Zinc selenites

Zinc selenite exits in two crystalline forms, a- and p-ZnSeOs. The a form appears to be metastable at ordinary temperatures. Sapozhnikov and Markovskii [66SAP/MAR] de- 

Gospodinov and Bogdanov [83GOS/BOG] studied the behaviour of a-ZnSe03 on heating. A transition of the a form to the P form occurs at (600 5) K according to measurements with differential scanning calorimetry. The enthalpy of the transition observed was (ZnSeOs, a, 600 K) = (13.22 0.13) kJ-mol. Fusion occurs at (913 5) K 

The agreement between the two investigations is poor, particularly at high temperatures. The results for Reaction (V.77) will not be accepted. 

An estimate of the standard entropy of P-ZnSe03 is calculated as the average from [64BAK/BUK] and [72POL/MAK] to be (ZnSeOj, P, 298.15 K) = (122.2 15.0) J K mol. Being an estimate this value does not quality for selection. 

37 X 10 T ) J-K mol. This result is not accepted since the value of C° and its temperature variation appear to be too large. 

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One-dimensional zinc selenite. [C3N2H 2]4[Zn4(Se03)8] is formed by the connectivity of the strictly alternating Zn04 and Se03 units which results in the formation of corner sharing 4-membered ring chains, similar to that observed in metal phosphates and sulfates.90... 

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 view of the scarce and conflicting information available the review cannot select a value of the solubility product of zinc selenite, but logi A" , ((V.78), 298.15 K) < - 7.25 is indicated for an aged precipitate of zinc selenite. 

The primary solubility data and the calculation of the solubility products, defined in the usual way, are presented. This presentation contains some unexpected results. The total solubility of the metal ion and selenite are approximately equal in water and in the inert salt solutions for the magnesium and manganese selenites. This is the expected result for a simple dissolution reaction. For calcium selenite, the metal ion concentration was about 100 times greater than the total selenite concentration whereas for zinc selenite the opposite was found. There is no comment in the paper on these results, which contradict the equilibrium reactions used in the paper to define the reported solubility products for calcium and zinc selenite. The review also noted that the calculation of the magnesium and selenite activities from the total concentrations introduces activity coefficients between 0.1 to 0.01 at moderate ionic strengths. Thus the values of these coefficients appear unreasonably small. On the whole, the activity coefficient corrections introduced appear to vary in an erratic way between the various systems studied. 

LES/SEL2] Leshchinskaya, Z. L., Selivanova, N. M., Heat of formation of zinc selenite (ZnSe03.H20), Russ. J. Phys. Chem., 38, (1964), 527-528. From a citation in this Bibliography... 

POL/MAK] Polukarov, A. N., Makhmetov, M. Z., Dissociation pressure of zinc selenite, Zh. Fiz. Khim., 46, (1972), 1059, in Russian, English translation in [72POL/MAK2]. Cited on pages 261,262. 

EBE/M1C] Ebert, M., Micka, Z., Uchytilova, M., Zinc selenites The... 

Zinc selenite OjSeZn 621 46.4 Benzoic acid CaHaO, 122.35 18.02... 

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