Selenic acid-selenate system

The result of this measurement seems to form the basis of later evaluations of the standard enthalpy of formation of selenic acid. 

Trzil [58TRZ] calculated Af//° (H2Se04, aq 1 1200) by the thermochemical cycle shown in [58TRZ] in Appendix A to be -601.65 kJ mol. This cycle has also been employed by the review to obtain this datum. The auxiliary data introduced in the calculation are shown in the same reference. The result is - (590.80 1.67) kJ-mok . 

The value reported in [82WAG/EVA] is -587.0 kJ-moP. The more negative value calculated by Trzil is mainly caused by an earlier accepted value of the enthalpy of formation of Se02(cr), which was about 10 kJ-mol too exothermic. The review adopts  

The standard enthalpy of formation of H2Se04(cr) is obtained by combining this value with heats of dissolution of the acid. 

cited in [58TRZ], measured integral heats of dissolution of H2Se04(cr) and obtained (uncertainty added here)  

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SEL/SOB] Selivanova, N. M., Sobol, L. G., Gorokhov, V. K., The sodium selenate-selenic acid-water system at 25°C, Zh. Neorg. Khim., 16, (1971), 2800-2803, in Russian, English translation in... 

The situation in selenate systems is somehow complicated since 100% selenic acid shows different properties. In contrast to the respective sulfate systems, the systems M2Se04/H2Se04/H20 are not stable at low water concentrations and temperatures above 150 C. This requires working at lower temperatures. However, it takes much longer in comparison to sulfate systems, due to the considerably higher viscosity of selenate systems. In view of this difficulty, it is often a problem to perform the crystallization properly. 

In searching to formulate a mechanism of CuInSc2 phase formation by one-step electrodeposition from acid (pH 1-3) aqueous solutions containing millimolar concentrations of selenous acid and indium and copper sulfates, Kois et al. [178] considered a number of consecutive reactions involving the formation of Se, CuSe, and Cu2Se phases as a pre-requisite for the formation of CIS (Table 3.2). Thermodynamic and kinetic analyses on this basis were used to calculate a potential-pH diagram (Fig. 3.10) for the aqueous Cu+In-i-Se system and construct a distribution diagram of the final products in terms of deposition potential and composition ratio of Se(lV)/Cu(ll) in solution. 

In addition to sulphate, selenate (J. M. Bigham, unpubl.) and chromate (S. Regen-spurg unpubl.) can also be incorporated in the tunnels of synthetic schwertmannite. Whether or not two different Se-O distances (based on EXAFS) attributable to surface and tunnel selenate, respectively, exist in the Se-form is still under discussion (Waychunas et al., 1995, 1995 a). The Cr form has the bulk composition Fei6Oi6(OH)i0.23(CrO4)2.gg. In fact, synthetic schwertmannite formed in the sul-phate/arsenate system tolerates arsenate only up to a As/(As-rS) mole ratio of ca. 0.5, and it is likely that most of this arsenate is surface-bound. Above this ratio, a new, very poorly ordered Fe-hydroxy arsenate with two broad XRD peaks at ca. 0.31 and 0.16 nm and BhfS at 4.2K and ca. 1.5 K of 41.6 and 47.3T, respectively, forms (Carlson et al. 2002). From this one may conclude that, whereas the tetrahedral oxyanions with hexavalent central cations (S Se Cr) can be accomodated in the tunnel positions, the pentavalent cations can not, or not as easily. Schwertmannite from acid mine water contained between 6 and 70 g kg As (Carlson et al. 2002). 

In both these systems selenium is introduced into the ring using an aqueous solution of selenous acid. Thus, the diaminopyrimidines (191) produce the [l,2,5]selenadiazolo[3,4-[Pg.510]

ALP/SAY] Macalpine, G. B., Sayce, L. A., The system cupric selenate-water-selenic acid at 25°, J. Chem. Soc., (1932), 1560-1565. Cited on pages 146,295. 

K1N] King, G. B., The system ammonia-selenic acid-water at 30°C, J. 

FUR/KIN] Furukawa, H., King, G. B., The system magnesium selenate-selenic acid-water at 30 C, J. Phys. Chem., 48, (1944), 174-178. Cited on page 397. 

HYDROGEN BONDING SYSTEMS IN ACID METAL SULFATES AND SELENATES... 

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