TOF‐SIMS characterization of an aluminovanadate oxide catalyst

Time‐of‐flight Secondary Ion Mass Spectrometry (TOF‐SIMS) has been employed to characterize the surface physical and chemical state of aluminovanadate oxide catalyst precursors (‘V? Al? O’) precipitated at different pH values in the range of 5.5…10. The reference oxide V₂O₅ has also been studied for comparison purposes. It is shown that the analysis of molecular ion emission yields valuable information on the surface elemental and phase composition. Increasing pH values while precipitating from aqueous precursor solutions are found to result in a monotonic variation of the surface composition, in a progressive hydroxylation of aluminium and vanadium and in an increasing dispersion of vanadium oxide species. SIMS data evaluated on the basis of Plog's valence model of molecular ion emission reveal reduced V⁴⁺ states, the fraction of which is dependent on the pH value. The SIMS results are supported by XPS data. The enhancement of the catalytic activity in oxidative propane dehydrogenation over V? Al? O prepared at high precipitation pH is in good correlation with the measured surface characteristics. Copyright © 2007 John Wiley & Sons, Ltd.     

Gravimetric determination of uranium(vi) with n-benzoylphenylhydroxylamine

Uranium(VI) can be quantitatively precipitated by N-benzoylphenylhydroxylamine at pH 5.4. The precipitate is weighed after ignition to U₃O₈, or directly as U0₂(C₁₃H₁₀O₂N)₂. Cerium(III). thorium, lead and bismuth can be masked with magnesium-EDTA complex. Iron(IlI), titanium (IV) zirconium and molybdenum(Vl) and small amounts of aluminium can be precipitated with the reagent and filtered before uranium(Vl) is determined in the filtrate by proper adjustment of acidity. Fluoride, carbonate and organic acids interfere.     

5: 6-Benzoquinaldinic acid as an analytical reagent: Determination of thorium and zirconium

Thorium and zirconium are determined in the presence of rare earths, alkaline earths and magnesium, when precipitated from weakly acid solution with 5:6-benzoquinaldinic acid. Thorium is completely precipitated at pH 3.0 as Th(C₁₄H₈O₂N)₄, but though zirconium is precipitated at the lower pH of 1.8, its composition is not stoichiometric and hence is ignited to the oxide before weighing. Co-precipitation of magnesium and alkaline earths is prevented by the addition of ammonium chloride.     

Ion-selective sensors based on molybdenum bronzes

New pH- and sodium ion-sensitive metal-oxide-type sensors have been developed and tested with a direct solid state contact method. Performance was demonstrated at ambient temperature with single crystals of several molybdenum bronzes (i.e. Na_0.9Mo₆O₁₇, Li_0.9Mo₆O₁₇, Li_0.33MoO₃ and K_0.3MoO₃). The pH sensors with Na-molybdenum-oxide bronzes show near ideal Nernstian behavior in the pH range 3–9. The response is not affected by the direction of the pH change. The response time of most molybdenum bronze pH sensors is less than 5 s for 90% response. The sodium molybdenum bronze sensor responded reproducibly and fast to changes of Na⁺ concentration in the range 1–10^–4 mol dm^–3. Cross sensitivity tests to other ions such as H⁺ or K⁺ have shown that the new sodium ion sensor may be used when the concentration of other ions is an order of magnitude smaller than the Na⁺ concentration. pH sensors with single crystals of molybdenum oxide bronzes can be used to follow pH titrations. Electronic Publication     

Preparation of Forsterite by the Geopolymer Technique—Gel Compositions as a Function of pH and Crystalline Phases

Mg-bearing silicate precursor gels have been prepared by mixing 0.74 mol/L sodium metasilicate and 1.48 mol/L magnesium nitrate solutions. Caustic soda solution of 1.0 mol/L concentration was introduced to regulate pH. The magnesium nitrate solution was added dropwise to the sodium silicate solution in equi-volume at various pH values. Raw and heat-treated gels were characterized by XRF, TG-DTA, XRD and FE-SEM. As a result, gel compositions were dependent on pH values of mixing solutions. The pH value yielding stoichiometric forsterite composition, MgO/SiO₂ = 2 was reached at pH 9.3. In addition, this value was pH 8.4 for stoichiometric enstatite composition, MgO/SiO₂ = 1. With decreasing pH from 9.3, the ratio became less than 2 and forsterite and enstatite precipitated by heating the gels. With increasing pH from 9.3, the ratio became more than 2 and forsterite and periclase precipitated by heating the gels. DTA curves showed a characteristic exothermic peak centered at 700–900^∘C, indicating relatively low temperature formation of crystalline phases due to the presence of polycondensed frame works of silicates in the precursor gels.     

Determination and comparison of stability constants of tungsten(VI) and molybdenum(VI) with nitrilotriacetic acid and glutamic acid at different ionic strengths

The formation constants of species formed in the systems H⁺?+?W(VI)?+?nitrilotriacetic acid (NTA) and H⁺?+?NTA have been determined in aqueous solution for pH?=?4–9 at 25°C and different ionic strengths ranging from 0.1 to 1.0?mol?dm^?3 NaClO₄, using potentiometric and spectrophotometric techniques. It was shown that tungsten(VI) forms a mononuclear 1?:?1 complex with NTA of the type WO₃L^3? at pH?=?7.5. The composition of the complex was determined by the continuous variations method. The complexation of molybdenum(VI) with glutamic acid was investigated in aqueous solution ranging in pH from 4 to 9, using polarimetric, potentiometric and spectrophotometric techniques. The composition of the complex was determined by the continuous variations method. It was shown that molybdenum(VI) forms a mononuclear 1?:?1 complex with glutamic acid of the type MoO₃L^2? at pH?=?6.0. The dissociation constants of glutamic acid and the stability constants of the complex were determined at 25°C and at ionic strengths ranging from 0.1 to 1.0?mol?dm^?3 sodium perchlorate. In both complex formation reactions the dependence of the dissociation and stability constants on ionic strength is described by a Debye-Huckel type equation. Finally, a comparison has been made between the patterns of ionic strength dependence for the two complexes and the results have been compared with data previously reported.     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

An attempt to obtain aluminium hydroxide that could give aluminium oxides of increased thermal stability was made. Aluminium hydroxide was precipitated during a hydrolysis of aluminium chloride in ammonia medium. The influence of preparative conditions, such as a dosing rate of aluminium precursor, pH, duration of the precipitate refluxing and temperature of calcination, on the properties of obtained hydroxides and oxides was investigated. The materials were studied with the following methods: thermal analysis, IR spectroscopy, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours. Precipitated boehmites had high values of S _BET determined from nitrogen adsorption (220–300 m²g^–1), good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. It has been proved that a high pH value during the precipitation of aluminium hydroxide favoured better crystallisation of boehmite structure, higher temperature of its dehydroxylation into γ-Al₂O₃, and delayed transformation of γ phase into α-Al₂O₃. Aluminium oxides derived from the hydroxides precipitated at a high pH were the most stable at high temperatures, and were characterised with the best surface properties. The online version of the original article can be found at     

Activation pH and Gating Dynamics of Influenza A M2 Proton Channel Revealed by Single‐Molecule Spectroscopy

Because of its importance in viral replication, the M2 proton channel of the influenza A virus has been the focus of many studies. Although we now know a great deal about the structural architecture underlying its proton conduction function, we know little about its conformational dynamics, especially those controlling the rate of this action. Herein, we employ a single‐molecule fluorescence method to assess the dynamics of the inter‐helical channel motion of both full‐length M2 and the transmembrane domain of M2. The rate of this motion depends not only on the identity of the channel and membrane composition but also on the pH in a sigmoidal manner. For the full‐length M2 channel, the rate is increased from approximately 190 μs⁻¹ at high pH to approximately 80 μs⁻¹ at low pH, with a transition midpoint at pH 6.1. Because the latter value is within the range reported for the conducting pK _a value of the His37 tetrad, we believe that this inter‐helical motion accompanies proton conduction.     

Buffer Standards for the Physiological pH of the Zwitterionic Compound,DIPSO, from 5 to 55 °C

The values of the second dissociation constant, pK ₂, and related thermodynamic quantities of 3-[N,N-bis (2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) have already been reported over the temperature range 5 to 55 °C including 37 °C. This paper reports the pH values of four NaCl-free buffer solutions and four buffer composition containing NaCl salt at I=0.16 mol⋅kg⁻¹. Conventional pa _H values are reported for all eight buffer solutions. The operational pH values have been calculated for four buffer solutions recommended as pH standards, at 25 and 37 °C after correcting the liquid junction potentials with the flowing junction cell.     

Extractive spectrophotometric determination of molybdenum(V) in molybdenum steels

An extraction spectrophotometric method has been developed for the determination of traces of molybdenum present in molybdenum steels which is based on the extraction of the orange-red molybdenum-thiocyanate-acetonethiosemicarbazone complex into chloroform from hydrochloric acid medium. The complex has an absorption maximum at 472 nm with a molar absorptivity of 1.9 × 10⁴ liters mol⁻¹ cm⁻¹. Beer's law is valid over the concentration range 0.1–9.5 ppm of molybdenum with an optimum concentration range of 0.4–9 ppm. The equilibrium shift method indicates 1:4:2 composition for molybdenumthiocyanate-acetonethiosemicarbazone complex. The effect of acidity, reagent concentrations, temperature, and interferences from various ions are reported.     

Coulometric titration of zinc with ferrocyanide

A method has been developed for the automatic coulometric titration of zinc ion with ferrocyanide ion generated by the reduction of ferricyanide ion at a platinum cathode. The composition of the zinc precipitate was found to depend on the pH. Over the pH range from about 1 to 3 it corresponds exactly to K₂Zn₃[Fe(CN)₆]₂, but at higher pH values the precipitate contains a smaller proportion of ferrocyanide to zinc. At the optimum pH of 2 quantities of zinc from 0.3 to 30 mg in 8o ml were titrated automatically with an average error smaller than ± 1% The ferriferrocyanide couple should be applicable to other coulometric titrations involving both redox and precipitation reactions.     

Some observations on the use of a new matrix modification electrothermal atomization-atomic absorption spectrophotometric method for the determination of micro- and sub-microgram amounts of molybdenum in human teeth

A very sensitive and selective electrothermal atomization-atomic absorption Spectrophotometric (ETA-AAS) matrix-modified method for the determination of micro- and submicrogram amounts (0–25 μ g⁻¹ molybdenum) in whole human one-rooted teeth has been developed. Hydrazine sulfate, (NH₂)₂ · H₂SO₄, which has been used as a matrix modifying reagent (MMR) is found to be very influential in removing matrix interference effects such as calcium which is present in hydroxy apatite (the main mineral constituent of tooth), at pH 2.0–2.2. Beer's law is obeyed over the range 0–1.5 ng molybdenum 5/μl injected solution. The absolute sensitivity and detection limit of the method are respectively 6.46 and 1.32 pg molybdenum/5 μl injected sample solution. The recovery percentage and RSD% are also determined. Compared to the neutron activation method, the proposed method is rapid, more available, and less expensive and requires no grinding of the tooth with metals, mortar, or mills. It is more sensitive and simpler than flame techniques. Background correction is not necessary. No separation or preconcentration of molybdenum is required. The method has been applied for the determination of molybdenum in teeth taken from representative districts of Baghdad.     

Interaction of Molybdenum(VI) Oxyanions with +2 Metal Cations

The association of molybdenum(VI) oxyanions with metal cations was investigated in solutions of low ionic strength, such as those prevailing in most natural waters. Potentiometric titrations were carried out for the systems containing molybdenum(VI) anions and divalent metal cations (M = Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg and Pb). This selection includes the major cations and some other cations of high environmental relevance. The interaction of iron(III) with Mo(VI) anions was also studied. At neutral and basic pH values and for those systems where the solubility of the molybdate salt is high enough, ionic species pairs such as [M(MoO₄)] predominate. At acidic pH values, [M(HMoO₄)]⁺ and [M(Mo₇O₂₄)]^4– are formed, the latter species are only relevant for total molybdenum concentrations higher than 1 mmol·L^?1. These results provide the basis for molybdenum speciation in natural aquatic systems, on which the environmental fate, bioavailability and toxicity of the element depend.     

Copolymerization of Methacrylic Acid and N-Vinylpyrrolidone in Aqueous Solution

The monomer reactivity ratios for the copolymerization of methacrylic acid (MA) and N-vinylpyrrolidone (NVP) in aqueous media at 30°C were determined as a function of pH (range 2-10), by use of both the modified differential (YBR) and integrated copolymerization equation to process the data at high conversions (< 70% by weight). The reactivity ratio r₁ (for MA) ranges from 0.92 to 8.3 and that for NVP (r₂) is very small except at pH 7 and 8. The ri values show two minima: 2.9 at pH 4 and 0.92 at pH 8, nearly corresponding to the pKa values of the monomer MA and the polymer, respectively. Addition of 1 M sodium chloride results in an increase of n values, and the values are still lower than those of the undissoeiated acid. The trend of r_xwith pH is seen to follow that of the homopolymerization behavior of MA reported in the literature. The r₁ and r₂ are of the same order as those obtained in dimethylformamide in the literature.     

The application of thiosalts in analysis : Estimations based on decomposition of thiosalts part B. estimation of molybdenum and tin

1. It has been shown that molybdenum can be precipitated quantitatively by adding excess of concentrated ammonium sulphide to a molybdate solution and then decomposing the thiosalt with excess of hydrochloric acid. 2. The weight of the precipitate corresponds to the formula MoS₃.2H₂O and the quantity of metal can be estimated by multiplying the weight of the precipitate by a constant factor. 3. The same method is also applicable to the gravimetric estimation of tin. The precipitate of stannic sulphide is easily filterable and its weight corresponds to the formula SnS₂.2H₂O. 4. The general method in its simple form has not been found to be applicable to vanadium, platinum and gold.     

Interaction of arsenite ions with molybdate in aqueous solution

Interaction of arsenite ions with molybdate ions in aqueous HClO₄ at pH of 3–5 were studied using spectrophotometry. A heteropoly anion (HPA) with the AsMo₄ stoichiometry was revealed. Elemental analysis, X-ray photoelectron spectroscopy, and IR spectroscopy were used to characterize the composition and structure of the cesium salt of As(III)-Mo(VI) HPA precipitated from solution. The comparison of the IR spectra of cesium salts of the synthesized HPA and dimethylarsenite HPA (CH₃)₂AsMo₄O₁₅H^2?, whose structure is known, implies that AsMo₄O ₁₅ ^3? has a similar structure, where the AsO ₂ ^? group is linked by four connected molybdenum octahedra (Mo₄O₁₂)O^2?.     

Effect of pH on the reactivity ratios in aqueous solution copolymerization of acrylic acid and N-vinylpyrrolidone

Acrylic acid (AA) and N-vinylpyrrolidone (NVP) were copolymerized in aqueous solution at 30°C in the pH range 4–9 and the monomer reactivity ratios (r₁ for AA and r₂ for NVP) were determined as a function of pH from the high conversion data by using both the differential (YBR) and the integrated copolymerization equations. The value of r₁ decreased from 5.2 at pH 4 to a minimum of 1.3 at pH 5 and then increased to 8.1, 6.6, and 7.2 at pH 7, 8, and 9, respectively. Addition of 1M NaCl at pH 6.5 restored the r₁ nearly to that at pH 4, the predominantly un-ionized acid. The r₂ values for NVP were nearly zero at all pH values except at pH 5. The variation of the reactivity ratios with pH was examined in terms of the electrostatic interactions between the ionized monomer molecules, the rate of homopolymerization of acrylic acid, and the cation binding to the poly(acrylic acid) molecules. The r₂ values for NVP compared favorably with the literature values reported in bulk and organic solvent systems, although r₁ values are much higher.     

The interfacial electrochemistry of goethite (α-FeOOH) especially the effect of CO2 contamination

The surface charge density/pH function of goethite is shown to be very similar to the surface charge density/surface potential functions reported for mercury. This finding differs from all other reports and we attribute the difference to the experimental method. This paper describes a method of titrating oxide suspensions which reduces pH drifts and suspension and stirring effects to negligible values and also a method for effectively removing CO₂ from oxide suspensions. When goethite which had been stored as a suspension in contact with air was titrated in N₂-purged NaI solution the point of zero charge (pzc) was within the range found by previous workers, i.e. pH 8.75 and the σ/pH function did not resemble reported σ/ψ₀ functions of mercury but when goethite from the same preparation was purged with N₂ for 2 months and then titrated in N₂-purged NaI the pzc was pH 9.3 and the σ/pH function did resemble the σ/ψ₀ function of mercury.     

Preparation and ionic conductivity of (100−x)(0.8Li2S·0.2P2S5)·xLiI glass–ceramic electrolytes

The glass–ceramic electrolytes of (100?x)(0.8Li₂S·0.2P₂S₅)·xLiI (in mole percent; x?=?0, 2, 5, 10, 15, 20, and 30) were prepared by mechanical milling and subsequent heat treatment. Crystalline phases analogous to the thio-LISICON region II or III in the Li₂S–GeS₂–P₂S₅ system were precipitated. The thio-LISICON III analog phase was mainly precipitated at the composition x?=?0, and the thio-LISICON II analog phase was precipitated in the composition range from x?=?2 to 15. The X-ray diffraction peaks of the thio-LISICON II analog phase shifted to the lower diffraction angle side with increasing the LiI content. High conductivities above 2?×?10^?3?S?cm^?1 at room temperature were observed in the glass–ceramics at the wide composition range from x?=?2 to 15. The glass–ceramic electrolyte at x?=?5 with the highest conductivity of 2.7?×?10^?3?S?cm^?1 showed a wide electrochemical window of about 10 V. The addition of LiI to the 80Li₂S·20P₂S₅ (in mole percent) glass was effective in crystallizing the thio-LISICON II analog phase with high conductivity from the glass.     

Sorption of uranyl ions on silica: Effects of contact time, pH, ionic strength, concentration and phosphate

The sorption of UO₂ ²⁺ and phosphate on silica were simultaneously studied. The effect of contact time between the solid phase and aqueous solution, pH and ionic strength on the UO₂ ²⁺ sorption in the absence and the presence of phosphate was investigated. The effect of contact time between the solid phase and aqueous solution, pH and ionic strength on the phosphate sorption was investigated too. The isotherms of UO₂ ²⁺ and phosphate sorption at different pH values were determined. It was found that as compared with the sorption in the absence of phosphate, the sorption of UO₂ ²⁺ on silica in the presence of phosphate is increased at low pH and decreased at high pH; the abruptly increased with increasing pH in the pH range 3-6; the sorption is gradually decreased with increasing pH in the pH range 2-12; the sorption insensitive and the sorption of phosphate is sensitive to ionic strength.