J. Chem. Thermodynamics 43 (2011) 750–753 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct The standard molar enthalpies of formation of Pb 2 CrO 5 (s) and Pb 5 CrO 8 (s) by acid solution calorimetry Sulata Kumari Sahu a , Rajesh Ganesan a , T.G. Srinivasan b , T. Gnanasekaran a, ⇑ a b Liquid Metals and Structural Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Fuel Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India a r t i c l e i n f o Article history: Received 29 October 2010 Accepted 27 December 2010 Available online 8 January 2011 Keywords: Acid solution calorimetry Enthalpy of formation Lead chromates a b s t r a c t The standard molar enthalpies of formation, D f H m (298.15 K) of Pb 2 CrO 5 (s) and Pb 5 CrO 8 (s) were deter- mined using an isoperibol calorimeter. The enthalpies of solution of pure compounds, viz., Pb 2 CrO 5 (s), and Pb 5 CrO 8 (s) as well as mixtures viz., {PbCrO 4 (s) + PbO(s)} and {PbCrO 4 (s) + 4PbO(s)} in HNO 3 (aq., 8 mol dm 3 ) at T = 298.15 K were measured. Using these values, D f H m (298.15 K) of Pb 2 CrO 5 (s) and Pb 5 CrO 8 (s) were determined as (1171.5 ± 1.7) kJ mol 1 and (1850.6 ± 8.7) kJ mol 1 , respectively. O 2011 Elsevier Ltd. All rights reserved. 1. Introduction A study of the interaction of liquid lead with components of steel in the presence of trace amount of oxygen is needed in the context of lead-cooled nuclear reactors and accelerator dri- ven sub-critical systems (ADSS) [1]. Corrosion in the (lead + steel) systems is mitigated by the formation of an adherent oxide layer on the structural steels. A detailed knowledge of the composition and the thermochemical data of this oxide layer are needed to exploit this process. Understanding the thermo- chemistry of ternary compounds of the (Pb + Cr + O) and (Pb + Fe + O) systems is of considerable interest in this regard. Preparation and thermal stability of the compounds existing in (Pb + Cr + O) system have been reported in the literature [2–8]. Three stable compounds, viz., PbCrO 4 (s), Pb 2 CrO 5 (s) and Pb 5 CrO 8 (s) exist in this system. The enthalpy of formation of PbCrO 4 (s) at T = 298 K, D f H m 298K , had been determined by Dellien et al. [9] by solution calorimetry with NaOH and Na 4 EDTA as the solvents. No corresponding data for Pb 2 CrO 5 (s) and Pb 5 CrO 8 (s) are available in the literature. Recently, the partial phase dia- gram of (Pb + Cr + O) system was established at the author’s lab- oratory by long term equilibration studies [7] and also the standard molar Gibbs free energy of formation of Pb 5 CrO 8 (s), Pb 2 CrO 5 (s), and PbCrO 4 (s) were determined [8] by measuring equilibrium oxygen pressures over appropriate ternary phase fields either by manometry or by solid oxide electrolyte based emf cells. In the present work, the enthalpies of formation of ⇑ Corresponding author. Tel.: +91 44 2748 0302; fax: +91 44 2748 0065. E-mail address: gnani@igcar.gov.in (T. Gnanasekaran). 0021-9614/$ - see front matter O 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2010.12.015 Pb 2 CrO 5 (s) and Pb 5 CrO 8 (s) at T = 298 K have been determined using acid solution calorimetry. 2. Experimental 2.1. Chemicals High purity PbO (mass fraction purity >0.999 on metal basis, M/s Aldrich Chem. Co., USA) and Cr 2 O 3 (mass fraction purity >0.99999 on metal basis, M/s Johnson Matthey Materials Tech- nology, UK) powders were used for preparation of ternary compounds. Thermo gravimetric analyses showed that PbO(s) absorbs carbon dioxide (0.1 wt%) and Cr 2 O 3 (s) absorbs moisture (1.8 wt%) on prolonged exposure to ambient air. Hence, PbO(s) and Cr 2 O 3 (s) were calcined for 3 h under flowing argon gas at temperatures of (673 and 973) K, respectively, stored in desicca- tors and handled with very short exposure to ambient air. Doubled distilled AR grade HNO 3 and HCl which were diluted with the distilled water to the required concentrations were used as the calorimetric solvents. 2.2. Compound preparation Pb 5 CrO 8 (s), Pb 2 CrO 5 (s) and PbCrO 4 (s) were prepared by solid state reaction between powders of PbO(s) and Cr 2 O 3 (s) taken in the required stoichiometric ratios. The conditions used for prepara- tion of these ternary compounds are described in our earlier pub- lication [7]. The compounds were characterised by X-ray diffraction using a Siemens D500 X-ray powder diffractometer with Cu K a radiation and graphite monochromator. The XRD pat- terns obtained for Pb 5 CrO 8 (s), Pb 2 CrO 5 (s) and PbCrO 4 (s) matched