Application of microwave digestion to the preparation of sediment samples for arsenic speciation

Several extraction procedures are described allowing arsenic speciation in sediments. The extraction of organometallic compounds such as dimethylarsinic acid or monomethylarsonic acid is quite simple since these compounds are stable in the different extraction media (HCl/ HNO₃, H₃PO₄, ammonium oxalate) and are easily released independent of the extraction mode (magnetic stirring or microwave solubilization). Extraction yields are higher than 96% for these two arsenic forms. An HCl/HNO₃ microwave solubilization procedure allows the quantitative solubilization of mineral arsenic, but the differentiation between the two oxidation states is not possible owing to the oxidation of As(III) to As(V). Extractions with orthophosphoric acid or ammonium oxalate allow the solubilization of mineral arsenic with extraction yields ranging from 90 to 95% and the differentiation between As(III) and As(V). Nevertheless, the amount of As(III) is underestimated owing to its partial oxidation. The usefulness and advantages of microwave solubilization compared with conventional extraction procedures are discussed. Received: 17 May 1996 / Revised: 19 September 1996 / Accepted: 25 September 1996     

CAROTENOID PIGMENT ADMINISTRATION and DELAY IN DEVELOPMENT OF UV-B-INDUCED TUMORS

—The results of two studies are reported. In the first study, the carotenoid pigments, β-carotene. canthaxanthin and phytoene were administered to mice after one UV-B-induced skin tumor had developed, to see if carotenoid administration would delay the development of subsequent tumors. Canthaxanthin significantly delayed the development of subsequent tumors. In the second study, the same pigments were administered starting 10 weeks before, and for 24 weeks after, exposure to a large single fluence (8 × 10⁴J/m²) of UV-B radiation, to determine the effect of the pigments on tumor development. Phytoene significantly prevented the development of tumors to this fluence of radiation.     

Synthese regiospecifique,d'alcenylfluorosilanes et de cetones β, γ- et γ, δ-ethyleniques siliciees a partir de sila-1 cyclopentenes-3 et de sila-3 bicyclo[3.1.0] Hexanes

Studies of the electrophilic substitution on two 1-sila-3-cyclopentenes and their cyclopropane homologues revealed the important role of silicon and provided a new route to alkenylfluorosilanes and novel β,γ or especially γ,δ-unsaturated silylated ketones.     

PHYTOENE AS A PROTECTIVE AGENT AGAINST SUNBURN (>280nm) RADIATION IN GUINEA PIGS

Abstract— Phytoene, the colorless triene precursor of β-carotene (17 mg/g body weight) or a placebo was given to guinea pigs by daily intraperitoneal injection for 14 days, after which the animals were exposed to radiation (> 280 nm). It was found that the animals receiving phytoene developed significantly less erythema to the radiation than did the animals receiving placebo.     

Etude thermoanalytique de conditions de formation du gallate de sodium,II

Résumé L'étude par TG, ATD, analyse chimique et IR du système Na₂O₂/Ga₂O₃ a montré qu'au début du chauffage, sous l'action de la soude, il se formait de l'hydroxogallate qui se transformait en gallate vers 130–150C. Entre 287 et 350, Na₂O₂ et NaOH attaquent conjointement Ga₂O₃. NaGaO₂ obtenu possède des groupements GaO₄. La sensibilité de l'ATD permet de différencier les domaines de réactivité maximale des deux composés sodiques. Audelà de 350, une nouvelle réaction de Na₂O₂ produit principalement du gallate à groupements GaO₆. L'attaque de Ga₂O₃ au cours de ces différentes phases dépend essentiellement de sa variété cristalline.

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Thermogravimetric, differential thermoanalytical and infrared spectroscopic studies of the system Na₂O₂/Ga₂O₃ have shown that in the reaction with sodium hydroxide hydroxogallate is formed, which transforms to gallate in the range of 130 to 150. Between 287 and 350 Na₂O₂ and NaOH attack Ga₂O₃ together, and the NaGaO₂ obtained contains GaO₄ groups. The DTA method permitted to distinguish the ranges of maximum reactivity of the two sodium compounds. Above 350 gallate containing GaO₆ groups is mainly formed in a new reaction of Na₂O₂. The attack of Ga₂O₃ in the course of the different steps depends mainly on its crystal structure.

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Zusammenfassung Thermogravimetrische, differentialthermoanalytische und infrarotspektroskopische Untersuchungen des Systems Na₂O₂/Ga₂O₃ zeigten, daß bei Einwirkung von Natriumhydroxyd Hydroxogallat gebildet wurde, das zwischen 130–150 in Gallat überging. Zwischen 287 und 350 greifen Na₂O₂ und NaOH gemeinsam das Ga₂O₃ an, das erhaltene NaGaO₂ enthält GaO₄ Gruppen. Durch DTA gelang es die Gebiete der maximalen Reaktivität der zwei Natriumverbindungen zu differenzieren. über 350 entsteht durch eine neue Reaktion von Na₂O₂ hauptsächlich Gallat mit GaO₆ Gruppen. Der Aufschluß von Ga₂O₃ hängt während dieser verschiedenen Phasen im wesentlichen von seiner kristallinen Beschaffenheit ab.

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Na₂O₂/Ga₂O₃ , , , 130–150. 287 350 Na₂O₂, NaOH, Ga₂O₃ NaGaO₂ G₄-. . Na₂O₂ 350, , G₆-. G₂₃ .

     

Friedel-crafts reaction of α-chloroaldimines : possible intermediacy of α-imidoylcarbenium ions

α-Chloroaldimines are arylated at the α-position under Friedel-Crafts conditions via intermediacy of α-imidoylcarbenium ions.     

Dimorphisme et structure du disulfure de lanthane LaS2

The stoichiometric lanthanum disulfide LaS₂ presents a reversible phase transition at about 750°C. The α low-temperature form is monoclinic with the LaSe₂ type. All the crystals are twinned with the same twin law (100). The cell parameters are a = 8.18, b = 8.13, c = 4.03Å, γ = 90°, space group $\text{P2}{}_1\text{a}$. The β high-temperature form has the orthorhombic structure previously described with the parameters a = 8.13, b = 16.34, c = 4.14 Å; space group Pnma. The two structures are compared.     

A general ultrasound-assisted access to room-temperature ionic liquids

The replacement of common organic solvents by room-temperature ionic liquids (RTILs) is a topical subject in both academia and industry. In the last decades, the number of applications for RTILs has followed an exponential curve and spilled over the boundaries of chemistry. Still, one of the main drawbacks of these compounds is their difficult access. The present ultrasound-assisted method affords a general and easy access to a large variety of room-temperature ionic liquids.