Two polymorphs of bis(2‐carbamoylguanidinium) fluorophosphonate dihydrate

Two polymorphs of bis(2‐carbamoylguanidinium) fluorophosphonate dihydrate, 2C₂H₇N₄O⁺·FO₃P²⁻·2H₂O, are presented. Polymorph (I), crystallizing in the space group Pnma, is slightly less densely packed than polymorph (II), which crystallizes in Pbca. In (I), the fluorophosphonate anion is situated on a crystallographic mirror plane and the O atom of the water molecule is disordered over two positions, in contrast with its H atoms. The hydrogen‐bond patterns in both polymorphs share similar features. There are O—H...O and N—H...O hydrogen bonds in both structures. The water molecules donate their H atoms to the O atoms of the fluorophosphonates exclusively. The water molecules and the fluorophosphonates participate in the formation of R⁴₄(10) graph‐set motifs. These motifs extend along the a axis in each structure. The water molecules are also acceptors of either one [in (I) and (II)] or two [in (II)] N—H...O hydrogen bonds. The water molecules are significant building elements in the formation of a three‐dimensional hydrogen‐bond network in both structures. Despite these similarities, there are substantial differences between the hydrogen‐bond networks of (I) and (II). The N—H...O and O—H...O hydrogen bonds in (I) are stronger and weaker, respectively, than those in (II). Moreover, in (I), the shortest N—H...O hydrogen bonds are shorter than the shortest O—H...O hydrogen bonds, which is an unusual feature. The properties of the hydrogen‐bond network in (II) can be related to an unusually long P—O bond length for an unhydrogenated fluorophosphonate anion that is present in this structure. In both structures, the N—H...F interactions are far weaker than the N—H...O hydrogen bonds. It follows from the structure analysis that (II) seems to be thermodynamically more stable than (I).     

Bis(tetraethylammonium) hydrogensulfate dihydrogenphosphate at 292 and 150 K

The title compound, 2C₈H₂₀N⁺·HSO₄⁻·H₂PO₄⁻, was crystallized in a desiccator over P₄O₁₀ from a water solution of stoichiometric amounts of tetraethyl­ammonium hydroxide and sulfuric and phosphoric acids. The compound is deliquescent. The structure contains two symmetry‐independent cations in nearly the same conformation, as well as two symmetry‐independent anions, the central atoms of which are equally occupied by P and S. The anions are interconnected by short O⃛O hydrogen bonds into one‐dimensional chains. The distances and angles between some of the methyl or methyl­ene groups and anionic O atoms indicate the presence of C—H⃛O hydrogen bonds. The structure was determined from data at 292 (2) and 150 (2) K. These room‐ and low‐temperature structures are virtually the same, with the exception of the localization of the H atoms that participate in the symmetry‐restricted O⃛O hydrogen bonds. A differential scanning calorimetry experiment indicated no phase transition below the temperature at which the compound started to decompose (353 K), down to 93 K.     

Energies of electronic states of promoter ions in hydrodesulfurization catalysts

Poly-vinylpyrrolidone-anchored (PVP) supported platinum catalyst (PVP-Pt) and rare earth metals (Sm³⁺, Pr³⁺, Ce³⁺, Nd³⁺and La³⁺) promoted PVP-Pt catalysts were prepared and used in the hydrogenation of o- and m-chloronitrobenzene (CNB) in ethanol at 303 K and atmospheric pressure. It was found that rare earth metals improved the selectivity of chloronitrobenzene to chloroaniline (CAN), while the hydrogenation rate of CNB decreased. In the presence of NaOH, PVP-Pr-Pt and PVP-La-Pt catalysts exhibit the highest selectivity in the hydrogenation of CNB. At the same time different support materials of Pt catalyst (such as polyethylene glycol (PEG), ethyl cellulose (EC)) possessed great influence on catalytic activity and selectivity of p-CAN.This revised version was published online in December 2005 with corrections to the Cover Date.     

A Study of PbTiO3 Crystallization in Pure and Composite Nanopowders Prepared by the Sol-Gel Technique

 In this investigation the crystallization of PbTiO₃ upon annealing of pure nanopowders and PbTiO₃–SiO₂ (1:1 v/v) nanocomposite powders prepared by the sol-gel technique was studied. Using X-ray diffraction phase analysis, the start of PbTiO₃ crystallization in pure PbTiO₃ powders was detected at 400°C. Distinct crystallization of PbTiO₃ in PbTiO₃–SiO₂ nanocomposites starts at 700°C, whereas SiO₂ remains amorphous. There are indications that an interface interaction between the PbTiO₃ and the SiO₂ phase plays an important role in hindering the crystallization of PbTiO₃. The particle size (size of coherently scattering regions) was estimated from the broadening of the X-ray diffraction line profiles. The average size of PbTiO₃ nanocrystallites increases with temperature and time of annealing, the influence of temperature being more significant than that of the annealing time. Differential scanning calorimetry confirmed the results of the X-ray diffraction with respect to the start of the crystallization. Laser beam scattering and scanning electron microscopy provided the statistical distribution of the grain size and the morphology of the powder grains, showing that each grain of the powders contains several nanocrystallites (coherently scattering regions).     

Operation of a proficiency testing scheme of trace elements in sewage sludge with reference values

A proficiency test (PT) was carried out to investigate the quality of laboratory results measuring trace elements in sewage sludge. The scheme relied on reference values, established through flame atomic absorption spectroscopy and instrumental neutron activation analysis. Within the framework of the project, ”Proficiency testing in Central and Eastern European Countries”, the measurements were done in parallel using induced coupled plasma mass spectrometry. This paper discusses the design of the PTS, the metrological concepts behind it and the way in which it was operated. The discussion includes the estimation of measurement uncertainty of the reference values obtained, homogeneity testing and its uncertainty evaluation, as well as all relevant quality assurance aspects. From the results, it can be concluded that all measurement methods involved agree within their respective uncertainties. Furthermore, it can be concluded that it is feasible to operate a proficiency test of trace elements in sewage sludge using reference values. The agreement between the reference values and the consensus values is satisfactory.     

Two phases of bis­(tetraethyl­ammonium) di‐μ‐chloro‐bis­[di­chloro­palladium(II)]

A phase transition was found to occur at ∼153 K in the title compound, (C₈H₂₀N)₂[PdCl₆]. The structures of the two phases are reported at 292 and 130 K. The low‐temperature phase is twinned. The phase transition is accompanied by a minor displacement of the ions. There are C—H⋯Cl interactions as short as ∼2.80 Å, indicating the existence of hydrogen bonds, and this was confirmed by vibrational spectroscopy. The [Pd₂Cl₆]²⁻ anion occupies sites of mmm and 2/m symmetry in the room‐temperature and low‐temperature phases, respectively.     

The use of stable isotopes for Cr(VI) determination in silty-clay soil solution

In assessing the environmental hazard of Cr(VI) present in soil, exchangeable Cr(VI) is important, since it can be easily washed out from the upper part of the soil into subsurface soil, surface and ground water, and taken up by plants. The aim of this study was to evaluate the degree of species interconversion that may occur during the extraction of exchangeable Cr(VI) from silty-clay soil with phosphate buffer in order to establish an extraction method that would be effective, accurate and with minimal or no species interconversions. The Cr(VI) concentration in soil extracts was determined by speciated isotope dilution inductively coupled plasma mass spectrometry (SID-ICP-MS). The study was performed on soil samples from a field treated with tannery waste for 17 years. Samples were spiked by enriched stable isotopic solutions of ⁵⁰Cr(VI) and ⁵³Cr(III) that were added to phosphate buffers (0.1 M KH₂PO₄-K₂HPO₄ (pH 7.2) and/or 0.1 M K₂HPO₄ (pH 8)). To optimize extraction, mechanical shaking and/or ultrasound-assisted extraction were compared. The separation and detection of Cr species was performed by high-performance liquid chromatography (HPLC) ICP-MS. When mechanical shaking was applied, 90 % reduction of Cr(VI) was induced by extraction with 0.1 M KH₂PO₄-K₂HPO₄, while with 0.1 M K₂HPO₄ reduction was around 40 %. To shorten the extraction time and the possibility of species interconversions, ultrasound-assisted extraction was further applied only with 0.1 M K₂HPO₄. For total extraction of exchangeable Cr(VI) with a maximum 10 % reduction of Cr(VI), five consecutive ultrasound-assisted extractions were needed.

Preparation of a sewage sludge laboratory quality control material for butyltin compounds and their determination by isotope-dilution mass spectrometry

The characterisation of a laboratory quality control material (QCM) for dibutyltin (DBT) and tributyltin (TBT) in sewage sludge is described. The reference values were determined by the use of two different types of isotope-dilution mass spectrometry: gas chromatography–mass spectrometry and gas chromatography–inductively coupled plasma mass spectrometry. To avoid possible analytical errors such as non-quantitative extraction and species degradation during sample preparation, different extraction methods were tested (microwave- and ultrasound-assisted extraction and mechanical stirring). The reference values were based on the unweighted means of results from the homogenisation and characterisation studies. The reference values obtained were 1,553 ± 87 and 534 ± 38 ng Sn g^-1 for DBT and TBT, respectively. In the uncertainty budget estimation, the sample inhomogeneity and between-method imprecision were taken into account. The concentrations of DBT and TBT in QCM are similar to those in the harbour sediment certified reference material PACS-2. Likewise, the levels of DBT and TBT are in the range of these compounds normally present in sewage sludge worldwide. In the future, the QCM will be used for an intercomparison study on DBT and TBT in sewage sludge, and as a day-to-day QCM during studies concerning the application of sewage sludge as an additive to artificial soil or as a raw material in civil engineering construction.