Initial Rate Method Research Articles

Kinetic Potentiometry as a Method for Studying the Interactions of Antioxidants with Peroxyl Radicals.

This work presents a new method using kinetic potentiometry to study the thermodynamic and kinetic parameters of the reactions of antioxidants with peroxyl radicals. The rate constants of the reaction of antioxidants with radicals have been determined, and the groups of "fast" and "slow" antioxidants have been conventionally distinguished. Fast antioxidants include ascorbic, uric, gallic, chlorogenic, caffeic acids, glutathione, L-cysteine, and catechol with constant values from (1.05-9.25) × 103 M·s-1; "slow" antioxidants are α-tocopherol (in aqueous media), ionol, 2,6-ditretbutylphenol, and compounds of the azoloazine series, modified with polyphenolic fragments, with constant values from (4.00-8.50) × 102 M·s-1. It is shown that the value of the rate constant is directly related to the type of kinetic dependence of the potential recorded when an antioxidant is introduced into the solution of the radical initiator. It is shown that the method with the determination of the induction period is difficult in the study of "slow" antioxidants. It has been established that the area above the curve of the kinetic dependence Exp(∆E) is directly related to the amount of inhibited peroxyl radicals and can be used to assess the inhibitory properties of an antioxidant from a thermodynamic point of view. "Fixed time method" and "Initial rate method" were used. Positive correlations between the described method have been established. The utility of the parameter of the area above the curve of the kinetic dependence Exp(∆E) in the study of objects of complex composition is shown.

Hydrolysis and condensation kinetic studies of mercaptopropyl trimethoxysilane using in-situ Raman spectroscopy

The hydrolysis and condensation kinetic parameters of mercaptopropyl trimethoxysilane (MPTMS, HS(CH2)3Si(OCH3)3) have rarely been quantitatively studied due to the fast reaction rate with the presence of catalysts. In this work, we employed the in-situ Raman spectroscopy to monitor the hydrolysis and condensation reactions of MPTMS in real time. First, with CH3OH as the solvent and NH4OH as the catalyst, the hydrolysis kinetics of MPTMS was investigated via the initial rate method. The reaction orders with respect to MPTMS, H2O, and NH4OH are 1.0, 1.0, and 1.0, respectively. The activation energy (Ea) and pre-exponential factor (A) of MPTMS hydrolysis reaction are (28.4 ± 1.1) kJ/mol and 938.1 (mol/L)-2·min−1, respectively. Then, an ‘acid-catalyzed hydrolysis and base-catalyzed condensation’ two-step strategy was employed to decouple the hydrolysis and condensation reactions. With the hydrolysates, trihydroxy silanols HS(CH2)3Si(OH)3, as the starting reactant, the early-stage condensation reaction kinetics was studied. The reaction orders with respect to HS(CH2)3Si(OH)3 and NH4OH are 1.4 and 2.8, respectively. The activation energy (Ea) and pre-exponential factor (A) of the condensation reaction are (52.2 ± 0.4) kJ/mol and 7.5 × 1013 (mol/L)-3.2·min−1, respectively. The kinetic parameters determined in this work could provide quantitative guidance for the design and control of MPTMS reaction process.

Utilization of a kinetic isotope effect to decrease decomposition of ceftriaxone in a mixture of D2O/H2O

The discovery of cephalosporin and demonstration of its improved stability in aqueous solution, as well as enhanced in vitro activity against penicillin-resistant organisms, were major breakthroughs in the development of β-lactam antibiotics. Although cephalosporins are more stable with respect to hydrolytic degradation than penicillins, they still experience a variety of chemical transformations. The present study offers an insight into the rates and mechanisms of ceftriaxone degradation at the therapeutic concentration in water, a mixture of water and deuterium oxide, and deuterium oxide itself at the neutral pH.Specific ceftriaxone degradation products were observed in aged samples (including a previously unreported dimer-type species), and by comparing the degradation rates in H2O and D2O, the observation of a kinetic isotope effect provided some valuable insight as to the nature of the initial ceftriaxone degradation. The effect of protium to deuterium isotope change on the degradation kinetics of ceftriaxone was evaluated using the method of initial rates based on HPLC analysis as well as by quantitative 1H NMR spectroscopy. Moreover, computational analysis was utilized to get a molecular insight into chemical processes governing the ceftriaxone degradation and to rationalize the stabilizing effect of replacing H2O with D2O.

Validation of an Accurate and Expedient Initial Rates Method for Characterizing Mechanophore Reactivity

Understanding structure-mechanochemical reactivity relationships is important for informing the rational design of new stimuli-responsive polymers. To this end, establishing accurate reaction kinetics for mechanophore activation is a key objective. Here, we validate an initial rates method that enables the accurate and rapid determination of rate constants for ultrasound-induced mechanochemical transformations. Experimental reaction profiles are well-aligned with theoretical models, which support that the initial rates method effectively deconvolutes the kinetics of specific mechanophore activation from the competitive process of nonspecific chain scission.

Kinetic Spectrophotometric Determination of Ascorbic Acid by Using Phenanthroline Agent in Eruca sativa

Ascorbic acid is used for treatment in hypovitaminosis, nutritional deficiency, scurvy, gingivitis, Barlow and Basedow diseases. In this study, a simple and sensitive kinetic spectrophotometric method was developed for the determination of ascorbic acid in Eruca sativa. The rate of formation of the product formed as a result of the reaction of ascorbic acid with iron (III) sulfate and phenanthroline reagents was determined by measuring the absorbance at 510 nm wavelength at 5 minute intervals for 60 minutes. The measurement curve prepared from standard ascorbic acid solution is linear in the concentration range of 4.0-6.0 mg/mL. The regression equation obtained from the graph of logarithms of molar concentrations versus logarithms of slopes was found as log (rate) = 2.5721 logC - 9.461 (r=0.9999) according to the initial rate method. The developed method was applied to Eruca sativa samples and the amount of ascorbic acid in fresh leaves was found to be 72.80 mg/100 g.

Explicit Solutions for Real-time Reversible Inhibition Kinetics using Lambert W Function: Towards Progress Curve Analysis

Accurate estimation of kinetic parameters is challenging due to the dynamics and mathematical nature of the chemical systems. This paper presents simple, yet efficient closed-form solutions for the enzymatic conversion of the substrate to the product in real-time derived using the Lambert W function. The real values of the Lambert W function were calculated from the Lambert package as implemented in MATLAB. The expressions exhibit remarkable robustness in estimating the parameters for randomized data at 1% to 4% variation in noise levels. Furthermore, unlike the initial rates method, the expressions estimate chemical kinetic parameters utilizing a full range of experimental data, thus minimizing the risk of missing information that would be detected at an extended time-span. Thus, the implementation of closed-form solutions presented in this paper for the estimation of kinetic parameters eliminates common pitfalls imposed by the initial rates and double reciprocal methods.
 Keywords: Enzyme, Lambert W function, Reversible inhibition, Closed-form solutions

NOVEL FAST ANALYTICAL METHODS FOR THE ANALYSIS OF FLUOXETINE IN PURE AND PHARMACEUTICAL DOSAGE FORM

Novel and accurate analytical methods were developed and validated for the characterization of Fluoxetine in its pure and pharmaceutical dosage form Prozac®. Fluoxetine was determined by IBA techniques (PIGE, PIXE and RBS). It has been also analyzed spectrophotometrically at 610 nm after oxidation with potassium permanganate in alkaline medium. In addition, Fluoxetine was kinetically determined using the initial rate method, the fixed absorbance method and the fixed time method. Moreover, a Gas chromatography - mass spectrometry technique is proposed for the investigation of Fluoxetine without a prederivatization phase. The spectrophotometric method was performed with a concentration array of 2-10 μg/mL at 610 nm and a regression coefficient (r) of 0.996. The fixed time method was the most suitable one to determine Fluoxetine with correlation coefficient value (r) of 0.9966. The Gas chromatography - mass spectrometry investigated the drug in a concentration range of 20-100 μg/mL and a regression coefficient (r) of 0.999. IBA analysis presented a precision of less than 3% and a very low limit of detection. Consequently, these proposed methods would be useful tools for determining Fluoxetine as all the assay results exposed satisfactory sensitivity, accuracy and reproducibility

Quantitative determination of base-catalyzed hydrolysis kinetics of methyltrimethoxysilane by in-situ Raman spectroscopy

A thorough understanding of the hydrolysis reaction of alkoxysilanes is of great significance to the synthesis of silicone resins, however, the determination of the reaction kinetics is pretty hard. This is because the hydrolysis of alkoxysilanes is fast under the alkaline conditions, especially methyltrimethoxysilane (MTMS), and the reaction is difficult to terminate. Meanwhile, there is a reverse hydrolysis reaction in the system. In this work, we developed a fast and quantitative method by applying in-situ Raman spectroscopy to determine the base-catalyzed hydrolysis kinetics of MTMS. The short measuring time, high sensitivity, and low water interference enable Raman spectroscopy to be a powerful tool for the real-time monitoring of rapid hydrolysis reaction. In order to avoid the influence of the reverse reaction and condensation reaction, the initial rate method was employed to determine the reaction kinetic parameters. The reaction orders with respect to MTMS, H2O, and NH4OH are 0.8, 0.9, and 0.7, respectively. The activation energy (Ea) and pre-exponential factor (A) are 26.02 kJ/mol and 1088.94 (mol/L)−1.4·min−1, respectively. Furthermore, the solubility of MTMS in H2O was measured, and a mathematical model of the heterogeneous hydrolysis process of MTMS microdroplets was established. This model can predict the time required for the disappearance of MTMS microdroplets, thereby providing quantitative guidance for the process design and product quality control.

Using Linear Flow Parameter Initial Rate Method to Optimize Completion Design of Western Canada’s Montney Tight Gas Reservoir

Since 2007, the horizontal well staged fracturing technology has been widely used in the development of tight sandstones in the Deep Basin Montney reservoir of Western Canada Basin. The horizontal length of the horizontal well, the amount of proppant added to the fracturing and completion, the frac stage spacing, and the choice of open-hole completion or casing completion have become important factors in the optimization of completion design. LFPIR (Linear Flow Parameter Initial Rate) is observed through the double logarithmic curve of the natural gas production data of the well. After the development enters the stable linear flow stage, the linear flow corresponding to the ideal situation can be obtained when the initial production rate in this stage is inversely extended to tD≈0. The LFPIR depends on the stimulated volume of the reservoir after fracturing, and the completion effect directly affects the size of the stimulated reservoir. The completion quality of the well can be judged by studying the LFPIR of the developed Montney horizon well in the Heritage area. By comparing the LFPIR of wells with different completion designs, optimize the completion design in the tight gas reservoirs.

Highly dispersed and stable Fe species supported on active carbon for enhanced degradation of rhodamine B through peroxymonosulfate activation: Mechanism analysis, response surface modeling and kinetic study

Herein, highly dispersed Fe species supported on activated carbon (CA-Fe-C) was fabricated with the aid of citric acid, showing excellent catalytic performance and stability for the degradation of rhodamine B (RhB) through peroxymonosulfate (PMS) activation. The radical quenching study, electron paramagnetic resonance (EPR) measurement and PMS consumption analysis confirmed the RhB degradation mechanism involving reactive radical species attacking RhB and non-radical oxidation pathway. The main degradation intermediates were identified by LC-MS, and the possible RhB degradation pathways were deduced. A multivariate quadratic polynomial model was developed using the response surface methodology (RSM) to examine how the removal of RhB was influenced by several quantifiable factors, with the objective of optimizing RhB removal. The analysis of variance (ANOVA) was used to evaluate the adequacy and significance of the proposed model. The effects of solution pH values, initial concentration, inorganic anions, PMS dosage, catalyst dosage, and metal loading amount on RhB removal were investigated. The method of initial rates was employed to determine the intrinsic reaction rate law, and the reaction was identified to follow the Langmuir-Hinshelwood (L-H) kinetics model.

Kinetic Insights into Cyanosilylation of Aldehydes Catalyzed by a Covalently Bridged Dinuclear (Salen)titanium Complex

AbstractEnantioselective addition of trimethylsilyl cyanide (TMSCN) to aldehydes is one of the most extensively studied organic reactions in asymmetric catalysis. Herein, we report our intensive kinetic investigation on the asymmetric addition of TMSCN to benzaldehyde, catalyzed by a covalently bridged dinuclear (salen)titanium complex 2, which has been one of the most efficient artificial chiral catalysts reported so far for this reaction. It was found that the method of initial rates for kinetic investigation is not appropriate in this case because of the presence of a significant incubation period in the catalysis, while the method of progress rates proved to be more reliable and efficient for judging the kinetic orders of this catalytic system. The kinetic results revealed that the reaction follows first order with respect to the catalyst and is nearly independent of concentrations of both benzaldehyde and TMSCN. A detailed catalytic mechanism for cyanosilylation of benzaldehyde in the presence of 2 was proposed, wherein the key active dinuclear species works in a cooperative manner for dual activation of both reactants.

Production of Biogas from Kitchen Wastes Generated from the Mess of NIT Agartala

A review has been done on the formulation of mathematical models for designing anaerobic batch reactor for the producing of biogas from solid wastes. Using material balance analysis, the design parameters, especially biokinetic behavior of solid wastes are studied. Hydrolysis constants and reaction orders at both low total solids concentrations and high total solids concentrations were thoroughly reviewed by initial rate method. Population growth model and first order hydrolysis model are also studied. It has been found that if sufficient reaction time is given, in terms of reaction kinetics, the hydrolysis process in kitchen waste fermentation can be described as a first order reaction. An attempt is done by utilizing the biodegradable kitchen waste collected from Aryabhatta hostel, NIT Agartala for generation of biogas and demonstrating the possibilities of energy recovery. A simple anaerobic digester has been fabricated to capture the generated biogas. To produce methane as a clean fuel the use of cow dung as substrate is studied to ferment the kitchen wastes.

Strontium isotope fractionation during precipitation of strontianite in aqueous solutions as a function of temperature and reaction rate

In order to study Strontium (Sr) isotope fractionation during the precipitation of strontianite (SrCO3) as a function of the specific precipitation rate (R*) and temperature (T), strontianite was precipitated at 12.5, 25.0 and 37.5 °C by diffusing NH3 and CO2 gases into aqueous solutions. The specific precipitation rate R* (mol/cm2.h) for every sample was determined by applying the initial rate method. The mean isotope difference between bulk solution and precipitate (∆88/86Srstrontianite-solution) was found to be −0.279 ± 0.005‰ (2σmean) independent of both rate and temperature. Hence, Sr isotope fractionation in strontianite is completely different from that in calcite and aragonite, where a strong dependency from both rate and temperature can be observed. The latter is interpreted to reflect the competition between Sr2+ and Ca2+ ions for incorporation into the calcium carbonate crystal lattice, which is absent during the precipitation of pure strontianite. The isotope difference between strontianite and bulk solution then simply reflects the intermolecular forces in the aqueous solutions as well as the kinetic effect. The difference in the (∆88/86Srstrontianite-solution between experiments then reflects the dehydration energy of Sr ions in the adsorption layer of SrCO3.

Use of heterogeneous activated carbon supported copper catalyst for catalytic wet oxidation of biomethanated spent wash: Reaction kinetics, catalyst stability, catalyst deactivation kinetics and biochemical methane potential

In the present study, the potential of activated carbon (AC) supported copper catalyst was investigated during catalytic wet oxidation (CWO) of pretreated high strength biomethanated spent wash (BMSW) stream which is originated from alcohol distillery. Prior to wet oxidation (WO) treatment, the BMSW was centrifuged to remove suspended solids. During CWO treatment at 220 °C temperature, the chemical oxygen demand and color removals of 72% and 97%, respectively, were observed from the centrifuged BMSW (catalyst dose = 3.4 g L−1, reaction time = 3 h). The analysis of treated liquid samples confirmed the formation of acetic acid in large concentrations which is one of the major precursors of methane during post anaerobic treatment. The kinetic expressions for WO and CWO processes were also developed using initial rate method. However, the recovered and regenerated catalyst could only be reused twice in CWO runs. The biochemical methane potential (BMP) test results showed significant production of biogas from the treated BMSW. Based on the findings, it can be inferred that the CWO of BMSW can be a potential treatment method before its disposal in the environment.

Comparing hydrogen absorption kinetics of the samples of intermetallic compound and metal hydride compact on its basis

This work is devoted to an experimental study and comparison of the kinetics of hydrogen absorption by an intermetallic compound LaNi4.4Al0.3Fe0.3 in form of pure intermetallic compound free backfill and a compact based on it obtained by cold pressing with a spiral matrix of nickel-foam. To calculate the kinetic parameters of the hydrogen absorption reaction, the initial rates method is used. The PCT absorption isotherms are measured at temperatures of 313, 333, and 353 K. The experimental data are described with quite high confidence by the chosen model, which assumes that the reaction rate controlling step is the dissociative absorption of hydrogen on the surface of the a-phase. The rate of hydrogen absorption increases with increasing pressure drop and temperature. It is shown that the rate of hydrogen absorption by the sample of pure IMC is significantly less dependent on temperature compared to the compact sample. In addition, the reaction rate at temperatures of 313 and 333 K is higher for the free backfill sample, and at 353 K it is higher for the metal hydride compact. The values of the absorption constant and the activation energy of the hydrogen absorption reaction are determined for both samples.

Oxidation of hexacyanoferrate(II) ion by hydrogen peroxide: evidence of free radical intermediacy

The redox reaction between hexacyanoferrate(II) ion as the reducing agent and hydrogen peroxide as the oxidizing one, in slightly acid (pH 4.36–6.65) aqueous solutions containing phosphate ions, has been studied by means of an UV–Vis spectrophotometer monitoring the formation of Fe(III) at 420 nm. The initial hydrolysis of the Fe(II)-cyanide complex reactant to yield a pentacyanoaquaferrate(II) intermediate, as well as a decrease in the solution pH (acid catalysis), had both a positive effect on the reaction rate, whereas an increase in the concentrations of Fe(III), phosphate and chloride ions, and d-mannitol had a negative effect (inhibition). A computer program, specifically designed for this purpose, allowed arriving at a complex eight-coefficient experimental rate law by application of the initial rate method, accounting for the dependences of the reaction rate on the concentrations of Fe(II), H2O2 and Fe(III). The reaction was characterized by a low value of the apparent activation energy (28.9 ± 2.7 kJ mol−1). Finally, a multi-step reaction mechanism, involving the participation of a penta-coordinated Fe(II) complex, and hydroxyl, superoxide and other free radicals as intermediates, has been proposed. This mechanism was supported by the available experimental information and confirmed by numerical simulations performed via the fourth order Runge–Kutta integration method.

Redox Reaction between an Oxo-Bridged Diiron Complex and Hydrazine: A Kinetic Study

Objective: Mechanistic investigation of the reaction between the complex ion [FeIII 2(μ-O)(phen)4(H2O)2]4+ (1) (phen = 1,10-phenanthroline) and its hydrolytic derivatives [FeIII 2(μ-O)(phen)4(H2O)(OH)]3+ (1a) and [FeIII 2(μ-O)(phen)4(OH)2]2+ (1b) which coexist in rapid equilibria in the range pH = 3.00 to pH = 5.00 (pKa1 = 3.71 ± 0.03, pKa2 = 5.28 ± 0.07) with N2H5 + ion to produce [Fe(phen)3]2+ has been covered in this study. Methods: Rise in absorbance of the product with time was measured and rates have been determined using initial rate methods. Dependence of rate on pH and concentration of N2H5 + are studied in detail. Results: In the presence of excess phenanthroline, the reaction follows simple first-order exponential profile. Interestingly, in the absence of any added phenanthroline, the reaction becomes faster and the reaction profile changes to nearly linear with curvature near the end of the reaction. Slight autocatalytic nature has been observed only for the slower reactions. The observed rate constants obtained using initial rate method show first-order dependence on the concentration of N2H5 +. An increase in rate with increasing pH has been noticed. The plot of rate versus 1/[H+] shows saturation beginning near pH 4.6, suggesting that the mono-deprotonated form of the complex (i.e. 1a) is the active oxidant in this pH range. Conclusion: Presumably, the reaction becomes faster in the absence of added phenanthroline because ligand dissociation from the parent complex becomes facilitated when there is no external phenanthroline ligand present. This produces a coordinatively unsaturated Fe(III) species which is a faster oxidant.

Hydrogen production by supercritical water gasification of methylhydrazine in continuous system

Supercritical water gasification technology has ability to process unsymmetrical dimethylhydrazine (UDMH) wastewater derived from launching spacecraft, missile or satellite realizing both harmless treatment and resource utilization due to its unique property. Owning advantages such as compact structure, highly reduced reactor volume, improved heat transfer efficiency and fully mixed reactants, a spiral continuous reactor was proposed based on the characteristics of UDMH wastewater. Methylhydrazine (MMH) was used as model compound on account of UDMH’s commercial unavailability. The degradation of MMH in supercritical water is determined as an apparent first-order reaction using initial rate method. The effect of different parameters including temperature (450–550 °C), feeding flow rate (10–75 g/min), feeding concentration (5–30 %), and catalyst type on gasification were systematically investigated and subsequently the migration of carbon and nitrogen was also concerned. Results showed that addition of KOH could make the carbon gasification efficiency approach 100 % with the maximum hydrogen yield up to 21.6 mol/kg under the condition of 500 °C, 23 MPa, feeding flow rate of 30 g/min, feedstock concentration of 10 %. The proportion of organic carbon in total carbon tended to decrease with the increase of temperature. Most of nitrogen (up to 99.7 % at most) was gasified within our experimental range.

Role of Glycerol Oxidation Pathways in the Reductive Acid Leaching Kinetics of Manganese Nodules Using Glycerol.

Manganese nodules from ocean bed are potential resources of Cu, Ni, and Co for which land-based deposits are scarce in India. The present work describes a novel approach of using glycerol, a nontoxic biomass-derived reductant, for the reductive acid leaching of manganese nodules. Parameters such as acid concentration, time, temperature, and pulp density were optimized for leaching. The optimal leaching conditions were found to be 10% (w/v) pulp density and 10% (v/v) H2SO4 at 80 °C with 1% (v/v) glycerol yielding >95% of Ni and >98% Cu, Co, and Mn extraction within an hour. Kinetic analysis of the data based on the initial rate method showed that the leaching process was chemical reaction-controlled with an apparent activation energy of 55.47 kJ/mol. Various oxidation intermediates of glycerol formed during leaching were identified using mass spectrometry and Raman spectroscopy, and a probable oxidation pathway of glycerol during the leaching process has been elucidated based on the analysis. Glycerol was oxidized to glyceraldehyde, glyceric acid, tartronic acid, dihydroxyacetone, hydroxy pyruvic acid, glyoxalic acid, oxalic acid, and finally converted to CO2 during leaching. The fast reaction kinetics, near-complete dissolution of manganese, and other associated metals in the nodule can be attributed to the participation of all intermediate products of glycerol oxidation in redox reactions with MnO2, enhancing the overall reduction leaching efficiency.

Determination of penicillamine, tiopronin and glutathione in pharmaceutical formulations by kinetic spectrophotometry.

A novel and simple method for the determination of penicillamine (PEN), tiopronin (mercaptopropionyl glycine, MPG) and glutathione (GSH) in pharmaceutical formulations by kinetic spectrophotometry has been developed and validated. It is based on the redox reaction where the thiol compound (RSH) reduces CuII-neocuproine complex to CuI-neocuproine complex. The non-steady state signal of the formed CuI- neocuproine complex is measured at 458 nm. The initial rate and fixed time (at 1 min) methods were validated. The calibration graph was linear in the concentration range from 8.0 × 10‒7 to 8.0 × 10‒5 mol L-1 for the initial rate method and from 6.0 × 10‒7 to 6.0 × 10-5 mol L-1 for the fixed time method, with the detection limits of 2.4 × 10-7 and 1.4 × 10‒7 mol L-1, resp. Levels of PEN, MPG and GSH in pharmaceutical formulations were successfully assayed by both methods. The advantages of the presented methods include sensitivity, short analysis time, ease of application and low cost.