Kinetic analysis of complex reactions

TG and DSC techniques have been extensively used to study complex solid state reactions. For complex reactions (constituting of all exothermic or all endothermic one step first order individual reactions), it has been shown that the results of TG and DSC instruments may not be identical. This is because the TG instrument is incapable of correctly recording the true effective reaction rate of complex reactions (if the reaction rate is not proportional to the total amount of reactants). This may happen when the reaction rates of the individual reactions in the composite reaction mixture are significantly different. In this communication it has also been suggested that the Friedman analysis of obtaining activation energy (E) may be inapplicable for complex or composite reaction due to the fact that, there may be no unique effective constant conversion at various heating rates.

---

Zusammenfassung TG- und DSC-Techniken wurden in großem Umfang zur Untersuchung komplexer Festkörperreaktionen herangezogen. Für (alle exotherme oder alle endotherme individuelle Einschritt-Reaktionen erster Ordnung in sich einschließende) komplexe Reaktionen wurde gezeigt, daß die Ergebnisse von TG- und DSC-Geräten nicht notwendigerweise identisch sind, weil das TG-Gerät die wahre effektive Reaktionsgeschwindigkeit komplexer Reaktionen nicht korrekt zu registrieren imstande ist (wenn die Reaktionsgeschwindigkeit nicht proportional der Gesamtmenge der Reaktanten ist). Dies kann der Fall sein, wenn die Reaktionsgeschwindigkeiten der individuellen Reaktionen im Reaktionsgemisch significant unterschiedlich sind. In der vorliegenden Publikation wird auch darauf hingewiesen, daß die Friedman-Analyse zur Ermittlung der Aktivierungsenergie (E) bei komplexen Reaktionen nicht unbedingt anwendbar ist, da die Konversion von der Aufheizgeschwindigkeit abhängig sein kann.

---

. , , , . , , . , . , E . , .

---

---

The author would like to thank the reviewer for his comments.     

Flow-reversal flow-injection analysis. Enhancement of flow-injection titrations

Most FIA applications involve simple unidirectional continuous-flow regimes. This paper continues the development of the flow-reversal (frFIA) approach and its applications. Flow-reversal flow programming achieves flexibility in operation without requiring manifolds to be refitted. The sensitivity of FIA titrations can be increased by use of frFIA.     

Radiometric flow-injection analysis

The initiation, present status and future trends of radiometric flow-injection analysis is described.     

Sandwich standardization in flow-injection analysis

The use of internal coupling of valves as an easy and handy way to intersperse calibration and determination is shown. The advantages of derivative over normal recordings are considered.     

Laminar dispersion in flow-injection analysis

Simple expressions are given for the dispersion and the travel times of samples in simple flow-injection analysis systems. The sum of these two quantities is the total residence time of the sample in the system. The expressions are based on numerical solutions of the diffusion-convection equation. Preliminary experiments are in agreement with the derived simple expressions, as are peak curve shapes. Diffusion coefficients can be obtained in a straightforward manner.     

Use of complex displacement reactions in photometric analysis

The theoretical requirements for the quantitative photometric determination of trace amounts of metal ions by use of complex displacement reactions are discussed and a calculation method using conditional equilibrium constants and side-reaction functions defined and used by Ringbom is recommended. As an example, the requirements in the determination of erbium(III) by using copper(II)-EDTA complex and PAN indicator in ammonium lactate buffer solutions are calculated.     

Prussian-Blue-based amperometric biosensors in flow-injection analysis

Optimisation of the electrodeposition of Prussian Blue onto mirrored glassy carbon electrodes yielded a modified electrode practically insensitive to oxygen reduction. At the same time the electrode activity towards hydrogen peroxide reduction was extremely high. This allowed the detection of hydrogen peroxide by electroreduction over a wide potential range. Flow-injection investigations of this electrode inserted into a flowthrough electrochemical cell of the confined wall-jet type showed that the response for hydrogen peroxide is limited by diffusion. Glucose and alcohol biosensors were made by immobilisation of glucose oxidase and alcohol oxidase respectively, within a Nafion layer, onto the top of the Prussian-Blue-modified electrodes. By increasing the density of Nafion and decreasing the measuring potential the glucose biosensor was made completely insensitive to both ascorbate and acetominophes.     

Determination of thorium with DBF-arsenaz by flow-injection analysis

A sensitive,selective,rapid,simple,and convenient method for the determination of thoriumwas developed.The method was based on the color reaction of thorium with DBF-arsenazo in astrong acidic medium (4 mol/L HCI).The flow-injection technique was used with an automated flow-injection analyzer.Beer's law was obeyed in the range of 0—1.4 μg/mL.The relative standarddeviations were less than 1%.     

Automated prevalve dilution in flow-injection analysis

Summary An automated method for the determination of chloride in surface, ground and domestic water based on the concept of flow-injection analysis is described. Samples of a widely varying chloride content up to 800 mg/l are analysed by using an automated prevalve dilution technique. The sampling rate is 92 samples per hour. The method is suitable for the analysis of chloride with a coefficient of variation of better than 1.2%. The measurement is based on the redcoloured iron thiocyanate complex.

---

Automatische Verdünnung vor dem Ventilsystem bei der FließinjektionsanalyseAutomatische Chloridbestimmung in Oberflächen-, Grund- und Leitungswasser

Zusammenfassung Wasserproben mit stark schwankenden Chloridgehalten bis zu 800 mg/l wurden mit Hilfe der automatischen Verdünnungstechnik analysiert. Der Probendurchsatz betrug 92 je Stunde. Der Variationskoeffizient ist besser als 1,2%. Die Messung beruht auf der Bildung des rotgefärbten Eisenthiocyanatkomplexes.

     

Micellar-catalyzed reactions for flow-injection systems : Determination of pyridoxal

The advantages of applying the solubilization and catalytic properties of aqueous micelle solutions to reactions taking place in flow-injection systems are demonstrated. The reaction of pyridoxal (a B₆ vitamin) with cyanide was investigated in both aqueous and micellar cetyltrimethylammonium bromide (CTAB) solution. Higher sensitivities and lower limits of detection were obtained for the micellar carrier, with the pseudo-first-order reaction rate increasing by a factor of two in 0.05 M CTAB relative to water carrier. Because the micellar aggregates also increase fluorescence quantum yields, use of fluorescence detection gave further signal enhancement, with the limit of detection lowered by a factor of three. Measurement of dispersion in the two systems was also investigated and compared. A new method of characterizing dispersion in flow-injection systems, based on moment analysis of exponentially modified Gaussian peak shapes, is described.     

Speciation studies by flow-injection analysis

A survey of the use of flow-injection analysis for speciation studies is presented. Comments are made about the future potential of the technique, and suggestions are made for further research.     

A diamagnetic dititanium(III) paddlewheel complex with no direct metal-metal bond

Reaction of Ti[N(But)Ar]3 (Ar = 3,5-C6H3Me2 or Ar' = C6H5) with CO2 at -40 degrees C produces diamagmetic Ti(III) paddlewheel complexes with long Ti-Ti separations (>3.4 Angstrom), thus excluding direct Ti-Ti bonding. 1H NMR spectroscopy shows that the compounds are diamagnetic in solution in the temperature range of -65 to +70 degrees C. In the solid state, the diamagnetism was found to persist between 2 and 300 K. Calculations at the density functional theory level suggest that the diamagnetism results from antiferromagnetic coupling by superexchange through the ligand pi system.     

Substitution reactions of metal-metal quadruply-bonded tetrachlorotetrakis(phosphine)dimolybdenum compounds

Summary The substitutions of dimolybdenum compounds of the type [Mo₂Cl₄L₄] (where L=PEt₃, PEt₂Ph or PEtPh₂), together with the preparation and characterization of the metal-metal quadruply bonded [Mo₂Cl₄(triphos)₂] obtained from K₄Mo₂Cl₈] are described.     

流动注射胶束化学发光法测定曲克芦丁

基于HCl介质中曲克芦丁与KIO4-MnSO4反应产生强的化学发光,且吐温-80的存在能使这一反应的化学发光强度大大增强。结合流动注射技术,建立了流动注射化学发光法测定曲克芦丁的新方法。在优化的实验条件下,曲克芦丁的质量浓度在2.0×10-7~8.0×10-5g/mL范围内与化学发光强度呈良好的线性关系,检出限为7×10-8g/mL。对1.0×10-6g/mL曲克芦丁进行11次平行测定,方法的相对标准偏差为1.8%。用于曲克芦丁片剂和针剂中曲克芦丁的测定。     

Spectrophotometric determination of iron with ferrozine by flow-injection analysis

Two methods for the determination of iron by normal FIA and reversed FIA were developed using sodium 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-4',4'-disulphonate (ferrozine). The reagent formed a chelate with Fe(II) in hexamethylentetramine buffered medium at pH 5.5. In one previous reaction coil Fe(III) was reduced to Fe(II) by ascorbic acid and in the other reaction coil the complexation reaction was developed. The linear range of the determination was 0.5-6 and 0.1-5 mug ml(-1) of iron for normal FIA and reversed FIA respectively. The proposed method was sensitive (detection limit 0.012 and 0.010 mug ml(-1)), rapid and reproducible (RSD 0.3 and 0.28%). The method was satisfactorily applied to the determination of iron in waste water, toadstool tissue, potato leaves, human hair and bauxites at a sampling rate of 90 and 50 samples h(-1) for normal FIA and reversed FIA respectively.     

Chronocoulometric flow-injection analysis with solvent polymeric membrane ion sensors

A method to carry out chronocoulometric measurements with solvent polymeric membrane ion sensors in flow-injection systems has been developed. For this, a double potential step was synchronised to the passage of the sample plug through the detector cell. A four-electrode potentiostat with ohmic drop compensation and a new flow-through cell to incorporate the four-electrode and the membrane were developed. A plasticized poly(vinyl chloride) (PVC) membrane containing TBATBP was used and the procedure was applied to the determination of tetraethyl-ammonium. The effect of the electrochemical and flow-injection variables was studied. In the selected conditions, a linear relationship between the quantity of electricity and tetraethyl-ammonium concentration was obtained in the range 5×10⁻⁷-5×10⁻⁵ M. The detection limit was 7×10⁻⁸ M. Good repeatability and between day reproducibility were obtained. The potential application to other quaternary ammonium ions including acetylcholine was also studied.     

Determination of pentachlorophenol by flow-injection analysis with spectrophotometric detection

Two FIA methods for the determination of pentachlorophenol, based on its oxidizing-condensation reaction with 4-aminoantipyrine in aqueous medium and spectrophotometric detection in the conventional and stopped-flow modes, are described. In both cases the calibration graphs are linear in the range from 1.0 to 60.0 mug/ml pentachlorophenol, with sampling rates of 48 and 28 samples/hr for triplicate analysis, respectively. Interferences from phenol and other chlorophenols have been evaluated. The stopped-flow method is particularly useful for determining pentachlorophenol in commercial formulations containing no more than 15% of other chlorophenols.     

Metal speciation by flow-injection analysis

A two-channel switching valve is incorporated in the flow-injection manifold for on-line control of the metal speciation of Fe(II)/Fe(III) and Cr(III)/Cr(VI). 1,10-Phenanthroline is used for the iron determinations and diphenylcarbazide for chromium. The absorbances are measured at 512 and 540 nm, respectively. When a 30-mul injection loop is used, the response is linear for 0.5-30.0 ppm Fe and 0.5-40.0 ppm Cr. The relative standard deviation in each case is approximately 1%. The method allows at least 180 injections per hour.     

Processing and error analysis of signals in flow-injection analysis

The processing and error analysis of signals in flow-injection systems were systematically studied by simulation and experimental measurements. The content includes an error analysis for peak-height and peak-area signal, a least-squares filtering procedure applied to the flow-injection curve and a peak recognition to remove interferences from air bubbles. Simulation results were obtained by statistical processing of peak-height and peak-area values from Gaussian curves to which noise had been added. The experimental measurements were done by an automatic flow-injection device to obtain detailed information for each individual point of a peak. 2-(2-Arsenophenylazo)-7-(2,6-dichlorophenylazo-4-sulphonic acid)-1,8-dihydroxynaphthalene-3,6-disulphonic acid (DCSA) was used for measuring physical dispersion alone, and Fe(II)-o-phenanthroline for the measurement of both physical dispersion and chemical reaction. The results from computer simulation and experiments agreed well.     

Reduction of injection variance in flow-injection analysis

In order to achieve maximum sensitivity in flow-injection analysis, sample dispersion must be kept to a minimum. This dispersion process, however, is not well understood. Studies of the dispersion process have concentrated on dispersion within the flow manifold while dispersion due to the injection process has been largely ignored. Here sample injection loops packed with inert glass beads and a Serpentine II (distorted) empty loop were constructed and compared to traditional empty sample loops. Digitization of the response curves and subsequent calculation of the statistical moments were used to compare the contribution of each sample loop type to the total system dispersion. Both packed and Serpentine II sample loops were shown to decrease dispersion and increase throughput in flow-injection systems. Plots of peak variance vs. injection volume show variance increasing 1.67 times faster with traditional open sample loops compared to packed loops. When combined with other peak width minimization techniques, this method should further lower concentration limits of detection.