Appendix Selenite

The data have been reconsidered by the review and the following observations were made, see also Appendix A. The solid selenium used in the experiments was obtained by reduction of a selenite solution by thiosulphate. The selenium might therefore not be in its standard state. The activity of the specimen is most likely elose enough to the standard state activity, however, since the precipitate was kept at boiling temperature for several hours. A recalculation of the side-reactions with more recent values of the auxiliary equilibrium constants made little difference to the result. The analytical data are not always consistent with the stoichiometry of Reaction (V.24) and the known initial composition of the test solution. The authors also observed this and held oxidation of iodide by initially present oxygen responsible for the discrepancies. Flowever, in some instanees the deviations from the expected concentrations are remarkably large. The deviations do not invalidate the results if equilibrium prevails, which was tested. 

Chukhlantsev and Tomashevsky [57CHU/TOM] prepared lead selenite by mixing an acidified 0.2 M solution of lead acetate and a 0.1 M sodium selenite solution in stoichiometric amounts. The precipitate was crystalline. Chemical analysis confirmed the 1 1 ratio between Pb(Il) and Se(IV). No X-ray diffraction measurements were performed. The solubility of the specimen in dilute solution of nitric or hydrochloric acid was measured at 293 K. The experiments were performed and the data recalculated as described in Appendix A, [56CHU]. The result for ... 

Popova, Slavtscheva, and Gospodinov [86POP/SLA] determined the solubility of aluminium selenite in dilute nitric acid. The preparation and composition of the selenite is not discussed. It is assumed to have the formula Al2(Se03)3-6H20(cr) by the review. The solubility product calculated in the paper, see Appendix A, for Reaction (V.65),... 

Tananaev, Volodina, Selivanova and Bol shakova, [76TAN/VOL], [76TANA OL2] prepared a number of gallium selenites and determined their standard enthalpies of formation from calorimetric measurements of enthalpies of dissolution in HCl(aq, 1 8). The experimental data have been re-evaluated, see Appendix A, to yield ... 

In the aqueous phase, Umland and Wallmeier [80UML/WAL] studied the reduction of selenite at the mercury electrode in the presence of Zn by polarography, see Appendix A. The solubility product of ZnSe(s) was obtained from the position of the half-wave potential of the second reduction step, HgSe(s) + Zn "" + 2e" ZnSe(s) + Hg(l), to be log (ZnSe, s, 298.15 K) = -(23.212.0). Combined with CODATA for Zn and the selected value for Se l it corresponds to AfG° (ZnSe, s, 298.15 K) = - (151.0 11.8) kJ mol. The value is in disagreement with the value obtained by extrapolation of high temperature data. Since the validity of the polarographic method has not been documented, the value of AfG°(ZnSe, a, 298.15 K) obtained from the extrapolation of the high temperature data has been selected. 

As described in Appendix A, Ripan and Vericeanu [68RIP/VER] studied the solubility of zinc selenite by conductivity measurements. A calculation based on the equilibrium constants in [76BAE/MES] and the total concentrations provided in the paper, mean 1.40 x 10 M, leads to the conclusion that about 30% of dissolved Zn(ll) would be present in hydrolysed forms, if the solvent were conductivity water. No reliable solubility product can therefore be derived from the conductivity data. The value logio = -(7.71+0.05) was estimated by Masson, Lutz and Engelen [86MAS/LUT] from the data, but the composition of the solid phase is apparently not known. 

In the aqueous phase, Umland and Wallmeier [80UML/WAL] studied the reduction of selenite at the mercury electrode in the presence of Cd by polarography, see Appendix A. The solubility product of CdSe(s) was obtained from the position of the half-wave potential of the second reduction step, HgSe(s) + Cd + 2e CdSe(s) +... 

Ripan and Vericeanu [68R1P/VER] studied the solubility of CuSe03-2H20(s) by conductivity measurements as described in Appendix A. An equilibrium analysis performed as outlined there on the total concentrations provided in the paper, about 1.8 X 10 M, leads to the conclusion that almost 80% of the copper(II) would be present as hydroxo complexes at equilibrium. Hence, a conductivity measurement is unsuitable for the determination of the solubility of copper selenite and no reliable value of the solubility product can be calculated from the data in [68R1PA ER]. Masson et al. [86MAS/LUT] calculated log (, = - (7.49 0.10) from the data neglecting hydrolysis of Cu. ... 

Feroci et al. [97FER/F1N] studied the system Cu -SeOj by polarography. No shift in /2, but a steady decrease in the limiting current, was observed with increasing selenite concentration. It was assumed from these results that solid copper selenite was formed, although no precipitate was observed in the electrode vessel. The solubility product was reported as log Al, ((V. 106), 0.15 M NaNOa, K) = - 8.42. As discussed in Appendix A, the result of this investigation can not be considered as reliable. 

Mehra and Gubeli [68MEH], [69MEH/GUB] made extensive measurements of the solubility of Ag2SeO ,(s) in aqueous solutions at 298.15 K as a function of pH and total selenite concentration. From these measurements, which are presented and evaluated in Appendix A, the review accepts the results log /f, ((V.l 14), 1 M NaCI04, 298.15 K) =- (15.40 + 0.35) and the stability constant of the reaction ... 

Selivanova, Leshchinskaya, and Klushina [62SEL/LES] measured the enthalpy change when crystalline silver selenite was formed from AgN03(cr) and a solution of sodium selenite in a calorimetric experiment. Their result is used in Appendix A to estimate the standard enthalpy of formation of Ag2Se03(cr) to be Af//° (Ag2Se03, cr, 298.15 K) = - (363.44 1.02) kJ-mol. This value agrees well with [82WAG/EVA] but differs appreciably from the result in the paper, - 345.0 kJ-moE. ... 

Pyatnitskii and Durdyev [66PYA/DUR] attempted to determine the stability constants of cobalt-selenite complexes from solubility measurements in selenite solution. As discussed in Appendix A, their equilibrium model is likely to be incorrect. The proposed equilibrium constant of the reaction ... 

Rai, Felmy, and Moore [95RA1/FEL3] made a serious attempt to determine the solubility product of iron(lll) selenite as described in Appendix A. Their work shows that Fe2(Se03)3-6H20 is the stable phase in contact with an aqueous phase with a pH below 4. Above this value the selenite starts to transform into other, basic phases of unknown composition. 

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 ... 

Maier, 8uponitskii, and Karapet yants [67MAI/8UP] made a calorimetric determination of the enthalpy change of the reaction between an aqueous solution of ce-rium(III) chloride and Na28e03(cr). The cerium selenite formed was amorphous. As no other datum exists for the enthalpy of formation of this compound the result of the investigation is included in the review but not selected. The calorimetric measurement is re-evaluated in Appendix A on the assumption that the selenite contains 10 molecules of water of crystallisation. The result is Af//° (Ce2(8eO3)3-10H2O, am, 298.15 K) = -(5732.8 +5.3) kJ-mol. ... 

Maier, Suponitskii, and Karapet yants [71MA1/SUP] prepared anhydrous crystalline lanthanum selenite and measured its enthalpy of dissolution in HCl(aq, 1 18.5) in a calorimeter. The data are evaluated in Appendix A and yield... 

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