Studies on the determination of dissolved oxygen in water

Studies on the decomposition rates of the Mn(III) complex of cyclohexanediaminetetra-acetate (DCTA) in light and in darkness have shown that this complex is more stable than the one derived from ethylenediaminetetra-acetate. The optimum pH range for the determination of dissolved oxygen by means of the Mn(III)-DCTA complex is found to be between 3 and 4. The absorbance of this complex is independent of the amount of DCTA used (in the range 0.2–1.0 g) with water samples containing a maximum of 3.2 ppm of dissolved oxygen. Significant interferences are caused by the presence of CO²⁻₃, HCO⁻₃, S₂O²⁻₃, PO³⁻₄, I⁻, NO⁻₂, SO²⁻₃, Ca²⁺, Fe²⁺ and Fe³⁺ at 500 times the oxygen concentration.     

Structure and stability of ammonium-sulfate and guanidium-sulfate complex

Ab initio (HF/6-31G^** and B3LYP/6-31 + + G^**) methods have been used to study the stability and structure of complexes between CH₃SO⁻₃ and CH₃NH⁺₃ or C(NH₂)⁺₃. Results show that no hydrogen jump is involved in the complex formations, which is different from previous work studying complexes between CH₃COO⁻ and CH₃NH⁺₃. In addition, we have studied complexes between CH₃SO⁻₃ and HC(NH₂)⁺₃ or ⁺H₃NC(NH₂)₃, all of which have a cage structure.     

Superoxide ion formation process on reduced MoO3---TiO2

Two novel Mo⁵⁺ ions, different from either the D_2d symmetry Mo⁵⁺ on anatase or the D_2hMo⁵⁺ on rutile, appeared on partially reduced molybdenum oxide supported on -titanic acid. The oxygen molecule is held on one of these novel Mo⁵⁺ ions but is not reduced to the superoxide ion. Oxygen is reduced to O⁻₂ entirely by the Mo⁴⁺ ion formed from the D_2d symmetry ion and the O⁻₂ is subsequently stabilized after moving to a Ti⁴⁺-site.     

Non-empirical molecular orbital studies of the possibility of ferromagnetic ordering in isoelectron-substituted ferrocarbon modifications

The vertical singlet-triplet (ST) splitting in the C₃H₆ molecule and C₂BH⁻₆ and C₂NH⁺₆ ions has been investigated by means of ab initio calculations. The molecular geometry was either taken as that corresponding to the UHF/6-31G^* or UHF/6-31+G^* energy minimum of the triplet configuration (for C₃H₆, C₂BH⁻₆, C₂NH⁺₆), or was extracted from the ferrocarbon crystal (for C₃H₆). Polarized split-valence basis sets (6-31G^*, 6-31+G^* and 6-311G^**) and fourth-order Møller-Plesset perturbation theory at the MP4SDTQ level of approximation provides more reliable ST splitting values, than the MP4SDQ level, the latter being the most accurate method used in our previous work. These calculations prove the presence of ferromagnetic ordering within the same quasi-graphite plane of ferrocarbon crystal. The presence of such ordering has been shown within the same quasi-graphite plane upon substitution of sp³ carbon atoms by ions B⁻, N⁺ and so on.     

Theoretical Study on Ionic Liquid Based on 1-Ethyl-3-Methyl- Imidazolium Cation and Hexafluorophosphate or Tetrafluoroborate

The Hartree-Fock and DFT/B3LYP methods have been employed to investigate the electronic structures of 1-ethy1-3-methyl-imidazolium cation(EMIM~ ),BF_4~-,PF_6~-,EMIM~ -BF_4~-,and EMIM~ -PF_6~- using the Gaussian-94 soft-package at 6-31 G(d,p)basis set level for hydrogen,carbon,nitrogen,boron, phosphorus,and fluorine atoms.Comparison of the electronic structures of the lowest energy of EMIM~ - BF_4~- and EMIM~ -PF_6~- pairs,and single EMIM~ ,BF_4~- and PF_6~- showed that the optimized EMIM~ -BF_4~- and EMIM~ -PF_6~- pair conformers were BF_4~- and PF_6~- outside the 5-ring plane between the ethyl group and the methyl group.The cohesion of C—H…F hydrogen bond between cation and anion is reinforced by charge assistance.The interaction energy between EMIM~ and PF_6~- is 328.8 kJ/mol at the B3LYP level and 326.6 kJ/mol at the Hartree-Fock level,whereas that between EMIM~ and BF_4~- is 353.5 kJ/mol at the B3LYP level and 350.5 kJ/mol at the Hartree-Fock level.The low energy interactions caused by bulky asymmetric EMIM~ ,and charge dispersion of cation and anion give rise to the low melting point of ionic liquid EMIM~ -BF_4~- and EMIM~ -PF_6~-.The two hydrogen bonding models of single hydrogen bond formation,and the hydrogen transfer between C_2 in EMIM~ and F in BF_4~- or PF_6~- were principally depicted.     

The stereochemistry of quaternary N-methyl sparteinium cations, their derivatives, stereoisomers and analogues : Part II. Molecular and crystal structure, and IR spectra of methiodide and methperchlorate of 2-oxosparteine

The crystal and molecular structures of two quarternary salts of 2-oxosparteine (II), the methiodide (IICH⁺₃ • I⁻) and the methperchlorate (IICH⁺₃ • ClO⁻₄) have been determined on the basis of X-ray and IR data. The studies were performed by analogy to previously investigated quaternary salts of sparteine (I), the methiodide (ICH⁺₃ • I⁻) and the methperchlorate (ICH⁺₃ • ClO⁻₄). As expected, the configurations and conformations of cationic parts within the two pairs of quaternary salts are identical, except for the structure of their A/B fragments, which in ICH⁺₃ cations have the character of tertiary amines, but in IICH⁺₃ that of lactams.

On the basis of accumulated X-ray and IR data the similarities and differences in the modes of interaction of perchlorate and iodide anions with quaternary cations, and especially with their N⁺---CH₃ groups are discussed. In this discussion are also included the methiodide and methperchlorate of -isosparteine: IIICH⁺₃ • X⁻ (X⁻ = I⁻ or ClO⁻₄) where N⁺---CH₃ groups are cisoidally oriented to the basic nitrogen atoms. The most interesting observations are as follows: (i) When N⁺---CH₃ groups are easily accessible for direct quasi hydrogen bonding interactions with counter anions and when other positive charged groups, for instance lactam groups, are absent in quaternary cations, perchlorate anions interact more strongly than the iodide anions and in consequence introduce conformational changes into the ring with N⁺---CH₃ group as well as into further rings. (ii) Perchlorate and iodide anions interact with N⁺---CH₃ groups similarly and very weakly if at all, when the N⁺---CH₃ groups are for steric reasons inaccessible to counter anions or when in quaternary cations there are additional groups which attract the counter anions electrostatically. The last mechanism operates in both quaternary salts of 2-oxosparteine and this is the reason why their monocrystals are isosteric and IR spectra almost identical. (iii) The sterically hindered N⁺---CD₃ groups in both IIICD⁺₃ • X⁻ salts give rise in their IR spectra to two doubles of sharp, well resolved bands which indicate the presence of two different rotamers stabilized by two modes of weak intramolecular hydrogen bonds with basic N atoms. (iv) In IIICH⁺₃ • X⁻ and IIICD⁺₃ • X⁻ salts the perchlorate and iodide anions do not interact at all with the rotating and vibrating N⁺---CH₃ (N⁺---CD₃) group but the structures of these salts are not isosteric since the perchlorate anions interact more strongly than iodide anions with the A/B fragment of the IIICH⁺₃ cations. This is visible from the shapes and intensities of the so-called “trans” band in the IR spectra of both salts.     

Role of axial ligation on potentiometric response of Co(III) tetraphenylporphyrin-doped polymeric membranes to nitrite ions

Two types of Co(III) tetraphenylporphyrins, Co(III)TPPX (I) and Co(III)(N)TPPX (II), where X = C1⁻ or NO⁻₂ and N = C₅H₅N or C₆H₅CH₂C₅H₄N, are used as ionophores to prepare nitrite responsive polymeric membrane electrodes. The influence of the initial axial ligand (X⁻ and N) on the operative ionophore mechanism of these metalloporphyrins within the solvent polymeric membranes is examined. Results from potentiometric and electrodialysis experiments suggest that in the presence of nitrite in the test sample and internal solution, both types of Co (III) porphyrins studied (I and II) act as neutral carriers and that the addition of lipophilic cationic sites (e.g., tridodecylmethylammonium ions (TDMA⁺)) to the organic membrane is essential to improve the selectivity and long term stability of sensors prepared with these species. Membranes formulated with (I) or (II) in the nitrite form along with TDMACl in plasticized PVC films exhibit the following selectivity sequence: SCN⁻ > NO⁻₂ ˜ C1O⁻₄ > Sal⁻ > NO⁻₃ > Br⁻ > C1⁻. Membrane electrodes with added lipophilic cationic sites are shown to exhibit rapid, fully reversible and Nernstian response towards nitrite ions in the concentration range of 10⁻¹–10⁻⁵ M, with good long term stability.     

Enthalpies of transfer of several ions from water to water-n-propanol mixtures at 298.15 K

Measurements were made of the dissolution heats of NaBPh₄ and Ph₄PCl in water-n-propanol mixtures over the whole range of compositions. Assuming the equality of ΔH_tr+(Ph₄P⁺) and ΔH_tr+(BPh⁻₄), the transfer enthalpies of several ions from water to water-n-propanol mixtures at 298.15 K were calculated.     

Ion chromatographic determination of traces of some oxoanions with direct spectrophotometric detection

Investigation of the use of a single analytical procedure using the non-suppressed ion chromatographic method with direct spectrophotometric detection capable of determining eight oxoanions simultaneously is presented in this paper. Potassium phosphate was found to be the most suitable eluent for UV absorbance detection at 205 nm. Oxoanions AsO³⁻₃, SeO²⁻₃, AsO³⁻₄, VO⁻₃, SeO²⁻₄, WO²⁻₄, MoO²⁻₄ and CrO²⁻₄ were detected at ng ml⁻¹ levels with well separated peaks at retention time < 25 min. The working range is in the range ng ml⁻¹ to 50 μg ml⁻¹. The method is sufficiently sensitive to determine As (V), V(V), Mo(VI) and Cr(VI) anions (and NO⁻₃) directly in a river water sample. The accuracy of these results was established by comparison with conventional atomic absorption methods.     

Field investigation of major and minor ions along Summit (Central Greenland) ice cores by ion chromatography

As a part of the European EUROCORE and GRIP (Greenland Ice Core Project) operations aimed at recovering deep ice cores at Summit (Central Greenland), we have for the first time successfully performed ion chromatography measurements in the field and investigated in detail the soluble impurities, including Na⁺, NH⁺₄, K⁺, Mg²⁺, Ca²⁺, F⁻, CH₃COO⁻, CH₂ OHCOO⁻, HCOO⁻, CH₃SO₃⁻, Cl⁻, NO⁻₂, SO₄²⁻ and C₂O₄²⁻, trapped in ice deposited over some 200 000 years in Greenland.     

The structure and potentials of the interaction of complexes of polyatomic anions with Li, Na, and the molecules of aprotic solvents

The semiempiric CNDO method with the modified potential of core-core repulsion is used for quantum-chemical calculations of the potential surface cross-sections, charge distribution and the electrostatic field in complexes of the type A⁻…M⁺ (A⁻ = ClO⁻₄, NO⁻₃, SCN⁻, BF⁻₄, AsF⁻₆; M⁺ = Li⁺, Na⁺) and A⁻…M⁺…Mol (acetonitrile, pyridine, dimethylformamide, dimethylsufloxide, nitromethane, tetrahydrofuran). It is established that the anion-cation interaction potential has the form of a complex function with several minima which correspond to possible types of coordination between anions and cations. The effect of solvent molecules leads to the weakening of the interionic bond and the decrease of the potential barrier between configurations with different types of anion-cation coordination.     

Untersuchungen zur atomspektroskopischen spurenanalyse in AB-halbleiter-mikroproben—II : Untersuchungen zur bestimmung von Ga-spuren in In und InAs und von In-spuren in Ga, GaAs und GaP durch AAS mit elektrothermischer atomisierung

The atomic-absorption determination of traces of Ga in In, P, As and InAs and of traces of In in Ga, P, As, GaAs and gap is described. The mechanism of the evaporation of the trace element according to the medium used (HCl or HNO₃) is discussed. Nitric acid medium is recommended for analytical determinations. The evaporation of the trace elements and matrices was investigated by means of absorbance vs. time curves. It is shown that a thermal fractionation of AsO³⁻₄ and PO³⁻₄ matrices and Ga and In traces is possible in the ashing step. In the case of Ga the thermal fractionation of the matrices (In³⁺, In³⁺/AsO³⁻₄) from the trace element is possible in the atomization step. The thermal fractionation of In traces from the matrices (Ga³⁺, Ga³⁺/AsO³⁻₄, Ga³⁺/Po³⁻₄) is impossible. The results and the reasons for the non-specific absorption are discussed. The absolute detection limits are Ga 45 pg, In 35 pg. The relative detection limits are 1 ppm in a 0.1 mg sample.     

An ab initio molecular orbital study on the gas-phase rapid reaction of the silicon cation Si with ammonia

The gas-phase rapid ion-molecule reaction Si⁺ (²P) + NH₃→ SiNH₂⁺ + H is theoretically investigated by the ab initio molecular orbital methods. Several possible pathways (A, B, C) on its potential energy surface have been examined, discussed and compared. Theoretical calculations indicate that pathway A is favourable in energy and that the reaction begins by forming a collision complex of the ion-dipole molecule Si-NH⁺₃, which forms with no barrier into the first energy well of the reaction coordinate. Migration of an H atom from an N atom to a Si atom forms the intermediate HSi-NH⁺₂, which corresponds to the second energy well and can fragment to the observed product SiNH⁺₂ by losing an H atom from the Si atom. The barriers for migration and fragmentation are 52.5 and 38.6 kcal mol⁻¹ respectively. Pathway A has a negative activation energy of −42.1 kcal mol⁻¹.     

Classical trajectory calculation for ion-pair formation in K+O2 collisions

Three-dimensional trajectory surface hopping calculations were performed on two diabatic energy surfaces. The covalent surface describes the K(²S) + O₂(³Σ⁻_g) state and the ionic surface K⁺(¹S) + O⁻₂(²Π_g). Transitions from one surface to another were computed through the Landau—Zener model. At small deflection angles, the energy loss distribution exhibits two peaks, as observed, due to O⁻₂ in its electronic ground state and to vibrationally excited O⁻₂.     

Quenching of the dissociative electron attachment resonances of O2 physisorbed on amorphous ice

Measurements of electron stimulated desorption (ESD) yields of O⁻, at incident electron energies below 20 eV, from 0.15 monolayers (ML) of O₂ physisorbed at 20 K on a variety of molecular solids have been performed. It is observed that for O₂ condensed on 4 ML of H₂O, the O⁻ signal from dissociative electron attachment (DEA) to O₂ is entirely absent. We attribute this to a complete quenching of the dissociative ²Π_u, ²Σ⁺_g, and ²Σ⁺_u, resonances of O⁻₂ by the adjacent water molecules.     

The enthalpy of mixing of liquid NaF and NaMgF3 from drop calorimetry

The enthalpy of mixing of three liquid mixtures of NaF and NaMgF₃ has been measured by drop calorimetry and was found to be negative. This energy release is attributed to a change in the equilibrium

Mg_1/4[MgF²⁻₄]_3/4+f⁻ å MgF²⁻₄ to the formation of complex MgF²⁻₄-ions. A ΔH^M diagram for the system NaF-MgF₂ up to 50 mol % MgF₂ has been constructed.     

Untersuchungen zur atomspektroskopischen spurenanalyse in AB-halbleiter-mikroproben—I : Verdampfbarkeit und unspezifische absorption von AB-verbindungen und ihren komponenten in der graphitrohrküvette

The evaporation of the species Ga³⁺, In³⁺, PO³⁻₄, AsO³⁻₄, Ga³⁺/AsO³⁻₄, Ga³⁺/PO³⁻₄ and In³⁺/AsO³⁻₄ in HCl and HNO₃ medium was investigated by measurement of the non-specific absorption at 250 nm, in a graphite cuvette, with a continuum light source. The maximum of the non-specific absorption is given for the ashing and atomizing phases as a function of the ashing temperatures. Ashing temperatures for analytical determinations can be derived from the results. The mechanisms of evaporation of the substances were investigated by means of extinction-time curves. Absorption spectra of the matrices and of the pure acids used were measured between 190 and 330 nm in the graphite cuvette at different ashing and atomizing temperatures. The InCl- and GaCl-bands of the C-system and new bands of GaO- and InO-molecules were found. The PO-band at 246 nm was detected. The results are discussed and can be applied for thermal fractionation and for background correction by the two-line method in trace analysis by electrothermal AAS.     

Kinetic–catalytic–spectrophotometric determination of low concentrations of molybdenum in white wines

The paper describes the determination of the molybdenum content in white wines based on its catalytical action on the kalium iodide oxidation by hydrogen peroxide in acid medium.

The optimum reaction conditions (the catalyst, KI and H₂O₂ concentrations, the pH value, the order of the reagent additions, the temperature) have been found by studying the effect of the reaction variables.

The influence of some metallic ions (Ca²⁺, Mg²⁺, Zn²⁺, Cd²⁺, Fe²⁺ and Fe³⁺) and complexing anions (F⁻, C₂O²⁻₄, EDTA⁴⁻) on the catalyzed reaction rate was elucidated.

The molybdenum concentration was estimated by the tangent, fixed-time and fixed-absorbance method. The obtained average values for molybdenum content in white wines are within the 1.77×10⁻⁷–1.83×10⁻⁷ mol l⁻¹ range.     

Gas-phase stability of cluster ions SF m + (SF6) n with m = 0–5 and n = 1–3

The gas-phase stabilities of cluster ions SF⁺_m (SF₆)_n with m = 0−5 were determined by using a high pressure mass spectrometer. The bond energies of SF⁺_m (SF₆)₁ were found to be less than 10 kcal/mol and to decrease with m = 0 → 5. There appear to be rather large gaps in the bond energies between n = 1 and 2 for the clusters SF⁺_m (SF₆)_n with m = 0−4. The structures of SF⁺₅, SF⁺ (SF₆)₁, SF⁺₃ (SF₆)₁, and SF⁺₅ (SF₆)₁ were investigated by ab initio molecular orbital calculations. For SF⁺₅, the D_3h geometry is found to be most stable andC_4v is a transition state of the Berry pseudorotation. For the ion-molecule complexes, the “on-top hat” models were found to be the most stable structures.     

Handling of electronic absorption spectra with a desk-top computer-II: calculation of stability constants from spectrophotometric titrations

A fully automatic system for combined spectrophotometric and pH titrations was described in Part I. Its performance in the titration of weak acids and metal complexes is discussed, along with a computer program for numerical treatment of the data, based on Marquardt's modification of the Newton—Gauss non-linear least-squares method. The deprotonation of p-nitrophenol at concentrations of 4 × 10⁻⁵ and 4 × 10⁻⁶M was studied in order to test the sensitivity. Results identical within the reproducibility of the pH-meter were obtained: pK^H = 7.00 ± 0.01 and 7.02 ± 0.01, respectively. Three complexation reactions were studied: (1) the interaction of SCN⁻ with the Co²⁺ complex of 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane (TMC); five independent experiments gave pK [CoTMC (SCN)⁺ CoTMC²⁺ + SCN⁻] = 3.099 ± 0.003: (2) the deprotonation of the Cu²⁺ complex of 3,7-diazanonanediamide (DANA); five experiments gave pK (CuDANA²⁺ CuDANAH⁺₋₁ + H⁺) = 7.14 ± 0.01 and pK (CuDANAH⁺₋₁ CuDANAH₋₂ + H⁺) = 8.38 ± 0.01: (3) for the reaction of Cu²⁺ with 1,3,7-triazacyclodecane (L), data from different ligand: metal ratios had to be combined to obtain pK (CuL²⁺ Cu²⁺ + L) = 16.19 ± 0.01, pK (CuL²⁺₂ CuL²⁺ + L) = 10.30 ± 0.01, and pK (Cu₂L₂ (OH)²⁺₂ 2 CuL²⁺ + 2 OH⁻) = 14.58 ± 0.03. Titration curves with a total change in absorbance of as little as 0.03 units could be analysed satisfactorily, extending considerably the useful range of concentrations for spectrophotometric titrations. In combined spectrophotometric/pH titrations the accuracy of the glass electrode is normally the limiting factor. Other equilibrium constants can easily be reproduced with standard errors of less than 0.01 log unit.