Sensor film for Vitamin C determination based on absorption properties of polyaniline

An analytical method based on the absorption changes of chemically polymerised polyaniline at 700 nm is proposed for the determination of Vitamin C. Vitamin C produces a polyaniline film reduction, originating changes in its absorbance proportional to the Vitamin C concentration. The optimum reaction conditions and the analytical characteristics have been studied. The linear response of the method ranged from 0.10 to 1.0 mg l⁻¹ for a 6 min reaction time and from 1.0 to 8.0 mg l⁻¹ for a 2 min reaction time. Reproducibility, expressed as the coefficient of variation, was 0.8% (6 min reaction time) and 2.3% (2 min reaction time) (n = 10). The method has been applied to Vitamin C determination in pharmaceutical preparations and commercial fruit juices. The results were compared with those obtained by the 2,6-dichlorophenolindophenol titration method (the AOAC Official Method) and no systematic errors were observed.     

Kinetic-Spectrophotometric Determination of Trace Quantities of Thiocyanate Based on Its Landolt Effect on the Reaction of Bromate with Hydrochloric Acid1

A new method for the rapid and sensitive determination of trace quantities of thiocyanate based on its Landolt effect on the bromate-hydrochloric acid reaction was developed. The induction period of the reaction is proportional to the SCN^– concentration. The decolorization of methyl orange by the reaction products was used to monitor the reaction spectrophotometrically at 525 nm. We were able to determine thiocyanate in the range 2 × 10^–7–4 × 10^–5 M by this method. The relative standard deviation for 10 determinations of 1.5 × 10^–6 M thiocyanate ion is 0.19% and the detection limit of the method was 7.00 × 10^–8 M. The method was applied to the determination of thiocyanate in human blood serum and of saliva samples with satisfactory results.     

Flow-Injection Spectrophotometric Determination of Hydrazine

A flow-injection spectrophotometric method for the determination of hydrazine is described. The method is based on the inhibitory effect of hydrazine on the reaction of thionine with nitrite in acidic media. The decolorization of thionine by the reaction with nitrite was used to monitor the reaction spectrophotometrically at 602 nm. The variables that affected the reaction rate were fully investigated and the optimum conditions were established. Hydrazine can be determined in the range 2.0–40.0 μg/ml with a limit of detection of 1.0 μg/ml. The relative standard deviation for 10 replicate determinations of 7.0 μg/ml hydrazine is 3.3%. The method is simple, rapid, and widely applicable.     

Microwave-assisted synthesis of α-hydroxy aromatic ketones from α-bromo aromatic ketones in water

A reaction ofα-bromo aromatic ketones in water with microwave irradiation gave the correspondingα-hydroxy aromatic ketones in good yields.The use of microwaves was found to significantly improve yields and shorten the reaction time.This reaction afforded a very clean,convenient method for the synthesis ofα-hydroxy aromatic ketones.     

Kinetic Spectrophotometric Determination of Hydrazine with Neutral Red-Nitrite System

 A new kinetic spectrophotometric method for the determination of hydrazine is described. The method is based on the inhibitory effect of hydrazine on the reaction of neutral red with nitrite in acidic media and 28 °C. A product from the reaction of neutral red with nitrite was used to monitor the reaction spectrophotometrically at 352 nm. Hydrazine can be determined in the range 4.7×10⁻⁶∼3.1×10⁻⁵mol/L with a detection limit of 3.1×10⁻⁶mol/L. The method was applied to the determination of hydrazine in water samples. Received July 23, 1999. Revision January 10, 2000.     

Kinetic study and analytical application of the iodate-arsenite reaction in strongly acidic solutions

The iodate-arsenite reaction in strongly acidic solutions was studied by stopped-flow spectrophotometric techniques. The molar absorptivity of iodine and the activation energy of the reaction were determined. A method for determining arsenite was developed on the basis of this reaction. The method was checked in the concentration range 5.0·10^-4–1.0·10^-2M, the mean error being about 3%.     

Indirect Kinetic–Spectrophotometric Determination of Resorcinol, Catechol, and Hydroquinone

A rapid, simple, and sensitive method for the determination of trace amounts of resorcinol, catechol, and hydroquinone in aqueous media has been proposed. The method is based on their reaction with nitrite. The excess nitrite is then determined by a kinetic method based on its reaction with Neutral Red. The reaction is monitored spectrophotometrically by measuring the decrease in the absorbance at 530 nm by a fixed time method. Resorcinol, catechol, and hydroquinone could be determined by the proposed method in the ranges of 0.1–2.0 mg/mL, 0.25–2.0 mg/mL and 0.1–3.0 mg/mL, respectively. The method was applied to the determination of resorcinol and catechol in pharmaceutical formulations with satisfactory results.     

Radical deoxygenation of alcohols and intermolecular carbon-carbon bond formation with surfactant-type radical chain carriers in water

An efficient and mild method is developed for radical deoxygenation of alcohols and formation of carbon-carbon bonds in water without adding additives such as surfactants. The reaction was applied to synthesis of 2′,3′-didehydro-2′,3′-dideoxynucleosides that are potent anti-HIV agents. The reaction afforded environmentally benign reaction conditions.     

Study on the reaction kinetics between PBT and epoxy by a novel rheological method

In the present study, the reaction kinetics of polybutylene terephthalate (PBT) and epoxy system was studied by a novel rheological method. The reaction process was determined by rheological test and the results showed that there were three stages in the reaction between PBT and epoxy, which were reaction-controlling stage (stage I), reaction-stagnation stage (stage II) and diffusion-controlling stage (stage III). In addition, the stage I was selected to study the reaction kinetics by the rheological method. The results showed that the reaction between PBT and epoxy could be classified as a pseudo-first-order reaction due to the excessive amount of epoxy group. Furthermore, the reaction apparent activation energy of the stage I determined by the rheological method was 143 kJ/mol. To confirm these results, the reaction kinetics was also evaluated by the endgroup determination method, and the results showed that the reaction could also be classified as a pseudo-first-order reaction. Moreover, the apparent activation energy of the reaction was 116 kJ/mol, which was similar to that of the value obtained by the rheological method.     

4-氟苯甲醛、β-萘胺和Meldrum酸一锅反应生成1-(4-氟苯基)-1,2-二氢苯并[f]喹啉-3(4H)-酮的反应机理理论研究

采用密度泛函理论(DFT)研究了4-氟苯甲醛、β-萘胺和Meldrum酸一锅反应生成1-(4-氟苯基)-1,2-二氢苯并[f]喹啉-3(4H)-酮的微观反应机理.在B3LYP/6-311G*基组水平上优化了反应物、过渡态、中间体及产物的几何构型,通过振动分析确认了过渡态的结构,并用内禀反应坐标(IRC)确认反应途径.应用分子中的原子理论(AIM)分析了这些物质的成键特征.采用SCRF(PCM)方法研究了反应体系的溶剂化效应.报道了可能的反应路径,其中Re→TS1→IM1→TS2→IM2→TS3→IM3→TS4→IM5→TS7→IM9→TS13→IM10→TS14→P3具有相对较低的活化能,是反应的主要通道,理论预测的主要产物与实验吻合.     

Flow-injection chemiluminescence determination of dihydralazine sulfate based on hexacyanoferrate(III) oxidation sensitized by eosin Y

A novel flow-injection chemiluminescence (CL) method for the determination of dihydralazine sulfate (DHZS) is described. The method is based on the reaction between DHZS and hexacyanoferrate(III) in alkaline solution to give weak CL signal, which is dramatically enhanced by eosin Y. The CL emission allows quantitation of DHZS concentration in the range 0.02-2.8 μg ml⁻¹ with a detection limit (3σ) of 0.012 μg ml⁻¹. The experimental conditions for the CL reaction are optimized and the possible reaction mechanism is discussed. The method has been applied to the determination of DHZS in pharmaceutical preparations and compared well with the high performance liquid chromatography (HPLC) method.     

Solvent-free asymmetric direct aldol reactions organocatalysed by recoverable (Sa)-binam-l-prolinamide

The combination of (S_a)-binam-l-Pro (5 mol %) and benzoic acid (10 mol %) was used as catalysts in the direct aldol reaction between different aliphatic ketones and 4-nitrobenzaldehyde under solvent-free reaction conditions. Three different procedures are assayed: magnetic stirring (method A), magnetic stirring after previous dissolution in THF and evaporation (method B), and ball mill technique (method C), methods A and B being the simplest. These reaction conditions allowed us to reduce the amount of required ketone to 2 equiv to give the aldol product in similar reaction times and regio-, diastero-, and enantioselectivities than in organic or aqueous solvents.     

Ultrasound-assisted rapid synthesis of <Emphasis Type="Italic">β</Emphasis>-aminoketones with direct-type catalytic Mannich reaction using bismuth(III) triflate in aqueous media at room temperature

An innovative, powerful, efficient and relatively rapid method was developed to synthesise various β-aminoketone derivatives from cyclohexanone, substituted aromatic amines and aromatic or hetero-aromatic aldehydes via ultrasound-assisted direct-type catalytic Mannich reaction using bismuth(III) triflate in water. Good yields of the desired β-aminoketones were obtained at room temperature by ultrasound-assisted reaction within 1–2 h. The major advantages of the proposed method are undemanding conditions, easy operation, low toxicity, shorter reaction time, anti selectivity and higher yields in comparison with conventional methods.     

A novel and specific fluorescence reaction for uracil

Facile and specific methods to quantify a nucleobase in biological samples are of great importance for diagnosing disorders in nucleic acid metabolism. In the present study, a novel fluorogenic reaction specific for uracil has been developed. The reaction was carried out in an alkaline medium containing benzamidoxime and K₃[Fe(CN)₆] which were heated for 2.0 min. Under the optimum reaction conditions, strong fluorescence was produced only from uracil, not from other many biogenic compounds tested such as cytosine, thymine, adenine, guanine, nucleobases, nucleosides, nucleotides, amino acids, saccharides, creatine, creatinine and urea. The sensitivity of this method was compared with a known fluorogenic reaction using phenacylbromide which does not react with uracil but reacts with cytosine, adenine and their analogues. The proposed uracil-specific reaction showed approximately 400-fold higher sensitivity than the phenacylbromide reaction. The lower detection limit of uracil by the present method was 100 pmol mL⁻¹, and a good linearity of the calibration curve was obtained up to 100 nmol mL⁻¹ uracil. Due to its high sensitivity and specificity, the quantitative determination of uracil was possible by the proposed fluorimetric method.     

Kinetic Spectrophotometric Determination of Oxalic Acid Based on the Catalytic Oxidation of Bromophenol Blue by Dichromate

 A sensitive catalytic method is developed for the spectrophotometric determination of oxalic acid. It is based on the catalytic action of oxalic acid on a new indicator reaction – the oxidation of Bromophenol Blue by dichromate in dilute sulfuric acid medium. The reaction rate is monitored spectrophotometrically by measuring the absorbance at 600 nm after quenching the reaction with sodium hydroxide. A calibration graph from 0.1 to 8.0 μg mL⁻¹ of oxalic acid and a detection limit of 0.04 μg mL⁻¹ was obtained. The applicability of this method was demonstrated by the determination of oxalic acid in water extracts from vegetables such as spinach, mushrooms and fresh kidney beans. Received October 18, 1999. Revision June 14, 2000.     

RISM-SCF study of the free-energy profile of the Menshutkin-type reaction NH3+CH3Cl→NH3CH3++Cl− in aqueous solution

The free-energy profile for the Menshutkin-type reaction NH₃ + CH₃Cl → NH₃CH₃ ⁺ + Cl⁻ in aqueous solution is studied using the RISM-SCF method. The effect of electron correlation on the free-energy profile is estimated by the RISM-MP2 method at the HF optimized geometries along the reaction coordinate. Solvation was found to have a large influence on the vibrational frequencies at the reactant, transition state and product; these vibrational frequencies are utilized to calculate the zero-point energy correction of the free-energy profile. The computed barrier height and reaction exothermicity are in reasonable agreement with those of experiment and previous calculations. The change of solvation structure along the reaction path is represented by radial distribution functions between solute-solvent atomic sites. The mechanisms of the reaction are discussed from the view points of solute electronic and solvation structures. Received: 26 June 1998/Accepted: 28 August 1998 / Published online: 2 November 1998     

Kinetic Spectrofluorimetric Determination of Nitrite in Water Samples and Nitrogen Dioxide in the Atmosphere Sampled by the Liquid Droplet Method

 A simple, sensitive and selective kinetic spectrofluorimetric determination of NO₂ sampled by the droplet method in the atmosphere was proposed on the basis of the reaction of safranin O with nitrite. By this reaction, a diazonium salt is formed, which causes a fluorescent reddish-orange dye color of the solution to change into a non fluorescent blue color. The reaction was monitored fluorimetrically by measuring the decrease in fluorescence intensity of safranin O at (λ_ex/λ_em = 536 nm/579 nm) by a fixed-time method. The experimental conditions were optimized. Under the optimum conditions in the concentration range of 7.5–400 ng/ml, a linear calibration curve (r² = 0.9978) was obtained with a detection limit of 7.5 ng/ml. The method was applied successful ly to the determination of nitrite in spiked water and NO₂ in the atmosphere, as sampled by a liquid droplet method. Received August 23, 1999. Revision February 29, 2000.     

Highly sensitive spectrofluorimetric kinetic determination of ultratrace amounts of vanadium(V) based on the oxidation of 1,8-diaminonaphthalene by bromate

A highly sensitive catalytic quenching spectrofluorimetric method was described for the determination of V(V) based on its catalytic effect on the oxidation of 1,8-diaminonaphthalene by potassium bromate with Tiron as an activator in weakly acidic medium and the reaction mechanism was investigated. The reaction was followed spectrofluorimetrically by measuring the fluorescence intensity of 1,8-diaminonathphlene (DAN) (λ_ex=356 nm, λ_em=439 nm) at a fixed time of 5 min from initiation of the reaction. Under the optimum conditions, vanadium(V) can be determined in the range 0.05-50.0 ng ml⁻¹ with a S.D.=0.024 for 15 times measurements. The detection limit of the method was down to 0.0088 ng ml⁻¹ and the catalytic reaction activation energy was found to be 43.92 kJ mol⁻¹. The proposed method was tested for the determination of vanadium(V) in rice and natural water samples.     

Development of a spectrophotometric Sequential Injection Analysis (SIA) procedure for determination of ammonium: A Response Surface Methodology (RSM) approach

This paper describes the optimization and use of a Sequential Injection Analysis (SIA) procedure for ammonium determination in waters. Response Surface Methodology (RSM) was used as a tool for optimization of a procedure based on the modified Berthelot reaction. The SIA system was designed to (i) prepare the reaction media by injecting an air-segmented zone containing the reagents in a mixing chamber, (ii) to aspirate the mixture back to the holding coil after homogenization, (iii) drive it to a thermostated reaction coil, where the flow is stopped for a previously established time, and (iv) to pump the mixture toward the detector flow cell for the spectrophotometric measurements. Using a 100 μmol L^− 1 ammonium solution, the following factors were considered for optimization: reaction temperature (25 - 45 °C), reaction time (30 - 90 s), hypochlorite concentration (20 - 40 mmol L^− 1), nitroprusside concentration (10 - 40 mmol L^− 1) and salicylate concentration (0.1 - 0.3 mol L^− 1). The proposed system fed the statistical program with absorbance data for fast construction of response surface plots. After optimization of the method, figures of merit were evaluated, as well as the ammonium concentration in some water samples. No evidence of statistical difference was observed in the results obtained by the proposed method in comparison to those obtained by a reference method based on the phenol reaction.     

Investigation of reaction products of sulphuric acid with ilmenite

The reaction of sulphuric acid with titanium raw materials runs violently with simultaneous emission of gases. Such run of reaction creates danger of explosion. This process is very complicated from the reason of complexity of reactions and variety of compounds in reaction mixture. To determine safe conditions of reaction with high efficiency, it is necessary to investigate phase composition products of reaction. Products of reaction were investigated by using X-ray diffraction. However reaction products show high amorphy and this is the reason of difficulties to determine all phases. For this reason also was used additional method — ‘drop’ calorimeter. This method is used for determination of average specific heat of liquid or solid samples. In this case, this method was used for verification of phase composition of products of reaction. On the basis of investigation the phase composition of reaction product is following: titanyl sulfate, iron(III) sulfate, monohydrated iron(II) sulfate, magnesium sulfate and unreacted remainders of ilmenite and magnesium silicate.