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81.
Carbon and gold microdisc electrodes (30 and 10 μm, respectively) have been tested as substrates for in situ bismuth film plating from unstirred solutions of variable acetate buffer content and were subsequently used in the anodic stripping voltammetry determination of Pb(II) and Cd(II) ions. The effects of Bi(III) concentration, analyte accumulation time, stirring as well as supporting electrolyte content have been studied. Under optimal conditions good voltammetric responses were obtained by means of square wave anodic stripping voltammetry in unstirred analyte solutions of 5 × 10−8 to 10−6 M, even in the absence of added buffer. In an indicative application, Pb(II) ion levels were determined in tap water using bismuth-plated carbon microdisc electrodes. 相似文献
82.
A new rapid and sensitive FI assay is reported for the simultaneous direct spectrophotometric determination of trace Cr(VI) and Cr(III) in real samples. The method is based upon the reaction of Cr(VI) with chromotropic acid (CA) in highly acidic medium to form a water-soluble complex (λmax = 370 nm). Cr(III) reacts with CA only after its on-line oxidation to Cr(VI) by alkaline KIO4. The determination of each chromium species in the sample was achieved by absorbance differences. The calibration curves were linear over the range 3-4000 μg l−1 and 30-1200 μg l−1 for Cr(VI) and Cr(III), respectively, while the precision close to the quantitation limit was satisfactory in both cases (sr = 3.0% for Cr(VI) and 4.0% for Cr(III) (n = 10) at 10 and 50 μg l−1 level, respectively). The method developed proved to be adequately selective and sensitive (cL = 1 and 10 μg l−1 for Cr(VI) and Cr(III), respectively). The application of the method to the analysis of water samples (tap and mineral water) gave accurate results based on recovery studies (93-106%). Analytical results of real sample analysis were in good agreement with certified values. 相似文献
83.
Garrido NM Jorge M Queimada AJ Macedo EA Economou IG 《Physical chemistry chemical physics : PCCP》2011,13(20):9155-9164
A methodology is proposed for the prediction of the Gibbs energy of solvation (Δ(Solv)G) based on MD simulations. The methodology is then used to predict Δ(Solv)G of four solutes (namely propane, benzene, ethanol and acetone) in several solvents of different polarities (including n-hexane, n-hexadecane, ethylbenzene, 1-octanol, acetone and water) while testing the validity of the TraPPE force field parameters. Excellent agreement with experimental data is obtained, with average deviations of 0.2, 1.1, 0.8 and 1.2 kJ mol(-1), for the four solutes respectively. Subsequently, partition coefficients (log P) for forty different solute/solvent systems are predicted. The a priori knowledge of partition coefficient values is of high importance in chemical and pharmaceutical separation process design or as a measure of the increasingly important environmental fate. Here again, the agreement between experimental data and simulation predictions is excellent, with an absolute average deviation of 0.28 log P units. However, this deviation can be decreased down to 0.14 log P units, just by optimizing partial atomic charges of acetone in the water phase. Consequently, molecular simulation is proven to be a tool with strong physical basis able to predict log P with competitive accuracy when compared to the popular statistical methods with weak physical basis. 相似文献
84.
We consider a Markovian clearing queueing system, where the customers are accumulated according to a Poisson arrival process and the server removes all present customers at the completion epochs of exponential service cycles. This system may represent the visits of a transportation facility with unlimited capacity at a certain station. The system evolves in an alternating environment that influences the arrival and the service rates. We assume that the arriving customers decide whether to join the system or balk, based on a natural linear reward-cost structure. We study the balking behavior of the customers and derive the corresponding Nash equilibrium strategies under various levels of information. 相似文献
85.
86.
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87.
88.
In this paper we present a detailed analysis of queueing models with vacations and impatient customers, where the source of impatience is the absence of the server. Instead of the standard assumption that customers perform independent abandonments, we consider situations where customers abandon the system simultaneously. This is, for example, the case in remote systems where customers may decide to abandon the system, when a transport facility becomes available. 相似文献
89.
The transport properties of GeSe indicate an antiferromagnetic character up to the temperature of 376 K. Above that temperature a metallic behaviour is materialized. Magnetic measurements confirm these findings indicating a Néel temperature at 381 K above which GeSe becomes paramagnetic. The magnetic moment is to be attributed to spin only angular momentum processes. 相似文献
90.
The Hubbard model is approximated by a static, random, self-consistent one-particle Hamiltonian incorporating magnetic order. Using this approximation and techniques available from the theory of disordered systems, thermodynamic and transport properties are obtained. 相似文献