Z4-Kerdock Codes, Orthogonal Spreads, and Extremal Euclidean Line-Sets

When m is odd, spreads in an orthogonal vector space of type⁺(2m + 2,2) are related to binary Kerdock codes and extremalline-sets in 2^{m + 1} with prescribed angles. Spreads in a 2m-dimensionalbinary symplectic vector space are related to Kerdock codesover Z₄ and extremal line-sets in with prescribed angles. These connections involve binary, realand complex geometry associated with extraspecial 2-groups.A geometric map from symplectic to orthogonal spreads is shownto induce the Gray map from a corresponding Z₄-Kerdock codeto its binary image. These geometric considerations lead tothe construction, for any odd composite m, of large numbersof Z₄-Kerdock codes. They also produce new Z₄-linear Kerdockand Preparata codes. 1991 Mathematics Subject Classification:primary 94B60; secondary 51M15, 20C99.     

On the thermal decomposition mechanism for dehydroxylation of alkaline-earth hydroxides

The activation energy for thermal dehydroxylation in vacuum of alkaline-earth hydroxides has been calculated from thermogravimetric data. The experimental results of Mg(OH)₂ Ca(OH)₂ and Sr(OH)₂ are in agreement with an unimolecular decay law and their activation energies are similar to the values of enthalpies of decomposition. In contrast, as the dehydroxylation process of Ba(OH)₂ takes place in liquid phase and the BaO does not dissolve into the molten Ba(OH)₂, a kinetic of zero order describes the reaction rate and the activation energy is lower than the enthalpy of decomposition.

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Zusammenfassung Die Aktivierungsenergie zur thermischen Dehydroxilierung von Erdalkali-Hydroxiden im Vakuum wurde aus thermogravimetrischen Daten errechnet. Die Versuchsergebnisse bezüglich Mg(OH)₂, Ca(OH)₂ und Sr(OH)₂ sind mit einem unimolekularen Zersetzungsgesetz in Übereinstimmung und die entsprechenden Aktivierungsenergien sind den Werten der Zersetzungsenthalpien ähnlich. Im Gegensatz hierzu wird im Falle von Ba(OH)₂, da der Dehydroxylierungsvorgang in der flüssigen Phase stattfindet und das BaO sich nicht im geschmolzenen Ba(OH)₂ löst, die Reaktionsgeschwindigkeit durch eine Kinetik nullter Ordnung beschrieben und die Aktivierungsenergie liegt hierbei niedriger als die Zersetzungsenthalpie.

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Résumé L'énergie d'activation de la déshydroxylation sous vide des hydroxydes des métaux alcalins est calculée à partir des données thermogravimétriques. Les résultats expérimentaux obtenus avec Mg(OH)₂, Ca(OH)₂ et Sr(OH)₂ sont en accord avec une loi de décomposition unimoléculaire et les énergies d'activation correspondantes sont du même ordre de grandeur que les enthalpies de décomposition. Par contre, du fait que la déshydroxylation de Ba(OH)₂ a lieu en phase liquide et que BaO ne se dissout pas dans Ba(OH)₂ fondu, la vitesse de la réaction est décrite par une cinétique d'ordre zéro et l'énergie d'activation est plus faible que l'enthalpie de décomposition.

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. Mg(OH)₂, Ca(OH)₂ Sr(OH)₂ . , Ba(OH)₂ BaO , , .

     

Synthesis of Iron Oxide Nanoparticles Used as MRI Contrast Agents: A Parametric Study

Colloidal iron oxides play an important role as magnetic resonance imaging (MRI) contrast agents. The superparamagnetic particles actually used are constituted by solid cores (diameter of 5-15 nm), generally coated by a thick polysaccharidic layer (hydrodynamic radii of 30-100 nm), and formulated by direct coprecipitation of iron salts in the presence of polymeric material. To better control the synthesis, we attempted to formulate new stable uncoated superparamagnetic nanoparticles. Colloids were generated by coprecipitation of an aqueous solution of iron salts and tetramethylammonium hydroxide (TMAOH) solution. The influence of parameters such as media composition, iron media, injection fluxes, Fe and TMAOH concentrations, temperature, and oxygen on size, magnetic and magnetic resonance relaxometric properties, and colloidal stability of particles were evaluated. We have determined the relative importance of these parameters as well as the optimal conditions for obtaining uncoated stable particles with an average size of 5 nm and interesting relaxivities. The interpretation of the observed limits takes into account diffusibilities of reactants and product, feeding rates of reactants, and surface properties of nanoparticles. A model of synthesis, related to spontaneous emulsification of suspensions, is proposed. Copyright 1999 Academic Press.     

On the kinetic analysis of thermoanalytical diagrams obtained with the “quasi-isothermal” heating technique

A comparative study of “n-order”, Avrami—Erofeev and diffusion-controlled reactions has been carried out using thermoanalytical data, calculated by assuming a “quasi-isothermal” heating technique. The results reported support the hypothesis that a single diagram obtained with this technique allows the discernment of Avrami-Erofeev, “n-order” and diffusion-controlled reactions. This does not occur when a linear heating program is used, as was reported in previous papers. In addition, it is shown that a single thermoanalytical curve obtained with a “quasi-isothermal” heating program does not permit the determination of the actual value of n of those reactions following “n-order” kinetics. On the other hand, it is proved that the kinetic analysis of two thermoanalytical curves obtained with a linear heating program and a “quasi-isothermal” heating technique, respectively, would provide an excellent procedure for discerning the proper mechanism of solid-state reactions.     

Continuous flow spectrophotometric determination of paracetamol in pharmaceuticals following continuous microwave assisted alkaline hydrolysis

This paper reports a continuous flow-based spectrophotometric method for the determination of paracetamol in pharmaceuticals. Dilute samples containing paracetamol are continuously hydrolysed in an alkaline medium, using a household microwave oven, to p-aminophenol, which reacts with o-cresol in 3.5 M NaOH. The blue derivative thus formed exhibits an absorbance maximum at 620 nm. The detection limit of the method is 0.2 mug ml(-1) and the system obeys Beer's law from 0.6 to 20 mug ml(-1). Repeatability was checked by calculating the R.S.D. for 11 standard solutions containing a paracetamol concentration of 4 mug ml(-1) each and found to be 1.8%. The proposed method was validated by determining paracetamol in various pharmaceutical preparations with average R.S.D. of 2.4%.