Charge transfer across a polymer gel/liquid interface: determination of ionophores

An electrochemical method for the determination of the ionophores monensin and lasalocid was developed, based on the polarization of an agar gel/nitrobenzene electrolyte interface. The measured current corresponding to the facilitated ion transfer across this interface is directly proportional to the concentration of an ionophore dissolved in the organic phase. Using cyclic voltammetry in a three-electrode system the detection limit for both ionophores is about 3 × 10^?5 M.     

Thermal properties of hydrated lanthanum and yttrium polyvanadates

The study of thermal properties of La(HV₆O₁₆)₃.19.5H₂O and Y(HV₆O₁₆)₃.22H₂O showed, that the dehydration of both compounds is discontinuous. After release of last water amount, the parent structure is destabilized and products of thermal decomposition, V₂O₅ and corresponding orthovanadates, start to be formed. The decomposition of La(HV₆O₁₆)₃.19.5H₂O results in formation of V₂O₅ and LaVO₄, the temperatures of crystallization of which differ. The decomposition products of yttrium salt crystallize simultaneously. All compounds formed melt at 685°C. By crystallization of the melt, the mixtures V₂O₅-LaVO₄ and V₂O₅-YVO₄, respectively, are formed which are stable up to 1000°C.     

Inexpensive device for measurements with ion-selective and pH electrodes

The design and construction of a simple device for measuring ionic concentrations (or pH) with ion-selective electrodes are described. The automated system includes a special electronic circuit with an operational amplifier, a signal conditioner and a personalcomputer. A digital multimeter can be used if automation is not required. The results obtained in tests with iodide-, chloride- and nitrate-selective electrodes and glass electrodes show very good agreement with those obtained with sophisticated commercial apparatus.     

Combinations of hydroxyl-terminated polybutadiene and boron

Mixing and curing experiments with suspensions of boron in hydroxyl-terminated polybutadiene (HTPB) have been performed. In kneading experiments on a plastograph very high viscous masses with bad curing properties were obtained. The experiments indicated that HTPB is attached with its OH groups to acidic impurities on the surface of the boron powder. By adding a basic substance, such as octadecyl amine, mixing and curing were undertaken with much less difficulty.     

Esterification of C1−C8 carboxylic acids to n-butyl esters in aqueous solutions for their gas chromatographic analysis

Summary The esterification of C₁–C₈ fatty acid in aqueous solutions and in the presence of sulfuric or hydrochloric acid and with n-butanol is described. It has been established that the esterification can be used for the quantitative determination of these fatty acids in the concentration ratio range of [H₂O]/[n-BuOH]=0.01–5.3. In the concentration ratio range of 0.01–0.8 the water present does not interfere. In the concentration range of above 0.8 anhydrous sodium sulfate is used for binding the water, in the amount of [Na₂SO₄.anh.]/[H₂O]0.2.     

Conformational behaviour of 2,2′-bipyrrole

The rotational potential around the interannular bond in 2,2-bipyrrole has been calculated making use of standard minimal STO-3G and split valence 4-31G basis sets. Geometrical optimization concerning the most significant interannular internal parameters has been performed with both basis sets. The trans conformer is predicted to be more stable than the cis. The minimal basis set predicts the existence of a cisoid-gauche minimum which after limited optimization becomes very shallow and it seems to be an artifact of the rigid rotor approximation. At 4-31G level, both the trans and cis conformers represent maxima in the potential curve and two gauche minima appear at =46.0° and =147.6°, the latter being the absolute minimum. The absolute maximum of the potential curve corresponds to the cis conformer.     

Gross alpha and gross beta radioactivity of water samples from ININ and Maquixco in the State of Mexico

The conditions to measure the gross alpha and gross beta radioactivity in water samples from the surrounding of the Nuclear Center (Instituto Nacional Investigaciones Nucleares, (ININ), and Collection, Treatment and Storage Center for Low Level Radioactive Waste (Maquixco) in the State of Mexico were established. The samples were collected from 1987 to 1996. The gross alpha and gross beta radioactivity in the samples were determined by a gas-flow proportional counter. The results indicated that the gross alpha and gross beta radioactive contamination in water samples were below the maximum contaminant level (MCL).     

Extraction of microamounts of strontium by a nitrobenzene solution of bis-1,2-dicarbollylcobaltate in the presence of 15-crown-5

Extraction of⁸⁵Sr by a nitrobenzene solution of bis-1,2-dicarbollylcobaltate in the presence of 15-crown-5 (15C5, L) has been investigated. The equilibrium data and the typical maxima concerning the dependencies of the Sr distribution ratios on the analytical concentration of 15C5 in the system under study can be explained assuming that the HL⁺, HL₂ ⁺ and SrL₂ ²⁺ species are extracted into the nitrobenzene phase. The values of the corresponding extraction and stability constants of the extracted species in nitrobenzene saturated with water have been determined.     

HPLC-Electrochemical detection with graphite-poly (tetrafluoroethylene) electrode Determination of the fungicides thiram and disulfiram

The suitability of composite graphite-poly(tetrafluoroethylene) (Teflon) electrodes as amperometric indicator electrodes in HPLC detection is demonstrated. The determination of the fungicides thiram and disulfiram in the presence of ziram has been chosen as an analytical problem. The optimization of working conditions, such as the choice of the organic solvent used in the mobile phase as well as its percentage, the potential applied to the composite electrode, and the time elapsed between mixing the carbamates and the injection, has been accomplished by using the wall-jet flow-cell configuration. The effect of the acetonitrile percentage used in the mobile phase on the retention of thiram, disulfiram, ziram and phenol was evaluated. Resolution up to the baseline can be achieved with 45% acetonitrile. The sensitivity of the determination of thiram and disulfiram in the presence of a constant concentration of ziram is slightly better when using a wall-jet cell; however, the background current is higher, as well as the baseline noise and the time necessary to achieve stabilization of the baseline before the injection. Lower limits of detection for both fungicides, as well as a better repeatability, were obtained when using a thin-layer flow cell configuration. As an application, the determination of thiram in spiked apple samples, at a level of 0.5 mg thiram kg(-1) apple, has been carried out with a mean recovery of 97 +/- 3% for a significance level of 0.05.     

The effect of carbon dioxide on xylose fermentation byPichia stipitis

Carbon dioxide was found to reduce the xylose fermentation rate of two different strains ofPichia stipitis (CBS 5773 and CBS 5776) significantly in anaerobic batch fermentations. The maximum specific fermentation rate in a carbon dioxide atmosphere was about 45% lower than the fermentation rate in nitrogen atmosphere. Further-more, the fermentation rate was found to be correlated to the growth rate. It is probable that the carbon dioxide influences the fermentation rate partly through decreasing the growth rate. It was also found that the fermentation rate of CBS 5773 was higher than for CBS 5776 and that the specific fermentation rate was lower at a higher cell density.