The atmospheric oxidation of ethyl formate and ethyl acetate over a range of temperatures and oxygen partial pressures

The Cl‐atom‐initiated oxidation of two esters, ethyl formate [HC(O)OCH₂CH₃] and ethyl acetate [CH₃C(O)OCH₂CH₃], has been studied at pressures close to 1 atm as a function of temperature (249–325 K) and O₂ partial pressure (50–700 Torr), using an environmental chamber technique. In both cases, Cl‐atom attack at the CH₂ group is most important, leading in part to the formation of radicals of the type RC(O)OCH(O^?)CH₃ [R = H, CH₃]. The atmospheric fate of these radicals involves competition between reaction with O₂ to produce an anhydride compound, RC(O)OC(O)CH₃, and the so‐called α‐ester rearrangement that produces an organic acid, RC(O)OH, and an acetyl radical, CH₃C(O). For both species studied, the α‐ester rearrangement is found to dominate in air at 1 atm and 298 K. Barriers to the rearrangement of 7.7 ± 1.5 and 8.4 ± 1.5 kcal/mole are estimated for CH₃C(O)OCH(O^?)CH₃ and HC(O)OCH(O^?)CH₃, respectively, leading to increased occurrence of the O₂ reaction at reduced temperature. The data are combined with those obtained from similar studies of other simple esters to provide a correlation between the rate of occurrence of the α‐ester rearrangement and the structure of the reacting radical. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 397–413, 2010     

Time-resolved stimulated emission spectroscopy in the ultrashort domain through pump-probe experiments

The photoemission properties of fluorescent chromophores have a widespread application in many fields ranging from chemical-physics and biology to organic light emitting devices. These systems usually display high fluorescence conversion efficiency, which makes them suitable for transient/gain experiments also in liquid solutions, thin films and eventually in protein environments.Pump and probe methods have been widely employed for wavelength-resolved spectroscopy in the subpicosecond time scale. In our group, we have recently assembled a new experimental setup for pump and probe spectroscopy: preliminary tests on the Rhodamine B dye in ethanol have been performed in order to optimize the setup. The dynamic response of photoinduced changes of the chromophore dispersed into a suitable solvent has been studied with a subpicosecond time resolution.The optically prepared initial state of the Rhodamine B in ethanol solution appears to evolve on a timescale of few picoseconds into a successive state, which could be attributed to an intramolecular charge transfer state.     

Flow injection spectrophotometric determination of reducing sugars using a focalized coiled reactor in a domestic microwave oven

A flow injection (FI) spectrophotometric procedure for determining reducing sugars content in sugar cane juices using a focalized PTFE coiled reactor positioned at the output antenna of a domestic microwave oven at 700 W is proposed. In this system, sample solution converge to 1.0 mol l(-1) NaOH and 5.2 mmol l(-1) K(3)Fe(CN)(6) solutions previously mixed and the decrease of hexacyanoferrate(III) concentration was monitored at 420 nm. Under best analytical conditions, there was a direct relationship between absorbance decrease and reducing sugar content (fructose plus glucose concentrations) in the concentration range from 50 to 1200 micromol l(-1) with a detection limit of 15 micromol l(-1). The relative standard deviations (rds) were less than 1.4% for ten injection of 400 and 800 micromol l(-1) fructose solution and the analytical frequency was 70 h(-1). A paired t-test showed that all results obtained for sugar cane juices using this FI procedure and the Somogyi-Nelson batch procedure agree at the 95% confidence level.     

Simultaneous voltammetric determination of paracetamol and caffeine in pharmaceutical formulations using a boron-doped diamond electrode

A simple and highly selective electrochemical method was developed for the single or simultaneous determination of paracetamol (N-acetyl-p-aminophenol, acetaminophen) and caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) in aqueous media (acetate buffer, pH 4.5) on a boron-doped diamond (BDD) electrode using square wave voltammetry (SWV) or differential pulse voltammetry (DPV). Using DPV with the cathodically pre-treated BDD electrode, a separation of about 550 mV between the peak oxidation potentials of paracetamol and caffeine present in binary mixtures was obtained. The calibration curves for the simultaneous determination of paracetamol and caffeine showed an excellent linear response, ranging from 5.0 × 10⁻⁷ mol L⁻¹ to 8.3 × 10⁻⁵ mol L⁻¹ for both compounds. The detection limits for the simultaneous determination of paracetamol and caffeine were 4.9 × 10⁻⁷ mol L⁻¹ and 3.5 × 10⁻⁸ mol L⁻¹, respectively. The proposed method was successfully applied in the simultaneous determination of paracetamol and caffeine in several pharmaceutical formulations (tablets), with results similar to those obtained using a high-performance liquid chromatography method (at 95% confidence level).     

Determination of the chemical oxygen demand (COD) using a copper electrode: a clean alternative method

A clean alternative method for the determination of chemical oxygen demand (COD) was developed using a copper electrode as an electrocatalytic sensor. The measuring principle is based on oxidation current of organic compounds in the wastewater. The effects of important experimental conditions, such as electrolytic solution concentration and potential scan rate, on analytical performance have been investigated. Analytical linear range of 53.0–2,801.4 mg l⁻¹ COD with detection limit of 20.3 mg l⁻¹ COD was achieved. The procedure was successfully applied to the COD determination in wastewater from soft industries. The results obtained using the proposed method were in good agreement with those obtained using the conventional (i.e., dichromate) COD method. In this fashion, the COD value of a sample can be determined in a simple, rapid, accurate manner, and the end products do not contain toxic metals.     

Phenol removal from aqueous solution by carbon xerogel

Carbon xerogel (CX) was used for phenol adsorption from aqueous solution. CX was synthesized by sol?Cgel polycondensation of resorcinol with formaldehyde using sodium carbonate (Na₂CO₃) as catalyst. Then, it was dried by convective drying technique and pyrolyzed under inert atmosphere. Phenol adsorption kinetics was very fast, what was attributed to the presence of open pore structure. The kinetic studies showed that the adsorption process could be fitted to a pseudo-second-order model and the particle diffusion process is the rate-limiting step of the adsorption. The phenol removal was maximum and unaffected by pH changes when the initial pH of the phenol solution was in the range of 3?C8. The optimum adsorbent dose obtained for phenol adsorption onto CX was 0.075?g/50?cm³ solution. The Langmuir model described the adsorption process better than the Freundlich isotherm model and the monolayer adsorption capacity is 32?mg?g^?1. Among the desorbing solutions used in this study, the most efficient desorbent was EtOH (100?%) which released about 87?% of phenol bound with the CX.     

Determination of Atrazine in Natural Water Samples by Differential Pulse Adsorptive Stripping Voltammetry Using a Bismuth Film Electrode

A new method using differential pulse adsorptive stripping voltammetry for the determination of atrazine (ATZ) in natural water samples using a bismuth film electrode (BiFE) is proposed. The calibration curve was linear in the atrazine concentration range from 6.7×10^?7 to 2.0×10^?5 mol L^?1, with a limit of detection (LOD) of 1.4×10^?7 mol L^?1. The proposed electrode was applied for atrazine determination with satisfactory results compared with a high‐performance liquid chromatography method (HPLC).