Selenite, stability

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 8 (13,17,18,22). The binary compounds with carbon, boron, nitrogen, silicon, and sulfur are not included these are of interest, however, because of their stability at high temperatures. A large number of ternary compounds, including numerous oxyhalides, and more complicated compounds have been synthesized and characterized. These include many intermediate (nonstoicliiometric) oxides, and besides the nitrates, sulfates, peroxides, and carbonates, compounds such as phosphates, arsenates, cyanides, cyanates, thiocyanates, selenocyanates, sulfites, selenates, selenites, teflurates, tellurites, selenides, and tellurides. 

In sediments and soils, the chemistry of selenium differs from that of sulfur in that the stability of selenite is similar to that of sulfate. The reduction of selenite to elemental selenium, which tends to immobilise selenium in soils and water, is an important process in the natural environment. Selenates are only stable under alkaline oxidising conditions and have been found, for example, in the Chilean nitrate deposits. 

Techniques developed for the determination of selenite and selenate involve a succession of several analytical steps (e.g. reduction, separation, detection) which are often far from being validated. In addition, the knowledge related to the stability of the species is still very scarce. A project has hence been launched within the BCR programme with the aim to evaluate the stability of Se-species in solution [42] this feasibility study has been continued by an interlaboratory study for the evaluation of method performance [43]. Both investigations were designed to improve the state-of-the-art of Se-speciation prior to the tentative certification of solution candidate reference materials as described in this section. As a follow-up, artificial freshwater solutions containing inorganic Se-species were prepared (RMs 602 and 603) [40,41]. 

Many problems occur in Se-speciation analysis, owing e.g. to risks of adsorption on container walls, instability of species or contamination, insufficient separation efficiency of the chromatographic techniques, problems of conversion yield of selenite to selenate etc. Prior to conducting an interlaboratory project on this topic, it was hence decided to assess the stability of selenite and selenate according to various factors (effects of container materials, additives, temperature and light). The study focused on tests of effects of physicochemical parameters on solutions stored in polyethylene and PTFE containers. Container volumes were 100 and 500 mL for polyethylene and 500 and 1000 mL for PTFE. Stock and initial working solutions were prepared in 1 and 5 L polyethylene containers previously cleaned with nitric acid (at pH 2) and rinsed with Milli-Q water. The stock solutions were prepared with sodium selenite and sodium selenate (purity >98%). 

The effect of storage at 40°C was studied in 100 mL vessels (instead of 500 mL as used in the other experiments). Surprisingly, the stability was found to be much better for both species in solutions stored at pH 2 and pH 6 in polyethylene containers (with and without addition of chloride). Tests performed with samples stored in the dark and exposed to sunlight demonstrated that light had no significant effect on the stability of selenite and selenate for the period tested. 

At this stage, the conclusions of the feasibility study were that the optimum temperature at which there is no risk of selenium losses at the 10 and 50 pg L levels over 12 months is -20°C. The stability of the species at both 20 and 40°C depends upon the pH and the container type. Generally, both selenite and selenate stored in polyethylene containers at 40°C appeared to be more stable than at room temperature, particularly at pH 6. The presence of chloride tended to stabilize both species. 

The conclusions of the overall study were that a CT concentration of 2000 mg L or more is suitable to stabilize selenite but samples have to be opened only at the time of analysis to ensure a complete stability. This recommendation was clearly stressed to participants in the first interlaboratory study (see below). 

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 ... 

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 ... 

In the early 1970 s, Mehra and Gubeli published a series of papers [70MEH/GUB], [70MEH/GUB2], [71MEH/GUB] on selenide complexation and precipitation with Ag, Mn and Hg. The three works all originate from a thesis by M. C. Mehra, Studies on the stabilities of some metal selenite, sulphide, and selenide complexes in solution [68MEH], and constitute the only studies in the literature where soluble selenide complexes have been indicated. 

Mehra, M. C., Studies on the stabilities of some metal selenite, sulphide and selenide complexes in solution, Ph. D. Thesis, Laval University, Quebec, Canada, (1968). Cited on pages 135, 282, 301, 302, 303, 304, 338, 341, 514, 529. 

Diamines. Chromatography has been used to isolate three isomers of trans- and cis-[Co(CN)2 (RR)-cyclohexane-l,2-diamine 2] and five isomers of the corresponding propylenediamine complexes. Mer- and /ac-isomers of tris(meso-pentane-3,4-diamine)cobalt(iii) have been prepared and separated using column chromatography. The rates of aquation of three isomers of [CoCl(tmd)(dien)] and one isomer of [CoCl(tmdXdpt)] have been measured and the kinetic parameters calculated [dpt = NH2(CH2)3NH(CH2)3NH2, tmd = NH2(CH2)3NH2]. The interaction of [Co(dien)2] with sulphate, thiosulphate, sulphite, selenite, tellurite, and carbonate ions has been studied potentiometrically and stability constants determined for the outer-sphere complexes. The i.r. spectrum of octahedral... 

The project was started in 1992 by a feasibility study on the optimal storage conditions for solutions containing selenate and selenite [142], The verified stability of Se species in solutions enabled the organization of an interlaboratory study in 1993-94 [143], which was followed by a certification campaign in 1994-95 [144,145]. 

The solutions were stable at — 20°C in all conditions tested (pH 2 and 6, with and without chloride, without addition of acid). At ambient temperature, samples stored at pH 2 in polyethylene containers showed instability of selenite after one month storage whereas selenate remained stable this difference in behaviour was attributed to possible adsorption of selenite onto the container walls. The stability was better at pH 6 but instability of selenite was still observed after two months storage, whereas selenate remained stable. When a FIFE container was used, dramatic losses of selenite were observed at pH 6. 

Two concentration levels of selenite and selenate solutions were prepared a low concentration solution of 6 gg selenite + selenate (solution A) and a high concentration solution of 50 pgL selenite + selenate (solution B). Sodium chloride (2000 mg L ) was added to stabilize the inorganic species (at pH 6). 

The stability of the materials was tested at +20 °C over a period of 12 months and selenite and selenate were determined at the beginning of the storage period and after 1, 3, 6, 9 and 12 months. Samples were analysed using the same procedures as for the homogeneity study. Selenite and selenate were each determined in quadruplicate (one replicate analysis in each of four bottles) at each occasion of analysis. The evaluation of the stability was based on the procedure described in Chapter 3, using the results of the homogeneity study (performed immediately after bottling) as reference for the samples analysed at the various occasions. 

Consequently, it was decided to test other polypropylene bottles with tighter caps for the storage of the reference materials. This additional study was carried out 24 months after the initial stability study and led to the detection of instability problems of the Se species over a long-term period. A clear decrease in selenite content was observed in the new polypropylene bottles after 8 months storage, which was particularly acute for the low-concentration reference materials (Table 8.5), whereas selenate remained stable over the same period. On the basis of these results, if was found necessary to control the... 

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