高浓度V-H2O体系的溶解组分优势区域图和Pourbaix图

对25 ℃, 总V浓度cT(V)=1.0×10^-3 mol·L-1时的V-H2O 体系可能存在的组分进行热力学和电化学分析, 计算了各溶解组分的浓度, 运用“浓度比较法”作为判据, 确定液相和液相、液相和固相以及各固相间的边界线, 绘制了cT(V)=1.0×10^-3 mol·L-1时V-H2O 体系的溶解组分优势区域图和Pourbaix图. 与低浓度(cT(V)=1.0×10^-5 mol·L-1)的优势区域图比较发现, 在一定的温度和压力下, 总钒浓度对于钒阳离子的优势区影响不大, 但对钒酸根阴离子的优势区影响显著; 随着总钒浓度的增大和酸化程度的逐步加强, 钒酸根阴离子会发生一系列复杂的质子化和聚合反应; 总钒浓度越高、酸性越强, 多核钒酸根阴离子越稳定.     

Extraction and separation of mercury using 3,4,4a,5-tetrahydro-3, 3,4a-trimethyl-7-(substituted)-pyrimido [1,6-a] benzimidazole-1-thiol (PBT) and its application to industrial effluents

A selective method for the extraction of mercury(II) from hydrochloric acid solution with 3,4,4a,5-tetrahydro-3,3,4a-trimethyl-7-(substituted)-pyrimido [1,6-a]-benzimidazole-1-thiol, (PBT) is proposed. The extraction ability of the system was studied both in strongly acidic (6 mol dm^–3 HCl) and alkaline (pH 12) media. The optimum extraction conditions were evaluated from a critical study of the effect of pH, PBT concentration and metal ion concentration. The probable composition of the species has been deduced from lg-lg plots and an extraction mechanism is suggested. A separation scheme is proposed. The method has been applied for the determination of Hg (II) in industrial waste water.     

Speciation of selenite and selenate using living bacteria

In this work, a reliable method is described for speciation of soluble inorganic selenium ions, Se(IV) and Se(VI), which combines an uptake process by using living bacterial cells and electrothermal atomic absorption spectrometry (ETAAS). A selective retention of either Se(IV) or Se(IV) plus Se(VI) can be carried out by using the uptake system made up of either Pseudomonas putida or Escherichia coli strains cultivated in a culture medium based on glucose (P. putida) and glucose plus dipotassium phosphate (E. coli) mixed together with the original sample solution containing the selenium species. Discrimination between inorganic selenium species is possible by combining the optimization of the bacterial cell, the growth conditions and the relative rates of their retention from the sample. In the general procedure, an equilibrium between the analyte in the solution and the uptake system is allowed to be established, and then the concentration of selenium is determined directly in the biomass by slurry sampling ETAAS. Nonetheless, a theoretical model is proposed to describe the retention process by the living bacterial cells, which also provides a feasible quantification of the extraction process before the adsorption equilibrium is reached and whenever the agitation conditions and the sampling time are under control. The detection limits for the inorganic selenium species at the best retention conditions are of 5.7 ng Se(IV) ml(-1) for P. putida and 6.1 ng Se(IV) ml(-1) and 6.3 ng Se(VI) ml(-1) for E. coli. The relative standard deviations of the adsorption/determination process are 2.9-6.3%.     

Mapping concentration profiles within the diffusion layer of an electrode: Part III. Steady-state and time-dependent profiles via amperometric measurements with an ultramicroelectrode probe

A platinum-disk ultramicroelectrode is used to monitor amperometrically the concentrations of the electroactive substrate and of its electrogenerated product(s) inside the diffusion layer created by a larger working electrode. This allows a direct monitoring of the target species concentration profiles without any assumption even when diffusion coefficients differ significantly. The validity of the method is established experimentally through the study of the one-electron reversible oxidation of the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ couple in aqueous KCl, under steady-state or under transient diffusion conditions. Under steady-state conditions, the results compare excellently with those we obtained by means of the potentiometric method reported in Part II of this series. Under transient diffusion conditions, the measured concentration profiles match perfectly those predicted for planar diffusion, which demonstrates the interest of the method for the analysis of dynamic diffusion-kinetic problems. The validity of the method in a complex diffusional situation is established as previously (Part II) by the investigation of the effect of a conproportionation reaction taking place during the second reduction of tetracyanoquinodimethane (TCNQ) in DMF.     

Vanadium in Italian waters: monitoring and speciation of V(IV) and V(V)

In this work, a highly sensitive method was developed to separate vanadium (IV) from vanadium (V), which are both contained in water at trace levels. A suitable strong anionic exchange column (SAX) loaded with disodium ethylendiaminetetraacetic acid (Na₂EDTA) was used to trap both vanadium species dissolved in 10–100 ml of water at pH 3. The vanadyl ion was selectively eluted by means of 15 ml of an aqueous solution containing Na₂EDTA, tetrabutylammonium hydroxide (TBA⁺OH⁻), and isopropanol (ⁱPr-OH) and was subsequently determined by atomic absorption spectroscopy with electrothermal atomization. The concentration of vanadate ion was calculated by subtracting the vanadyl concentration from the total concentration of vanadium. The optimal conditions for a selective elution were evaluated. The recovery of vanadium (IV) was 95% or better. The proposed method provides a simple procedure for the speciation of vanadium in aqueous matrices. The collection of the two forms could easily be carried out at the sampling site. Therefore, the risk of changing the concentration ratio between vanadium species was widely reduced. The detection limits were 1 μg/l for both species, when a 10-ml sample was eluted through the column. The method was applied successfully to vanadium speciation on different kinds of Italian volcanic water: Mount Etna (Sicily), Lake Bracciano and Castelli Romani (Latium).     

Simultaneous determination of trace arsenic(III), antimony(III), total arsenic and antimony in Chinese medicinal herbs by hydride generation-double channel atomic fluorescence spectrometry

A new method was developed for simultaneous determination of trace arsenic and antimony in Chinese herbal medicines by hydride generation-double channel atomic fluorescence spectrometry with a Soxhlet extraction system and an n-octanol-water extraction system, respectively. The effects of analytical conditions on the fluorescence intensity were investigated and optimized. A water-dissolving and methanol-water-dissolving capability were compared. The contents of different species in five Chinese herbal medicines and their decoctions were analyzed. The concentration ratios of n-octanol-soluble As or Sb to water-soluble As or Sb were related to the kinds of medicine and the acidity of the decoction. Soxhlet extraction was found to be an effective method for plants pretreatment for determination of arsenic and antimony species in Chinese herbs; the interferences of coexisting ions were evaluated. The proposed method has the advantages of simple operation, high sensitivity and high speed, with 3σ detection limits of 0.094 μg g⁻¹ for As(III), 0.056 μg g⁻¹ for total As, 0.063 μg g⁻¹ for Sb(III) and 0.019 μg g⁻¹ for total Sb in a 1.0 g of the sample.     

Kinetic study of competing first-order processes: simultaneous quantification of hemoglobin and methemoglobin in whole blood

The development and evaluation of a kinetic method for the simultaneous quantification of methemoglobin and hemoglobin is described. A unique feature of the reaction/monitoring system is that the two species produce signals that change in opposite directions. Attempts to optimize the reaction conditions, procedures for preparing reliable standards, a new multiwavelength method used for comparison purposes and results for synthetic standards and blood samples to which known amounts of methemoglobin are added are described. Combined effects of competing directions of signal change, large concentration differences and conditions required for pseudo-first-order behavior results in a relativity narrowly defined set of conditions that permit simultaneous determinations. Even so, results for methemoglobin were in reasonable agreement between the kinetic and spectrophotometric methods (slope 1.05, intercept 0.17 mmol l^?1); difficulties in achieving pseudo-first-order behavior yielded less reliable results for hemoglobin with a least-squares slope of 1.16 for a comparison of results by kinetic and spectrophotometric methods. This latter problem had an adverse effect on comparison of percentages of methemoglobin in samples even though concentration values were in good agreement.     

An optimised single-reagent method for the speciation of chromium by flame atomic absorption spectrometry based on surfactant micelle-mediated methodology

The toxicity of chromium in the environment is dependent on the species in which it exists. This paper outlines a method for the analysis of the oxidation states of Cr employing a suitable chelating agent and the cloud point phenomenon for Cr(VI) and total Cr analysis. The method involves preconcentration of metal chelates followed by air-acetylene flame atomic absorption spectrometric analysis. The chelating agent chosen for this task is the ammonium pyrrolidinedithiocarbamate, which reacts with either Cr(VI) or total Cr under specific experimental conditions. The condensed surfactant phase with the metal chelate(s) is introduced into a flame atomic absorption spectrometer, whereby discrimination of Cr species is feasible by calculating the Cr(III) concentration from the difference between total Cr and Cr(VI). A multivariate design was employed to study the variables affecting the overall analytical performance for total Cr assay. The analytical curves are rectilinear up to 100 micrograms l-1 for both oxidation states of the metal. The limits of detection are 0.6 microgram l-1 and the relative standard deviation (n = 5) at a concentration of 30 micrograms l-1 for both species is around 2.0%. The method was validated by analysing BCR 544 reference material certified for both Cr species. High recoveries in the range 96-107% were attained for the environmental and biological samples tested.     

A chemical concentration modulation sensor for selective detection of airborne chemicals

A method for selective detection of airborne chemicals based on concentration-modulated signals is described. A variable energy source (e.g., a heated filament) causes a modulated chemical reaction in a flowing gas stream, and a chemical sensor monitors the resultant modulated concentration signal. A parameter that is specific for a particular chemical species (i.e., a pseudo-activation energy) can be determined. Because this parameter is unique for a particular chemical and set of operating conditions, devices based on this method offer selectivity in sensing chemical vapors. A single detector capable of selectively identifying a large number of chemicals is possible using this approach.     

Spectrophotometric determination of uranium(VI) using a synergic mixture of ethyl acetoacetate and pyridine

A method is proposed for the extractive spectrophotometric determination of uranium(VI) using the synergic combination of ethyl acetoacetate (EAA) and pyridine (Py) as extractants. The optimum conditions have been evaluated by varying pH and ethyl acetoacetate and pyridine concentration. The nature of the extracted species has been determined by 1g D-1g C plots and the extracted species was found to be UO₂(EAA)₂.Py. The method has been applied for the separation and determination of uranium(VI) from synthetic mixtures.     

Speciation analysis of inorganic vanadium (V(IV)/V(V)) by graphite furnace atomic absorption spectrometry following ion-exchange separation

A sensitive and simple method of ion-exchange resin separation and graphite-furnace atomic absorption spectrometry (GFAAS) detection was proposed for the determination of inorganic vanadium species. Methylene Blue (MB) was used as a chelating agent of V(V) for ion-exchange separation. The complex of V(V) and MB could be trapped by ion-exchange resin at pH 3.0 and eluted by 1.0?mol?L^?1 NaOH. The vanadium species was determined subsequently by GFAAS. The concentration of V(IV) was calculated by subtracting the V(V) concentration from the total concentration of vanadium. Under the optimized experimental conditions, the detection limit of V(V) is 0.48?µg?L^?1 with RSD of 2.6% (n?=?5, c?=?2.0?µg?L^?1). In order to verify the accuracy of the method, a certified reference soil sample was analyzed, and the results obtained were in good agreement with the certified values. The range of recovery for V(IV) and V(V) was 97.8–99.3% and 101.7–103.6%, respectively. The proposed method was applied to the speciation analysis of vanadium in lake-water samples.     

Speciation of arsenic in mussels by the coupled system liquid chromatography-UV irradiation-hydride generation-inductively coupled plasma mass spectrometry

A method has been developed for the determination of seven arsenic species in mussel tissues by liquid chromatography-hydride generation-UV photo-oxidation and detection by inductively coupled plasma mass spectrometry. In order to determine the different species, two ion-exchange columns (anionic and cationic) were used with phosphate and nitric acid/ammonium nitrate as mobile phases, respectively. The optimisation of the conditions for separation, photo-oxidation and hydride generation is described. For each of these species, the limits of detection and repeatability are reported with the entire system coupling. This system was applied to the analysis of certified reference material (CRM 278) and mussels collected from Barcelona harbour. Extractions were achieved in methanol/water (1:1) using low-power focused microwaves as leaching process. As expected, arsenobetaine was the main compound extracted from both materials; the typical concentrations found were between 1 and 7 mg kg(-1). Other organoarsenical compounds, probably arsenosugars, were extracted in a concentration range of 0.3-1.5 mg kg(-1) in both cases. Amounts of dimethylarsinate (DMA) were found to be significant in the CRM 278, but very low in mussels from Barcelona harbour. The low limits of detection of the coupled system allow us to quantify low contents of other species (As(V), arsenocholine and monomethylarsonate (MMA)).     

Use of the differential pulse cathodic adsorptive stripping voltammetric method for the simultaneous determination of trace amounts of cadmium and zinc

A selective and sensitive method for the determination of cadmium and zinc is presented. The method is based on the adsorptive accumulation of the complexes of Cd(II) and Zn(II) ions with 4-amiono-5-methyl-2.4-dihydro-3H-1,2,4-triazol-3-tion (MMTT) onto hanging mercury drop electrode (HMDE), followed by the reduction of the adsorbed species using a voltammetric scan using differential pulse modulation. The ligand concentration, pH, potential and time of accumulation, scan rate, and pulse height were optimized. Under the optimized conditions, a linear calibration curve was obtained for the concentration of Cd(II) and Zn(II) in the range of 5–450 and 5–850 ng/mL, respectively, with a detection limit of 1.7 ng/mL Cd(II) and 1.3 ng/mL Zn(II). The ability of the method was evaluated by analysis of cadmium and zinc in water and alloy samples The text was submitted by the authors in English.     

Necessary and sufficient conditions for the appearance of extrema on concentration distribution curves in complex equilibrium systems

It is shown that species distribution curves with at least one minimum may be found in most equilibrium systems with three or more components. Whether such concentration minima are actually observed then depends on the values of the equilibrium constants and on the total (analytical) concentrations of the components. A general algorithm is given for the necessary and sufficient conditions for the appearance of extrema in multicomponent systems. Three-component systems are studied in more detail and special attention is given to the limiting case of a horizontal inflection, i.e., the point where the concentration minimum just disappears. Two well-studied chemical examples, the Cu(2+)-diethylenetriamine-OH(-) and Hg(2+)-Cl(-)-OH(-) systems are discussed, along with a simple model system showing as many as five extrema on a single distribution curve.     

Sol-gel polycondensation of methyltrimethoxysilane in ethanol studied by <Superscript>29</Superscript>Si NMR spectroscopy using a two-step acid/base procedure

Sol-gel polymerization of methyltrimethoxysilane (MTMS) in ethanol using a two-step acid/base catalyzed procedure (B2) is followed by ²⁹Si NMR spectroscopy. Analysis of the structural evolution of the B2 system shows that esterification of monomeric and end silicon species is rate-limited while that of linear and cyclic species is able to reach pseudoequilibrium in the second basic step. Condensation reactivity is reduced with increasing network connectivity, however, to a much less degree under B2 conditions than MTMS polymerization under acidic conditions. Steric effects as well as many other factors are attributed to this trend. The concentration of cyclic and polycyclic species of the B2 system is nearly 3 times lower compared to the acid-catalyzed system. The empirical degree of condensation at the gel point is determined to be 0.88. The effects of cyclization and phase separation on MTMS gelation are discussed for both B2 and acid-catalyzed systems. Based on these results it is believed that MTMS-based gels form for B2 and not acid-catalyzed conditions due to reduced cyclization, rapid hydrolysis and condensation, effective use of functional groups, and effective contribution of branched and polycyclic species as crosslinking points to connect polymeric chains in the B2 system.     

457—The mechanism of low frequency a.c. electrochemical disinfection

The mechanism of low frequency a.c. electrochemical disinfection was studied. A random oriented graphite fiber-epoxy matrix composite material was employed to fabricate the electrodes. This material was found to have very good electrochemical stability when cycled anodically and cathodically in solutions containing NaCl or NaBr.The lethal species was identified as HClO and HBrO in solutions containing Cl^? and Br^?, respectively, although the free halogen molecule may contribute to the observed disinfection activity under certain experimental conditions.The efficiency of disinfection increases with increasing concentration of Cl^? (or Br^?) and with decreasing pH. Deaerating the solution decreases the efficiency of disinfection significantly. NaBr was found to be effective at much lower concentrations than NaCl at all pH values but the effect is enhanced at pH > 7 due to ionization of HClO.The unique advantage of the method described in this paper is that a high transient concentration (in time and space) of the lethal species is generated, enough to destroy the most resistant microorganisms, while the average concentration of active halogen in the effluent liquid remains well below the objectionable level.     

Propagation rate constant in cationic polymerization of cyclic dienes. I. Polymerization of cyclopentadiene with titanium tetrachloride–trichloroacetic acid

Cationic polymerization of cyclopentadiene induced by titanium tetrachloride–trichloroacetic acid was investigated in a toluene solution at ?69 to ?77°C. All manipulations were handled under vacuum conditions. Time–conversion curves were determined accurately by following the exothermicity of the fast reaction in an adiabatic system. The polymerization kinetics were developed on the basis of a fast initiation reaction and a nonstationary-state concentration (diminishing concentration) of active species, and the propagation rate constant k₂ was determined by substituting either an initial rate of polymerization or a final conversion for the kinetic equations. k₂ for the present system was determined to be 350 l./mole-sec, which is larger than those so far reported for some vinyl monomers in cationic polymerization. The present method can be commonly applied to reactive monomers for the determination of k₂. The nature of termination reaction is discussed in connection with the determination of k₂.     

Liquid-liquid extraction of mercury(II) with triphenylphosphine sulphide: Application to medicinal and environmental samples

A method is proposed for the separation and determination of mercury(II) in environmental samples after its extraction from salicylate solution using triphenylphosphine sulphide (TPPS) as an extractant. The extraction conditions are optimized and reported. The nature of the extracted species is ascertained by a plot of log of distribution ratio versus log of concentration of salicylate and TPPS.     

Speciation analysis of vanadium in natural water samples by electrothermal vaporization inductively coupled plasma optical emission spectrometry after separation/preconcentration with thenoyltrifluoroacetone immobilized on microcrystalline naphthalene

A sensitive and simple method for low temperature electrothermal vaporization inductively coupled plasma optical emission spectrometry (ETV-ICP-OES) determination of V(IV) and V(V) after separation/preconcentration by a micro-column packed with immobilized thenoyltrifluoroacetone (TTA) on microcrystalline naphthalene has been developed. Thenoyltrifluoroacetone was used as both a chelating agent for micro-column separation/preconcentration and a chemical modifier for ETV-ICP-OES determination of vanadium. Both vanadium species could be trapped by micro-column at pH 4.0, and the vanadate (VO₂⁺) ion could be collected selectively at pH 2.4. Solid material loaded with analyte in the micro-column was dissolved with 100 μL of acetone containing 2.0 mmol L⁻¹ TTA and the vanadium was determined subsequently by ETV-ICP-OES. The concentration of vanadyl (VO²⁺) ion was calculated by subtracting the vanadate concentration from the total concentration of vanadium. Under the optimized experimental conditions, the detection limit (3σ) for the preconcentration of 5 mL of aqueous solution is 0.068 μg L⁻¹ for both species and the relative standard deviations were 4.3% for vanadium(V) and 4.8% for vanadium(IV) (c=10 μg L⁻¹, n=7), respectively. The method was applied successfully to the determination of vanadium(IV) and vanadium(V) in natural water samples.