mumbai 400085, India e-mail: smruti@barc.gov.in 123 J Therm Anal Calorim DOI 10.1007/s10973-012-2742-3