Praseodymium selenites

Aleksandrovich and Serebrennikov [62ALE/SER] measured the solubility of Pr2(Se03)3(s) at (298.15 + 0.05) K in HCl, HNO3, and H2SO4 of unspecified concentration. The experiments and the evaluation of the solubility product of the reaction  

Maier et al. [69MAI/SOL], [70KAR/MAI], [71MAI/SUP] prepared a number of praseodymium selenites. The enthalpies of dissolution of the selenites, PrCl3-7H20(cr), and H2Se03(cr) in HCI(aq, 1 18.5) were measured and have been used in Appendix A to calculate the following enthalpies of formation  

Since the calculation of the formation enthalpies from the experimental results involved non-TDB auxiliary data, they are included in Appendix E. 

The standard enthalpies of formation of Pr2(Se03)3-3Se02, (-3545.2), Pr2(Se03)3 Se02, (-3108.3), and Pr203-Se02, (-2302.4) kJ-mol evaluated from [70KAR/MA1] are not accepted as the solid phases were not sufficiently well characterised in the paper. 

DTA and weight loss curves show that on heating in air H2O is lost at 160°C and Se02 at 600°C in a single step. Intermediate decomposition products are not observed [2,10]. 

It was calculated from the solution enthalpy -14.58 0.14 kcal/mol In 10% HCl at 25°C at a dilution of 1 4000 and from literature data [2]. The enthalpy of formation AH298= -701.95 (or -690.5) kcal/mol is calculated for the amorphous product from the reaction enthalpy of Na2Se03 with PrCl3 (1 3000) in aqueous solution and by considering different literature data on the formation enthalpy of PrCl3-nH20, Maier et al. [3]. 

The anhydrous compound is formed on heating the dihydrate to 145°C, Savchenko etal. [1], or 170°C. Interplanar spacings and line intensities of the X-ray powder diagram are given. The standard enthalpy of formation AH298=-523.50 1.04 kcal/mol is calculated according to  

Pr(cryst) + 3Se(cryst) -1- V202(gas) + V2H2(gas) Pr(HSe03)3(cryst) 

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KAR/MA1] Karapet yants, M. K., Maier, A. 1., Soldatova, T. A., Enthalpies of formation of certain praseodymium selenites, Izv. Akad. Nauk SSSR, Neorg. Mater., 6, (1970), 1118-1122, in Russian, English translation in [70KAR/MA12]. Cited on pages 378, 521. 

Maier, Suponitskii, and Karapet yants prepared anhydrous crystalline lanthanum, praseodymium, and neodymium selenite by keeping the amorphous salts at an elevated temperature in a sealed tube. The salts are denoted by M2(Se03)3. The standard enthalpies of formation of the compounds have been calculated from the experimental results and auxiliary data in Table A-87. The solvent (sin) employed in the calorimetric dissolution experiments was HCI(aq, 1 18.5). Dissolution was made to the appropriate concentrations and the composition of the solutions will be denoted simply as dilute (dil). 

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