Selenite in Solution

First method 0.1 mL of sample was mixed with 0.5% NaBH4, followed by addition of HCl (6 mol L ) and preconcentration (10 mL to 1 mL), followed by HPLC separation (Hamilton PRP-XlOO). Calibration was by matrix matching, using Na2Se03 calibrant. Final detection was by quartz furnace AAS. 

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While the sorption curves are almost linear on a log log scale, the model fits a gentle curve as this is consistent with a bigger body of information (Fig. 9.). At any given level of sorption, the concentration of selenite in solution decreases with time and with increasing temperature. It is this decrease that is modelled as due to diffusive penetration. Selenate differs in that the sorption curves are steeper (as also shown in Fig. 9.) and, importantly, that the effects of time, though detectable, are much smaller. These two species therefore provide a test for the argument that apparent non-reversibility of sorption occurs because of the continuing reaction. 

Selenites in solutions of mineral acids may be detected by reduction to... 

Separation of Se and Te can also be achieved by neutralizing the alkaline selenite and tellurite leach with H2SO4 this precipitates the tellurium as a hydrous dioxide and leaves the more acidic selenous acid, H2Se03, in solution from which 99.5% pure Se can be precipitated by S02 ... 

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]. 

The bioavailability of selenium to a benthic deposit-feeding bivalve, Macoma balthica from particulate and dissolved phases was determined from AE data. The selenium concentration in the animals collected from San Francisco Bay was very close to that predicted by a model based on the laboratory AE studies of radiolabelled selenium from both particulate and solute sources. Uptake was found to be largely derived from particulate material [93]. The selenium occurs as selenite in the dissolved phase, and is taken up linearly with concentration. However, the particle-associated selenium as organoselenium and even elemental selenium is accumulated at much higher levels. The efficiency of uptake from the sediment of particulate radiolabelled selenium was 22%. This contrasts with an absorption efficiency of ca. 86% of organoselenium when this was fed as diatoms - the major food source of the clam. The experiments demonstrated the importance of particles in the uptake of pollutants and their transfer through the food web to molluscs, but the mode of assimilation was not discussed. 

Selenite in aqueous solution can be measured by colorimetric or fluoromet-ric methods. Selenite reacts with 2, 3-diaminonaphthalene to form a brightly colored fluorescent derivative that is extracted with hexane. The absorbance can be measured by a spectrophotometer at 480 nm, or the fluorescence may be measured by a fluorometer at 525 nm. 

Further, if the difference in entropy between selenite and hydroxyl is the same on the surface as it is in solution, the value for the entropy of water (16.7 e.u.) indicates that (x — y) = 1—i.e., a molecule of water is displaced from the surface during exchange of SeOs2 for OH". The... 

Selenites are likewise reduced to selenium. If, however, the solution of the selenite in concentrated sulphuric acid is treated with sodium sulphite, selenium separates but the tellurite is unaffected the latter can be detected in the solution after eliminating the sulphur dioxide. Salts of silver, copper, gold, and platinum must be absent for they are reduced to the metal by the reagent. 

Phosphate, silicate, borate, arsenate, selenite, chromate, and fluoride are anions for which ligand exchange is important. Nitrate, chloride, bromide, and perchlorate are not held, while sulfate and selenate may be weakly held. As a consequence, leaching of nitrate and sulfate from soil in drainage water can be significant, but very little phosphate is lost in solution. Of the trace metals, Co, Cu, Ni, and Pb are strongly held on oxide surfaces by chemisorption, but the process is much less important for Cd and Zn. 

Ammonium selenite, (NH4)2Se03.—Concentration of an alcoholic or aqueous solution of selenious acid saturated with ammonia yields the selenite in four-sided columns or laminae.6 On heating, it decomposes with separation of selenium, and evolution of nitrogen. 

Iron Selenites.—Although metallic iron does not appear to be soluble in selenous acid, yet selenites of iron are readily obtained in a variety of ways. When sodium selenite is added to ferrous sulphate solution, a white precipitate of ferrous selenite, FeSe03, is obtained.4 This becomes darker on exposure to air in consequence of oxidation. If the white precipitate is dissolved in hydrochloric acid, a portion of the selenium separates out, whilst ferric chloride and selenous acid remain in solution. Thus —... 

Complex Uranyl Selenites.—A series of crystalline compounds, rich in selenious acid, have been prepared" by decomposing uranyl salts in solution by means of selenious acid or an alkali selenite. The products are yellow- in colour and insoluble in %vater. Compounds of the follow-ing composition have been prepared ... 

Figure 13. Effect of period of incubation on the concentration of selenite (a) and selenate (b) in solution after adding respectively selenite or selenate to a soil. Intermediate points have been omitted for clarity [80].

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]. 

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. 

Popova et al. [86POP/SLA] determined the solubility of gallium selenite in dilute nitric or perchloric acid with concordant results. The preparation and composition of the selenite is not discussed. It is assumed to have the formula Ga2(Se03)3-6H20 by the review. The solubility product calculated from the total concentration of Ga(lII) in the saturated solution is accepted and selected, cf. Appendix A. 

Chukhlantsev and Tomashevsky [57CHU/TOM] prepared mercury(l) selenite by mixing 0.1 M solutions of mercury(I) nitrate and sodium selenite in stoichiometric amounts. Chemical analysis confirmed the 2 1 ratio between Hg(I) and Se(IV). No X-ray diffraction measurements were performed. The solubility of the specimen in dilute solution of nitric or sulphuric acid was measured at 293 K. The experiments were performed and the data recalculated as described in Appendix A, [56CHU]. The result for ... 

Slavtscheva, Popova, and Gospodinov [93SLA/POP] determined the solubility of copper selenite in dilute nitric acid and calculated its solubility product from measurements of pH and total Cu(ll) concentration in the equilibrium solution. No primary... 

Selivanova, Leshchinskaya, and Klushina [62SEL/LES] measured the enthalpy change when crystalline silver selenite was formed from AgN03(cr) and a solution of sodium selenite in a calorimetric experiment. Their result is used in Appendix A to estimate the standard enthalpy of formation of Ag2Se03(cr) to be Af//° (Ag2Se03, cr, 298.15 K) = - (363.44 1.02) kJ-mol. This value agrees well with [82WAG/EVA] but differs appreciably from the result in the paper, - 345.0 kJ-moE. ... 

The only experiment, which leads to a determination of the standard enthalpy of formation of sodium selenite, appears to be that of Thomsen [1882THO]. He measured the enthalpy of the reaction between one mole of selenious acid and two moles of sodium hydroxide in solution. The data are used to calculate the standard enthalpy of formation ofNaSeOsCcr) in Table A-1. 

The selenites were generally prepared by mixing a 0.1 to 0.2 M solution of the metal ion and a 0.1 M solution of sodium selenite in stoichiometric proportions. In some cases the precipitate was aged in the mother liquor before separation and drying. The specimens were analysed but only the ratio metahselenium is reported. X-ray diffraction patterns were not registered. It is therefore not clearly established whether the preparations were crystalline or (aged) amorphous phases. 

The authors reacted Na2Se03(cr) with a copper sulphate solution in an electrically calibrated calorimeter and measured the enthalpy change of the reaction. The product was CuSe03-2H20(cr) as shown by chemical analysis and X-ray diffraction. Crystalline anhydrous copper selenite was also prepared and the integral enthalpies of dissolution of the two selenites in 8% HMO3 (HNO3, aq 1 40, and denoted sin below) were determined. The data have been used in Table A-45 to calculate standard enthalpies of formation of the copper selenites. 

The authors reacted 9 x 10 moles of MgS04(aq, 1 2000) with an equivalent amount of Na2Se03(cr) in 325 grams of water in an electrically calibrated calorimeter. Crystalline MgSe03 6H20 was formed. From the solubility product of magnesium selenite the review estimates that about 6% of the Mg remained in solution after the reaction. A correction has been applied, which changes the enthalpy of Reaction 1 in Table A-51 from - (47.49 0.13) in the paper to - (48.25 0.25) kJ-mol. ... 

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