Kinetic treatment of unsegmented flow systems : Part 1. Subjective and semiquantitative evaluations of flow-injection systems with gradient chamber

A variable-time kinetic model is used to evaluate a single-channel flow-injection system with gradient chamber that has been identified as a continuous-flow titration. A physical model, mathematical equations, computed concentration vs. time profiles, experimental data, and formal definitions are used to identify qualitative and quantitative features of the method that have not been apparent from the titration model for the system. It is shown that determinations can be performed with and without reactant in the flow stream and when reactant is in the flow stream, with and without reactant in the gradient chamber when the sample is introduced. It is shown that lowest concentrations with shortest cycle times can be achieved when determinations are performed without reagent in the gradient chamber initially. Characteristics unique to each of three different data processing options are used to evaluate the validity of equations presented. It is suggested that some methods previously identified as continuous-flow titrations are most accurately identified as variable-time kinetic methods, and it is shown that this semantic differentiation can provide improved insight into the methods and can expand the scope of the methods by suggesting new experimental approaches with potential advantages relative to previously described procedures.     

Kinetic treatment of unsegmented flow systems : Part 5. Comprehensive Treatment for Systems with Gradient Chamber

A variable-time kinetic model is used to derive a comprehensive set of equations for an unsegmented flow system with a gradient chamber for the situations in which the analyte, reactant or product is monitored. Equations are presented for single-channel systems with excess analyte and excess reactant. These response equations are used to develop relationships that permit one to calculate whether analyte or reactant is in excess, to understand the merits and limitations of dispersion coefficients and to relate measured time intervals to initial analyte concentration for both single- and dual-channel systems. Relationships between time intervals (Δt) and the logarithm of initial analyte concentration (C) are inherently non-linear and include terms for the reference-point concentrations between which time intervals are measured. Special cases for which Δt vs. In C relationships are linear and do not include the reference-point concentrations are discussed and the assumptions that lead to these simplified equations are identified. Computed response curves are included to illustrate behavior in different phases of the processes.     

Kinetic treatment of unsegmented flow systems : Part 3. Flow-injection system with gradient chamber evaluated with a linearly responding detector

A variable-time kinetic model is evaluated for a flow-injection sample-processing system that includes a gradient chamber. Quantification of triiodide, including reaction with thiosulfate, is used as a model system. A thin-layer electrochemical detector consisting of a platinum working electrode and glassy carbon counter electrode with a 200-mV difference imposed between them yields linear current response for triiodide concentrations between 0.1 and 1.2 mM. Equations based on the kinetic model are evaluated for situations in which neither the flow stream nor gradient chamber contain reactant (thiosulfate) initially, only the flow stream containes reactant, and both the flow stream and gradient chamber contain reactant. Both calibration data and response curves exhibit very good agreement between theory and experiment except for the situation in which only the flow stream contains reactant. In this case, the theoretical treatment accurately predicts the general nature of responses but predicts slightly longer time intervals for completion of the process than are observed experimentally.     

Peroxidase-catalysed rupture of the CF bond as and indicator reaction for enzyme immunoassay : Kinetic determination of human immunoglobulin G, α-fetoprotein and placental lactogen

A fluoride ion-selective electrode is utilized as a sensor for the kinetic determination of peroxidase label in enzyme immunoassays. The method is based on a sandwich enzyme-linked immunosorbent assay (ELISA) technique, the peroxidase-catalysed rupture of the covalent CF bond in 4-fluorophenol and the subsequent release of fluoride ions. The determination of human immunoglobulin G (lgG), human α-fetoprotein (AFP) and human placental lactogen (HPL) was investigated. The potentiometric measurement of the rate of release of fluoride ion within 5 min provided a direct correlation with the concentration of analyte present in the sample. The concentration ranges investigated for the analytes were IgG 30 μg l^?1–10mg l^?1, AFP 5–500 μg l^?1 and HPL 60 ng l^?1–1 mg l^?1. Under the given experimental conditions, the detection limits were IgG 30, AFP 12.8 and HPL 1 μg l^?1. Replacing the rate method with the fixed-time mode (15–30 min) did not improve the detection limits. The performance of the present method was found to be comparable to that of the spectrophotometric detection technique.     

Scaling Up of a Photoreactor for Formic Acid Degradation Employing Hydrogen Peroxide and UV Radiation

A model for scaling up a homogeneous photoreactor was developed and experimentally verified in a pilot‐plant‐size apparatus. The procedure is exemplified by the oxidation of dilute aqueous HCOOH solutions with UV radiation (254 nm) and H₂O₂. First, the kinetic model and the kinetic parameters of the HCOOH degradation were obtained in a well‐stirred, small, batch flat‐plate photoreactor (volume=70 ml). The method employed in the analysis of the experimental results yielded reaction‐rate expressions for HCOOH and H₂O₂ that were independent of the reactor configuration. These kinetic equations and the corresponding kinetic constants were then used in a mathematical, fully deterministic model of a continuous‐flow, 2‐m‐long, annular reactor (0.0065 m² of cross section for flow) operating in a laminar‐flow regime to predict exit concentrations of HCOOH. Irradiation was provided in both cases by two different types of germicidal lamps. No additional experiments were made to adjust the reactor‐model parameters. Theoretical predictions from the representation of the reactor performance obtained were compared with experimental data furnished by experiments in the much‐larger‐size, cylindrical‐flow reactor. Results showed good agreement for the range of variables explored; they corresponded to expected operating conditions in water streams polluted with low concentrations of organic compounds.     

Determination of boron in water by flow injection/inductively coupled plasma emission spectrometry

A method is described involving flow injection and inductively coupled plasma emission spectrometry for the determination of boron (0.5–25 mg l^?1) in water at a sampling rate of 320 h^?1. An 11-ml capacity cloud chamber, with a tangential aerosol outlet, was used to introduce the nebulized sample to the plasma. The sample volume injected was 300 μl. The relative standard deviation for peak height was 3% for 10 mg l^?1 of boron at a carier flow-rate of 3.5 ml min^?1. The wash-out between samples was improved by using the 11-ml cloud chamber rather than a conventional 110-ml chamber.     

The steady-state approximation: Fact or fiction?

The kinetic equations for an 81-reaction model of a photochemical smog chamber have been solved using a complete numerical integration as well as a quasi-steady-state approximation (QSSA) procedure. The two sets of results differ markedly in their prediction of experimentally significant factors such as the hydrocarbon depletion rate and the ozone and (NO)_x peaking times. The sources of the discrepancy are traced to the fact that the assumed steady-state conditions were not satisfied, leading to errors in the concentrations of intermediate radicals which in turn affect critical rates in the reaction model. The occurrence of such discrepancies in various types of reaction models, and with different QSSA strategies, is discussed, and it is concluded that the extent of such errors in QSSA calculations cannot be reliably predicted. Their impact on conclusions regarding reaction mechanisms and rate constants can surpass uncertainties in experimental data; conversely the credibility of predictions derived through QSSA calculations becomes highly suspect. Since recently devloped methods for complete numerical integration of systems of kinetic equations are now available, it is recommended that these be adopted in future work, and that the use of QSSA be abandoned.     

Continuous potentiometric determination of sulphate in a differential flow system

Continuous monitoring of sulphate in a differential flow system equipped with two lead ion-selective electrodes is described. All solutions contained 75% methanol and were adjusted to pH 4. In the flow cell, a standard solution of lead(II) is pumped past the first sensing electrode and is mixed with the sample stream containing sulphate in a small mixing chamber; the mixture containing excess of lead(II) and lead sulphate precipitate then flows through the second sensing electrode chamber. The potential difference depends on the sulphate content in the sample. The effects of lead electrode passivation and the interferences of calcium and chloride are discussed. The system is useful for routine sulphate determination in the range 30–400 mg l^-1 with an accuracy of ±5%     

Chemiluminometric determination of the pesticide 3-indolyl acetic acid by a flow injection analysis assembly

A new method is proposed for the chemiluminescent determination of the pesticide 3-indolyl acetic acid by means of an flow injection analysis system. The chemiluminescence emission is obtained by oxidation of the analyte with Ce (IV) in nitric acid and presence of β-cyclodextrine.The continuous-flow method allows the determination of 159 samples h⁻¹ of 3-indolyl acetic acid in an interval of concentrations over the range 0.5-15.0 mg l⁻¹. The limit of detection was 0.1 μg l⁻¹ and the R.S.D. (n, 17) at 2.0 mg l⁻¹ of the pesticide level was 2.7%. The method was applied to water samples.     

Investigation of the differentiation of kinetic models

The differentiation of known kinetic models in appropriately chosen coordinate systems has been investigated. A new method of defining the control of chemical reaction rate has been presented, the suggested method being based on the use of known kinetic model equations. It consists in comparing the respective model courses with the experimental. For isothermal conditions, the model curves, f(α) vs. α, are compared with the experimental DTG vs. TG. For conditions of linear temperature increase, however, model courses of the type (1/f(α) (df(α)/dα) vs. α and corresponding experimental curves {[(DDTG/DTG²)—(Eγ/RT²)](1/DTG)} vs. TG are used for comparison: such visual comparison allows a rapid estimation of the closeness of experimental results and predicted models. This method was verified with positive results with experimental material under isothermal conditions (for the thermal decomposition of PbSO₄ and for the oxidation of copper in air) and for conditions of linear temperature increase (for the oxidation of copper in air).     

Multiphase flow modeling in centrifugal partition chromatography

The separation efficiency in Centrifugal Partition Chromatography (CPC) depends on selection of a suitable biphasic solvent system (distribution ratio, selectivity factor, sample solubility) and is influenced by hydrodynamics in the chambers. Especially the stationary phase retention, the interfacial area for mass transfer and the flow pattern (backmixing) are important parameters. Their relationship with physical properties, operating parameters and chamber geometry is not completely understood and predictions are hardly possible. Experimental flow visualization is expensive and two-dimensional only. Therefore we simulated the flow pattern using a volume-of-fluid (VOF) method, which was implemented in OpenFOAM^®. For the three-dimensional simulation of a rotating FCPC^®-chamber, gravitational centrifugal and Coriolis forces were added to the conservation equation. For experimental validation the flow pattern of different solvent systems was visualized with an optical measurement system. The amount of mobile phase in a chamber was calculated from gray scale values of videos recorded by an image processing routine in ImageJ^®. To visualize the flow of the stationary phase polyethylene particles were used to perform a qualitative particle image velocimetry (PIV) analysis. We found a good agreement between flow patterns and velocity profiles of experiments and simulations. By using the model we found that increasing the chamber depth leads to higher specific interfacial area. Additionally a circular flow in the stationary phase was identified that lowers the interfacial area because it pushes the jet of mobile phase to the chamber wall. The Coriolis force alone gives the impulse for this behavior. As a result the model is easier to handle than experiments and allows 3D prediction of hydrodynamics in the chamber. Additionally it can be used for optimizing geometry and operating parameters for given physical properties of solvent systems.     

A flow sampling strategy for the analysis of oil samples without pre-treatment in a sequential injection analysis system

The present work describes a sequential injection analysis (SIA) system adapted to direct analysis of oil samples. In this way, the flow system was designed by incorporation, in the SIA system, of an injection valve that was responsible for the sample insertion. With the developed sampling strategy the sample pre-treatment outside the flow system is avoided and also the problems associated with viscous samples in flow systems.The developed SIA system was applied to the determination of iron (Fe(III)) in edible oil samples and was based on the formation of a red complex (λ = 510 nm) between Fe(III) and thiocyanate in organic medium. A mixture of methanol:chloroform (85:15) was used as carrier solution and possible refractive index associated with the spectrophotometric detection was avoided by introduction of a mixing chamber in the flow system. The presented methodology produced 4.2 ml of effluent and consumed 150 μl of sample and 0.95 mg of thiocyanate per determination.Linear calibration plots were obtained for Fe(III) concentrations up to 25 mg l⁻¹ and the detection limit of the determination was 0.31 mg l⁻¹. The developed methodology exhibited a good precision, with an R.S.D. < 3.5% (n = 15) and a determination frequency of 20 determinations h⁻¹. The results of the analysis were evaluated by recovery studies (96.5-104.5%) and by the analysis of two AOCS reference samples.     

The kinetic determination of clinically significant enzymes in an automated flow-injection system with fluorescence detection

An automated flow-injection manifold is described for the kinetic determination of enzyme activities by a stopped-flow procedure with fluorescence detection. The linear calibration range for alkaline phosphatase is 0/2-250 U l^?1 with a precision of 2%; sample thoughput is 35/2-40 h^?1. Linear responses were also obtained for lipase (0/2-100 U l^?1), acethylcholinesterase (0/2-500 U l^?1) and chymotrypsin (0/2-200 U l^?1). The advantages of this approach to the determination of plasma enzyme activities include sensitivity and the small sample and reagent volumes needed.     

Spectrophotometric determination of micro amounts of cationic polymeric flocculants by flow injection analysis

Cationic polymeric flocculants in water are determined spectrophotometrically with the anionic compound 3-(2-hydroxy-3-carboxyanilide-1-azonaphthalene)-4-hydroxybenzenesulfonic acid at pH 10 by flow injection analysis. The calibration graph at 680 nm is rectilinear from 0 to 20 mg l^?1 under optimal conditions. The relative standard deviation for 20 injections of Cat-Floc (10 mg l^?1) was 1.2%; the sample throughput was 60 h^?1.     

Galvanostatic Fast Charging of Alkali-Ion Battery Materials at the Single-Particle Level: A Map-Driven Diagnosis

In this work, we develop a new tool to provide a diagnostic map for alkali-ion intercalation materials under galvanostatic conditions. These representations, stated in the form of capacity level diagrams, are built from hundreds of numerical simulations representing different experimental conditions, summarized in two dimensionless parameters: a kinetic parameter denominated Ξ and a finite diffusion parameter l. To lay the theoretical and methodological foundations, a general model is used here. This model can be adapted to the thermodynamic and kinetic framework of specific systems. We provide two representative examples.     

Determination of ethanol by air-stream separation with flow injection and electrochemical detection at a nickel oxide electrode

Ethanol is separated by an air stream from a 1-ml sample with collection in 1 ml of 0.2 mol l^-1 sodium hydroxide. Measurement is by voltammetry at a tubular, catalytic nickel oxide electrode with 30-μl sample injected into a continuous flow stream. Relative standard deviation for repetitive measurements is 1.8% for synthetic samples. Serum samples extracted for 100 s and then measured yielded linear results for concentrations of 2 × 10^-4–5 × 10^-2 mol l^-1 ethanol (0.001–0.23%), with relative standard deviation of 2.0%. The time per determination was about 2 min.     

Effect of various types of equations of state for prediction of simple gas hydrate formation with or without the presence of kinetic inhibitors in a flow mini-loop apparatus

This paper compares the effects of using various types of equations of state (PR,¹ SRK,² ER,³ PT⁴ and VPT⁵) on the calculated driving force and rate of gas consumption based on the Kashchiev and Firoozabadi model for simple gas hydrate formation for methane, carbon dioxide, propane and iso-butane with experimental data points obtained in a flow mini-loop apparatus with or without the presence of kinetic inhibitors at various pressures and specified temperatures. For this purpose, a laboratory flow mini-loop apparatus was set up to measure gas consumption rate when a hydrate forming substance (such as C₁, C₃, CO₂ and i-C₄) is contacted with water in the presence or absence of dissolved inhibitor under suitable temperature and pressure conditions. In each experiment, a water blend saturated with pure gas is circulated up to a required pressure. Pressure is maintained at a constant value during experimental runs by means of the required gas make-up. The total average absolute deviation was found to be 15.4%, 16.3%, 15.8%, 17.8% and 17.4% for the PR, ER, SRK, PT and VPT equations of state for calculating gas consumption in simple gas hydrate formation with or without the presence of kinetic hydrate inhibitors, respectively. Comparison results between the calculated and experimental data points of gas consumption were obtained in flow loop indicate that the PR and ER equations of state have lower errors than the SRK, VPT and PT equations of state for this model.     

Hydride generation atomic absorption spectrometry with different flow systems and in-atomizer trapping for determination of cadmium in water and urine-overview of existing data on cadmium vapour generation and evaluation of critical parameters

An overview of literature data on vapour generation techniques for cadmium and comparison with own experiments by means of several different types of hydride generation-electrothermal atomic absorption spectrometric systems (HG-ETAAS) (batch, semi-batch (SB), continuous-flow (CF) and flow-injection (FI) as well as different gas-liquid separators (GLS) exhibits apparent variations and inconsistency. However, if data for optimal chemical conditions are re-plotted in another coordinates: C_HCl (mol l⁻¹) vs. the ratio of reductant-to-acid molar input rates (i.e. millimoles per minute), [BH₄⁻]:[H⁺], much better consistency of data is revealed: more than half of data are clustered around 0.2-0.3 mol l⁻¹ HCl which appears an optimal acidity at moderate BH₄⁻ concentrations; the tetrahydroborate molar input rates should always be in excess versus the H⁺ molar input rates (1.1 to tenfold); relatively high flow rates of argon purge gas are required (≥120 ml min⁻¹); special attention to the blank control at ng l⁻¹ levels as well as to the construction of gas-liquid separator and vapour transfer lines should be paid. ‘Milder’ conditions for HG could be provided with some of the examined systems and GLSs, thus minimizing reagent consumption, blanks, vigorous reactions, foaming, aerosol production and drift in measurements: e.g. 0.4 mol l⁻¹ HCl—3% m/v NaBH₄ with the semi-batch system and 0.25 mol l⁻¹ HCl—2% m/v NaBH₄ in continuous flow mode. Experimental system is based on the Transversely Heated Graphite Atomizer coupled with flow injection system FIAS 400. Integrated platforms are treated for permanent modification with Zr (110 μg) or W (240 μg) and then with Ir (8 μg). Temperatures of trapping, pyrolysis and atomization are 350, 500 and 1300 °C, respectively. The best overall efficiency of HG, transportation and trapping is 41%. The characteristic mass for peak area measurements is m_o=2.8 pg and the limit of detection is 0.002 μg l⁻¹. The long-term stability of characteristic mass (within-day, 8 h) is m_o=2.8±0.1 pg (R.S.D. 4.0%, n=8), whereas the corresponding between-day figures (1 mo) are m_o=2.8±0.2 pg (R.S.D. 6.6%, n=6). The linear range is 0.002-0.12 μg l⁻¹ with a sample loop of 1.8 ml, being strongly impaired with smaller sample volumes in FI mode. The sample throughput rate is 10 h⁻¹ with the semi-batch system. Applications to real human and bovine urine samples and CRMs of sea water (CASS-3), river water (SLRS-1 and SLRS-3) and urine (SRM 2670) are presented.     

The amperometric response of tubular electrodes applied to flow-injection determinations

The classical treatment by Taylor for the dispersion of mass injected into a fluid stream in a linear, tubular channel is applied for amperometric detection in flow-injection systems with a tubular, flowthrough electrode. Peak current is predicted to be dependent upon the $\text{l}\text{3}$rd root of flow rate for low dispersion and the —$\text{1}\text{6}$th root of flow rate for high dispersion. Agreement between experimental results and theory is excellent for flow rates of 0.5 ml min^-1 or less in a linear, tubular channel with an inner radius of 0.0483 cm. As examples, experimental and predicted currents agree to better than 0.1 μA throughout a peak with a maximum value near 2 μA, and effects of retention volume, sample volume, and flow rate on the ratio of peak current to steady-state current are predicted with errors of 9% or less.     

Simultaneous determination of silicate and phosphate in boiler water at power plants based on series flow cells by using flow injection spectrophotometry

A new flow injection spectrophotometric method is described for the simultaneous determination of silicate and phosphate. Effects on the sensitivity of the method of the wavelength, temperature, length of reaction coils, pump rates, acidity, sampling volume, concentration of the chromogenic reagent, etc. were also investigated. The optimum conditions were ascertained.The principle of the method is that total concentration of silicate plus phosphate is determined when a injected sample plug is passing through the first flow cell and then the concentration of silicate is serially) determined at a second flow cell of the same detector after continuously masking the yellow molybdophosphate in the sample zone. Finally, the concentration of phosphate is obtained by difference.Silicate and phosphate are determined in boiler water at power plants; 60-120 samples h⁻¹ be analyzed. Determination ranges are 0.05-22 mg l⁻¹ for silicate and 0.1-24 mg l⁻¹ for phosphate. Relative standard deviations for metasilicate and orthophosphate were ≤1.2 and 1.3%, respectively. Recovery ranges of silicate and phosphate in the samples are 98-103%.