Transition behaviors and hybrid nanofibers of poly(vinyl alcohol) and polyethylene glycol–POSS telechelic blends

We report solution properties of the blend solutions of poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG)–POSS telechelic and its corresponding hybrid nanofibers prepared by electrospinning. The morphologies, microstructures, and wettability of the resulting PVA/PEG3.4k–POSS hybrid nanofibers are studied. The morphologies of the resultant PVA/PEG3.4k–POSS nanofibers are regular with the fiber diameter ranging from 610 ± 110 to 810 ± 280 nm. When the content of PEG3.4k–POSS telechelic increases above 20 wt.%, the beaded fiber morphologies are observed due to severe aggregations of the PEG3.4k–POSS telechelics as well as increased viscosity at higher concentration. In addition, the solution properties of pure PEG3.4k–POSS telechelic solution (ca. 3–5 wt.%) and PVA/PEG3.4k–POSS solutions blended with PVA are explored, and found to show the reversible turbid-to-transparent transition behavior with respect to the solution temperature. Water contact angle measurement of the PVA/PEG3.4k–POSS nanofiber membranes demonstrates an enhanced hydrophobic nature due to the incorporated POSS moieties.     

Simultaneous First Derivative Spectrophotometric Determination of Palladium and Nickel Using 2-(2-Thiazolylazo)-5-Dimethylaminobenzoic Acid as an Analytical Reagent

 A simple, rapid, selective, sensitive and economical method has been developed for the simultaneous determination of trace amounts of palladium and nickel in aqueous methanolic medium using 2-(2-thiazolylazo)-5-dimethylam inobenzoic acid as an analytical reagent by first derivative spectrophotometr y. Palladium is determined by measuring base to peak distance at λ=695.0 nm while nickel is estimated by zero crossing method in the mixture. The linearity is maintained between 0.12–1.75 μg mL⁻¹ for palladium and 0.07–1.60 μg mL⁻¹ for nickel in the pH range 2.8–7.2 and 3.4–8.8 respectively. Seven replicate determinations of 1.0 μ g mL⁻¹ of palladium and 0.8 μg mL⁻¹ of nickel in a mixture give a mean signal height of 0.391 for Pd and 0.541 for Ni with relative standard deviations of 0.9% and 1.2%, respectively. The sensitivity of the proposed method is 0.391 (dA/dλ)/(μg mL⁻¹) for palladium and 0.685 (dA/dλ)/(μg mL⁻¹) for nickel. Various parameters have been optimised for the simultaneous determination of palladium and nickel in various complex samples. Received March 30, 1999. Revision November 25, 1999.     

Application of Tungsten-Rhodium Permanent Chemical Modifier in Slurry Analysis: Determination of Cadmium

 A tungsten-rhodium coating on the integrated platform of a transversely heated graphite atomiser (THGA) was used as a permanent chemical modifier for the determination of Cd in sediment slurries by electrothermal atomic absorption spectrometry. Slurries were ultrasonicated during 20 s before being delivered to the previously W-Rh treated platform. The permanent W-Rh modifier remains stable by approximately 250 measurements when 20 μl of slurries containing up to 1.0% m/v are delivered into the atomiser. In addition, the permanent modifier increases the tube lifetime up to 720 analytical firings. Also, when the W-Rh permanent modifier was employed, there was less variation of the slope of the analytical curves during the total atomiser lifetime, resulting in a decreased need of re-calibration during routine analysis, increasing the sample throughput. The atomiser lifetime was limited to the THGA wall durability, because the W-Rh treated platform was intact after more than 720 analytical firings. Detection limits based on integrated absorbance for 1.0% m/v slurries were 1.5 ng g⁻¹ Cd for 250 μg W +200 μg Rh permanent modifier and 11.5 ng⁻¹ Cd for 5 μg Pd +3 μg Mg(NO₃)₂. Results for the determination of cadmium in sediment slurries using the W-Rh permanent modifier were in agreement with those obtained with dissolved sample solutions by using Pd + Mg(NO₃)₂, since no statistical differences were found by the paired t-test at the 99% level. Received September 6, 1999. Revision December 1, 1999.     

Dissolution behavior of anodic oxide films formed in sulfanic acid on aluminum

Chemical dissolution of the barrier layer of porous oxide films formed on an aluminum foil (99.5% purity) in 1.5 M sulfanic acid after immersion in a 2 mol dm⁻³ sulphuric acid at 50 °C was studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing technique. Re-anodizing was conducted in 0.5 mol dm⁻³ H³BO³/0.05 mol dm⁻³ Na²B⁴O⁷ solution. We found that the change in the porous oxide growth mechanism was observed at the anodizing voltage of 30 V. Taking into account this result chemical dissolution behaviour of the barrier layer of porous films formed at 20 V and 36 V and also the influence of annealing of oxide films at 200 °C were studied. We showed the interplay between the dissolution rates and charge distribution across the barrier layer. We conclude that the outer and middle layers have negative space charges and the inner layer has positive space charges.     

Cage Structure Formation of Singly Doped Aluminum Cluster Cations Al<Subscript><Emphasis Type="BoldItalic">n</Emphasis></Subscript><Emphasis Type="BoldItalic">TM</Emphasis><Superscript>+</Superscript> (<Emphasis Type="BoldItalic">TM</Emphasis> = Ti,V, Cr)

Structural information on free transition metal doped aluminum clusters, Al _n TM ⁺ (TM = Ti, V, Cr), was obtained by studying their ability for argon physisorption. Systematic size (n = 5 – 35) and temperature (T = 145 – 300 K) dependent investigations reveal that bare Al _n ⁺ clusters are inert toward argon, while Al _n TM ⁺ clusters attach one argon atom up to a critical cluster size. This size is interpreted as the geometrical transition from surface-located dopant atoms to endohedrally doped aluminum clusters with the transition metal atom residing in an aluminum cage. The critical size, n _crit , is found to be surprisingly large, namely n _crit = 16 and n _crit = 19 – 21 for TM = V, Cr, and TM = Ti, respectively. Experimental cluster–argon bond dissociation energies have been derived as function of cluster size from equilibrium mass spectra and are in the 0.1–0.3 eV range.     

A chitosan-templated monolithic siliceous mesoporous-macroporous material

 We synthesised a porous siliceous material via hydrothermal hydrolysis of sodium silicate, using chitosan as a template. As far as we know, this is the first synthesis of siliceous porous material using chitosan as a template in a hydrothermal way. A fibrous material was obtained, whose macroscopic fibres were formed by a spongelike siliceous network with pores having a radius of 0.57 μm. The siliceous walls of the pores were, in turn, of the form of a microporous–mesoporous material; the pore radius distribution was polymodal with maxima at 0.84, 1.0, 1.2 and 1.5 nm and a broad band between 3 and 10 nm. This structure may be due to the aggregation of the hydrated chitosan helices in bundles of parallel fibres with different size and the gelation of the system. The aggregation process might be induced by the addition of silicate. Received: 12 January 2000/Accepted: 7 March 2000     

Electrochemical etching of silicon carbide

Both n- and p-type SiC of different doping levels were electrochemically etched by HF. The etch rate (up to 1.5 μm/min) and the surface morphology of p-type 6H-SiC were sensitive to the applied voltage and the HF concentration. The electrochemical valence of 6.3 ± 0.5 elementary charge per SiC molecule was determined. At p-n junctions (p-type layer on a n-type 6H-SiC substrate) a selective etching of the p-type epilayer could be achieved. For a planar 6H-4H polytype junction (n-type, both polytypes with equal doping concentrations) the 4H region was selectively etched under UV illumination. Thus polytype junctions could be marked by electrochemical etching. With HCl instead of HF no etching of SiC occurs, but a SiO₂ layer (thickness up to 8 μm) is formed by anodic oxidation. Received: 29 October 1998 / Accepted: 27 January 1999     

Composite film coatings based on epoxy-polysiloxane systems of cationic polymerization and their anticorrosion properties

The anticorrosion properties of epoxy-polysiloxane coatings on the surface of the aluminum alloy D-16 were studied by a potentiodynamic method. It was established that the use of the hybrid coating led to an increased corrosion resistance from 0.250 kΩm·cm² for uncoated alloy to 0.396-0.996 kΩm·cm² for the coated aluminum support. The yield of the sol fraction, the micro hardness, and the glass transition temperature of the polymers were determined.     

Sensing method based on impedance variation of minicolumn packed with cation-exchanger under electric field

Voltage-induced impedance variation of the minicolumn (i.d. 0.53 mm, length 2 mm) packed with cation exchanger was investigated to develop a sensing method. An aqueous sample solution containing the metal cations was continuously supplied to the minicolumn during the impedance measurement with the simultaneous application of both alternating current voltage (amplitude, 1.0 V; frequency, 200 kHz to 6 Hz) and direct current (DC) offset voltage (0.1 to 1.0 V). On a complex plane plot, the profile of the column impedance consisted of a semicircle (200 kHz to 100 Hz) and a straight line (<100 Hz), of which slope varied with the magnitude of the applied DC offset voltage (V _DC). The slope–V _DC relation depended on the kind of the metal cation and its concentration; in particular, the slope–V _DC relations of monovalent cations (Na⁺ and K⁺) and divalent ones (Mg²⁺ and Ca²⁺) were significantly different. With the change in the concentration of minor divalent salt of MgCl₂ or CaCl₂ (60 to 140 μM) in the sample solution containing 10 mM NaCl, the slopes showed almost linear relationships between those with application of V _DC = 0.1 V and 1.0 V both for magnesium and calcium additions. In the case of plural addition of both MgCl₂ and CaCl₂ to the solution, the data points in the slope_{0.1  V}–slope_{1.0  V} plot were located between the two proportional lines for single additions of magnesium and calcium, reflecting both the mixing ratio and net concentrations of the divalent cations. Thus, simulations determination of Mg²⁺ and Ca²⁺ can be attained on the basis of the slope_{0.1  V}–slope_{1.0  V} relation obtained by the impedance measurements of the minicolumn. Actually, the contents of both magnesium and calcium cations in the bottled mineral waters determined simultaneously using the proposed method were almost equivalent to those obtained by the atomic absorption spectrometric measurement.     

Colorimetric Microdetermination of Mercury(II) with Some Quinoxaline Azo Compounds in Presence of an Anionic Surfactant

Summary.  A new simple, rapid, sensitive, and selective method is proposed for the microdetermination of mercury. Mercury(II) forms insoluble complexes with 2,3-dichloro-6-(2-hydroxy-3,5-dinitrophenylazo)-quinoxaline (1), 2,3-dichloro-6-(5-amino-3-carboxy-2-hydroxy-phenylazo)-quinoxaline (2), 2,3-dichloro-6-(2,7-dihydroxynaphth-1-ylazo)-quinoxaline (3), and 2,3-dichloro-6-(3-carboxy-2-hydroxy-naphth-1-ylazo)-quinoxaline (4) in aqueous acidic medium; the complexes can be made soluble by the action of an anionic surfactant. The solution of the pink coloured compounds is stable for at least 24 h. Beer’s law is obeyed over the concentration range from 0.1 to 2.8 μg · cm⁻³ of mercury. For a more accurate analysis, Ringbom optimum concentration ranges were found to be 0.25–2.5 μg · cm⁻³. The molar absorpitivity, Sandell sensitivity, and relative standard deviations were also calculated. A slight interference from Pd²⁺ and Cd²⁺ is exhibited by the first three ligands, whereas the last one is only negligibly affected by these metal ions. Strong interference from Ag(I) is evident for all ligands, whereas alkali, alkaline earth, and other transition metals tested posed negligible interference. 15 μg · cm⁻³ of Cd²⁺ and Pd²⁺ or 10 μg · cm⁻³ of Ag⁺ can be tolerated if 1.0 mg of potassium bromide and 2.0 mg of citrate as masking agents are added for the determination of 1.5 μg · cm⁻³ of mercury(II). The method was applied to the determination of methyl- and ethylmercury chloride and the analysis of environmental water samples. Received August 7, 2000. Accepted (revised) October 18, 2000     

Transition structures and exo/endo stereoselectivities of concerted [6 + 4] cycloadditions with density functional theory

Density functional theory transition structures were located for three concerted [6 + 4] cycloaddition reactions involving cis-hexatriene and butadiene, cyclopentadiene and cycloheptatriene, and cyclopentadiene and tropone. Geometries, energies, and entropies were computed at the Becke3LYP/6-31G* level. The activation energy of the concerted [6 + 4] cycloaddition of hexatriene and butadiene is 33.3 kcal/mol, about 8 kcal/mol above the activation energy of the butadiene plus ethylene [4 + 2] cycloaddition. The endo concerted [6 + 4] transition state is 1.1 kcal/mol higher than the exo. The [6 + 4] reaction of cyclopentadiene and cycloheptatriene has a barrier of 25.9 kcal/mol, while the cyclopentadiene–tropone barrier drops to 20.7 kcal/mol. Received: 3 December 1998 / Accepted: 18 February 1999 / Published online: 7 June 1999     

Semiconducting properties of polypyrrole films in aqueous solution

The influence of the nature of the anions on the conductivity of polypyrrole films in aqueous solution was investigated by photocurrent spectroscopy combined with electrochemical impedance spectroscopy in dependence on the potential. As demonstrated, the conductivity of polypyrrole films at negative potentials can vary from a semiconducting to an ionic conducting state, depending on the size of the charge-compensating counter-anion incorporated during the electropolymerization. The reduced polypyrrole shows semiconducting properties when small anions are inserted, releasing the polymer matrix during the reduction process. The polymer can than be considered as a two-layer system, consisting of a semiconducting layer and a porous layer. Measurements at different thickness of polypyrrole films have shown that the position of the semiconducting layer is in the electrode/polymer interface. The ohmic resistance of the semiconducting layer is in the range 1–5 kΩ, the capacitance approaches a value of 100–500 nF and the flatband potential is −0.62 V_SCE. If large anions are incorporated, cation insertion takes place during reduction, the electrolyte content in the polymer then is relatively high and the polymer's behaviour is similar to that of an ionic conductor. The results are presented and discussed together with the example of methylsulfonate as a relatively small anion and polystyrenesulfonate as a large anion. Received: 28 July 1998 / Accepted: 22 February 1999     

Spectrophotometric Microdetermination of Some Pharmaceutically Impor tant Aminoquinoline Antimalarials, as Ion-Pair Complexes

 A simple, rapid, accurate and sensitive spectrophotometric method for the microdetermination of some pharmaceutically important aminoquinoline antimalarials, namely amodiaquine dihydrochloride (I), chloroquine phosphate (II) and primaquine phosphate (III) is described. The method is based on the interaction of these drugs with calmagite indicator to give highly coloured ion-pair complexes which exhibit maximum absorption at 663, 665 and 666 nm, respectively, Beer’s law is obeyed in the concentration ranges 1.0–25.0, 1.0–28.0 and 1.0–33.0 μg/ml for the drugs I, II and III, respectively. For more accurate analysis, the Ringbom optimum concentration ranges are 2.5–22.5, 2.0–26.0 and 3.0–30.0 μg/ml, respectively. The apparent molar absorptivities were calculated. Statistical treatment of the experimental results indicates that the method is sufficiently accurate and precise. The accuracy of the method is indicated by the recovery (99.8±1.4%) and the precision by the relative standard deviation (>1.5%). The proposed method has been applied to the determination of these drugs in certain formulations, with results that compared favourably with those obtained by the official methods. Received November 2, 1998. Revision February 29, 2000.     

Quantitation of Some Recently Introduced Antibacterial Drugs Using Sudan III as Chromogenic Reagent

 A simple, rapid, accurate and sensitive spectrophotometric method for the determination of norfloxacin (NRF), ofloxacin (OFL) and ciprofloxacin (CPF) is described. This method is based on the formation of an ion pair with sudan III in aqueous-acetone medium [40% (v/v) acetone]. The coloured products are measured at 567, 565 and 566 nm for NRF, OFL and CPF, respectively. The optimization of various experimental conditions is described. Beer’s law is obeyed in the range 0.4–12.0, 0.4–8.8 and 0.4–10.4  ;μg mL⁻¹ of NRF, OFL and CPF, respectively. For more accurate results, Ringbom optimum concentration ranges were 0.8–11.2, 0.6–8.5 and 0.8–10.0 μg mL⁻¹, respectively. The results obtained showed good recoveries of ±1.2, ±1.5 and ±1.7% with relative standard deviations of 0.67, 0.83 and 1.08% for NRF, OFL, and CPF, respectively. The molar absorptivity and Sandell sensitivity were also calculated. Applications of the proposed method to representative pharmaceutical formulations are successfully presented. Received April 30, 1999. Revision November 25, 1999.     

Flow-injection analysis for the determination of total inorganic carbon and total organic carbon in water using the H2O2–luminol–uranine chemiluminescent reaction

In the presence of carbonate and uranine, the chemiluminescent intensity from the reaction of luminol with hydrogen peroxide was dramatically enhanced in a basic medium. Based on this fact and coupled with the technique of flow-injection analysis, a highly sensitive method was developed for the determination of carbonate with a wide linear range. The method provided the determination of carbonate with a wide linear range of 1.0 × 10⁻¹⁰–5.0 × 10⁻⁶ mol L⁻¹ and a low detection limit (S/N = 3) of carbonate of 1.2 × 10⁻¹¹ mol L⁻¹. The average relative standard deviation for 1.0 × 10⁻⁹–9.0 × 10⁻⁷ mol L⁻¹ of carbonate was 3.7% (n = 11). Combined with the wet oxidation of potassium persulfate, the method was applied to the simultaneous determination of total inorganic carbon (TIC) and total organic carbon (TOC) in water. The linear ranges for TIC and TOC were 1.2 × 10⁻⁶–6.0 × 10⁻² mg L⁻¹ and 0.08–30 mg L⁻¹ carbon, respectively. Recoveries of 97.4–106.4% for TIC and 96.0–98.5% for TOC were obtained by adding 5 or 50 mg L⁻¹ of carbon to the water samples. The relative standard deviations (RSDs) were 2.6–4.8% for TIC and 4.6–6.6% for TOC (n = 5). The mechanism of the chemiluminescent reaction was also explored and a reasonable explanation about chemical energy transfer from luminol to uranine was proposed. Figure Chemiluminescence profiles in batch system. 1, Injection of 100 μL of K₂CO₃ into 1.0 mL luminol-1.0 mL H₂O₂ solution; 2-3 and 4-5, Injection in sequence of 100 μL of K₂CO₃ and 100 μL of uranine into 1.0 ml luminol-1.0 mL H₂O₂ solution; C_luminol = 1.0 × 10⁻⁷ mol/L, C_H2O2 = 1.0 × 10⁻⁵ mol/L, C_uranine = 1.0 × 10⁻⁵ mol/L, C_K2CO3 = 1.0 × 10⁻⁷ mol/L except for 4-5 where C_K2CO3 = 1.0 × 10⁻⁴ mol/L     

Polycarbazole-based electrochemical transistor

A polycarbazole conducting polymer transistor has been constructed having the dimensions 1 cm × 2 cm × 1 mm. Polycarbazole film used here has a redox potential of 1.30 V. Polymer-coated platinum plates were used as the source and drain. The inter-electrode spacing of the device is typically of the order of 200–500 μm to minimise the internal resistance. The high saturation current region of the transistor in the most positive bias voltage (1.3 V), with negligible hysteresis and greater stability, appears to give a device that is superior to other conducting polymer transistors. Received: 27 May 1997 / Accepted: 17 September 1997     

Performance and scaling of a 1.2 V/1.5 Ah nickel/metal hydride cell to a 6 V/1.5 Ah battery

A 1.2 V/1.5 Ah positive-limited nickel/metal hydride cell has been studied to determine its charge-discharge characteristics at different rates in conjunction with its AC impedance data. The faradaic efficiency of the cell is found to be maximum at ∼70% charge input. The cell has been scaled to a 6 V/1.5 Ah battery. The cycle-life data on the battery suggest that it can sustain a prolonged charge-discharge schedule with little deterioration in its performance. Received: 17 November 1998 / Accepted: 5 February 1999     

Study on the Catalytic Determination of Vanadium (V) Using the Rhodamine 6G-Periodate Redox Reaction and its Analytical Application

 A fluorescence quenching method for the determination of vanadium (V) based on the vanadium- catalyzed oxidation of rhodamine 6G (R6G) with periodate in the presence of ethylenediaminetetraacetic acid disodium salt (EDTA) in sulfuric acid medium is described. The fluorescence was measured with excitation and emission wavelengths of 525 and 555 nm, respectively. The calibration graph for vanadium (V) had linear ranges of 3.0 × 10⁻⁹–1.5 × 10⁻⁸ mol/l and 1.5 × 10⁻⁸–4.0 × 10⁻⁸ mol/l, respectively. The detection limit was 1.7 × 10⁻⁹ mol/l. The proposed method was successfully applied to the determination of vanadium (V) in river water, rain water and cast iron samples. Received June 29, 2001 Revision October 9, 2001     

Development of membrane electrodes for selective determination of some antiepileptic drugs in pharmaceuticals,plasma and urine

Newly developed, simple, low-cost and sensitive ion-selective electrodes have been proposed for determination of some antiepileptic drugs such as lamotrigine, felbamate, and primidone in their pharmaceutical preparations as well as in biological fluids. The electrodes are based on poly(vinyl chloride) membranes doped with drug–tetraphenyl borate (TPB) or drug–phosphotungstic acid (PT) ion-pair complexes as molecular recognition materials. The novel electrodes displayed rapid Nernstian responses with detection limits of approximately 10⁻⁷ M. Calibration graphs were linear over the ranges 5.2 × 10⁻⁷–1.0 × 10⁻³, 1.5 × 10⁻⁶–1.0 × 10⁻³, and 2.6 × 10⁻⁷–1.0 × 10⁻³ M for drug–TPB and 5.8 × 10⁻⁷–1.0 × 10⁻³, 1.8 × 10⁻⁷–1.0 × 10⁻³, and 6.6 × 10⁻⁷–1.0 × 10⁻³ M for drug–PT electrodes, respectively, with slopes ranging from 52.3 to 62.3 mV/decade. The membranes developed have potential stability for up to 1 month and proved to be highly selective for the drugs investigated over other ions and excipients. The results show that the selectivity of the ion-selective electrodes is influenced significantly by the plasticizer. The proposed electrodes were successfully applied in the determination of these drugs in pharmaceutical preparations in four batches of different expiry dates. Statistical Student’s t test and F test showed insignificant systematic error between the ion-selective electrode methods developed and a standard method. Comparison of the results obtained using the proposed electrodes with those found using a reference method showed that the ion-selective electrode technique is sensitive, reliable, and can be used with very good accuracy and high percentage recovery without pretreatment procedures of the samples to minimize interfering matrix effects. Figure Structure of lamotrigine, felbanate and primidone     

Simultaneous determination of phenylenediamine isomers and dihydroxybenzene isomers in hair dyes by capillary zone electrophoresis coupled with amperometric detection

The simultaneous determination of three isomers of phenylenediamines (o, m, and p-phenylenediamine) and two isomers of dihydroxybenzenes (catechol and resorcinol) in hair dyes was performed by capillary zone electrophoresis coupled with amperometric detection (CZE–AD). The effects of working electrode potential, pH and concentration of running buffer, separation voltage, and injection time on CZE–AD were investigated. Under the optimum conditions the five analytes could be perfectly separated in 0.30 mol L⁻¹ borate–0.40 mol L⁻¹ phosphate buffer (pH 5.8) within 15 min. A 300 μm diameter platinum electrode had good responses at +0.85 V (versus SCE) for the five analytes. Their linear ranges were from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol L⁻¹ and the detection limits were as low as 10⁻⁷ mol L⁻¹ (S/N = 3). This working electrode was successfully used to analyze eight kinds of hair dye sample with recoveries in the range 91.0–108.0% and RSDs less than 5.0%. These results demonstrated that capillary zone electrophoresis coupled with electrochemical detection using a platinum working electrode as detector was convenient, highly sensitive, highly repeatable and could be used in the rapid determination of practical samples. Figure Electropherograms obtained from 10 mg mL⁻¹ hair dye sample solutions at a platinum working electrode under optimum CZE–AD conditions: (a) natural black (I), (b) golden: (1) p-phenylenediamine, (2) m-phenylenediamine, (3) o-phenylenediamine, (4) resorcinol, and (5) catechol