Selenite, determination

It was stressed by the participants using ICP-MS that polyatomic interferences from Cl can be removed to improve the accuracy of selenite determination ways to do so are (i) to dilute the original sample after pre-concentration of the selenite species, (ii) to modify the plasma conditions by adding N2 carrier to the plasma or (iii) to use anion-exchange chromatography to separate chloride from the selenium species. When diluting and pre-concen-trating on a pre-column, care should be taken that selenite is quantitatively recovered. The use of silver nitrate to remove chloride is suspected to affect selenite. 

Liquid chromatography/MS has also very recently been used for selenate and selenite determination and selenomethionine. And another recent liquid chromatographic hyphenated method, liquid chromatography-hydride generation atomic fluorescence spectrometry, has been used to determine selenite, selenate, selenocysteine, and selenomethionine in a single run. Reverse-phase and ion-exchange columns were used in series to accomplish the complex separation of all four of these analytes. And microchip capillary electrophoresis has most recently been used for determination of various Se-amino acids with negligible sample consumption . An excellent US Center for Disease Control summary of Se analytical methods is available. ... 

Discussion. This gravimetric determination depends upon the separation and weighing as elementary selenium or tellurium (or as tellurium dioxide). Alkali selenites and selenious acid are reduced in hydrochloric acid solution with sulphur dioxide, hydroxylammonium chloride, hydrazinium sulphate or hydrazine hydrate. Alkali selenates and selenic acid are not reduced by sulphur dioxide alone, but are readily reduced by a saturated solution of sulphur dioxide in concentrated hydrochloric acid. In working with selenium it must be remembered that appreciable amounts of the element may be lost on warming strong hydrochloric acid solutions of its compounds if dilute acid solutions (concentration <6M) are heated at temperatures below 100 °C the loss is negligible. 

Measures and Burton [556] used gas chromatography to determine selenite and total selenium in seawater. 

In coastal environment, detrital and authigenic Fe and Mn oxides, which accumulate in oxic surface sediments, play a pivotal role in determining the geochemical behaviour of arsenic (Mucci et al., 2000) and selenium (Belzile et al., 2000). Arsenic and selenium differ in their affinities for metal oxide surfaces. Although both adsorb onto iron oxides, arsenate (As(V)) adsorbs more strongly than arsenite (As(lll)), and selenite (Se(IV)) adsorbs more strongly than selenate (Se(VI)) (Belzile et al., 2000). 

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. 

The atomic absorption spectrophotometric methods discussed in section 12.10.2.2 [122, 124] has been applied to the determination of selenium in sediments. Itoh et al. [165] and Cutter [122] (section 12.10.2.2) have used hydrogen generation-atomic absorption spectrophotometric techniques to determine selenium in non-saline sediments. Cutter [122] was able to distinguish between selenite, selenate, total selenium and organic selenium in sediments. 

Direct in situ X ray (from synchroton radiation) adsorption measurements (EXAFS) (Hayes et al., 1987, Brown et al., 1989) permit the determination of adsorbed species to neighboring ions and to central ions on oxide surfaces in the presence of water. Such investigations showed, for example, that selenite is inner-spherically and selenate is outer-spherically bound to the central Fe(lll) ions of a goethite surface. It was also shown by this technique that Pb(II) is inner-spherically bound to 5-AI2O3 (Chisholm-Brause et al., 1989). 

Results of the second experiment (Table 3) show that Se-enriched broccoli sprouts have properties similar to enriched broccoli florets that contain SeMSC as the predominant form of Se (13). Consumption of Se from high-Se broccoli sprouts, as compared to Se from selenite, resulted in a significant decrease in the number of aberrant crypts. Additional experimentation is needed to determine whether the decrease in carcinogenesis is a result primarily of the presence of SeMSC, and if there is a correlation between SeMSC content in enriched plants and the reduction of carcinogenesis. If such a correlation is established, then the SeMSC content of various enriched plants could be used to screen for the greatest efficacy in tumor reduction. Se-enriched broccoli appears to be similar to enriched broccoli florets, for which the predominant form of selenium was also shown to be SeMSC (13). 

In 1847 Sacc determined the atomic weight by several methods, of which only two proved of value, namely (i) pure selenium dioxide was reduced with ammonium hydrogen sulphite and the resulting selenium weighed (ii) barium selenite was calcined with sulphuric acid and the resulting barium sulphate weighed. The results were as follows 5... 

With the exception of the calcium, strontium, barium and mercurous salts, the normal selenates are readily soluble in water. Barium chloride and mercurous nitrate are therefore convenient precipitation agents.6 Barium selenate is, however, more soluble than barium sulphate, and also differs from the latter salt in being slowly reduced to selenite by hydrochloric acid 7 for these reasons precipitation with barium chloride is not applicable to the quantitative determination of selenie acid. A concentrated solution of selenie acid which has been saturated with barium selenate deposits crystals of barium selenie acid, H2[Ba(Se04)2].8... 

Karlson and Frankenberger [60] have developed a simple column ion-chromatographic column method for the determination of selenite in soil extracts with the simultaneous determination of chloride, nitrite, nitrate and phosphate. Separation of the anions was conducted on a low-capacity ion-exchange column, and anions were quantified by conductiometric detection. The eluent stream consisted of 1.5 mmol/1 phthalic acid and adjusted to pH 2.7 with formic acid. 

A standard official method is based on fluorimetry [65-67]. In this method, the plant sample is digested with 60% m/m perchloric acid 70% nitric acid, 3 1 v/v, and the residue is dissolved in 2 M hydrochloric acid. Any selenate present in this solution is converted to selenite by boiling. The concentration of selenate is then determined fluorimetrically as a dekalin extract of the complex formed with 2,3-diaminonaphthalene. 

Cane sheath and neem leaf samples were used and the final acid digest was heated with concentrated hydrochloric acid to reduce any selenate to selenite. The solutions were finally made up to 100 ml and 5 and 10 ml aliquots were analysed. The samples did not contain any selenium, and recoveries were determined by the addition of 50 and 100 xg of selenium to the plant samples before decomposition. The results were all satisfactory. When these experiments were repeated with 1.0 and 2.5 xg of selenium as spikes, the recoveries were 80 -120% at the 1 xg level and 96-120% at the 2.5 pg level (per five grams of sample). 

Measures, C.I. and J.D. Burton. 1980. Gas chromatographic method for the determination of selenite and total selenium in sea water. Anal. Chim. Acta 120 177-186. 

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