; à 33°C, les réactivités relatives des disaccharides vis-à-vis du Cr(VI) sont les suivantes: Mel > Lac > Cel > Mal. En milieu acide, il se forme du Cr(V) comme intermédiaire qui réagit avec le substrat plus rapidement que le Cr(VI). Les spectres de RPE montrent qu'il se forme des intermédiaires penta-et hexacoordinés d'oxo-Cr(V) dans lesquels le disaccharide agit comme ligand bidentate. Les espèces pentacoordinées oxo-Cr(V) sont présentes à toutes les [H + ] alors qu'on n'observe les espèces hexacoordinées qu'à des pH < 2 et que dans ces conditions elles se décomposent rapidement en produits d'oxydoréduction. À des pH allant de 3 à 7 où on n'observe pas de formation des espèces hexacoordinées d'oxo-Cr(V), les complexes de Cr(V) sont suffisamment stables pour être observés en solution pour des périodes allant de plusieurs jours à plusieurs mois.

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The oxidation of d-glucitol and d-mannitol by Cr VI yields the aldonic acid (and/or the aldonolactone) and Cr III as final products when an excess of alditol over Cr VI is used. The redox reaction occurs through a Cr VI 3 Cr V 3 Cr III path, the Cr VI 3 Cr V reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by:where k G2 H (8.5 AE 0.2)´10 À2 m À3 s À1 and k GH (1.8 AE 0.1)´10 À2 m À2 s À1 , at 338. The slow redox steps are preceded by the formation of a Cr VI oxy ester with l max 371 nm, at pH 4.5. In acid medium, intermediate Cr V reacts with the substrate faster than Cr VI does. The EPR spectra show that five-and six-coordinate oxo-Cr V intermediates are formed, with the alditol or the aldonic acid acting as bidentate ligands. Pentacoordinate oxoCr V species are present at any [H ], whereas hexacoordinate ones are observed only at pH`2 and become the dominant species under stronger acidic conditions where rapid decomposition to the redox products occurs. At higher pH, where hexacoordinate oxo-Cr V species are not observed, Cr V complexes are stable enough to remain in solution for several days to months.1. Introduction. ± Chromium(VI) (Cr VI ) is a potential hazard both in a biological and an ecological context [1]. The observation of Cr V and Cr IV intermediates in the selective oxidation of organic substrates by Cr VI and their implication in the mechanism of Cr-induced cancers [2 ± 3] has generated a considerable amount of interest in their chemistry and biochemistry [4 ± 7]. The biological reduction of Cr VI to lower states has been observed with a wide variety of naturally occurring cellular reductants [8 ± 12]. Ligands that possess two O-atoms able to form five-membered rings about the metal ion, such as 1,2-diols and a-hydroxy acids, are effective as nonenzymatic reductants and complexation agents towards hypervalent chromium and can stabilize the labile oxidation states of chromium [13 ± 19]. For this reason, it is interesting to look at the ability of polyhydroxy compounds to reduce Cr VI to Cr III , in order to know the role they may play in the chemistry of Cr VI .We are studying the possible fate of Cr VI and Cr V in biological systems by examining reactions of Cr VI with low-molecular molecules [20 ± 34]. Our studies on the reduction of Cr VI and intermediate Cr V by aldoses [20] [30] [31] [33], deoxyaldoses [20] [23] [25] [30] [31], sugar acids [22] [27] [29], and methyl glycosides [34] showed that the relative redox reactivities of these saccharides toward chromate is based on the relative rates of the oxidation vs. complexation processes [31] [32]. Under conditions of

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En Argentina, no existen datos regionales de intervalos de referencia de minerales en cerdos según las líneas genéticas actuales y distintas categorías de producción en sistema intensivo. Por ello, el objetivo del presente estudio fue determinar intervalos de minerales: calcio, fósforo, magnesio, sodio, potasio, hierro, cobre y cinc, en suero de cerdos en establecimientos del centro de Santa Fe y de Entre Ríos. Se trabajó con 300 muestras de sangre de cerdos sanos de dos líneas genéticas diferentes en las categorías de lechones recién nacidos, lechones destetados, hembras nulíparas gestantes seleccionadas fenotípicamente para la reposición del plantel, y hembras multíparas gestantes. Se determinaron las concentraciones séricas de los minerales por espectrofotometría de absorción atómica (FAAS) con un equipo Perkin Elmer modelo Analys 200, con métodos oficiales de AOAC. Se analizaron también muestras de agua. Los intervalos de referencia se calcularon utilizando parámetros paramétricos o no paramétricos dependiendo de la distribución de los datos. Los valores medios, medianas, valores mínimo y máximo e intervalos de referencia, para los distintos minerales en cada categoría productiva, se presentan en tablas. Los intervalos de referencia calculados serán útiles para el diagnóstico de deficiencias mineral y la vigilancia nutricional en cerdo de producción de carne.