SUBJECTS selenium

Oxygen halides are dealt with in Chapter 11, p. 334. Sulphur, selenium and tellurium form many halides, and only a brief introduction to the subject is given here. 

This chapter is an attempt to present the important results of studies of the synthesis, reactivity, and physicochemical properties of this series of compounds. The subject was surveyed by Bulka (3) in 1963 and by Klayman and Gunther (4) in 1973. Unlike the oxazoles and thiazoles. there are few convenient preparative routes to the selenazoles. Furthermore, the selenium intermediates are difficult to synthesize and are often extremely toxic selenoamides tend to decompose rapidly depositing metallic selenium. This inconvenience can be alleviated by choice of suitable reaction conditions. Finally, the use of selenium compounds in preparative reactions is often complicated by the fragility of the cycle and the deposition of metallic selenium. 

Useful information on the major industrial applications of selenium and tellurium can be traced in the old (1942), yet still expedient review of Waitkins et al. [4], based on most of the early (pre-war) references on the subject. Not much has changed since then, at least with regard to the industrial uses of these elements. 

Supplementation with antioxidant micronutrients, for example, vitamin E and selenium, in the elderly and in dementia subjects (Tolonen et al., 1985) has indicated that such treatments may be of some limited benefit. The value of ascorbate as a cerebroprotective antioxidant against excitotoxic neuronal injury has been proposed (Griinewald, 1993). 

Necheles et al. (N4) first reported a genetically determined homozygous GSH-Px deficiency associated with neonatal jaundice and mild hemolysis. Spontaneous recovery from hemolysis was noted 3 months after birth. Thereafter, several cases with GSH-Px deficiency were reported. Newborn infants exhibit significantly lower red blood cell GSH-Px activity and serum selenium concentrations than adult control subjects, and a significantly positive correlation between selenium concentration and GSH-Px activity has been observed. Furthermore, the addition of selenium stimulates, both in vivo and in vitro, the GSH-Px activity. The neonatal red blood cell GSH-Px deficiency may be partially due to insufficient availability of selenium during pregnancy (P9). Therefore, the diagnosis of GSH-Px deficiency in newborn infants must be made carefully. 

Selenium (IV) adsorbed as selenotrisulfate was then eluted from the column with either 0.1 M penicillamine or 0.1M cysteine. The eluate was then subjected to an acid digestion procedure to reduce selenium to the tetravalent state with diaminonaphthalene for fluorometric determination. Approximate agreement with the tellurium coprecipitation method was obtained. The application of both methods to the analysis of estuarine waters permitted the separate determination of both selenium (IV) and selenium (VI), since the tellurium coprecipitation methods did not differentiate between the two species. 

Selenium has been the subject of many reviews (Rosenfeld and Beath 1964 Frost 1972 Sandholm 1973 Zingaro and Cooper 1974 Frost and Ingvoldstad 1975 Anonymous 1975 National Academy of Sciences [NAS] 1976 Harr 1978 U.S. Environmental Protection Agency [USEPA] 1980, 1987 Lo and Sandi 1980 Shamberger 1981 Wilber 1980, 1983 Fishbein 1977, 1983 National Research Council [NRC] 1983 Reddy and Massaro 1983 Eisler 1985 Lemly and Smith... 

Selenium has a complex chemistry in the environment because of its multiple oxidation states and variable surface adsorption properties. Qualitatively it is analogous to sulfur occurring in the oxidation states +6 (selenate, Se04 ), +4 (selenite, SeOs "), 0 (elemental selenium) and —2 (Se, selenide) The Se anion closely resembles S (radii 0.20 and 0.185nm, respectively) and is often associated with sulfide minerals. Also, like S, Se is subject to volatilization through biological methylation. 

An accidental spray of selenium dioxide into the eyes of a chemist caused superficial burns of the skin and immediate irritation of the eyes. Within 16 hours, the subject s vision was blurred and tbe lower portions of both corneas appeared dulled. Sixteen days after the accident, the corneas were normal. ... 

The charge transport in amorphous selenium (a-Se) and Se-based alloys has been the subject of much interest and research inasmuch as it produces charge-carrier drift mobility and the trapping time (or lifetime) usually termed as the range of the carriers, which determine the xerographic performance of a photoreceptor. The nature of charge transport in a-Se alloys has been extensively studied by the TOF transient photoconductivity technique (see, for example. Refs. [1-5] and references cited). This technique currently attracts considerable scientific interest when researchers try to perform such experiments on high-resistivity solids, particularly on commercially important amorphous semiconductors such as a-Si and on a variety of other materials... 

It should be noted that the last reaction is anomalous owing to the soft nature of the selenium such that the intermediate is an episelenonium species (i.e. less carbocat-ionic) and the attack of the counterion is subject to steric effects such as the 2,6-endo interaction. 

The three samples with the optimum reagent ratios (Ratio A, B, and C) were subjected to TCLP analysis and the leachate was analyzed for arsenic, barium, cadmium, chromium, cobalt, lead, mercury, nickel, selenium and silver. 

By the action of hydrazine hydrate on a dilute solution of selenic acid, hydrazine hydrogen selenate may be obtained as a colourless compound which is not decomposed by boiling water, but which, when dry, explodes with great readiness when subjected to heat, to shock, or to fumes of hydrogen chloride. For this reason, before hydrazine hydrate is used in the analysis of selenium compounds (see p. 307), it is essential that selenic acid and selenates should be reduced to selenites by means of hydrochloric acid.3... 

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