Search results for " kinetics"
showing 10 items of 285 documents
Ascorbic acid degradation kinetics in mushrooms in a high-temperature short-time process controlled by a thermoresistometer
2004
Abstract The degradation of ascorbic acid was studied in mushrooms heated at temperatures between 110 and 140°C, high-temperature short-time conditions, in a five-channel computer-controlled thermoresistometer. The kinetics parameters were calculated on the assumption that there are 2 degradation mechanisms, one aerobic (during the first few seconds of the process) and the other anaerobic. The 2 stages followed first-order reaction kinetics, with E a =46.36 kJ/mol for aerobic degradation and E a =49.57 kJ/mol for anaerobic degradation.
High accuracy [sup 18]O(p,α)[sup 15]N reaction rate in the 8⋅10[sup 6]–5⋅10[sup 9] K temperature range
2011
The 18O(p,α)15N reaction is of great importance in several astrophysical scenarios, as it influences the production of key isotopes such as 19F, 18O and 15N. In this work, a high accuracy 18O(p,α)15N reaction rate is proposed, based on the simultaneous fit of direct measurements and of the results of a new Trojan Horse experiment. In particular, we have focused on the study of the broad 660 keV 1/2+ resonance. Since Γ∼100–300 keV, it strongly influences the nearly‐zero‐energy region of the cross section by means of the low‐energy tail of the resonant contribution and dominates the cross section at higher energies. Here we provide a factor of 2 larger reaction rate above T∼0.5 109 K based ov…
Vergleichende reaktionskinetische Untersuchungen mit standardisiertem menschlichen Renin an vier angereicherten tierischen Reninsubstraten
1970
Zur Charakterisierung der Angiotensinbildung aus vier angereicherten tierischen Reninsubstraten (Rind, Schwein, Hund und Ratte) mit menschlichem Renin wurde das Temperatur- und pH-Verhalten untersucht, sowie die Michaelis-Menten-Konstanten und maximalen Reaktionsgeschwindigkeiten bestimmt.
Die thermische polymerisation von methylmethacrylat, 2. Bildung des ungesättigten dimeren
1980
The spontaneous thermal polymerization of methyl methacrylate (MMA) is accompanied by the production of serveral oligomers among which a linear unsaturated dimer H-1 (dimethyl 1-hexene-2,5-dicarboxylate) is predominant. The reaction kinetics of this dimer formation was investigated in bulk and in solution. The temperature dependence of the second order dimerization constant was determined as Reaction mechanisms for the thermal dimer formation of MMA are discussed.
Data Treatment in Kinetics
2000
Chemical kinetics is an important part of chemistry devoted to study how reactions proceed and quantify the rate of the process. The reaction mechanism is the chemical model that describes how the chemical change occurs. From the proposed mechanism, a mathematical model can be obtained to explain the evolution of the chemical species with time. In many cases, the mechanism can be simplified to a single rate law that relates the reaction rate with concentrations. In the last decades, the study of kinetic systems has benefited from the development of instrumentation, the increasing availability of specialized computer software, and the advances in data treatment techniques. A comprehensive re…
Study of solid state kinetics using voltammetry of immobilized particles. Application to tetragonal to monoclinic transition in nanoparticulate zirco…
2012
Abstract The voltammetry of immobilized particles methodology is applied to study solid state reaction kinetics on the basis of the electrocatalytic ability of solids toward selected electrochemical processes. Measurement of the time variation of catalytic current for oxygen evolution reaction in aqueous alkaline media provides a direct estimate of fractional conversion of the reactant in the course of the reaction for testing different reaction kinetic models. This methodology is applied to analyze the formation of monoclinic zirconia and praseodymia-doped zirconia from tetragonal precursors. Discrimination between competing and successive reactions mechanisms is obtained for reactions inv…
The influence of water and of alkali promotor on the carbon number distribution of fischer-tropsch products formed over iron catalysts
1987
The carbon number distribution of Fischer-Tropsch products formed over an alkalized precipitated iron catalyst has been studied as a function of the water vapor pressure of the synthesis gas. The carbon number distribution of formed hydrocarbons is characterized by a bimodal Schulz-Flory distribution of growth probabilities P1 ≈ 0.6 and P2 ≈ 0.87 attributed to unpromoted and promoted (alkalized) sites on the catalyst surface. Promoted sites are more stable with respect to oxidation (deactivation) by water than unpromoted sites. The growth probability of unpromoted sites decreases with increasing ratio P/P — Studies using Mossbauer spectroscopy have shown that iron foils treated with water c…
Chaotic dynamics in an unstirred ferroin catalyzed Belousov–Zhabotinsky reaction
2009
Abstract The Belousov–Zhabotinsky (BZ) reaction is the best known example of far from equilibrium self-organizing chemical reaction. Among the many dynamical behaviors that this reaction can exhibit, spatio-temporal chaos attracted particular interest, both for the ferroin and cerium catalyzed systems. In recent years transient chaos was found in the cerium catalyzed BZ reaction, when conducted in batch and unstirred reactors. It was established that the chaotic oscillations, originated by the coupling among chemical kinetics and transport phenomena, appeared and disappeared following a Ruelle–Takens–Newhouse scenario. In this Letter, we show results about the ferroin catalyzed system condu…
Convective mass transfer to partially recessed and porous electrodes
2003
Abstract The diffusional problem of a rotating porous electrode has been analysed based on the mass transfer equations for a partially blocked electrode. It is shown that the porous geometry leads to a dependence of the current on rotation rate identical to that corresponding to a coupled diffusion-activated electron transfer mechanism. The apparent rate constant, however, is related only to the geometry of the porous surface. It is shown that the characteristic diffusional length corresponds to the pore dimension modified by a term including the transition from linear to spherical diffusional geometry at the pore entrance. The theory is compared with experimental results for the reduction …
Cr(VI) sorption by intact and dehydrated Candida utilis cells: differences in mechanisms
2001
The Cr(VI) sorption kinetics by intact and dehydrated Candida utilis cells were studied. Yeast dehydration led to enhanced Cr(VI) sorption activity by cells during the first 15–20 min of rehydration. In experiments with K2Cr2O7 as a chromium source, [Cr2O7]2− was converted into [CrO4]2− during incubation with dehydrated biomass. An effect of the considerably enhanced Cr(VI) sorption activity by dehydrated cells in the buffered media at pH≈8.0 can serve as another confirmation of the different sorption mechanisms of intact and dehydrated C.utilis cells. This effect was not revealed in water solution at pH 8.0. This phenomenon will be studied in further investigations.