Arrhenius parameters determination in non-isothermal conditions for the uncatalyzed gasification of carbon by carbon dioxide

Non-isothermal thermogravimetric data were used to evaluate the Arrhenius parameters (activation energy, E, and pre-exponential factor, A) for the uncatalyzed gasification by carbon dioxide of two carbons, select as steam activated carbon (BPL) and SP-1 spectroscopically pure graphite. The paper reports on the application of the model-free isoconversional method (KAS/Vyazovkin linear method) for evaluating the activation energy of the gasification process. Activation energies have been calculated by this method were in good agreement with literature data for similar carbons. On the other hand, by means of the kinetic compensation relation between E and ln A, which was established by the model-dependent Coats–Redfern method, the value of the pre-exponential factor was estimated from the known value of the model-independent activation energy.     

Increasing stability of the fullerenes with the number of carbon atoms: the experimental evidence

The values of the molar standard enthalpies of formation, Delta(f)H(o)(m)(C(76), cr) = (2705.6 +/- 37.7) kJ x mol(-1), Delta(f)H(o)(m)(C(78), cr) = (2766.5 +/- 36.7) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), cr) = (2826.6 +/- 42.6) kJ x mol(-1), were determined from the energies of combustion, measured by microcombustion calorimetry on a high-purity sample of the D(2) isomer of fullerene C(76), as well as on a mixture of the two most abundant constitutional isomers of C(78) (C(2nu)-C(78) and D(3)-C(78)) and C(84) (D(2)-C(84), and D(2d)-C(84). These values, combined with the published data on the enthalpies of sublimation of each cluster, lead to the gas-phase enthalpies of formation, Delta(f)H(o)(m)(C(76), g) = (2911.6 +/- 37.9) kJ x mol(-1); Delta(f)H(o)(m)(C(78), g) = (2979.3 +/- 37.2) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), (g)) = (3051.6 +/- 43.0) kJ x mol(-1), results that were found to compare well with those reported from density functional theory calculations. Values of enthalpies of atomization, strain energies, and the average C-C bond energy were also derived for each fullerene. A decreasing trend in the gas-phase enthalpy of formation and strain energy per carbon atom as the size of the cluster increases is found. This is the first experimental evidence that these fullerenes become more stable as they become larger. The derived experimental average C-C bond energy E(C-C) = 461.04 kJ x mol(-1) for fullerenes is close to the average bond energy E(C-C) = 462.8 kJ x mol(-1) for polycyclic aromatic hydrocarbons (PAHs).     

Effects of activation conditions on the electrochemical capacitance of activated carbon nanotubes

The effects of different activation conditions (including KOH/CNTs ratio, activation temperature, activation time and nitrogen flow rate) on the electrochemical capacitance of activated carbon nanotubes (CNTs) have been investigated. All the four factors can affect the activated CNTs’ electrochemical capacitance. And the effects on the activated CNTs’ electrochemical capacitance carried out by virtue of their effects on activated CNTs’ BET specific surface area and graphitized degree.     

Applicability of emf measurements under external load resistance conditions for ion transport number determination

In order to evaluate the applicability of concentration cells for the ion transference number measurements with external load, the cell response was simulated with variable transport properties of the cell material, external load resistance, geometrical factors such as ion-conducting membrane thickness, and electrode kinetics. This technique is expected to be pertinent when ηF/RT<0.2, except possibly for conditions when the electrode kinetics is dependent on a relatively small limiting current density. In each particular case, the method validity can be verified by testing if the overpotential sum grows faster than current on decreasing the external load resistance. A pyrochlore-type material Gd_1.9Ca_0.1Ti₂O_7-δ with dominant oxygen ionic conductivity is used as a study case to demonstrate the criteria proposed to assess the applicability of emf measurements under short-circuit conditions.     

Quantitative determination of triacylglycerols separated on capillary columns according to acyl carbon number and level of unsaturation

The quantitative determination of triacylglycerols separated according to acyl carbon numbers and level of unsaturation was studied on two 10-m immobilised SE-54 columns. The fat samples were silylated before on-column injection. The 0.2 mm I.D. column showed higher resolving power and the 0.32 mm I.D. column better repeatability. The application to real quantitations was investigated with analyses of butter-fat solid and liquid fractions and of untreated and interesterified 70-30 mixtures of butter-fat solid fraction and hydrogenated rapeseed oil. Changes in the triacylglycerol composition induced by interesterification and fractionation are so pronounced that analysis on capillary columns can be recommended, in spite of only fair repeatability.     

Quartz crystal microbalance determination of vapors of volatile organic compounds on carbon nanotubes under batch conditions

Specific features of the sorption of vapors of monoatomic aliphatic alcohols and aromatic compounds on coatings formed from carbon nanotubes of various genesis studied by the method of piezoelectric micro weighing are considered. The morphology of the coatings is considered in dependence on the conditions of synthesis and aftertreatment of carbon nanotubes. A new method of the processing of signals from an array piezoelectric sensors is proposed; it allows an increase in the selectivity of the determination of volatile organic compounds in an equilibrium gas phase by 2–2.5 times. The advantages of the application of carbon nanotubes as sorption coatings in comparison to the standard polymeric sorbents for piezoelectric micro weighing are demonstrated: the detection limit decreases by 4–15 times and the duration of measurement by 5–55 times.     

Studies on elution conditions for the determination of anions by supressed ion-interaction chromatography using a graphitized carbon column

A new method has been developed for ion-interaction chromatography with suppressed conductivity detection and a new graphitized carbon packing, which is sintered from carbonic material at a high temperature. Combinations of various eluting agents, tetrabutylammonium hydroxide (TBA) and acetonitrile have been investigated to optimize the separation of eight common anions (F⁻, Cl⁻, NO₂⁻, Br⁻, NO₃⁻, SO₄²⁻, HPO₄²⁻ and I⁻). Calibration curves were linear from 0.5 to 10 μg/ml for F⁻, from 1.0 to 20 μg/ml for Cl⁻, NO₂⁻ and NO₃⁻, from 2.5 to 50 μg/ml for Br⁻ and SO₄²⁻ and from 5.0 to 100 μg/ml for HPO₄²⁻ and I⁻ with a correlation coefficient (r) of 0.999 or better. The relative standard deviations (R.S.D.s) of peak areas were between 0.2 and 0.9% for 10 repeated measurements. The application of this newly developed method was demonstrated by the determination of chloride, bromide and sulfate in pharmaceutical compounds using the direct injection method. The analytical results were within ±2% (relative) of the theoretical value, and thus in good agreement with the theoretical value for each sample.     

Precipitation chromatography. III. Optimization of the experimental conditions and determination of the polydispersity of fractions

The influence of the solvent–nonsolvent gradient and the temperature gradient on the efficiency of the precipitation chromatography is investigated with radically and anionically prepared polystyrene samples. Only a combination of both gradients gives optimal effectiveness of the column. The separating power of the column was determined by using mixtures of well defined samples. From the elution curves of the mixtures the nonuniformity U_fr of fractions can be estimated. For fractions of anionically prepared polystyrenes U_fr is on the average 0.001; this value is of the same order of magnitude as the statistical error. These results show that with this chromatographic method it is possible to determine the narrow molecular weight distributions of anionically prepared polystyrenes (or other polymers) with an accuracy sufficient for kinetic studies.     

Multivariate optimisation of the experimental conditions for determination of three methylxanthines by reversed-phase high-performance liquid chromatography

Caffeine (1,3,7-trimethylxanthine), theobromine (3,7-dimethylxanthine) and theophylline (1,3-dimethylxanthine) are the most important naturally occurring methylxanthines. Caffeine is a constituent of coffee and other beverage and included in many medicines. Theobromine and theophylline are formed as metabolites of caffeine in humans, and are also present in tea, cocoa and chocolate products.

In order to improve the chromatographic resolution (R_s) with a good analysis time, experimental designs were applied for multivariate optimisation of the experimental conditions of an isocratic reversed-phase high-performance liquid chromatographic (RP-HPLC) method used for the simultaneous determination of caffeine, theobromine and theophylline. The optimisation process was carried out in two steps using full three-level factorial designs. The factors optimised were: flow rate and mobile phase composition. Optimal conditions for the separation of the three methylxanthines were obtained using a mixture of water/ethanol/acetic acid (75:24:1%, v/v/v) as mobile phase and a flow rate of 1.0 mL min⁻¹. The RP-HPLC/UV method was validated in terms of limit of detection (LOD), limit of quantitation (LOQ), linearity, recovery and the precision, calculated as relative standard deviation (R.S.D.). In these conditions, the LOD was 0.10 μg L⁻¹ for caffeine, 0.07 μg L⁻¹ for theobromine and 0.06 μg L⁻¹ for theophylline. The proposed method is fast, requires no extraction step or derivatization and was suitable for quantification of these methylxanthines in coffee, tea and human urine samples.     

Effects of thermal activation conditions on the microstructure regulation of corncob-derived activated carbon for hydrogen storage

Activated carbons derived from corncob （CACs） were prepared by pyrolysis carbonization and KOH activation. Through modifying activation conditions, samples with large pore volume and ultrahigh BET specific surface area could be obtained. The sample achieved the highest hydrogen uptake capacity of 5.80 wt% at 40 bar and -196℃ The as-obtained samples were characterized by N2-sorption, transmission electron microscopy （TEM） and scanning electron microscopy （SEM）. Besides, thermogravimetric analysis was also employed to investigate the activation behavior of CACs. Detailed investigation on the activation parameters reveals that moderate activation temperature and heating rate are favorable for preparing CACs with high surface area, large pore volume and optimal pore size distribution. Meanwhile, the micropore volume between 0.65 nm and 0.85 nm along with BET surface area and total pore volume has great effects on hydrogen uptake capacities. The present results indicate that CACs are the most promising materials for hydrogen storage application.     

An alternative to the effective number of degrees of freedom

 In order to calculate the expanded uncertainty when one or more of the components of the standard uncertainty is based on a small number of degrees of freedom, the ISO Guide recommends utilising the 'effective number of degrees of freedom'. Calculating and utilising the 'effective number of degrees of freedom' to obtain a confidence interval both depend upon approximations, which, however, are adequate for determining an expanded uncertainty. This paper puts forward an alternative and simpler approximation which is also adequate but which is much easier to apply than the multi-step process given in the ISO Guide. Received: 2 July 1998 · Accepted: 12 August 1998     

Effect of plasmatron excitation conditions on the determination of traces

Optimal analysis conditions can be selected and errors caused by variations in the excitation conditions can be decreased or corrected based on the results of studying the temperature distribution along the plasma torch height and correlations between the analytical signal Y and the correcting signal Z that follows the temperature variation in the plasma torch. The obtained correlations can be used to create automated feedback between the spectrum recording system and the mode control system of the plasmatron.     

A method for experimental determination of effective resonance energies related to (n, γ) reactions

When making use of some single comparator or absolute standardization methods in reactor neutron and in epicadmium neutron activation analysis, the knowledge of the effective resonance energy ( ) is essential to correct for the effect of the nonideal epithermal flux distribution on the analysis result. can be calculated from neutron resonance data, but when these are incomplete, not accurate or even not known at all, experimental determination should be considered. Such a method, providing both and the resonance integral to 2200 ms^–1 cross-section ratio (Q_O), is described in this paper. Results are given for 11 isotopes.     

Use of experimental design and effective mobility calculations to develop a method for the determination of antimicrobials by capillary electrophoresis

A capillary zone electrophoresis (CZE) method for the determination of chloramphenicol (CLP), danofloxacin (DANO), ciprofloxacin (CIPRO), enrofloxacin (ENRO), sulfamethazine (SMZ), sulfaquinoxaline (SQX) and sulfamethoxazole (SMX) is described. For the development, the effective mobilities were estimated and a central composite design was performed. The method was in-house validated for CLP, CIPRO, ENRO and SMX determination in pharmaceuticals. In comparison with the HPLC method recommended by the United States Pharmacopoeia, this CZE method exhibited the same performance, with the advantage that seven different antimicrobials in pharmaceutical formulations could be simultaneously determined.     

The reduction of iron-containing particles deposited on carbon under the conditions of iron-catalyzed carbon hydrogenation

Mössbauer spectroscopy was used to study size changes of iron-containing particles deposited on carbon and formed in thermal treatment in hydrogen atmosphere under the conditions of iron-catalyzed hydrogenation of carbon.     

An experimental approach to find aniline-formaldehyde co-polymers as effective inhibitor on the interface of low carbon steel immersed in 0.5N HCl mixtures

The usefulness of aniline formaldehyde (AF), a modern water-soluble composite in 0.5 ?N hydrochloric acid as inhibitor of corrosion for mild steel, has been studied using weight reduction method, test of electrochemical impedance and potentiodynamic polarization methods. According to the findings by weight loss methods, 12ppm of AF co-polymer at room temperature i.e. about 35° ?± ?1°C for 3h duration shows best performance on metal surface and exhibit 93.44% Inhibitor efficiency. The above said results has also being reviled from other examination methods, which shows that the AF follows the Langmuir isotherm, as well as the adsorption properties of the sampling supports the results as maximum IE of 95.05%, using EIS. The tafel and linear polarization results of maximum IE was found to be 94.81% and 94.96% respectively which was well aligned with an atomic force microscope (AFM) for surface morphology and found AF to be best suited corrosion inhibitor showing mixed type of nature, at defined parameters.     

The titrimetric determination of carbon dioxide with special reference to the determination of carbon in organic compounds

The titrimetric barium carbonate method for the determination of. carbon in organic compounds has been examined and the various sources of error are discussed. The carbon dioxide from the combustion of the sample is absorbed in excess baryta containing 20% (w/v) barium chloride and the excess is back-titrated with hydrochloric acid in the presence of o-cresolphthalein indicator.The value of this method in conjunction with the previously described determination of water with succinyl chloride, for the simultaneous determination of carbon and hydrogen is assessed