Selenate, determination

The determination of selenate by difference between total inorganic selenium content and selenite was questioned. Although this method is quite commonly used and acceptable for selenate determination, this does not correspond to a direct measurement of the species hence, this method would not be accepted for certification and it was proposed that the results obtained by difference would be used as confirmative v ues of results obtained by techniques actually separating the Se species e.g. HPLC-ICP-MS or HPLC-HGAAS). 

It was recommended to use the term inorganic Se(IV) and Se(VI) (or selenite and selenate as used throughout this chapter) to make a clear distinction between these species and organic compounds. As stressed above, it was agreed that the method of selenate determination by difference would not be considered for certification the selenate certification should be based on the results of techniques involving a separation of the species. Standard addition methods or matrix matching are recommended. 

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. 

Lingane and Niedrach have claimed that the h-VI states of tellurium (or selenium) are not reduced at the dropping electrode under any of the conditions of then-investigation however, Norton et al. [42] showed that under a variety of conditions, samples of telluric acid prepared by several different procedures do exhibit well-defined (though irreversible) waves, suitable for the analytical determination of the element. The reduction of Te(H-VI) at the dropping electrode was found coulometri-cally to proceed to the -II state (whereas selenate, Se(-i-VI), was not reduced at the dropping electrode in any of the media reported). 

In fitting these data, we note that at pH 7.5 selenate is present almost exclusively as the SeO " oxyanion, and the species activity coefficient in the dilute fluid is nearly one. We can, therefore, take the species activity as equal to its dissolved concentration, in mol kg-1. If this had not been the case, we would need to account for the speciation and activity coefficient in determining the value of se04 for each experiment. 

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 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). 

Neutron diffraction examination of CsFe (S04)2 12H20 and CsFe (Se04)2 12H20 at 15 K showed a small but significant difference between Fe—O bond distances, 1.994(1) A in the sulfate, 2.002(1) A in the selenate. This difference is attributed to the different tilt an s of the coordinated water, 0.6° and 18.6°, respectively. A XRD structure determination on CsFe (8004)2 I2H2O gave Fe—0= 1.989(4) A (at room temperature). This selenate has the a-alum structure, whereas the corresponding sulfate has the /3-alum structure. These results have been placed in the context of X-ray and neutron diffraction studies of structures of hydrated cations in aqueous solution. ... 

Comprehensive accounts of the various gravimetric, polarographic, spectrophotometric, and neutron activation analytical methods have been published (1,2,5,17,19,65—67). Sampling and analysis of biological materials and oiganic compounds is treated in References 60 and 68. Many analytical methods depend on the conversion of selenium in the sample to selenous acid, H SeO and reduction to elemental selenium when a gravimetric determination is desired. 

A number of substances, such as the most commonly used sulfur dioxide, can reduce selenous acid solution to an elemental selenium precipitate. This precipitation separates the selenium from most elements and serves as a basis for gravimetry. In a solution containing both selenous and tellurous acids, the selenium may be quantitatively separated from the latter by performing the reduction in a solution which is 8 to 9.5 N with respect to hydrochloric acid. When selenic acid may also be present, the addition of hydroxy] amine hydrochloride is recommended along with the sulfur dioxide. A simple method for the separation and determination of selenium(IV) and molybdenum(VI) in mixtures, based on selective precipitation with potassium thiocarbonate, has been developed (69). 

In addition to the halide systems there are a number of thallium(I) salts of thallium(III) complex anions in which the overall stoichiometry implies, incorrectly, the presence of thallium(II) species. These include the sulfate, selenate,369 acetate370 and oxalate371 derivatives. An X-ray determination has confirmed that Tl(OAc)2 is indeed Tl[Tl(OAc)4], in which the anions are linked through seven-coordinate Tl+, with some evidence for a stereochemically active inert pair.370 In T1[T1(0H)(S04)2], the structure consists of sheets of linked anions with Tl+ ions.372... 

Fig. 2X3. Determination of phase angles in non-centrosymmctric crystals, a. Amplitudes and phases for corresponding reflections in isomorphous sulphate and selenate of strychnine. b. Knowledge of -Fguiphi aeb and ifse fs) gives magnitude but not sign of a c. Amplitudes and phases for a protein and two isomorphous derivatives with heavy atoms at different sites, d. Knowledge of Fp, Fp x p+Y fx> fy and gives unique...

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... 

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). 

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