Negative Thermionen-Massenspektrometrie von Selen

Summary Selenite and selenate were determined in ground waters with isotope dilution mass spectrometry (IDMS). This species analysis was possible by the use of an⁸²Se enriched selenite and selenate spike and a Chromatographic separation of both species after the isotope dilution step. In a column filled with the DEAE cellulose anion exchanger selenite could be separated with 1 mol/l formic acid, whereas selenate was eluted with 0.1 mol/1 nitric acid. The mass spectrometric isotope ratio measurement was carried out in a thermal ionization instrument using the formation of negative Se^– thermal ions for detection. Selenite, selenate and total selenium in ground water samples were determined in the concentration range of 0.2–20 n/g with relative standard deviations of 0.5%–5%. The selenate concentration was approximately ten to eighty times higher than the corresponding selenite concentration. There was always a difference of about 8% between the sum of the selenite and selenate concentrations and the total selenium concentration which can possibly be attributed to water-soluble selenides and elementary selenium, respectively.

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Herrn Prof. Dr. R. Neeb zum 60. Geburtstag gewidmet     

Mechanism of oxidation of selenium(IV) by cobalt(III) in perchloric acid — a kinetic study

Summary The kinetics of Co^III oxidation of Se^IV have been studied in aqueous HClO₄. The order with respect to Co^m is two the order with respect to Se^IV is one at low concentrations; two at high concentrations. The latter variation is attributed to the greater reactivity of the Se^IV dimier A mechanism involving complexation between oxidant and substrate is proposed. [CoOH]²⁺ is presumed to be the reactive Co^III species and H₂SeO₃ and HSeO ₃ ^– to be those of Se^IV. At 25° C, E_a, H and S for the monomeric path are 125.6±4.0 kJ mol^–1, 122.1±3.8 kJ mol^–1 and 206±12 JK^–1 mol^–1 respectively and those for the dimeric path are 88.6±3.6 kJ mol^–1, 85.9±3.4 kJ mol^–1 and 62.6±11.3 JK^–1 mol^–1 respectively.     

Electrochemical vapor generation of selenium species after online photolysis and reduction by UV-irradiation under nano TiO<Subscript>2</Subscript> photocatalysis and its application to selenium speciation by HPLC coupled with atomic fluorescence spectrometry

An online UV photolysis and UV/TiO₂ photocatalysis reduction device (UV–UV/TiO₂ PCRD) and an electrochemical vapor generation (ECVG) cell have been used for the first time as an interface between high-performance liquid chromatography (HPLC) and atomic fluorescence spectrometry (AFS) for selenium speciation. The newly designed ECVG cell of approximately 115 L dead volume consists of a carbon fiber cathode and a platinum loop anode; the atomic hydrogen generated on the cathode was used to reduce selenium to vapor species for AFS determination. The noise was greatly reduced compared with that obtained by use of the UV–UV/TiO₂ PCRD–KBH₄–acid interface. The detection limits obtained for seleno-DL-cystine (SeCys), selenite (Se^IV), seleno-DL-methionine (SeMet), and selenate (Se^VI) were 2.1, 2.9, 4.3, and 3.5 ng mL^–1, respectively. The proposed method was successfully applied to the speciation of selenium in water-soluble extracts of garlic shoots cultured with different selenium species. The results obtained suggested that UV–UV/TiO₂ PCRD–ECVG should be an effective interface between HPLC and AFS for the speciation of elements amenable to vapor generation, and is superior to methods involving KBH₄.     

Insoluble monolayers of irisresorcinol at the air/water interface

Monolayer properties of irisresorcinol [5-(cis-10-heptadecenyl) resorcinol] were measured at the air/water interface. TheA-T isobars of the monolayers at 10 and 15 mN/m gave two-dimensional thermal expansivities of 1.4 × 10^–4/K and 1.3 × 10^–4/K at a temperature span from 7–40 C, respectively. The- A isotherms of the material showed only a little dependence on temperature from 5–35 C and onpH except at highpH, where monolayers expanded by ionization of resorcinol headgroups. Some types of saccharose in the subphase exhibited a characteristic interaction with irisresorcinol in monolayers, and there is a possibility that this material will be used for molecular recognition of some saccharoses.     

Kinetics and mechanism of the interaction of thiourea with cis-diaquaethylene-diamineplatinum(II) perchlorate in aqueous medium

The kinetics of the interaction of thiourea with [Pt(en)(H₂O)₂]²⁺ have been studied spectrophotometrically as a function of [Pt(en)(H₂O)₂]²⁺, [thiourea] and temperature at a particular pH(4.0), where the substrate complex exists predominantly as the diaqua species and the thiourea ligand as a neutral molecule. The reaction proceeds via a rapid outer sphere association followed by two slow consecutive steps, the second step exhibiting first order dependence on the aqua ion and thiourea concentrations. The activation parameters for both the steps have been evaluated: (H ₁ = 54.8 ± 1.2 kJ mol^–1, S ₁ = –96 ± 4 J K^–1 mol^–1, H ₂ = 27.9 ± 0.8 kJ mol^–1 and, S ₂ = –183 ± 2.6 J K^–1 mol^–1). The low enthalpy of activation and large negative values of entropy of activation indicate an associative mode of activation for both consecutive steps.     

Molecular structure of N-(2-hydroxyethyl)pyridinium bromide

X-ray diffraction, structural analysis was employed to establish the structure of N-(2-hydroxyethyl)pyridinium bromide, C₇H₁₀NO⁺-Br^–. The plane of the pyridinium ring is twisted by 93 relative to the C-C bond of the side-chain. The N-C-C-O torsion angle is 63.6. The OH bond is oriented toward the heteroaromatic ring. The H-O-C-C torsion angle is 84.4. The molecular packing in the crystal lattice is such that the bromide ions are found between the heteroaromatic rings and the hydrogen bonded to the hydroxyl groups.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2617–2619, November, 1990.     

Chromium(VI), chromium(V) and chromium(IV) oxidation of 12-tungstocobaltate(II)

The reaction between Cr^VI and 12-tungstocobaltate(II) was carried out in 2.0 mol dm^–3 HCl and followed a simple second order rate law. The reaction was catalysed by hydrogen ion due to the formation of active H₂CrO₄ and was inhibited by chloride ion as, in its presence, conversion of the active species into inactive chlorochromate occurs. Chromium(V) and chromium(IV) were generated in situ by the use of Cr^VI—V^IV or Cr^VI—2-ethyl-2-hydroxybutyric acid and Cr^VI—i-PrOH reactions respectively, and the oxidation of 12-tungstocobaltate(II) by these atypical oxidation states, was also studied. The rate constants for the oxidation of 12-tungstocobaltate(II) by Cr^VI, Cr^V and Cr^IV were found to be in the ratio 1:1.2:5.2 respectively. The ionic strength did not affect the reaction, while decrease in the solvent polarity increased the rate of the reaction. The activation parameters were also determined and the values H , G and S were found to be 52.4 ± 6 kJ mol^–1, 100.8 ± 7 kJ mol^–1, –151.7 ± 10 J K^–1 mol^–1 respectively, supporting the mechanism proposed.     

Papierchromatographische Trennung, Identifizierung und Bestimmung von Selenit und Selenat

Zusammenfassung Es wird ein papierchromatographischer Nachweis beschrieben, wonach Selenit als blauer und Selenat als orangeroter Fleck entwickelt werden. Die Methode ist besonders zur Erkennung kleiner Mengen Selen(IV) neben viel Selen(VI) geeignet (11000). Die Nachweisgrenze liegt bei 0,2 g Se^IV bzw. 2,0 g Se^VI. Der Nachweis kann mit entsprechend geringerer Empfindlichkeit auch als Tüpfel- oder Fällungsreaktion ausgeführt werden.Zum Zwecke der quantitativen Bestimmung werden die Selenverbindungen mit Wasser aus dem Chromatogramm extrahiert, nachdem Selenat durch gasförmiges HCl vorher zu Selenit reduziert worden ist. Nach Zugabe von Kaliumjodid wird der Selengehalt der Lösungen photometrisch bestimmt. Die dabei erhaltenen Werte stimmen gut mit den Resultaten der Analyse im Makromaßstab überein. Die beschriebene Methode empfiehlt sich dort, wo Fremdsubstanzen die üblichen Bestimmungsverfahren für Selenit und Selenat nebeneinander stören.

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Summary A paper-chromatographic detection is described, by which selenite is developed as a blue and selenate as an orange-red spot. This method is especially suitable for the detection of small amounts of selenium(IV) in the presence of large amounts of selenium(VI) (11000). The detection limit is 0,2 g of Se^IV and 2,0 g of Se^VI, respectively. The reaction may also be carried out as a spot test or a precipitation with a correspondingly lower sensitivity.For the purpose of determination the selenium compounds are extracted from the paper by water; selenate is reduced to selenite by gaseous HCl before. After addition of KI the selenium content of the solutions is determined by photometric measurements.The values obtained are in good agreement with the results of quantitative analysis in macro scale. The method described is well suitable, if foreign substances interfere with the usual determination of selenite and selenate in the presence of each other.

     

Kinetics and mechanism of the oxidation of acetaldehyde by chromic acid in perchloric acid

Summary The oxidation of MeCHO by chromium(VI) has been studied in HClO₄ medium over a wide range of experimental conditions and has been found to obey the rate law;v=k[MeCHO][HCrO ₄ ^– ][H⁺]. The calculated H and-S values for the reaction are 30±2kJ mol^–1 and 171±7J mol^–1deg^–1, respectively. The mechanism is discussed in terms of carbon-hydrogen bond cleavage.     

Kinetics and mechanisms of the oxidation of the octacyanoniobate(III)ion by oxyanions in alkaline aqueous media

Summary The kinetics and mechanisms of the oxidation of Nb(CN) _inf8 ^sup5– by the oxyanions S₂O _inf8 ^sup2– , BrO _inf3 ^sup– , and IO _inf4 ^sup– have been investigated in alkaline aqueous media (pH 12). The second-order rate constant for the electron transfer reaction between Nb(CN) _inf8 ^sup5– and S₂O _inf8 ^sup2– at 25.0 °C, I = 0.36m (K⁺), is 11.1± 0.3 m ^–1 s ^–1 with H = 30 ± 2kJmol^–1 and S = - 125 + 7JK^–1 mol^–1. The rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by BrO _inf3 ^sup– at 25.0 °C, I = 0.20m (Na⁺), is 2.39 ± 0.08m ^–1 s ^–1 with H = 28 ± 2kJmol^–1 and S = -139 ± 7JK^–1mol^–1. The oxidation of Nb(CN) _inf8 ^sup5– by IO _inf4 ^sup– proceeds by two parallel pathways involving the monomeric IO _inf4 ^sup– ion and the hydrated dimer H₂I₂O _inf10 ^sup4– . The second-order rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by monomeric IO _inf4 ^sup– at 5.0 °C, I = 0.050m (Na⁺), is (3.3 ± 0.6) × 10³ m ^–1 s ^–1 with H = 75 ± 6 kJ mol^–1 and S = 94 ± 15 J K^–1 mol^–1, while the rate constant for the oxidation by H₂I₂O _inf10 ^sup4– is (1.8 ± 0.1) × 10³ m ^–1 s ^–1 with H = 97 ± 5 kJ mol^–1 and S = 166 ± 16 J K^–1 mol^–1 under the same reaction conditions. The rate constants for each of the oxidants employed display specific cation catalysis with the order of increasing rate constants: Li⁺ < Na+ < NH _inf4 ^sup+ < K⁺ < Rb⁺ < Cs⁺, in the same direction as the electronic polarizability of the cations. The results are discussed in terms of the outer-sphere electron-transfer processes and compared with the corresponding data and mechanisms reported for other metal-cyano reductants.     

Stoichiometry and kinetics of the oxidation of hydroxylammonium ion by 12-tungstocobaltate(III) anion in aqueous solution

Summary The stoichiometry and kinetics of the oxidation of hydroxylammonium ion by the 12-tungstocobaltate(III) anion has been studied in hydrochloric acid medium. The ratio of mols of oxidant consumed per mol of hydroxylammonium ion is 11 and the evolution of nitrogen is confirmed. In the 0.1–1.0 mol dm^–3 [H⁺] region, the oxidation is acid-independent and obeys the empirical rate law: –d[oxidant]/dt=k[oxidant] [reductant] where k=(3.51±0.18)×10^–4 mol^–1dm³s^–1 at 22.4±0.1C and I=2.0 mol dm^–3 (NaCl). Possible reaction steps and mechanism are suggested.     

Kinetics and mechanism of the reaction betweencis-dichlorobisbipyridineruthenium(II) and nitric acid in water-methanol solutions

Summary The kinetics of the reaction betweencis-dichlorobisbipyridineruthenium(II) and nitric acid have been investigated spectrophotometrically in the 25°–40° range in the presence of 0.03 to 0.2 mol dm^–3 HNO₃. The reaction proceeds with the stepwise formation of monoaqua and diaqua products. Only the formation of the monoaqua intermediate was followed as this species could not be obtained in a pure state. Aquation proceeds through a dissociative process. The second order rate constants are 11.8 (25°), 17.5 (30°); 30.0 (35°) l mol^–1 s^–1. Activation parameters are H 52±3 kJ mol^–1; S–108±8 JK^–1 mol^–1.     

Kinetics of oxidation of sulphite by hexachloroplatinate(IV) in acidic solution

Summary The kinetics of the oxidation of sulphite by hexachloroplatinate(IV) has been studied over wide range of experimental conditions. The reaction is first-order in substrate and in platinum(IV). The rate decreases with the increase in acidity. The effect of salt and of changing dielectric constants on the reaction rate have been studied. Values of H and S have been calculated and are 26.3 kJ mol^–1 and –35.9 JK^–1 mol^–1, respectively. On the basis of experimental evidence, a two-electron reduction mechanism is proposed.     

Thermal study of synthesis of cryptohalite

The present work represents a thermal study of synthesis of cryptohalite (Ammonium silicon hexafluoride) by sintering of quartz with ammonium fluoride using a derivatograph. The reaction products were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA curves indicate that the intensive formation of cryptohalite takes place at 125–155C by an endothermic reaction. Cryptohalite is unstable and dissociates at 320–335C as represented by the sharp and large endothermic peaks at these temperatures.The resulted cryptohalite is colorless in thin sections and crystallizes in cubic system, in the form of octahedral crystals with perfect (111) cleavage. The dimorph bararite is not detected in all runs.

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Zusammenfassung Mittels eines Derivatographen wurde die Synthese von Kryptohalit (Ammoniumsiliziumhexafluorid) durch Sintern von Quarz mit Ammoniumfluorid thermisch untersucht. Die Reaktionsprodukte wurden mikroskopisch und mit Hilfe eines Siemens-Crystalloflex Diffraktometers identifiziert. Die DTA-Kurven zeigen, da\ die intensive Bildung von Kryptohalit in einer endothermen Reaktion bei 125–155C abläuft. Wie durch die scharfen und intensiven endothermen Peaks bei 320–335C gezeigt wird, ist Kryptohalit bei dieser Temperatur instabil und dissoziiert.Das erhaltene Kryptohalit ist in dünnen Schnitten farblos und kristallisiert im kubischen System in der Form von oktaedrischen Kristallen mit perfekter (111) Spaltbarkeit. Das dimorphe Bararit konnte in keinem der Versuche beobachtet werden.

     

The effects of aqueous solvent structure on the mutarotation kinetics of glucose

The mutarotation rates of glucose in aqueous mixtures of tetrahydrofuran andtert-butanol in the mole fraction (x_i) range 0i<0.2 have been measured at 5° intervals in the range 5–35°C. The kinetic deuterium isotope effects have been determined for the same solvent compositions at 25 and 35°C. A statistical analysis of the Arrhenius plots indicates that the experimental errors, although small, are too large for the establishment of any compensation behavior between H and S which has often been claimed for reactions in mixed aqueous solvents. Nevertheless, it appears that H exhibits a complex solvent composition dependence, and the solvent effects on the measured rate constants differ markedly from those found for aqueous solutions ofN,N-dimethyl-formamide and dimethyl sulfoxide. There is a deuterium isotope rate effect for solvent mixtures in which 0i<0.1, over and above the usually observed kinetic isotope effect. This additional effect decreases with rising temperature. The results are discussed in terms of competing water ordering effects in ternary mixtures containing water, a hydrophilic solute, and a hydrophobic solute.     

The mechanisms of the acid-catalysed dissociation of tris-β-diketonatoruthenium(III) complexes

Summary The reactions of four -diketonatoruthenium(III) complexes in the presence of HNO₃ andp-MeC₆H₄SO₃H in the 45° and 57° range were followed spectrophotometrically in Me₂COH₂O mixtures. Dissociation of Ru(acac)₃ follows [H⁺]-dependent and [H⁺]²-dependent paths, whereas the bzac and F₃acacF₃ complexes follow only the [H⁺]-dependent path. The bzbz (Dibenzoylmethanate) complex is inert. Protonation of the bound ligand leads to its rupture from the metal ion. The bzac complex is kinetically more inert than the acac complex, because of extra stability arising from interaction of the (bzac) benzene ring with the pseudo-aromatic diketonate ring of the complex. Considering the kinetic labilities, the complexes may be arranged in the order Ru(F₃acacF₃)₃>Ru(acac)₃>Ru(bzac)₃>Ru(bzbz)₃.Activation parameters for [H⁺] dependent path are: H ₁ 86.5±7, 69±5, 121±7 kJ mol^–1, S ₂ –52±10, –107±10, 57±8 JK^–1 mol^–1 for acac, bzac and F₃acacF₃ complexes respectively and H ₂ 67±5 kJ mol^–1, S ₂ –92±8 JK^–1 mol^–1 for the acac complex only.     

Determination of freezable water content of beef semimembranous muscle DSC study

The freezable water contents of samples obtained from previously chilled semimembranous muscle of middle-aged beef carcasses after a 24 h cooling period a room at in 5±1C were determined by differential scanning calorimetry (DSC) at –5, –10, –15, –20, –30, –40, –50 and –65C. This was accomplished by freezing the samples at the above-mentioned temperatures, followed by thawing to 35C, and measuring the melting peaks of freezable water. The areas of these peaks were determined by using the peak integration method programs through a computer linked to the DSC, and they were then used to determine the latent heat of melting (H _m) in kJ kg^–1 at each freezing temperature. The resultant latent heat of melting per sample was divided by the latent heat for pure water to determine the amount of freezable water present in these samples. This amount of freezable water was divided by the total water content of the meat sample to determine the percentage of freezable water in the sample. The percentage of freezable water was subtracted from 100 to determine the percentage of bound water present in the sample.     

Kinetics and mechanism of the oxidation of thiosulphate by hexachloroplatinate(IV)

Summary Rate constants for the oxidation of thiosulphate by hexachloroplatinate(IV) have been measured. The kinetics of the oxidation of thiosulphate follow a second-order rate law, first order with respect to thiosulphate and first order with respect to platinum(IV). The influence of pH is small. The rates are found to depend on the nature and concentration of the cations and follow the order: Cs⁺>Rb⁺>K⁺>Na⁺>Li⁺. The activation parameters calculated from the temperature studies are: H=42.9 k J mol^–1 and S=–102 JK^–1 mol^–1. A mechanism of the reaction in terms of intermediate formation of free radicals followed by the formation of tetrathionate is postulated to explain the kinetic behaviour.     

Differential scanning calorimetric study of phase changes in poly(ethylene glycol) dodecyl ether-water systems

Phase changes in binary systems of poly(ethylene glycol) dodecyl ethers, C₁₂H₂₅(OC₂H₄)_xOH (x=5, 6 and 8) with water have been studied between –40 to 100 C by differential scanning calorimetry. A number of transitions, including the liquidliquid phase separation, were seen and the transition temperatures and enthalpy changes were determined. The observed temperatures were generally in good agreement with reported phase diagrams. In the C₁₂E₅ system, three of the four three-phase lines were seen and a more complete phase diagram is suggested for the water-rich part of the system. Most of the phase changes seen above 0 C are accompanied by small endothermic enthalpy changes of 0.7 to 0.9 kJ (mol C₁₂E_x)^–1, independent of system studied, type of transition and transition temperature. Water-rich solutions and liquid crystalline phases separate upon freezing into ice and crystals of hydrated amphiphile. Eutectics are developed at the following temperatures and compositions: C₁₂E₅ –3.0 C and 54 wt % C₁₂E₅; Q₂E₆ –4.5 C and 54 wt % C₁₂E₆, C₁₂E₈ –3.8 C and 49 wt % C₁₂E₈.     

Kinetics and mechanism of the interaction of DL-methionine with cis-diaquaethylenediamine platinum(II) perchlorate in aqueous medium

The kinetics of the interaction of DL-methionine with [Pt(en)(H2O)2]2+ have been studied spectrophotometrically as a function of [Pt(en)(H2O)22+], [DL-methionine], PH and temperature. The reaction proceeds via a rapid outer sphere association followed by two slow consecutive steps having first order dependence on the aqua ion and methionine concentrations. Activation parameters have been evaluated using the Eyring equation. The low H1 (15.6 kJ mol–1) and large negative value of S1 (–230 J K–1 mol–1) as well as H2 (19.5 kJ mol–1) and S2 (–226 J K–1 mol–1) indicate an associative mode of activation for both the aqua ligand substitution processes in the two consecutive steps.