Determination of antimony (III) and (V) by differential pulse anodic stripping voltammetry

Re-evaluation of DPASV procedures for determining low levels of Sb (III) and Sb (V) in solution identified several problem areas, e.g. anomalous ASV behaviour, possible formation of an intermediate valency state during the analytical cycle, and chemical interactions in acidified test solutions containing both valency states. Specific determination of Sb (III) can be achieved using base solutions composed of 0.2M HCl (detection limit 10 nM) or acetic acid/acetate buffer (detection limit 600 nM). For the determination of Sb (V), analysis in 2M HCl is recommended [with response in 0.2M HCl being used to correct for any Sb (III) present].     

Evaluation of "labile" metal in sediments by anodic stripping voltammetry

A procedure for determining the "labile" metal content of contaminated sediments (in different chemical environments) has been critically examined. The sediments were extracted overnight with different chemical solutions and the suspensions were analysed by differential pulse anodic stripping voltammetry. The extractants used have been recommended for soil/sediment speciation schemes, and by examination of the suspensions directly in the ASV cell, errors due to re-adsorption of released metal ion were minimized. The existence of different chemical forms of metal was signified by changes in peak shape and position or by the appearance of additional peaks. With complexing agents present the peak size was pH-dependent. The limitations of the ASV/suspension analysis technique have been carefully evaluated by using ten different extractants and seventeen sediments. The "lability" results obtained have been compared with the values obtained from a cation-exchanger transfer procedure. For characterizing the lability behaviour of the metal contents of sediments, preliminary extraction into a minimum of four base solutions is advisable, e.g., 0.02M nitric acid (low-pH labile); hydroxylamine in acetic acid (reducing conditions); acetic acid/acetate buffer (weakly sorbed and carbonate-bound) and 0.05M calcium chloride (exchangeable fraction at natural system pH), where the terms in parentheses describe the character of the fraction. The results are critically compared with those obtained by atomic-absorption analysis of the extracts and with those obtained by an earlier ion-exchange fractionation scheme. The advantages and limitations of the ASY systems are discussed.     

Processible Cyclopentadithiophene Copolymers for Photovoltaic Applications

We designed and synthesized a series of conjugated polymers containing alternating 4H-cyclopenta[2,1-b:3,4-b′]dithiophene units and comonomers consisting of 2,2′-bithiophene, 3^″, 4^″ -dihexyl-α -pentathiophene, 3,4-ethylenedioxythiophene and 5,5′ -bis(2-thienyl)-4,4′ -dihexyl-2,2′ -bithiazole. These polymers possess optical bandgaps in the range of 1.75 to 2.0 eV. The desirable absorption attributes of these materials make then excellent candidates for use in photovoltaic cells. Electrochemical studies indicate desirable HOMO-LUMO levels for use with fullerene derivatives as electron transporters. Field effect transistors made of these materials show hole mobilities in the range of 7.5 × 10^?4 cm²/Vs to 2.0 × 10^?3 cm²/Vs. Due to the combination of these characteristics, power conversion efficiencies up to 3.1% were achieved on devices made of bulk heterojunction composites of these materials with soluble fullerene derivatives.     

Untersuchung von Butter, Margarine, Tran und anderen Fetten

Ohne Zusammenfassung     

Monobenzylation Of 1,n-Diols via Dibutylstannylene Intermediates

Symmetrical primary l,n-diols, HO(CH_2)nOH, of any chain length from n = 2-10, can be selectively monobenzylated via sequential treatment with dibutyltin oxide and benzyl bromide.     

Reduction of vibrations by bang–bang controlled electrorheological dampers

Summary This article is concerned with the application of electrorheological fluids (ERF) to the reduction of vibrations by controlled damping. Many papers deal with mechanical characteristics of ERF, but only a few articles have been published about the technical applications of these fluids, although great expectations have been placed into it some years ago. Vibration reduction by damping control is a difficult task because forces can only be generated when relative velocities exist between both ends of the damper. The design of a control strategy cannot be investigated by the methods of linear control theory. It is a task for the dynamic programming. The main objective of this paper is to present different methods for the investigation of suitable control strategies, and to verify the theoretical results by simulation and experiments. By simulation, it can be shown that the bang–bang control of damping – that means switching between the highest and the lowest possible damping – is an efficient way for the reduction of vibrations in special systems. Nearly no additional power is necessary. This switching can be realised with controlled valves and bypasses or by the ERF. Experiments with the ERF prove the theoretical considerations; still, there are several side effects making it difficult to perform a useful experiment, for example, leakage in the damper or electric isolation problems or the dependence on whether the AC or the DC is used. Received 26 October 1998; accepted for publication 20 October 1999     

Hybrid density functional theory for pi-stacking interactions: application to benzenes, pyridines, and DNA bases

The suitability of a hybrid density functional to qualitatively reproduce geometric and energetic details of parallel pi-stacked aromatic complexes is presented. The hybrid functional includes an ad hoc mixture of half the exact (HF) exchange with half of the uniform electron gas exchange, plus Lee, Yang, and Parr's expression for correlation energy. This functional, in combination with polarized, diffuse basis sets, gives a binding energy for the parallel-displaced benzene dimer in good agreement with the best available high-level calculations reported in the literature, and qualitatively reproduces the local MP2 potential energy surface of the parallel-displaced benzene dimer. This method was further critically compared to high-level calculations recently reported in the literature for a range of pi-stacked complexes, including monosubstituted benzene-benzene dimers, along with DNA and RNA bases, and generally agrees with MP2 and/or CCSD(T) results to within +/-2 kJ mol(-1). We also show that the resulting BH&H binding energy is closely related to the electron density in the intermolecular region. The net result is that the BH&H functional, presumably due to fortuitous cancellation of errors, provides a pragmatic, computationally efficient quantum mechanical tool for the study of large pi-stacked systems such as DNA.     

The crystallization kinetics and morphology of nitric acid trihydrate

Crystallization kinetics of stable and metastable nitric acid trihydrate (NAT) were investigated by time dependent X-ray powder diffraction (XRD) measurements. Kinetic conversion curves were evaluated adopting the Avrami model. The growth and morphology of the respective crystallites were monitored in situ on the cryo-stage of an environmental scanning electron microscope (ESEM) under a partial pressure of nitrogen gas (0.3 Torr, 40 Pa). The results show a close relationship between the presence of ice in the sample and the crystallization mechanism of NAT, which results in different shapes and sizes of NAT crystal particles.