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Chemistry PROPERTIES OF ELEMENTAL SELENIUM Selenium (Se) was identified in 1818 by Berzelius as an elemental residue during the oxidation of sulfur dioxide from copper pyrites in the produc- tion of sulfuric acid. It is similar in properties to tellurium (discovered some 35 years earlier) and was named for the moon (serene in Greek) while tellurium had been named for the earth (tellus in Latin). Little was known about the biological action of selenium until its toxicity (Franke and Painter, 1936) and nutritional essentiality (Schwarz and Foltz, 1957) were recognized. Nevertheless, the discovery of selenium was followed by study of its chemistry, which led to many industrial uses for this element that is almost as rare as gold. Excellent reviews of the chemistry of selenium are available (Rosenfeld and Beath, 1964; Chizhikov and Shchastilivyi, 1968; Nazarenko and Ermakov, 1972; Klayman and Gunther, 1973; Zingaro and Cooper, 1974~. Selenium is classified in group VIA in the periodic table of elements. It has both metallic and nonmetallic properties and is considered a metal- loid. It is located between the metals tellurium and polonium and the non- metals oxygen and sulfur by group, and between the metal arsenic and the nonmetal bromine by period. The atomic properties and electronic config- uration of selenium are summarized in Table 1. Six naturally-occurring stable isotopes of selenium have been identified, and at least seven unsta- ble isotopes may be produced by neutron activation. Of the latter, 75Se, 77mSe, and Else may be used for the quantitative measurement of selenium 3

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4 Atomic weight Atomic number Electronic configuration Covalent radius, Atomic radius, Ionic radius, fL Atomic volume, w/d Common oxidative states Bond energy (M-M), kcal/mole Bond energy (M-H), kcal/mole Ionization potential, eV Electron affinity, eV Electronegativity Polarizability (M-2), cm3 X 10-25 pKa: MO(OH)2, aqueous MO2(OH)2, aqueous (H2M), aqueous (HM-), aqueous SELENIUM IN NUTRITION TABLE 1 Atomic Properties and Electronic Configuration of Selenium 78.96 34 3dlO4S24p4 1.16 1.40 1.98 amorphous: 18.55 monoclinic: 17.72 hexagonal: 16.31-16.50 -2, 0, +4, +6 44 67 9.75 -4.21 2.55 105 2.6 -3 3.8 11.0 by neutron activation analysis, and 75Se has proved to be particularly suit- able for biological experimentation because of its relatively long half-life (120 days). Like the other group VIA elements (sulfur and tellurium), selenium shows allotropy, existing in an amorphous state or in any of three crystal- line forms. Amorphous selenium is a freeflowing liquid at temperatures above 230C; its viscosity increases as the temperature is reduced to about 80C, followed by decreases in viscosity with further reductions in temper- ature. This phenomenon, like that demonstrated by amorphous sulfur, results from the formation at low temperatures of ring-shaped aggregates with lower viscosity; whereas selenium forms polymeric chains with greater viscosity at higher temperatures. Elemental selenium is, thus, vitreous at 31C-230C and is a hard and brittle glass below 31C. A red particulate form, colloidal amorphous selenium, can be prepared by the reduction of aqueous solutions of selenious acid; however, this form becomes crystalline at temperatures above 60C. Three crystalline forms of selenium occur: alpha-monoclinic, beta- monoclinic, and hexagonal. The monoclinic forms are composed of Ses rings and may be referred to as red (alpha-monoclinic) or dark red (beta

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Chemistry 5 monoclinic) selenium. Alpha-monoclinic selenium is composed of flat hex- agonal and polygonal crystals, whereas the crystals of beta-monoclinic se- lenium are needlelike or prismatic. Hexagonal selenium is called gray, black, metallic, gamma, or trigonal selenium. It is composed of spiral Sen chains. It is this form that is the most stable; amorphous selenium is trans- formed to the hexagonal form at 70-210C, and both monoclinic forms convert to the hexagonal form at temperatures above 110C. The physical properties of elemental selenium vary according to its allotropic form. These have been reviewed by Chizhikov and Shchastlivyi (1968) and Crys- tal (1972). Elemental selenium can be oxidized to +4 or +6 oxidation states. In the + 4 state, selenium exists as the dioxide (SeO2), selenious acid (H2SeO3), or selenite (SeO3-2) salts. Elemental selenium burns in air to form SeO2. This compound can also be formed by the oxidation of elemen- tal selenium by concentrated nitric acid. The production of SeO2 is impor- tant in the combustion of fossil fuels that may be rich in selenium. How- ever, SeO2 is easily reduced, and SeO2 formed by combustion is largely reduced back to the elemental state by sulfur dioxide produced concomit- tantly during that combustion. When amorphous selenium is oxidized in the presence of water, H2SeO3 is formed. The latter is a weakly dibasic acid that frequently acts as an oxidizing agent. Dissolved selenites are present as biselenite ions in aqueous solutions at pH 3.5 to pH 9. Selenite is readily reduced to elemental selenium at low pH by mild reducing agents such as ascorbic acid or sulfur dioxide. In the +6 state, selenium exists as selenic acid (H2SeO4) or selenate (SeO4-2) salts. Selenic acid is a strong acid formed by the oxidation of sele- nium or selenious acid by strong oxidizing agents such as NaBrO3 in NaHCO3 or by Br2, Cl2 or H202 in water. Most selenate salts are apprecia- bly more soluble than the corresponding selenite compounds. Their solubilities and stabilities are greatest in alkaline environments, and the conversion of selenates to the less stable selenites and to elemental sele- nium is very slow. Selenium reacts with halogens to form halides in which Se (+4) or Se (+6) are found (i.e., SeF6, SeF4, SeCl4, SeBr41. Selenium halides form acido complexes with the halogen derivatives of acids and with some of their salts. In its most reduced state ~-2) selenium exists as selenide. Hydrogen selenide (H2Se) is a fairly strong acid and is a colorless, highly toxic gas produced by hydrolysis of metal selenides or by heating (400C) elemental selenium in air. Hydrogen selenide rapidly decomposes in air to form ele- mental selenium and water. Whereas H2Se is fairly soluble in water, the selenides of metals have either low solubility (e.g., CuSe, CdSe) or are very insoluble (e.g., HgSe).

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6 SELENIUM IN NUTRITION CHEMISTRY OF SELENIUM-CONTAINING COMPOUNDS The chemistry of organic selenium compounds has been reviewed in detail by Klayman and Gunther (1973~. Numerous organoselenium compounds can be prepared from elemental selenium (usually amorphous selenium is used) by addition reactions: from H2Se or alkali selenides by addition or nucleophilic displacement reactions, from potassium selenocyanate by nu- cleophilic displacement or electrophilic substitution reactions, from phos- phorus pentaselenide by reactions with primary alcohols, and from sele- nium oxides by substitution reactions at carbon atoms or by electrophilic substitution reactions. Several reagents containing highly nucleophilic se- lenium anions are available. These reagents are prepared from elemental selenium and are all capable of nucleophilic attack on carbon with dis- placement of aliphatic halides or sulfonic esters, or of ring opening of epoxides or lactones. These reagents include potassium selenosulfate (K2SeSO3), solutions of selenium in aqueous sodium formaldehyde sulf- oxylate (NaSO2CH2OH) in the presence of sodium hydroxide, alkali selenides, and bistmethoxymagnesium) diselenide (CH3OMgSe)2. In ad- dition, selenium halides and oxyhalides may be used to prepare organose- lenium compounds by addition reactions to C C double bonds, or by elec- trophilic substitutions of hydrogen in aliphatic or aromatic species. A few organoselenium compounds with applicability for the formation of new C-Se bonds are selenourea, SeC(NH2~2, which is readily alkylated to give isoselenouronium salts in organic solvent; benzylselenol which, along with its anion, reacts as other selenium nucleophiles to produce the rather sta- ble benzyl alkyl monoselenides; and carbon diselenide (CSe2), which reacts with primary amines to give symmetrical selenoureas and with secondary amines to give N,N-dialkyldiselenocarbamic acids. Hydrogen selenide (H2Se) and the organoselenium compounds of inter- est in nutrition and health are the methylated forms of selenium, i.e., dimethyl selenide, (CH3~2Se; trimethylselenonium ion, (CH3~3Se+; the selenoamino acids, i.e., selenocysteine, selenocystine, selenomethionine, selenohomocystine; and the homocyclic and heterocyclic selenium com- pounds. The biological properties of these compounds in metabolism have *een discussed (Levander, 1976b). Although the chemistry of selenium is similar to that of sulfur, certain differences result in these elements being metabolized differently. First is the difference in the ease of oxidation of Se ~ + 4) and that of S ~ + 4), the former tending to undergo reduction and the latter tending to undergo oxidation. Thus, biological systems tend to re- duce selenium compounds and to oxidize sulfur compounds. Second is the difference in the relative strengths of acids H2Se and H2S, which is also seen in the acidic strengths of the hydrides of selenium and sulfur. The pK

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Chemistry 7 of the selenohydryl group of selenocysteine is 5.24, whereas that of the sulf- hydryl group of cysteine is 8.25. Therefore, at physiological pH the seleno- hydryl group of selenocysteine or other selenols exists largely in the dissoci- ated form, whereas the sulfhydryl group of cysteine or other thiols exists largely in the protonated form. METHODS OF ANALYSIS Selenium may be detected qualitatively by reduction to the elemental form (see Table 2~. The best reducing agents for selenites are thiourea and hy- droxylamine hydrochloride. Selenite can be determined in the presence of selenate by virtue of the different redox potentials for selenite and selenate (Se +~0.74V H2SeO3 +5 H2SeO4) in a strongly acid bromide solution, TABLE 2 Analysis of Selenium Detection Se Limit Interfering Reagent Detected Result of Reaction (~g/ml) Substances Thiourea Se+4 pink color or 5 Te, NO2-, Cu. Hg, red ppt. Hi, Au, Pt. Pd Hydroxylamine Se+4 pink color or 5 many elements HCl red ppt. except Te Iodide Se+4 red-brown ppt. 40 As+3, Ge+4 Mo+6 Thiocyanic acid Se+4 red-brown ppt. 2 As, Sb, Sn, Fe+2, MoO4 Pyrrole Se+4 pyrrole blue color 0.5 oxidizing elements, Se+6, Te+4 Te+6 Asymmetric Se+4 red color 2 oxidizing agents diphenyl hydrazine Methylene Se decolorization 3 oxidizing agents blue and NaS2 Ammonium Se+4 molybdate 3,3' . . d~am~no benzidine 2,3 . . clamlno naphthalene 3 PO4-3, SO4-2 Se+4 molybdenum selenium blue color yellow color or red 0.01 oxidizing agents, fluorescence Fe+3, Cu+2 Se+4 yellow color or green fluorescence 0.002 oxidizing agents SOURCE: Nazarenko and Ermakov ( 1972)

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8 S E LE NIUM IN NUTRITI ON wherein the oxidation of Se+4 to Se+6 is detected by the redox indicator, p-ethoxychrysoidine. The most sensitive methods of detecting selenium involve the formation of pia~selenols with orthodiamines (2,3-diaminonaphthalene; 3,3'-diamino- benzidine; 1,8-naphthalenediamine; 4-dimethyl-1,2-phenylenediamine; 4- methylthio-1,2-phenylenediamine). In the presence of these reagents in weakly acid solutions, selenites form piazselenols, which take on a straw- yellow color or, at higher levels of selenium, form brown-red precipitates. After extraction into organic solvent (e.g., cyclohexane, dioxane, toluene, benzene), pia~selenols fluoresce upon irradiation with ultraviolet light. Several methods have been employed for the quantitative determination of selenium. Among these are gravimetric procedures based upon the quantitative precipitation of selenium from selenites and selenates after reduction (Nazarenko and Ermakov, 1972~. The purest precipitates are formed when sulfurous acid is used as the reducing agent and when sele- nium is precipitated from concentrated hydrochloric acid. Other reducing agents (e.g., Fe+2, Sn+2, Cr+2 and v+2 salts, sodium hypophosphite, thiourea, glucose, lactose, ascorbic acid, thiosemicarbazide, sodium di- ethylthiocarbamate and mercaptobenzimidazole) have been employed in various gravimetric methods for determining selenium. The problem com- mon to all such procedures is that of production of precipitates free of con- taminating elements. Selenium can also be determined by electrolytic dep- osition with copper; however, the presence of tellurium interferes with this method. Milligram quantities of selenium can be determined by titration meth- ods, most of which are based on redox reactions. In such procedures, sel- enites and selenates are quantitatively reduced to selenium by sodium thiosulfate; iodide; or ferrous, chromous, and trivalent titanium salts. Se- lenium is then titrated by solutions of oxidants. Alternatively, selenites can be oxidized to selenate by excess KMnO4 or K2Cr2O7, with back titration of the excess by Fe+2. Small amounts of selenium can be determined by formation and colori- metric measurements of hydrosols. Hydrazine, SnCl2, and ascorbic acid are suitable reducing agents for the formation of selenium sols. Gum ara- bic, gelatin, or hydroxylamine hydrochloride can be used to stabilize the sol. The extinction density of selenium sols is measured at 260 nm. Among widely employed methods for the quantitative determination of low levels of selenium are: (a) photometric and fluorometric procedures based on the formation of piazselenols with aromatic o-diamines; (b) pro- cedures based on the formation of complexes with sulfur-containing or- ganic reagents (e.g., dithizone, bismuthiol II); (c) procedures based on the oxidation of organic compounds by Se+4 to diazonium salts, which react

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Chemistry 9 with aromatic amines to give intensely colored azo compounds; and (d) procedures based on the formation of complexes of Se-2 with phenyl-sub- stituted thiocarbazide or phenyl-substituted semicarbazide (e.g., 1,4- diphenylthiosemicarbazide). Of these procedures, the most widely used are reactions with o-diamines. The most selective and also most sensitive of these reagents is 2,3-diaminonaphthalene (DAN). Thus, the DAN proce- dure is most suitable for the determination of selenium in biological mate- rials (Olson et al., 1975~. It involves the reaction of DAN with selenious acid to form the selenodiazole 5-membered ring. Due to the intense fluo- rescence of piazselenol (maximum at 520 nm; excited at 390 nm or 366 nary), it is possible to determine 2 ng Se/ml by this procedure. Other procedures are less frequently employed. While photometric methods with sulfur-containing organic reagents have been used, they are relatively less selective; the diazonium salt procedures require preliminary elimination of interfering elements and of oxidizing and reducing agents; procedures in- volving the formation of complexes with selenium of lower valence show relatively poor sensitivity. Selenium can be determined by atomic absorption spectroscopy or by neutron activation analysis. These methods were reviewed by Watkinson (1967) and Olson (1976~. While these methods generally have been consid- ered less sensitive than that of the DAN procedure, some investigators have reported a sensitivity of 5 ng or less using neutron activation (McKown and Morris, 1978), flameless atomic absorption spectroscopy with a graphite furnace (Henn, 1975), hydride generation with condensation (Hahn et al., 1981) or gas chromatography (McCarthy et al., 1981~. Biological samples for analysis of submicrogram amounts of selenium have been prepared in various ways. Allaway and Cary (1964) described a procedure in which the sample is combusted in an oxygen atmosphere in a Shoniger flask. Subsequently, the selenium is separated by coprecipitation with arsenic, then dissolved in nitric acid and measured using the DAN method. Samples can also be "wet" digested using nitric and perchloric acids (Watkinson, 1966) or sulfuric and perchloric acids (Ewan et al., 1968a). A useful method for the determination of selenium in plant and animal tissues was reported by Olson (1969a). This method employs a di- gestion using nitric and perchloric acids followed by reaction with DAN. Upon extraction with decahydronaphthalene or cyclohexane, the piazsel- enol is measured fluorometrically. This procedure has become the official first action method of the Association of Official Analytical Chemists and has been improved and simplified (Olson et al., 1975~. Further modifica- tions have been made (Whetter and Ullrey, 1978) to reduce labor and equipment requirements and to increase the number of samples that can be analyzed per day.