Selenite ions, reactions

Pyatniskii and Durdyev measured the solubility of CoSe03-2H20 in water and as a function of the total selenite concentration at a pH of about 8.5. The temperature of the experiments is not mentioned. The primary data of the water solubility, which included the pH of the equilibrium solutions, have been used in a recalculation that yielded logid= -(8.1710.05), for the reaction CoSe03-2H20(cr) Co + SeO,"+ 2H2O. The result in the paper is - 7.64 calculated with an unspecified value of the protonation constant of the selenite ion. 

Reactions of selenite ions To study these reactions use a 0.1m solution of selenious acid, H2Se03 (or Se02), or sodium selenite, Na2Se03. 

The reduction of selenite ions by some selenothionate and thiosulphate ions has been studied. 8e03 reacts with 80830 in acidic solutions to form selenanemonosulphanedisulphonates with <6 8e atoms in the chain. The ions 805830 and 803830 were detected for the first time. 80 and 80830 reduce 8eO in acidic solutions to selenium metal but selenane-disulphonates are produced as intermediate products. In neutral systems, species exist with <4 Se atoms in the chain in acidic solutions species with <7 8e atoms in the chain were formed, including the new species 867830 . 8elenium metal is precipitated by the decomposition of the longest-chain intermediates, 8eO reacts with 820 to form 868 0 and 8406 . Excess SgO " reacts with 80840 in neutral solutions to form 8eS80 , which decomposes to selenium. In acidic solutions, this reaction is of less importance, and only minor precipitation of selenium is observed instead a sequence of build-up reactions takes place. 

Selenosulfate is an analogue of thiosulfate wherein one of the S atoms is replaced by a Se atom. Thiosulfate and selenosulfate anions are known to have tetrahedral structure as constituting the S and Se analogues, respectively, of the sulfate anion. The isomeric thioselenate anion SSeO " is not produced by the reaction of sulfur with selenite nor is the selenoselenate ion 86203 formed from selenium and selenite. Actually, SSeOj may be produced as a metal salt by boiling an aqueous solution of selenite with sulfur, but in aqueous solution thioselenates are not stable and isomerize to selenosulfates. 

In the case of CdSe formation using the codeposition methodology, a problem was encountered early on and studied by Skyllas-Kazacos and Miller [122]. It concerned the formation of selenide ions and their reaction with the selenite starting material to form elemental Se ... 

Redox reactions in soils are affected by a number of parameters, including temperature, pH (see Chapter 7), and microbes. Microbes catalyze many redox reactions in soils and use a variety of compounds as electron acceptors or electron donors. For example, aerobic heterotrophic soil bacteria may metabolize readily available organic carbon using NO3, NOj, N20, Mn-oxides, Fe-oxides and compounds such as arsenate (As04 ) and selenate (Se04 ) as electron acceptors. Similarly, microbes may use reduced compounds or ions as electron donors, for example, NH4, Mn2+, Fe2+, arsenite (AsCXj), and selenite (SeO ). 

The following interfere with the test strong reducing agents (hydrogen sulphide, dithionites, sulphites and selenites) V, U, Te, Hg, Bi, Au, Pd, Se, Te, Sb, Mo, W, Co and Ni. The reaction is not selective, but is fairly sensitive it can be used in the analysis of the Group IIB precipitate. Since iron(II) ions have no influence on the test, it may be applied to the tin solution which has been reduced with iron wire. 

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. 

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