Hydrophile Polymergele mit reaktiven Gruppen, 4. Mitt.: Chelatharze mit schwefelhaltigen Ankergruppen auf Basis von Saccharosemethacrylaten

Chelating resins with thioglycolate anchor groups have been synthesized by reaction of sucrosemethacrylate gels with thioglycolic acid. The capacities of the gels were found to be extremely high for Ag⁺ (5,3 mmol/g) and Hg²⁺ (4,9 mmol/g). About 1 mmol Hg²⁺/g could be recovered from the gel reversibly by treatment with hydrochloric acid. The removal of Hg²⁺ from aqueous 3M alkalichloride solutions was possible with capacities of 1 mmol/g.Sucrosemethacrylate gels with primary aromatic amino group were reacted with CS₂/NH₃ to yield gels with dithiocarbamate groups. Gels with thiorea groups were prepared by reaction of the amino groups with NH₄SCN/HCl. Diazotation of the amino groups, subsequent reaction with potassium xanthogenate and hydrolysis afforded gels with thiol groups. Thiol containing gels were synthesized also by reaction of the diazotised gels with Na₂S₂ and subsequent reduction with Na₂S. Reaction of the diazotised gels with mercaptans yielded resins with thioether anchor groups. The capacities of the sulfur containing gels were found to be max. 4,1 mmol/g for Hg²⁺ and 5,9 mmol/g for Ag⁺. About 35% of the bonded Hg²⁺ could be eluted resersibly with 3N-HCl.

     

Mercury analysis in environmental samples by EDXRF and CV-AAS

The analysis of total Hg in various environmental matrices was investigated using energy dispersive X-ray fluorescence (EDXRF) for both powdered and digested samples and cold vapor atomic absorption (CV-AAS) for digested samples. Several microwave decomposition procedures were evaluated with respect to the determination of Hg by CV-AAS and EDXRF. The use of different acid mixtures, microwave power settings and decomposition times were compared as well as the analysis on wet and freeze-dried samples. A set of standard reference samples were used to evaluate the procedures: NBS Citrus leaves (1572), NRC Pine needles (1575), NRC Dogfish muscle tissue (DORM-2), NRC Dogfish liver tissue (DOLM-2), BCR Human hair (397), BCR Coking coal (181) and NRC Marine sediment (PACS-1). An H₂SO₄/HNO₃/H₂O₂ acid mixture was found to be optimal for the complete mineralization and digestion of biological samples for Hg by CV-AAS whereas for EDXRF both the HNO₃/H₂O₂ and H₂SO₄/HNO₃/H₂O₂ acid mixture gave good results. The detection limit of EDXRF could significantly be decreased (from 0.2 μg/g to 0.058 μg/g) by performing the analysis with digested samples. A good agreement was obtained between CV-AAS and EDXRF analysis. The obtained results were also in good agreement with certified values. The methods were applied to environmental samples (coal, trees, leaves, spinach, fish, sediments) and human hair in a coal mining area and in places where they use coal for cooking in Vietnam. Received: 24 February 1997 / Revised: 13 June 1997 / Accepted: 18 June 1997     

High-pressure microwave dissolution of ceramics prior to trace metal determinations by microwave induced plasma atomic emission spectrometry

A commercial laboratory microwave acid digestion system was evaluated for the acid dissolution of ceramic powders (Al₂O₃, AlN, BN and Si₃N₄) prior to the determination of their trace element content by microwave induced plasma atomic emission spectrometry. Newly designed vessels, capable of withstanding internal pressures of over 110 bar, provide rapid and satisfactory results for sample dissolution. Sample preparation time was approximately 30 min (including the subsequent cooling time and preparation of the final solution). Results from conventional stainless-steel acid digestion vessel (Teflon bomb) dissolution are compared with the microwave bomb results of microwave plasma atomic emission spectrometry.     

Microwave synthesis of LaBO3 (B = Co,Fe) perovskites using graphite and citric acid additions

We studied the effects of graphite and citric acid additions on the formation of LaFeO₃ and LaCoO₃ perovskites upon microwave treatment of crystalline hydrates or nitrate solutions, respectively. The addition of graphite was shown to increase the yield of LaCoO₃ perovskite and to result in the formation of crystalline LaFeO₃ perovskite even at the microwave treatment stage. Subsequent thermal treatment at 800°C yields single-phase lanthanum ferrite with a high specific surface area (11 m²/g). Due to the addition of citric acid to nitrate solutions, a highly viscous gel forms, which allows preparation of single-phase perovskites with a high specific surface area (up to 34 m²/g) after microwave treatment and calcination. The samples obtained using the admixtures are characterized by a high catalytic activity in methane oxidation. No single-phase oxides form without introduction of these admixtures.     

Dissolution of sintered thorium dioxide in phosphoric acid using autoclave and microwave methods with detection by gamma spectrometry

A quantitative and fast method of dissolution of refractory thoria (ThO₂) was developed for the determination of thorium (Th) in a given sample. The dissolution of sintered ThO₂ powder, microspheres and pellets using 88% phosphoric acid was investigated. The conditions of quantitative dissolution of ThO₂ microspheres were optimized by conventional heating in autoclave and also by microwave heating. 100 mg of sintered ThO₂ microspheres were dissolved in 8 g of phosphoric acid in an autoclave, and heating at 170 °C for 3 h, in comparison to 5 g of phosphoric acid by microwave heating (375 W) at 220 °C for 1 h. Dissolution studies on the powder form of sintered ThO₂ were also performed. 1 g of sintered ThO₂ powder could be dissolved in 6.5 g of phosphoric acid in autoclave heating at 170 °C for 1 h. Strong complexing of (PO₄)³⁻ with Th⁴⁺ may be the influencing factor for quantitative dissolution of ThO₂.     

Microwave digestion of ancient peat and determination of Pb by voltammetry

Different acid compositions (HNO₃, H₂O₂, HF and HClO₄) and low pressure microwave digestion were applied to the microwave assisted dissolution of ancient peats. The digests were evaluated with respect to the decomposition of the inorganic and organic fraction of peats and to an optimised determination of Pb by differential pulse anodic stripping voltammetry (DPASV). Addition of hydrofluoric acid was necessary to achieve a complete dissolution of the resistant inorganic phase of the peat and to obtain the total Pb concentration. Addition of perchloric acid decreased the concentration of dissolved organic carbon significantly, enabling reliable voltammetric signals for Pb determinations with satisfying peak shape, baseline, sensitivity, and precision. The limit of detection for Pb in peats was 1 ng/mL in the acid digest, equivalent to 0.2 μg/g in solid peat. This allowed the determination of low, pre-industrial concentrations of Pb in peat bogs from various locations (Europe and SE Asia). A chelation column used to isolate the metals from the complex matrix to further improve the detection limits failed to provide good recoveries for the peats and plants. Received: 12 June 1998 / Revised: 24 July 1998 / Accepted: 28 July 1998     

Microwave digestion of ancient peat and determination of Pb by voltammetry

Different acid compositions (HNO₃, H₂O₂, HF and HClO₄) and low pressure microwave digestion were applied to the microwave assisted dissolution of ancient peats. The digests were evaluated with respect to the decomposition of the inorganic and organic fraction of peats and to an optimised determination of Pb by differential pulse anodic stripping voltammetry (DPASV). Addition of hydrofluoric acid was necessary to achieve a complete dissolution of the resistant inorganic phase of the peat and to obtain the total Pb concentration. Addition of perchloric acid decreased the concentration of dissolved organic carbon significantly, enabling reliable voltammetric signals for Pb determinations with satisfying peak shape, baseline, sensitivity, and precision. The limit of detection for Pb in peats was 1 ng/mL in the acid digest, equivalent to 0.2 μg/g in solid peat. This allowed the determination of low, pre-industrial concentrations of Pb in peat bogs from various locations (Europe and SE Asia). A chelation column used to isolate the metals from the complex matrix to further improve the detection limits failed to provide good recoveries for the peats and plants. Received: 12 June 1998 / Revised: 24 July 1998 / Accepted: 28 July 1998     

A Rapid Microwave‐Assisted Thermolysis Route to Highly Crystalline Carbon Nitrides for Efficient Hydrogen Generation

Highly crystalline graphitic carbon nitride (g‐C₃N₄) with decreased structural imperfections benefits from the suppression of electron–hole recombination, which enhances its hydrogen generation activity. However, producing such g‐C₃N₄ materials by conventional heating in an electric furnace has proven challenging. Herein, we report on the synthesis of high‐quality g‐C₃N₄ with reduced structural defects by judiciously combining the implementation of melamine–cyanuric acid (MCA) supramolecular aggregates and microwave‐assisted thermolysis. The g‐C₃N₄ material produced after optimizing the microwave reaction time can effectively generate H₂ under visible‐light irradiation. The highest H₂ evolution rate achieved was 40.5 μmol h⁻¹, which is two times higher than that of a g‐C₃N₄ sample prepared by thermal polycondensation of the same supramolecular aggregates in an electric furnace. The microwave‐assisted thermolysis strategy is simple, rapid, and robust, thereby providing a promising route for the synthesis of high‐efficiency g‐C₃N₄ photocatalysts.     

Study on the PTFE closed-vessel microwave digestion method in food elemental analysis

Summary A closed PTFE vessel microwave digestion method is described for the HNO₃/H₂O₂ digestion of food samples. 0.2–0.3 g of powdered samples are completely digested in 44 ml PTFE digestion vessels by 2 ml of 70% HNO₃ and 1 ml of 30% H₂O₂ within 2 min at 100% full power (500 W). After air-cooling for 10 min in a refrigerator at — 25°C the digestion solution is diluted to 25ml. The total elemental concentrations of Ca, Mg, Cu, Fe, Mn and Zn are determined by sequential ICP-AES. The accuracy of the procedure was tested by analysis of 3 SRMs. The results are in good agreement with certified values.

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Untersuchung zum Mikrowellenaufschluß im geschlossenen PTFE-Gefäß für die Elementanalyse in Lebensmitteln

     

Mittlere Schwingungsamplituden und Molekulargeometrie von IOF5

Mean amplitudes of vibration for IOF₅ have been recalculated from known spectroscopic data using the Method of the Characteristic Vibrations. The obtained values are in excellent agreement with those obtained from electron diffraction and microwave measurements and give also a correct picture of the molecular geometry.

     

Untersuchung von systematischen Fehlern bei der Bestimmung von Hg-Gesamtgehalten im Bereich < 10?5% in anorganischen und organischen Matrices mit zwei unabh?ngigen Verbundverfahren

Zusammenfassung Systematische Fehler, die bei der Bestimmung von Hg im ng/g und pg/g-Bereich vor allem bei der Probennahme, Probenvorbereitung und beim Probenaufschluß auftreten, wurden mit ²⁰³Hg Radiotracertechniken und mit zwei für den Spurenbereich entwickelten Verbundverfahren untersucht. Beim emissionsspektrometrischen Verfahren (OES-MIP) wird die Probe im mikrowelleninduzierten Sauerstoff Plasma (MIP) verascht und das Hg nach Abtrennung mit einem Au-Absorber in einem Argon-Plasma angeregt. Das emittierte Hg-Licht wird mit einer Photodiode registriert. Nachweisgrenze: 0,01 ng Hg; Variationskoeffizient: 5% für 1 ng Hg. Beim zweiten Verfahren wird Hg nach dem Probenaufschluß (HClO₃/HNO₃) mit einem mechanisierten Aufschlußsystem nach der Kalt-Dampf-Technik (Ascorbinsäure/SnCl₂ bzw. NaBH₄) freigesetzt, an einem Gold-Absorber gesammelt und anschließend durch flammenlose AAS bei 253,7 nm bestimmt. Nachweisgrenze: 0,5 ng Hg; Variationskoeffizient: 5% für 5 ng Hg.

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Study of systematic errors in the determination of total Hg levels in the range < 10^–5% in inorganic and organic matrices with two reliable spectrometrical determination procedures

Summary In the determination of Hg at ng/g and pg/g levels systematic errors are due to faults in the analytical methods such as intake, preparation and decomposition of a sample. The sources of these errors have been studied both with ²⁰³Hg-radiotracer techniques and two multi-stage procedures developed for the determination of trace levels. The emission spectrometric (OES-MIP) procedure includes incineration of the sample in a microwave induced oxygen plasma (MIP), the isolation and enrichment on a gold absorbent and its excitation in an argon plasma (MIP). The emitted Hg-radiation (253,7 nm) is evaluated photometrically with a semiconductor element. The detection limit of the OES-MIP procedure was found to be 0,01 ng, the coefficient of variation 5% for 1 ng Hg. The second procedure combines a semi-automated wet digestion method (HClO₃/HNO₃) with a reduction-aeration (ascorbic acid/SnCl₂), and the flameless atomic absorption technique (253,7 nm). The detection limit of this procedure was found to be 0,5 ng, the coefficient of variation 5% for 5 ng Hg.

Wir danken der Deutschen Forschungsgemeinschaft Bonn-Bad Godesberg für Personal- und Sachmittel, Herrn Professor Dr. E. Schlichting, Universität Hohenheim für Bodenproben und wertvolle Diskussionen, der Firma Kalle, Wiesbaden, für die Bereitstellung von Kunststoff-Folien sowie Fräulein R. Kolb für ihre Hilfe bei der Durchführung dieser Arbeit.     

Optimized microwave preparation procedure for the elemental analysis of aquatic sediment

 This paper summarizes several key points in applying the microwave preparation technique to the elemental analysis of aquatic sediments and reports systematic experiments in searching for an optimal microwave preparation procedure for element analysis in sediment samples. The determination of the elements Cu, Pb and Cd in a standard reference aquatic sediment sample (CRM 280, COMEUR) was achieved by first digesting the samples in a microwave oven equipped with PFA advanced composite vessels, followed by AAS measurement. The influence of microwave power, digestion time, various dissolution reagents and the HF removing conditions was studied. It has been shown that for a 0.1 g sediment sample the optimal microwave preparation conditions are: 4–5ml HNO₃/HF/H₂O₂ as solvent, digesting time 30 min with 100% microwave power and evaporating the residual acid within 8 min in an open vessel at 80 °C. The element recovery rates with AAS measurement can reach up to 92.4–100.6%. Received: 23 July 1996/Revised: 23 September 1996/Accepted: 25 September 1996     

Atomabsorptionsspektroskopische Bestimmung von Spuren Cr, Cu, Fe und Ni in Boroxid

Zusammenfassung Die Metalle Chrom, Kupfer, Eisen und Nickel werden in reinem Boroxid mittels Atomabsorptionsspektroskopie erfaßt und kalibriert. Dazu wird B₂O₃ in verdünnter Salzsäure gelöst und die Metalle direkt analysiert. Die Nachweisgrenzen liegen zwischen 0,005 und 0,01 g/ml für diese Metalle. Bei den Messungen wurde eine systematische Signaldepression der Extinktionswerte festgestellt. Die Beeinflussung durch verschiedene B₂O₃-Konzentrationen wird diskutiert.

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Determination of traces of Cr, Cu, Fe, Ni in boron oxide with atomic absorption spectroscopy

Summary The metals Cr, Cu, Fe and Ni are determined and calibrated in pure B₂O₃ with atomic absorption spectroscopy. B₂O₃ is dissolved in low-concentrated hydrochloric acid and the metals are analyzed directly. The detection limits are found to be in the range of 0.005 to 0.01 g/ml of these metals. The measurements showed a systematic signal depression of the absorbance values. The influence of different B₂O₃ concentrations is discussed.

     

Improved method for the quantitative determination of ultra trace level of Pu-239+240 in siliceous base samples using microwave assisted dissolution

Mobilisation of alpha emitting radionuclides from silicious base sample is one the challenging task for environmental radiochemist. During this study, rapid and complete dissolution of the siliceous base samples were carried out by optimizing temperature, pressure and power of the microwaves. The Pu-239+240 in digested samples was pre-concentrated by scavenging Fe as Fe(OH)₃. Pu-239+240 was isolated from the Fe(OH)₃ by co-precipitating Pu with Bi(PO₄) in HNO₃ medium at pH 2. Pu-239+240 was separated from Bi(PO₄) and other transuranics by passing through cation and anion exchange resin. Pu-239+240 was counted by alpha spectrometry after electroplating on stainless steel planchet. The detection limits achieved for Pu-239+240 was 60 μBq/g (2.6 × 10⁻¹⁴ g/g). Pu-242 was used as a tracer for the evaluation of recovery of Pu-239+240. Samples prepared after complete destruction of matrix in microwave, showed 10–20% higher concentration of Pu-239+240 compared to conventional acid leached. Consistent recovery in the range of 97–99% for Pu-242 were observed in microwave digested samples whereas inconsistent results were observed in acid leached samples where the recoveries were in the range of 75–86%. Siliceous matrix degradation was tracked by monitoring the surface morphology and composition of the residue left at various stages of digestion using Scanning Electron Microscope (SEM) coupled with Energy dispersive X-ray spectrometer (EDS).     

Microwave Hydrothermal Synthesis and Acidity of Nanosize Gallosilicate Mesoporous Molecular Sieves

A range of nanosize alkaline‐free gallosilicate mesoporous molecular sieves (GaMMS) were synthesized using microwave hydrothermal method and various analytical and spectroscopic techniques were employed to systematically investigate the structure, coordination geometry of gallium, acidic properties and catalytic activity. These nanosize GaMMSs exhibit high surface area (240 ～ 720 m²/g), pore volume (1.06 ～ 1.49 m³/g), narrow pore size distribution and nano‐particle size between 20 and 100 nm and four‐coordinated gallium site mainly, which were characterized by XRD, N₂ adsorption/desorption, TEM, ⁷¹Ga MAS‐NMR and ICP‐AES. Mesoporosity, mesostructure and nanosize can be controlled simply by microwave irradiation time and temperature. Pyridine‐IR result reveals the coexistence of Lewis and Brönstead acid sites in these nanosize GaMMS while NH₃‐TPD profiles suggest nanosize one has stronger acid site and contains more acid sites than the conventional one. Catalytic performance was evaluated using the catalytic cumene cracking test. The result indicates that the nanosize GaMMS shows a much higher activity than that of conventional GaMCM‐41, probably due to higher concentrations of H‐form sites, external surface and fast diffusion.     

Achieving Efficient Incorporation of π‐Electrons into Graphitic Carbon Nitride for Markedly Improved Hydrogen Generation

A rapid and highly efficient strategy for introducing C into g‐C₃N₄ involves copolymerizing π‐electron‐rich barbituric acid with melamine via a facile microwave‐assisted heating, thereby eliminating the issues in conventional electric furnace heating, such as the severe volatilization, owing to the mismatch of the sublimation temperatures of barbituric acid and melamine. The g‐C₃N₄ catalyst after optimizing the C‐doping content actively generates increased amounts of H₂ under visible light exposure with the highest H₂ generation rate of 25.0 μmol h^?1, which is nearly 20 times above that using g‐C₃N₄ produced by conventional electric furnace heating of two identical monomers (1.3 μmol h^?1). As such, the microwave‐assisted heating strategy may stand out as an extremely simple route to incorporating π‐electrons into g‐C₃N₄ with markedly improved photocatalytic performance.     

Rapid determination of boron traces in silicate materials

Summary This simple method for the determination of boron takes a total time of 20 min. Samples are mixed with CaF₂ suprapur in the ratio 31 and pyrohydrolized in a Leco induction furnace for 15 min under steam of H₂O. The BF₃ liberated is collected in a NaOH-Na₂CO₃ solution (10 g NaOH +1 g Na₂CO₃ per liter). This solution is filled up to 200 ml with twice distilled water; 2 ml of this solution is mixed with conc. sulphuric acid and carminic acid solution and filled up to 50 ml. The absorption is measured by a Zeiss Elko II photometer with filter S 59. Analyses of four internations reference samples show excellent precision with a relative standard deviation of less than 5% (e.g. for a boron content of 2.1 ppm in granite G-2).

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Schnelle Spurenbestimmung von Bor in silicatischem Material

Zusammenfassung Die Borbestimmung mit dieser schnellen und einfachen Methode benötigt ca. 20 min. Die Proben werden mit CaF₂ Suprapur im Verhältnis 31 vermischt und in einem Leco-Induktionsofen 15 min unter Einleitung von Wasserdampf pyrohydrolysiert. Das entstandene BF₃ wird in einer NaOH-Na₂CO₃-Lösung(10 g NaOH+1 gNa₂CO₃pro Liter) aufgefangen. Diese Lösung wird mit zweifach destilliertem Wasser auf 200 ml aufgefüllt; 2 ml dieser Lösung werden mit konz. Schwefelsäure und Carminsäure-Lösung versetzt und auf 50 ml aufgefüllt, bevor ihre Absorption mit einem Zeiss — Elko II — Photometer mit Filter S 59 gemessen wird. Analysen von 4 internationalen Referenzproben zeigen eine hohe Präzision mit einer relativen Standardabweichung von weniger als 5% (z. B. für einen Gehalt von 2,1 ppm Bor im Granit G-2 des USGS).

     

Prevention of loss of mercury(II) during storage of dilute solutions in various containers

Summary Losses of ionic mercury from various solutions of 30 ng Hg/ml and 1 g Hg/ml concentration stored in polyethylene, polypropylene, quartz and glass were studied for storage periods up to 57 days by means of the radiotracer technique. Distilled water solutions and solutions treated with HCl and HNO₃ lost substantial fractions of their mercury, whereby both adsorption and volatilization were involved. The largest losses (of up to 98%) occurred after the storage of water and nitric acid solutions in polyethylene. A combination of hydrochloric acid and hydrogen peroxide, also in the presence of nitric acid, proved to be excellently suited for preventing the loss of mercury. No detectable changes of mercury concentration were observed for solutions treated with 2% HCl + 2% H₂O₂ and stored in polyethylene for up to two months. After this storage period, the losses were < 2.5% for quartz, < 5% for glass and < 7% for polypropylene.

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Verhinderung von Quecksilber(II)-Verlusten bei der Lagerung verdünnter Lösungen in verschiedenen Behältern

     

Determination of total chromium in Marine Sediment Reference Material BCSS-1

The determination of Cr in National Research Council of Canada Marine Sediment Reference Material BCSS-1 is addressed. Mixed acid digestions utilizing HF, HClO₄ and HNO₃ were investigated. Single microwave assisted digestions in closed vessels at medium pressures (8 bar) were inappropriate (80% recovery). Double digestion at moderate pressure and digestions at high pressure (70 bar) or lengthy open-beaker hot plate dissolutions resulted in 93±5% recovery of certified Cr content. Flame atomic absorption and inductively coupled plasma atomic emission spectrometry were used for quantitation. In all cases, the method of standard additions was necessary to eliminate the approximately 10% enhancement in re- sponce relative to unmatched standards. Compared to a certified content of 123±7 g/g (mean and standard deviation), values of 116±6 were obtained using an open beaker digestion; 114±5 using a double microwave digestion at 8 bar; 113±2 for microwave digestion at high pressure and 111±4 at high pressure (81 bar) with triple microwave digestion in a Parr bomb. No acid dissolution procedure is adequate for Cr in this sample although recovery of total Cr is complete from National Institute of Standards and Technology Buffalo River Sediment (SRM 2704).