Initial Rate Methods Research Articles

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.

Thermoluminescence properties of beta particle irradiated Ca3Al2O6 phosphor relative to environmental dosimetry

Undoped Ca3Al2O6 phosphor was successfully synthesized through a gel-combustion method using different fuels. It was characterized by X-ray diffraction (XRD) technique and its cubic phase structure was confirmed from XRD pattern. TL data were recorded from room temperature (RT) to 500 °C in the heating rate of 2 °C/s. The glow curves of Ca3Al2O6 sample exposed to different beta doses (0–200 Gy) exhibited a significant glow peak at about 184 °C. The TL intensity of the glow peak exhibited very good linearity between 0.1 and 10 Gy. Following this, it was decreased at higher doses which was referred to this effect as monotonic dose dependence. Initial rise (IR), peak shape (PS), and variable heating rate (VHR) methods were used to estimate trapping parameters. Computerized glow curve deconvolution (CGCD) method via TLAnal software was also applied to estimate the number of peaks and kinetic parameters corresponding to the main glow curve in Ca3Al2O6 sample. The trapping activation energy of the main dosimetric peak was calculated to be around 1.30 eV for all methods. Present findings confirm that Ca3Al2O6 host is a promising candidate for applications in environmental dosimetry as one depicts good TL dose response with adequate sensitivity and linearity.

Oxy-Cracking Reaction for Enhanced Settling and Dewaterability of Oil Sands Tailings

Oil sands exploitation in Alberta relies on water-intensive processes that result in large amounts of oil sand process-affected water (OSPW). Tailings ponds are used to store OSPW in accordance with government regulations. One of the main challenges of these tailings ponds is the settling of fine particles which, without treatment, would remain suspended for decades. This study introduces a novel application for the oxy-cracking technique to enhance fine particle settling through simultaneous oxidation and cracking reactions: residual bitumen and hydrocarbons are cracked and solubilized in the liquid phase, freeing the fine particles to agglomerate and settle faster. The pathway for this process is determined by the kinetics parameters, and it releases minimal CO2. A full lumped kinetics model study is presented to describe the oxy-cracking reaction. Settling enhancement and dewaterability were studied using the initial settling rate (ISR) and capillary suction time (CST) methods. After oxy-cracking, the mudline was reduced by 65% within 24 h with no settling observed in the original sample. CST experiments resulted in fast and easy dewaterability of the consolidated solids after an oxy-cracking reaction of neat OSPW samples. The oxy-cracking process outperforms other processes reported in the literature for diluted samples treated with large amounts of coagulants. Oxy-cracking and settling enhancement were further confirmed by the characterization techniques of GC–MS, FTIR, XRD, TGA, and TOC analysis. The proposed oxy-cracking technique offers new opportunities for the wastewater treatment of OSPW generated by the oil sands industry.

Effect of particle size on the thermoluminescence dosimetric properties of household salt

This work investigates the effect of particle size on the thermoluminescence (TL) response of household salt that is classified into five size fractions between 63 and 250 μm. Irradiation induced two main TL peaks at 110 and 240 °C for each particle size fraction. In addition to these peaks, two additional glow peaks were observed at 80 and 300 °C for particles with small size (<90 μm). The complexity of the glow curve pattern increased with decreasing particle size (<125 μm). The intensity of the main dosimetric TL peak at 240 °C increased with increasing particle size. The maximum beta radiation sensitivity of the salt samples was obtained for the particle sizes of 125–180 and 180–250 μm. For different particle sizes, the stability of the dosimetric TL peaks at room temperature was studied over a storage period of 30 days. No systematic difference in the minimum detectable dose values observed for each grain size fraction and it was found to be 0.4 Gy for each grain size fraction. The kinetic parameters of the dosimetric TL peak were determined for different particle sizes using the initial rise and variable heating rate methods. Moreover, the variations in the glow peaks depending on the heating rate were also analyzed. While an expected behavior of low temperature TL at 110 °C was observed, an interesting heating rate behavior (anti-quenching) of the high temperature dosimetric TL peak at 240 °C was observed.

Thermoluminescence kinetic analysis and dosimetry features of MgSO 4 :Dy and MgSO 4 :Cu nano-rods

Abstract MgSO 4 :Dy and MgSO 4 :Cu nano-rods (NRs) were synthesized for the first time by semi co- precipitation method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were utilized for sample characterization. The optimum amount of dysprosium and copper concentrations were obtained both at 0.1 mol% in MgSO 4 :Dy and MgSO 4 :Cu NRs. T m -T stop and computerized glow curve deconvolution (CGCD) methods were used for identifying the number of component glow peaks and kinetic parameters of the synthesized NRs. Initial rise and variable heating rate methods were also used to ensure the reliability of obtained kinetic parameters. Results show that the TL sensitivity of MgSO 4 :Dy is about 7 times more than that of magnesium sulfate doped with Cu. The TL dose response of MgSO 4 :Dy and MgSO 4 :Cu NRs are linear up to absorbed dose of 10 KGy. Other TL dosimetry characteristics of the produced NRs are also presented and discussed.

Assessment of four loss methods in simulation of flood hydrograph case study: Kasilian Basin

The aim of this study is to evaluate the effect of loss methods application on run-off volume and hydrograph shape used by hydrologic models. In this research, four loss methods including constant loss (φ index), initial loss-loss rate, SCS and constant fraction approach were applied to be evaluated. To do this, Clark Rainfall-Runoff model was applied for 24 storm events from Kasilain basin. Then, characteristics of calculated hydrographs including peak discharge, runoff volume and time to peak were compared with the same characteristics of the same observed hydrographs and analysed statistically. Statistical analysis of the effect of mentioned loss methods on characteristics of hydrographs using mean relative error, coefficient of determination (R²) and root mean square error, showed the relative superiority of the SCS, initial loss-loss rate, constant loss, and constant fraction (proportional loss) methods, respectively. Therefore, the initial losses have a significant effect on the rainfall abstraction for flood hydrographs simulation in forest areas.

Trapping centers and their distribution in Tl2InGaSe4 single crystals by thermally stimulated luminescence

Thermoluminescence (TL) measurements in Tl2InGaSe4-layered single crystals have been carried out in the temperature range of 10–200 K at various heating rates (0.2–1.0 K s−1) to get information about the characteristics of traps. Two TL overlapping glow peaks related to defect levels have been clearly observed. Thermal cleaning procedure was applied to the glow curves to separate overlapped peaks. Initial rise, peak shape, and heating rate methods were used to calculate the activation energies of the revealed traps. The energy values of 5 and 28 meV were evaluated for the peaks observed at low and high temperatures, respectively. Moreover, heating rate dependence and traps distribution analysis were also investigated on the curve obtained after thermal cleaning. The activation energies of the distributed trapping centers were found to be increasing from 29 to 151 meV with increasing the illumination temperature from 42 to 80 K.

Kinetic spectrophotometric H-point standard addition method for the simultaneous determination of diloxanide furoate and metronidazole in binary mixtures and biological fluids

Simple, reliable, and sensitive kinetic spectrophotometric method has been developed for the simultaneous determination of diloxanide furoate and metronidazole using H-point standard addition method (HPSAM). The method is based on the oxidation rate difference of diloxanide and metronidazole by potassium permanganate in basic medium. A green color has been developed and measured at 610nm. Different experimental parameters were carefully optimized. The limiting logarithmic and the initial-rate methods were adopted for the construction of the calibration curve of each individual reaction with potassium permanganate. Under the optimum conditions, Beer’s law was obeyed in the range of 1.0–20.0 and 5.0–25.0μgml−1 for diloxanide furoate and metronidazole, respectively. The detection limits were 0.22μgml−1 for diloxanide furoate and 0.83μgml−1 for metronidazole. Correlation coefficients of the regression equations were greater than 0.9970 in all cases. The precision of the method was satisfactory; the maximum value of relative standard deviation did not exceed 1.06% (n=5). The accuracy, expressed as recovery was between 99.4% and 101.4% with relative error of 0.12 and 0.14 for diloxanide furoate and metronidazole, respectively. The proposed method was successfully applied for the simultaneous determination of both drugs in pharmaceutical dosage forms and human urine samples and compared with alternative HPLC method.

Kinetic spectrophotometric methods for the determination of alfuzosin hydrochloride in bulk and pharmaceutical formulations

Simple and sensitive kinetic spectrophotometric methods were established for the determination of alfuzosin hydrochloride in bulk and in its pharmaceutical preparations using alkaline potassium permanganate as an oxidizing agent. The methods involve determination of alfuzosin HCl by kinetic studies of its oxidation at room temperature for a fixed time of 15 min. The absorbance of the colored manganate ions was measured at 610 nm. Alternatively, the decrease in the absorbance of permanganate upon addition of the studied drug was also measured at 525 nm. The absorbance-concentration plots in both procedures were rectilinear over the range of 2.0–30.0 μg/mL. The different experimental parameters affecting the development were carefully studied and optimized. The determination of alfuzosin HCl by the fixed concentration and initial rate methods is also feasible with the calibration equations obtained but the fixed time method has been found to be more applicable. Both procedures were applied to the determination of alfuzosin HCl in formulations. The results obtained were in good agreement with those obtained using reference methods.

Kinetic spectrophotometric method for the determination of perindopril erbumine in pure and commercial dosage forms

A kinetic spectrophotometric method has been developed for the determination of perindopril erbumine in pure and commercial dosage forms. The method is based on the reaction of drug with potassium permanganate in alkaline medium at room temperature (30±1°C). The reaction was followed spectrophotometrically by measuring the increase in absorbance with time at 603nm and the initial rate, fixed time (at 8.0min) and equilibrium time (at 90.0min) methods were adopted for constructing the calibration graphs. All the calibration graphs are linear in the concentration range of 5.0–50.0μg/ml. The limits of detection for initial rate, fixed time and equilibrium time methods were 0.752, 0.882 and 1.091μg/ml, respectively. The activation parameters such as Ea, ΔH‡, ΔS‡ and ΔG‡ were also determined for the reaction and found to be 60.93kJ/mol, 56.45kJ/mol, 74.16J/Kmol and −6.53kJ/mol, respectively. The variables were optimized and the proposed methods are validated as per ICH guidelines. The method has been further applied to the determination of perindopril erbumine in commercial dosage forms. The analytical results of the proposed methods when compared with those of the reference method show no significant difference in accuracy and precision and have acceptable bias.

Thermoanalytical study of linkage isomerism in coordination compounds

The non-isothermal differential scanning calorimetry (DSC) experiments revealed that linkage isomerization of both pure trans-[Co(en)2(ONO)2]PF6 (dinitrito isomer) and trans-[Co(en)2(NO2)2]PF6 (dinitro isomer) occurs in the solid state at elevated temperatures. The process was found to be exothermic for the dinitrito isomer and endothermic for dinitro isomer. The pure isomers could be considered to be in metastable states at ambient temperatures which would be converted to an equilibrium mixture of both isomers (stable state) upon heating. Since the isomerization of both isomers may be described as a two stages process, the corresponding DSC peaks have been resolved into two peaks by means of nonlinear curve fitting tools of PeakFit® software. The resolution of the peaks helped us to determine enthalpy changes of stepwise isomerization of both isomers. A first-order initial rate and Kissinger methods have been employed to estimate kinetic parameters of the stepwise isomerization reactions. The Kissinger method provided more reliable kinetic results. The high activation energy and positive entropy changes of isomerization of both isomers are considered as indication of a dissociative mechanism in solid state.

Kinetic spectrophotometric determination of β-adrenergic antagonists using alkaline potassium permanganate

A new simple and sensitive kinetic spectrophotometric method is described for analysis of bisoprololfumarate (BSF) and propranolol hydrochloride (PRH) in pure and pharmaceutical formulations. The method involves the reaction of the drugs with alkaline potassium permanganate producing bluish-green manganate species. The reaction was followed spectrophotometrically by measuring the increase in the absorbance from the formation of MnO4 2- at 610 nm. The different experimental parameters affecting the development and stability of the colors were carefully studied and optimized. The fixed-time method is adopted for constructing the calibration curves, which were found to be linear over the concentration ranges of 2.0-16g/mL for BSF and 2.0-18g/mL for PRH. The determination of the studied drugs by initial rate, variable time and rate-constant methods was feasible with the calibration equations obtained but the fixed time method has been found to be more applicable. The method has been successfully applied for the determination of drugs in commercial dosage forms. The results were validated through inter-day and intra-day precision assays according to the ICH guidelines and also through recovery studies. Statistical comparison of the proposed method with that of official methods shows excellent agreement and indicates no significant difference in their accuracy and precision.

Kinetic Spectrophotometric Determination of N-Acetyl-L-cysteine Based on a Coupled Redox-Complexation Reaction

A novel simple kinetic spectrophotometric method for the determination of N-acetyl-L-cysteine (NAC) has been developed and validated. The proposed method is based on a coupled redox-complexation reaction, the first step of which is the reduction of Fe(3+) by NAC; the second one includes the complexation of Fe(2+), resulting from the preceding redox reaction, with 2,4,6-trypyridyl-s-triazine (TPTZ). The stable Fe(TPTZ)(2)(2+) complex exhibits an absorption maximum at lambda = 593 nm.The initial rate and fixed-time (at 5 min) methods were utilized for constructing calibration graphs. The graphs were linear in concentration ranges from 4.0 x 10(-6) to 1.0 x 10(-4) mol L(-1) for the initial rate method and 1.0 x 10(-6) to 1.0 x 10(-4) mol L(-1) for the fixed-time method, with detection limits of 1.0 x 10(-6) and 1.7 x 10(-7) mol L(-1), respectively. The proposed methods were successfully applied for the determination of NAC in its commercial pharmaceutical formulations.

Chemical stability of 4′-azidocytidine and its prodrug balapiravir

Background: R1479, a 4′-azidocytidine nucleoside analog, was developed for the treatment of Hepatitis C virus infection. Balapiravir (R1626) is the tri-isobutyrate ester prodrug of R1479 under clinical development to improve exposure of R1479 upon oral administration. Objective: The chemical stability and the rate of azide release of R1479 and balapiravir were studied. Methods: R1479 and balapiravir solutions were prepared at different pH values and stored at various temperatures. An ion pair high-performance liquid chromatography (HPLC) method with gradient elution was employed to analyze the prodrug, parent, and degradation products. Azide was measured using a reversed phase HPLC method with UV detection after formation of the 3,5-dinitrobenzoyl azide derivative with 3,5-dinitrobenzoyl chloride. The data were analyzed using initial rate and conventional first-order kinetic methods. Results: R1479 degrades to cytosine and azide in aqueous solutions, whereas balapiravir mainly degrades to R1479 and mono- and diesters of R1479. The rates of azide release from R1479 and balapiravir were generally comparable with the corresponding amount formed of cytosine. Conclusion: Azide release is pH dependent and is faster in acidic solutions than in neutral solutions. The amount of azide released is significantly less from balapiravir than that from R1479, suggesting a potential advantage of the prodrug over the parent drug.

Kinetic spectrophotometric determination of certain cephalosporins using iodate/iodide mixture

A simple, precise and accurate kinetic spectro-photometric method for determination of ce-fradine anhydrous, cefaclor monohydrate, ce-fadroxil monohydrate, cefalexin anhydrous and cefixime in bulk and in pharmaceutical formula-tions has been developed. The method based on a kinetic investigation of the reaction of the free carboxylic acid group of the drug with a mixture of potassium iodate and potassium iodide at room temperature to form yellow coloured triiodide ions. The reaction was followed up spectrophotometrically by measuring the increase in absorbance at 352 nm as a function of time. The initial rate, fixed time, variable time and rate-constant methods were adopted for constructing the calibration curves but fixed time method has been found to be more applicable. The analytical performance of the method, in terms of accuracy and precision, was statistically validated; the results were satisfactory. The method has been successfully applied to the determination of the studied drugs in commercial pharmaceutical formulations. Statistical comparison of the results with a well established reported method showed excellent ag- reement and proved that there is no significant difference in the accuracy and precision.

Kinetic spectrophotometric determination of certain cephalosporins using oxidized quercetin reagent

A simple, precise and accurate kinetic spectrophotometric method for determination of cefoperazone sodium, cefazolin sodium and ceftriaxone sodium in bulk and in pharmaceutical formulations has been developed. The method is based upon a kinetic investigation of the reaction of the drug with oxidized quercetin reagent at room temperature for a fixed time of 30 min. The decrease in absorbance after the addition of the drug was measured at 510 nm. The absorbance concentration plot was rectilinear over the range 80–400 μg mL −1 for all studied drugs. The concentration of the studied drugs was calculated using the corresponding calibration equation for the fixed time method. The determination of the studied drugs by initial rate, variable time and rate-constant methods was feasible with the calibration equations obtained but the fixed time method has been found to be more applicable. The analytical performance of the method, in terms of accuracy and precision, was statistically validated; the results were satisfactory. The method has been successfully applied to the determination of the studied drugs in commercial pharmaceutical formulations. Statistical comparison of the results with a well established reported method showed excellent agreement and proved that there is no significant difference in the accuracy and precision.

Influence of heating rate on thermoluminescence of Mg2SiO4 : Tb dosimeter

In this paper the influence of different heating rates on thermoluminescence (TL) kinetics, glow-peak temperatures, heights and integrated TL intensities of the main glow peak of recently produced Mg2SiO4 : Tb dosimeters are reported. The TL properties of the phosphor were investigated after β-irradiation at room temperature. Glow curve shape, peak shape, repeated initial rise and variable heating rate methods were used to analyse the TL kinetics of the main glow peak at 473 K. Results show that the TL glow peak of Mg2SiO4 : Tb phosphor obeys second-order kinetics. Analysis of the main dosimetric peak by using the methods mentioned above revealed that activation energy (E) is about 1 eV and the pre-exponential factor (s) is in the range 1010–1011 s−1. In addition it is ascertained that the glow-peak temperature increases with heating rate. The glow-peak height showed two maxima in the heating rate range investigated. The behaviour of the integrated TL intensities and the maximum glow-peak temperatures related to the heating rate have been discussed in terms of thermal quenching and clustering formation.

Kinetic and maximum-absorbance spectrophotometric methods for the determination of olanzapine

Two simple and sensitive spectrophotomet- ric methods were developed for the determination of olanzapine (OLZ) based on its oxidation with potassi- um iodate in a sulfuric acid medium to produce an intense violet-colored species exhibiting a maximum absorbance at 537 nm. The initial rate of formation and the maximum absorbance of the violet-colored oxidized product were monitored, in the first and sec- ond methods. The various experimental parameters affecting the rate of development and stability of the oxidized product were carefully studied and optimized. Beer 's law was obeyed up to 4.0 and 7.0 � gc m � 3 OLZ with correlation coefficients of 0.998 and 0.996 (n ¼ 6) and detection limits of 0.1 and 0.15 � gc m � 3 , for the initial rate and maximum absorbance methods. The proposed methods were conveniently applied to the determination of OLZ in its dosage forms and in spiked serum samples.

Development and Validation of Kinetic Spectrophotometric Method for the Determination of Losartan Potassium in Pure and Commercial Tablets

A validated kinetic spectrophotometric method has been developed for the determination of losartan potassium in pure and dosage forms. The method is based on oxidation of the losartan potassium with alkaline potassium permanganate at room temperature (25 ± 1 °C). The reaction is followed spectrophotometrically by measuring the increase in absorbance with time at 603 nm, and the initial rate, fixed time (at 12.0 min) and equilibrium time (at 90.0 min) methods are adopted for constructing the calibration graphs. All the calibration graphs are linear in the concentration range of 7.5–60.0 μg mL−1 and the calibration data resulted in the linear regression equations of n̈ = −6.422 × 10−7 + 1.173 × 10−5 C, A =3.30 × 10−4 + 5.28 × 10−3 C and A = −2.09 × 10−2 + 1.05 × 10−1 C for initial-rate, fixed time and equilibrium time methods, respectively. The limits of detection for initial rate, fixed time and equilibrium time methods are 0.71, 0.21 and 0.19 μg mL−1, respectively. The activation parameters such as Ea, ΔH‡, ΔS‡, and ΔG‡ are also determined for the reaction and found to be 87.34 KJ mol−1, 84.86 KJ mol−1, 50.96 JK−1 mol−1 and −15.10 KJ mol−1, respectively. The variables are optimized and the proposed methods are validated as per ICH guidelines. The method has been applied successfully to the estimation of losartan potassium in commercial tablets. The performance of the proposed methods was judged by calculating paired t- and F- values. The analytical results of the proposed methods when compared with those of the reference method show no significant difference in accuracy and precision and have acceptable bias.

Study of defect levels in CdTe using thermoelectric effect spectroscopy

We have studied the defect levels in as grown and post growth processed cadmium telluride (CdTe) using thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) techniques. We have extracted the thermal energy (Eth) and trapping cross section (σth) for the defect levels using the initial rise and variable heating rate methods. We have identified 10 different defect levels in the crystals. Thermal ionization energy values obtained experimentally were compared to theoretical values of the transition-energy levels of intrinsic and extrinsic defects and defect complexes in CdTe determined by first-principles band-structure calculations. On the basis of this comparison, we have associated the observed ionization levels with various native defects and impurity complexes.