Aluminium selenates

Most investigators [50BAS], [63SEL/SAM], [69ROS/PER], [870JK/STA] conclude that the aluminium selenate crystallising at room temperature contains 16 molecules of 

A phase diagram and related information are available for the system Al2(Se04)3-MnSe04-H20 at 298.15 K in [870JK/STA] Al2(Se04)3-MgSe04-H20 at 298.15 Kin [870JK/STA]. 

This datum is combined with selected enthalpies of formation to  

The enthalpies of dissolution of Al2(Se04)3-18H20(cr) and Al2(Se04)3(cr) in 0.5 M KOH were also measured. These measurements yield  

The evaluation in Appendix A resulted in enthalpies of formation considerably different from those obtained in the paper, which reports 8000.6 and - 2592.4 kJ-moP, respectively. It is not possible to analyse the causes of the difference as no auxiliary data are disclosed in the publication. Since no other data for aluminium sele-nates are available, the values calculated by the review will be accepted but not selected as they partly rest on nonselected auxiliary values. They will thus be included in Appendix E. It may be noted that the standard enthalpy of formation of the anhydrous salt is almost independent of the number of water molecules of crystallisation ascribed to the hydrate. 

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The authors also prepared anhydrous aluminium selenate by heating AIH(Se04)2 H20 to 600 K. The product will be denoted as solid only, as a check of its crystalline state is not mentioned. The enthalpy of dissolution of the two aluminium selenates in 0.5 M KOH to the same unspecified concentration was measured to be - (178.90 + 0.92) kJ-mol and - (442.30 1.02) kJ-mof with the result for the anhydrous compound mentioned last. From these data the standard enthalpy of formation of Al2(Se04)3(s) is calculated to be-(2561.68 17.11) kJ-moP. ... 

Mazepova, V. 1., Thermal decomposition of aluminium selenate, Zh. Neorg. Khim., 8, (1963), 1645-1653, in Russian, English translation in [63SEL/SAM2], Cited on page 231. 

SEL/ZAL2] Selivanova, N. M., Zalogina, E. A., Gorokhov, V. K., Heat of formation of aluminium selenate, Russ. J. Phys. Chem., 47, (1973), 1530. From a citation in this Bibliography... 

Johansson, G., 1962. The Crystal Structure of a Basic Aluminium Selenate. Arkiv forKemi,20 305. 

Most mixed and complex ammonium metal sulphates (and selenates) [948,949] lose NH3, H20 and S03 (or Se03) to form the simple metal sulphate (or selenate) some of the ammonia may be oxidized [949]. The basic aluminium ammonium sulphate [950], (NH4)20 3 A1203 4 S03 xH20 (x = 6—8), loses water at 473 K. Deammination and complete dehydration commences at >673 K, and S03 evolution starts at about 873 K to yield residual A1203 which contains traces of S03. a—Time data for most of the stages obeyed the contracting volume equation [eqn. (7), n = 3] [951]. 

This stated that compounds which crystallise in the same form (i.e. are isomorphous) are similar in atomic composition, and he quoted as examples the sodium phosphates and arsenates, in which an arsenic atom takes the place of a phosphorus atom. Mitscherlich used the fact that potassium selenate was isomorphous with potassium sulphate to deduce the formula of potassium selenate, and hence determine the atomic weight of selenium. Berzelius used the isomorphism of the sulphates and the chromates as evidence that the formula of anhydrous chromic acid was CrOj. He suggested that the lower oxide of chromium had a formula of Q2O3, and since this was isomorphous with the oxides of iron and aluminium, he was able to propose correct formulae for these compounds. 

Selenous acid produces the corresponding selenadiazole (191 X = Se). The condensed thiadiazole (191 X = S) is resistant to oxidation, but is reconverted into (190) (90%) by reduction with lithium aluminium hydride. The spectral properties (n.m.r., u.v.) and half-wave potentials of the compounds (191 X = S or Se) point to coplanarity of the phenyl substituent with the bicyclic heteronucleus. ... 

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