Selenium solvent extraction

Organ oselenium compounds, such as phosphine selenides, are being evaluated in solvent extraction systems for silver and gold (63). Also, potential pharmaceuticals containing selenium have been prepared (64). 

V. H. Aprahamian and D. G. Demopoulos, The Solution Chemistry and Solvent Extraction Behaviour of copper, iron, nickel, zinc, lead, tin, Ag, arsenic, antimony, bismuth, selenium and tellurium in Acid Chloride Solutions Reviewed from the Standpoint of PGM Refining, Mineral Processing and Extractive Metallurgy Review, Vol. 14, p. 143,1995. 

Other methods reported for the determination of beryllium include UV-visible spectrophotometry [80,81,83], gas chromatography (GC) [82], flame atomic absorption spectrometry (AAS) [84-88] and graphite furnace (GF) AAS [89-96]. The ligand acetylacetone (acac) reacts with beryllium to form a beryllium-acac complex, and has been extensively used as an extracting reagent of beryllium. Indeed, the solvent extraction of beryllium as the acety-lacetonate complex in the presence of EDTA has been used as a pretreatment method prior to atomic absorption spectrometry [85-87]. Less than 1 p,g of beryllium can be separated from milligram levels of iron, aluminium, chromium, zinc, copper, manganese, silver, selenium, and uranium by this method. See also Sect. 5.74.9. 

SCjSj, and of molecules of even higher selenium content was seen in the mass spectra taken from the samples of both quenched melts and of crystals obtained by solvent extraction of them. It must be taken into account, however, that even gentle heating of the samples may initiate disproportionation reactions. Also the volatihty of different ring molecules may be quite different. Therefore the composition of the vapour does not necessarily reflect the composition of the solid phase in all details. 

The element may be determined at 196.0 nm by A AS, using a nitrous oxide-acetylene flame (which is more transparent than air-acetylene at this low wavelength), or by AFS in a variety of flames.46,47 The detection limit of both techniques for selenium is around 1 mg 1 1, too low to be useful for environmental analyses. The element is therefore invariably determined by hydride generation techniques, coupled to AAS or AFS detection, as discussed in Chapter 6, section 2, or by furnace AAS, or occasionally by solution spectrofluorimetry using 2,3-diaminonaphthalene as a reagent. If direct flame AAS or AFS are to be used for some reason, then pre-concentration by solvent extraction is necessary.1 However, this approach is rarely used nowadays. 

P. Hocquellet, M. P. Candiller, Evaluation of microwave digestion and solvent extraction for the determination of trace amounts of selenium in feeds and plant and animal tissues by electrothermal atomic absorption spectrometry, Analyst, 116 (1991), 505-509. 

No aqua cations exist for sulfur or selenium. However, there is substantial evidence for a TeIV(aq) cation from measurements on the solubility of Te02 in acidic solutions with additional evidence coming from solvent extraction studies.206 Speciation studies show that a monocationic ion TeIV(aq) should persist in dilute solutions (Electrochemical reduction of TeIV(aq) is also consistent with the existence of a cationic form in strongly acidic solution. Whether this is Te(OH)3+(aq), TeO(OH)+(aq), or HTe02+(aq) is less certain. 

Kumpulainen J, Saarela KE. 1992. Determination of selenium in staple foods and total diets by electrothermal atomic absorption spectrometry without solvent extraction. J Anal Atomic Spectr 7(2) 165-170. 

Most selenium measurements are complicated by the presence of other elements in the sample. In some cases selenium can be separated from interferences by distillation as the bromide, by high pressure liquid chromatography, or by solvent extraction (1, 12, 13). The hydride generation technique provides good separation of selenium from interferences in the atomic absorption technique that has been developed. However, separation from arsenic and mercury is not accomplished, and these elements and the nitrate ion have been found to interfere (14,15). However, the effect of these species on the measurement is not significant for the method developed by the Project. 

In clinical chemistry the determination of PGM often requires selective enrichment procedures. Solvent extractions with organic complexing agents and anion exchangers are mostly used for this purpose. Collector precipitation, e.g., with easily reduced elements such as tellurium or selenium, and electrochemical enrichment techniques [26] are possible as well. These procedures have so far not been used in clinical chemistry. 

Equilibrium Partition Coefficients. Equilibrium solvent extraction experiments are a convenient way to screen various complexing agents for suitability as selenium extractants and to assess the potential for competition from innocuous ions like sulfate. The solvent extraction experiments allow the determination of reaction equilibrium constants and distribution or partition coefficients of the selenium species between the organic (membrane) phase and the aqueous (internal or external) phase. The equilibrium constant (K ) and distribution coefficient (K ) for the reaction of selenite with the complexing agent are defined in the equations given below,... 

Selenium(IV) reacts with substituted 1,2-diaminobenzene or 2,3-diamlnonaphthalene in acidic solution to form stable cyclic derivatives which can t>e extracted into an organic solvent and analyzed by gas or liquid chromatography [682,683]. With chloro-, bromo-, or nitro-substituents the plazselenols can be determined with an electron-capture detector at the low picogram level. Se(VI) does not form piazselenol derivatives so the reaction with diaminobenzene can be used to determine the concentration of Se(IV) and Se(VI). Selenium(VI) may be redu to Se(IV) with... 

Tertiary phosphine selenides have been prepared either by direct fusion of the tertiary phosphine with elemental selenium1 or by refluxing the phosphine with selenium in an inert solvent.2 In the case of a solid phosphine, such preparations often require numerous and time-consuming extractions to obtain the pure phosphine selenide. 

Bicyclo[4.3.0]nonan-8-one (6.9 g, 50 mmol) was added to a solution of 34% H202 (5 g, 50 mmol) and Se02 (100 mg, 1 mmol) in f-BuOH (60 mL). The mixture was heated at 80 JC for 7 h. After removal of selenium by filtration, the solvent was removed and the residue was taken up in Et20 (50 mL). The ethereal solution was extracted with sat. K2C03. The alkaline solution was acidified and extracted with Et20. After distillation of the solvent, the crude carboxylic acid was obtained yield 2.28 g (30%) bp 128 "C/0.8 Torr mp 80 C. amide mp 195 C (benzene). 

---