Analysis of relative rate of reaction/process

Kinetic or rather thermokinetic analysis of thermal decomposition with releasing gaseous products is a current subject of discussion in many works and still devotes much attention to the property and meaningfulness of the single kinetics triplet determination. Analysis of thermogravimetric data by the relative rate of reaction/process were used to examine the frequently studied process of thermal dissociation of calcium carbonate as a model compound and comparing with published data for chemically defined compounds (azo-peroxyesters) and complex polyolefins?Cliquid paraffin mixtures. Two methods of correlation between parameters of the model were observed: thermodynamic and analytical expression. It was shown that these relations depend on course of the process. For analyzed model substance, enthalpy of reaction can be deduced on the basis of relation between coefficients in three-parameter equation. For large values of coefficients it is not possible, but other type of correlation was proved. Also, the relative rate of CO₂ gasification (Boudouard reaction) of brown and bituminous coals was analyzed and compared to that of its pyrolysis.     

New technique for generating high concentrations of gaseous OH radicals in relative rate measurements

We have developed a technique for generating high concentrations of gaseous OH radicals in a reaction chamber. The technique, which involves the UV photolysis of O₃ in the presence of water vapor, was used in combination with the relative rate method to obtain rate constants for reactions of OH radicals with selected species. A key improvement of the technique is that an O₃/O₂ (3%) gas mixture is continuously introduced into the reaction chamber, during the UV irradiation period. An important feature is that a high concentration of OH radicals [(0.53–1.2) × 10¹¹ radicals cm^?3] can be produced during the irradiation in continuous, steady‐state experiment. Using the new technique in conjunction with the relative rate method, we obtained the rate constant for the reaction of CHF₃ (HFC‐23) with OH radicals, k₁. We obtained k₁(298 K) = (3.32 ± 0.20) × 10^?16 and determined the temperature dependence of k₁ to be (0.48 ± 0.13) × 10^?12 exp[?(2180 ± 100)/T] cm³ molecule^?1 s^?1 at 253–328 K using CHF₂CF₃ (HFC‐125) and CHF₂Cl (HCFC‐22) as reference compounds in CHF₃–reference–H₂O gas mixtures. The value of k₁ obtained in this study is in agreement with previous measurements of k₁. This result confirms that our technique for generating OH radicals is suitable for obtaining OH radical reaction rate constants of ～10^?16 cm³ molecule^?1 s^?1, provided the rate constants do not depend on pressure. In addition, it also needed to examine whether the reactions of sample and reference compound with O₃ interfere the measurement when selecting this technique. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 317–325, 2003     

Conductivity measurements and relative association constants of some 3:1 electrolytes

Conductance measurements are reported for dilute aqueous solutions at 25°C of potassium hexacyanoferrate(III) and of sodium, potassium, rubidium, and cesium octacyanotungstate(V). The results are interpreted in terms of ion-pair formation, and association constants for the formation of these ion pairs are calculated. For the ion pairs MW(CN) ₈ ^2– , the results are: M=Na, 12±8; K, 23±7; Rb, 37±10; and Cs, 51±4M ^–1, at 25°C and zero ionic strength.     

CN radical reaction rate measurements by time-resolved FM spectroscopy

Time-resolved frequency-modulation spectroscopy is shown to be an effective method for measuring the rates of gas-phase reactions. As an example, the rate constant for the reaction of CN with C₂H₄ at 298 K is measured to be 2.5 ± 0.2 × 10⁻¹⁰ cm³s⁻¹, in good agreement with other literature values. The efficiency and sensitivity of this technique will be of interest to the chemical kineticist. © 1997 John Wiley & Sons, Inc.     

The interpretation of relaxation rate measurements made in flow systems

Methods for analyzing reaction rate data obtained from fluorescent decays distorted by flow effects are discussed, with particular reference to the measurement of relaxation rates in flowing mixtures containing reactive species by the laser-induced fluorescence technique.     

Persulfate and perphosphate oxidation of 2-propanol: A relative rate study

A competitive reaction study for two isoelectronic peroxides (peroxodisulfate S₂O and peroxodiphosphate P₂O) interacting with the free radical ·Clpar;CH₃)₂OH is described. The radical formation is initiated by photolysis, the amounts of peroxide remaining analyzed volumetrically. It is found that persulfate reacts with the organic radical over 100 times more rapidly than does perphosphate. Mechanistic consequences in relation to previous work are briefly discussed.     

Determination of gas-phase ozonolysis rate coefficients of a number of sesquiterpenes at elevated temperatures using the relative rate method

The rates of ozonolysis of four sesquiterpenes, β-caryophyllene, α-humulene, isolongifolene and α-cedrene, are determined in the gas phase at an elevated temperature of 366 ± 3 K and a pressure of ~780 Torr using the EXTreme RAnge chamber (EXTRA). The experimentally obtained rate coefficients agree with extrapolated room temperature rate coefficients for isolongifolene and α-cedrene but not for β-caryophyllene and α-humulene, which were found to be three orders of magnitude slower than this in the literature. These new measurements support the hypothesis that operating under ambient conditions, kinetic measurements of condensable species can be influenced adversely by heterogeneous processes and should therefore be treated with caution.     

Comparisons of integration vs. rate measurements for oxidase enzymatic analyses

Results obtained by integration of net indicator-substance signals during the progress of a reaction are compared to those concurrently achieved by rate estimation with the time vs. signal data, for glucose oxidase assays and enzymatic glucose determinations. Computer-simulated integration results typically exhibit data quality superior to that of rate measurements, for the entire very wide ranges of oxidase activities and concentrations studied. Only at extreme instrument-noise levels and low analyte values did any of the rate procedures (the variable-time method) show some advantage over integration. The data quality, expressed as relative uncertainties, of the simulated results obtained by use of the integration method was typically up to 120 times better than that obtained by rate methods, with greater enhancement at higher enzyme activities or substrate concentrations. Experimental results spanning physiological concentrations showed similar trends, but the advantage was not as great as that shown in the simulations.     

Molecular beam measurements of small specific rate constants for unimolecular reactions

The investigation of unimolecular reactions with small rate constants is difficult owing to competing processes (inelastic collisions and bimolecular reactions) and the diffusion of reactant and product molecules out of the detection volume. For this reason, a new experimental approach for the measurement of specific rate constants in a molecular beam experiment has been exploited; instead of monitoring the temporal change of intensity as in a cell experiment, we monitor the spatial change along the molecular beam axis after laser excitation. For a given particle velocity the flight path between excitation and detection region defines the reaction time. By varying the distance the specific rate constant can be determined directly both from the decrease in the number density of reactant molecules as well as from the increase in product molecules. As a model system, the laser-induced (λ = 193 nm) photodissociation of mesitylene (trimethylbenzene) is studied. Previous experiments on the specific rate constant of mesitylene at this excitation energy differ between each other by about a factor of ten. By combining the new results with measurements at higher excitation energies, rate constants over a range of two orders of magnitude are now available for this reaction. The differences between the various experimental results are discussed within the framework of a statistical theory.     

Determination of relative rate of spectral events by novel modification of two-dimensional correlation spectroscopy

Sign of two-dimensional (2D) correlation peaks provides information on sequence of spectral events. This information is related to molecular mechanism of changes in a given system. Recently, few papers addressing the problems with interpretation of the sign of 2D correlation peaks have been published. To overcome these problems, a modification of the generalized 2D correlation method has been proposed. This method compares variations in the dynamic spectrum with a linear change at a reference point. The rates of spectral responses at individual wavenumbers are proportional to magnitudes of the peaks in the slice of asynchronous spectrum at the reference point. This way, analysis of complex 2D contour plots is replaced by a simple examination of one-dimensional (1D) slice spectrum. In spite of reduced ability of the resolution enhancement, in special cases the proposed method provides information not accessible from the classical 2D correlation analysis. At first, the principles of this method are shown with the synthetic data. Next, the influence of spectral separation, band width and position changes on the slice spectrum is evaluated. Finally, the proposed approach is applied to the experimental spectra of two hydrogen-bonded systems.     

Absolute fluorescence quantum yields by relative fluorescence and photoacoustic measurements of low level luminescence quenching

A method is reported which allows absolute fluorescence quantum yields Φ_F to be obtained from relative fluorescence and non-radiative emission (photoacoustic) data. Absolute calibration procedures, external standards, and total fluorescence quenching are eliminated through use of low concentrations of a heavy atom quencher. The relative decrease in fluorescence emission and increase in non-radiative emission as a function of the quencher concentration are related to Φ_F through a set of simultaneous equations. Fluorescence quantum yields of Φ_F = 0.59 ± 0.03 for cresyl violet perchlorate in EtOH and Φ_F = 0.55 ± 0.02 for quinine sulfate in 0.1 N H₂SO₄ are reported.     

Thermodynamic interpretation of three-parametric equation: Part II. The relative rate of reaction

The way of practical using of three-parametric equation transformed into linear relation between relative rate of decomposition and temperature was presented, basing on thermal decomposition of chemically defined and other compounds. Further possibilities of interpretation of that relation were presented. The meaning of slope (a ₂) was laid down, particularly with connection to thermodynamic considerations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nanoemulsion stability: Experimental evaluation of the flocculation rate from turbidity measurements

The coalescence of liquid drops induces a higher level of complexity compared to the classical studies about the aggregation of solid spheres. Yet, it is commonly believed that most findings on solid dispersions are directly applicable to liquid mixtures. Here, the state of the art in the evaluation of the flocculation rate of these two systems is reviewed. Special emphasis is made on the differences between suspensions and emulsions. In the case of suspensions, the stability ratio is commonly evaluated from the initial slope of the absorbance as a function of time under diffusive and reactive conditions. Puertas and de las Nieves (1997) developed a theoretical approach that allows the determination of the flocculation rate from the variation of the turbidity of a sample as a function of time. Here, suitable modifications of the experimental procedure and the referred theoretical approach are implemented in order to calculate the values of the stability ratio and the flocculation rate corresponding to a dodecane-in-water nanoemulsion stabilized with sodium dodecyl sulfate. Four analytical expressions of the turbidity are tested, basically differing in the optical cross section of the aggregates formed. The first two models consider the processes of: a) aggregation (as described by Smoluchowski) and b) the instantaneous coalescence upon flocculation. The other two models account for the simultaneous occurrence of flocculation and coalescence. The latter reproduce the temporal variation of the turbidity in all cases studied (380 ≤ [NaCl] ≤ 600 mM), providing a method of appraisal of the flocculation rate in nanoemulsions.     

Evaluation of reinforcement corrosion rate in concrete structures by electrochemical noise measurements

Experimental results on the polarization resistance estimation in a three-electrode setup that consists of two carbon steel electrodes and the silver reference electrode are presented. The corrosion rate varies with the conditions changing inside the concrete structure due to its irregular wetting in an aqueous solution of NaCl. The change of polarization resistance of a single steel electrode can be identified by voltage and current electrochemical noise measurements. The observed change does not exceed the factor of a few times of the originally recognized value. The newly proposed method of electrochemical noise analysis for monitoring of swift corrosion rate changes is presented in detail. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 5, pp. 611–616. The text was submitted by the authors in English.     

Double-cuvette ISES: in situ estimation of enantioselectivity and relative rate for catalyst screening

Described is a new method for the screening of an array of catalysts, in situ, to estimate enantioselectivity and relative rates. We term this approach "double-cuvette ISES (in situ enzymatic screening)". The Co(III)-salen mediated hydrolytic kinetic resolution (HKR) of (+/-)-propylene oxide is used as a model reaction to demonstrate proof of principle. In two parallel cuvettes, a lower CHCl3-based organic layer is loaded with the epoxide and the chiral salen catalyst. Aqueous reporting layers, containing distinct "reporting enzymes" and their nicotinamide cofactors, are layered above the organic layers. The 1,2-propanediol enantiomers formed by the chiral catalyst diffuse into the aqueous layer and are oxidized there by the reporting enzymes at rates dependent upon the diol concentration, the R:S ratio of the diol, and the enantioselectivity of the reporting enzymes. A focused chiral salen library was constructed from seven chiral 1,2-diamines, derived from amino acid, terpenoid, and carbohydrates skeletons, and seven salicylaldehyde derivatives. Double-cuvette ISES identified a couple of interesting combinatorial hits in this salen array, wherein either the sense or magnitude of enantioselection for a given chiral diamine depends significantly upon the choice of "salicylaldehyde" partner. A comparison of predicted ee's and relative rates using this new screening tool with those independently measured is provided.     

Overall rate constant measurements of the reaction of chloroalkylperoxy radicals with nitric oxide

The overall rate constants of the NO reaction with chloroalkylperoxy radicals derived from the Cl-initiated oxidation of several atmospherically abundant alkenes-ethene, propene, 1-butene, 2-butene, 2-methylpropene, 1,3-butadiene, and isoprene (2-methyl-1,3-butadiene)-were determined for the first time via the turbulent flow technique and pseudo-first-order kinetics conditions with high-pressure chemical ionization mass spectrometry for the direct detection of chloroalkylperoxy radical reactants. The individual 100 Torr, 298 K rate constants for each monoalkene system were found to be identical within the 95% confidence interval associated with each separate measurement, whereas the corresponding rate constants for 1,3-butadiene and isoprene were both approximately 20% higher than the monoalkene mean value. Our previous study of the reaction of hydroxylalkylperoxy radicals (derived from the OH-initiated oxidation of alkenes) with NO yielded identical rate constants for all of the alkenes under study, with a rate constant value within the statistical uncertainty of the value determined here for the NO reaction of chloroalkylperoxy radicals derived from monoalkenes. Thus, the reaction of NO with chloroalkylperoxy radicals derived from dialkenes is found to be significantly faster than the NO reaction with either chloroalkylperoxy radicals derived from monoalkenes or hydroxyalkylperoxy radicals derived from either mono- or dialkenes.     

An absolute and relative rate study of the reaction of oh radicals with dimethyl sulfide

The gas phase reaction of OH radicals with dimethyl sulfide (CH₃SCH₃, DMS) has been studied using both an absolute and a relative technique at 295 ± 2 K and a total pressure of 1 atm. The absolute rate technique of pulse radiolysis combined with kinetic spectroscopy was applied. Using this technique a rate constant of (3.5 ± 0.2) × 10^?12 cm³ molecule^?1 s^?1 was obtained. For the relative rate method, rate constants for the reaction of OH with DMS were found to increase with increasing concentrations of added NO. These results are compared with the large body of kinetic and mechanistic data previously reported in the literature.     

Analysis of pitting corrosion rate

A method for estimating pitting corrosion rate is proposed. The method is based on the analysis of anodic and cathodic voltammograms that were measured on the specimens of two types: on the test specimen coated with an oxide film, which formed under the open-circuit conditions, and on the specimen, whose surface was totally activated with aggressive ions. The use of Tafel portions of these voltammograms in various combinations enables us to estimate the corrosion potential and corrosion current for general corrosion proceeding in the induction period, for pitting corrosion at the initial stage of its development, for a highly developed pitting corrosion, when the major area of specimen surface is occupied with pits, and also in the intermediate case.