Iodate Reaction Research Articles

Complex Kinetics of a Landolt-Type Reaction: The Later Phase of the Thiosulfate−Iodate Reaction

The thiosulfate-iodate reaction has been studied spectrophotometrically in slightly acidic medium at 25.0 +/- 0.1 degrees C in acetate/acetic acid buffer by monitoring the absorbance at 468 nm at the isosbestic point of iodine-triiodide ion system. The formation of iodine after the Landolt time follows a rather complex kinetic behavior depending on the pH and on the concentration of the reactants as well. It is shown that the key intermediate of the reaction is I(2)O(2), its equilibrium formation from the well-known Dushman reaction along with their further reactions followed by subsequent reactions of HOI, HIO(2), S(2)O(3)OH(-), and S(2)O(3)I(-) adequately accounts for all the experimentally measured characteristics of the kinetic curves. A 19-step kinetic model is proposed and discussed with 13 fitted and 7 fixed parameters in detail.

Evaluation of the performance of a constructal mixer with the iodide–iodate reaction system

A novel fluidic mixer, which takes advantage of impingement mixing and is designed according to the constructal approach, is evaluated by the Villermaux/Dushman method. The effects of different configurations (including the structure of the fluid collector and the diameter of the nozzles on the fluid injector) on the mixing performance (i.e. the segregation index and the energy dissipation rate) are determined. The segregation index is smaller, or in other words the degree of mixing is better, when a branch type fluid collector or smaller nozzles are used, however, at the cost of higher energy dissipation rate. When the flow rate is sufficiently high, mixing caused by the impingement of streams is almost complete, rendering the mixing in the branched channels unnecessary. As a result, if very high degree of mixing is pursued, the collector with a simple empty space should be used in the mixer to reduce energy consumption.

On the scalability of microstructured mixing devices

Microchannel systems for multilamination mixers can be obtained by stacking and bonding microstructured foils. So-called V-type mixers have been manufactured and examined at the Institute for Micro Process Engineering by means of the Iodide Iodate Reaction Method. In previous work we showed that V-type mixers can be scaled by varying the number of micromachined foils and holding constant velocities of the fluids exiting the microchannel system. Applying the Iodide Iodate Reaction Method, constant triiodide concentrations, indicating constant ‘mixing qualities’, were obtained with mixer inlays with 6 foils, 12 foils, and 24 foils. In order to understand the influence of the top- and the bottom foils, which are different regarding their geometrical environment, examinations are presented with mixer inlays with 4 foils and 2 foils. The two newly examined inlays are fully ‘in-line’ with the scaling considerations from the previously examined devices. An influence of the boundary foils cannot be shown. Furthermore, it is shown that the arrangement of the microchannels at the interface between the microchannel system and the mixing chamber (‘shape of the exit window’) has a crucial impact on the mixing results: the larger the width of the exit window, the better the mixing results.

The iodide iodate reaction method: The choice of the acid

Mixing processes are frequently characterized with competing chemical reaction schemes. The most popular reaction scheme for continuous flow mixers is the iodide iodate reaction method, which traditionally uses sulfuric acid as proton source for both reactions involved in the scheme. Experimental evidence is provided which indicates that for a quantitative treatment of the experimental data either the experiments need to be carried out with a strong acid such as perchloric acid or the dissociation constants of sulfuric acid need to be included in the models linking the primary experimental results with quantitative measures such as mixing times. Another chemical test reaction system, the acetal cleavage method, traditionally carried out with hydrochloric acid was applied for comparison. In that case, the use of perchloric acid showed no significant impact on the experimental results.

A new impinging stream–rotating packed bed reactor for improvement of micromixing iodide and iodate

A novel concept of a liquid–liquid contact reactor, the so-called impinging stream–rotating packed bed (IS–RPB), was presented for improvement of micromixing iodide and iodate. The micromixing efficiency of IS–RPB was studied by adopting iodide–iodate reaction system. The effects of operational conditions (e.g. high gravity factor, liquid flow rate, liquid flow ratio and acid concentration) on micromixing efficiency (characterized by segregation index X s ) were investigated. In addition, the effects of different impinging distance and impinging angle on micromixing efficiency were also considered. Besides, IS, RPB and IS–RPB reactors were contrasted with micromixing characteristics in the same experimental conditions. Comparison with other mixing devices, IS–RPB reactor has better micromixing characteristics than other reactors, which has a good application prospect.

Micromixing efficiency of rotating packed bed with premixed liquid distributor

Rotating packed bed (RPB), in which high gravity is simulated by a centrifugal force, plays an important role in process intensification of fluid mixing and mass transfer. However, uneven initial liquid distribution in RPB leads to poor micromixing efficiency in local areas of the packing. Therefore, a premixed liquid distributor is proposed in this work to improve the liquid distribution in RPB. Micromixing efficiency of RPB with the premixed liquid distributor was studied by adopting the iodide–iodate reaction as working system and show better micromixing efficiency compared to that of RPB with non-premixed liquid distributor. Also, the effects of operating conditions (e.g. rotational speed, acid concentration, volumetric flow rate) and geometries using the premixed liquid distributor on micromixing efficiency (characterized by segregation index X S) were investigated. The results show that segregation index X S decreases with the increase of rotational speed, and the decrease of acid concentration and volumetric flow rate.

Characterization of mixing in a hollow fiber membrane contactor by the iodide–iodate method: Numerical simulations and experiments

Abstract This paper presents numerical simulations and experimental data on the iodide–iodate reaction in a hollow fiber membrane device. The principle of reactive mixing in a membrane device is the following. Component A flows through the inlet of the lumen side and component B comes from the membrane pores (shell side). Components A and B then mix and react inside the lumen side. Experimental measurements of the segregation index were carried out under various experimental conditions and for two different hollow fiber devices. The numerical and experimental segregation indexes, X S , were found in the range 10 −2 –10 −3 , indicating effective mixing. The numerical simulations showed that I 3 − ions were produced mainly in the first part of the lumen side of the hollow fiber. This confirmed our previous results on precipitation which showed that high supersaturation data and experimental fouling were obtained in this part of the hollow fibers.

Further comments on the Iodide Iodate Reaction Method for characterising micromixing

Further comments on the Iodide Iodate Reaction Method for characterising micromixing

Characterisation and comparison of the micromixing efficiency in torus and batch stirred reactors

Abstract The mixing at a molecular scale (micromixing) plays an important role on selectivity, yield and quality of final products of a large range of competing fast chemical reactions. In this study, we have compared, by the use of iodide–iodate reaction tests, the micromixing in two reactors, the first one is the standard batch stirred reactor and the second is the torus reactor. Various conditions of agitation and feed locations were used for this study. A comparative analysis of the micromixedness ratio (α) in the two reactors was carried out on the basis of the local rate of specific energy dissipation.

Micromixing efficiency of a novel rotor–stator reactor

Abstract The importance of inlet region of rotating packed bed (RPB) was confirmed recently. This enlightened us to design another novel reactor, named rotor–stator reactor (RSR), which has multiple inlet regions along radial direction. The micromixing efficiency of RSR was studied by adopting the iodide–iodate reaction as working system. The effects of operating conditions (e.g. rotor speed and reagent concentration) on micromixing efficiency (characterized by segregation index XS) were investigated. In addition, the effects of different rotor–stator combinations as well as the amount of rotor- and stator-ring on micromixing efficiency were also considered. Based on the incorporation model, the micromixing time of RSR was estimated to reach about 10−5 s, which is less than that of rotating packed bed (about 10−4 s).

Convective mixing and chemical reactions in microchannels with high flow rates

This work presents a theoretical and experimental investigation of convective micromixing in various mixer structures and combinations with the aim of high mixing intensity and a high throughput. Different mixing elements are integrated on a silicon chip to achieve a device for high flow rates above 20kg/h. Test structures are fabricated and characterized according to their flow behavior and mixing performance. Flow measurements with pH neutralization and indication by bromothymol blue confirm the numerical simulations of the flow characteristics and concentration fields. The integral mixing quality in the micromixer is measured with the iodide–iodate reaction (Villermaux–Dushman) and shows excellent values for high Re numbers. This offers the potential to use microstructures for new applications in the production of chemicals. With the help of the obtained experimental and theoretical results, a new class of dimensionless numbers is proposed which characterizes the effectiveness of a mixing device and of the mixing process and compares different mixing times.

Micromixing Efficiency in a Rotating Packed Bed: Experiments and Simulation

Micromixing plays a crucial role in fast chemical reactions. The micromixing efficiency in a rotating packed bed (RPB) was first studied via a throughway method with specially designed RPB structure accompanied by adoption of the iodide−iodate reaction system as index to the micromixing efficiency. With sampling along the radial position, micromixing efficiency in different packing zones of RPB was detected and analyzed and the importance of the inlet region of the RPB on micromixing was experimentally confirmed. The effects of different operating conditions (e.g., rotational speed, liquid flow rates, and reagent concentration) on segregation index (Xs) were also investigated. On the basis of micromixing theory and appropriate assumptions on energy dissipation rate, the micromixing time τm in the RPB was evaluated to reach about 10-4 s, almost equal to what we calculated based on the incorporation model. Compared with other mixing devices, the RPB has a distinct advantage in improving micromixing efficiency, and maybe of many novel applications in the chemical and related industries.

Study Of Micromixing in a Stirred Tank Using a Rushton Turbine: Comparison of Feed Positions and Other Mixing Devices

The consecutive-competing iodide–iodate reaction scheme has been used to study micromixing phenomena in a baffled vessel of 0.29 m diameter agitated by a Rushton turbine. It has been confirmed that, by using successive injections, this reaction scheme is very efficient for such a study. Four agitator speeds giving mean specific energy dissipation rates, e T from ∼0.2 to ∼ 1.2 Wkg −1 have been used, with sub-surface feeding at one of four points. For a given speed, addition at each of these four points gave different local values of e ¯ T, ranging from less than e ¯ T very close to the top of the liquid to much greater close to the impeller. The point closest to the impeller was chosen to be such that feeding was estimated to be at the point of maximum e T. For the maximum speed, the segregation index, as a measure of the amount of ’waste product’, was ∼20% with feed at the top of the liquid (as preferred industrially because of its convenience). This ’waste’ was reduced to ∼5% by feeding at the point of maximum e T close to the impeller. A comparison was also made with results reported in the literature using the same reaction for two new devices developed for improved micromixing. By feeding at the carefully chosen position close to the impeller, the results with the Rushton turbine were as good as or better than with the special devices even at the comparatively low e ¯ T of ∼ l.2 Wkg−1. It was estimated that the maximum local specific energy dissipation rate was ∼70 times the mean, in reasonable agreement with a very recent study where the same pair of reactions and LDV were both used.

Catalytic Spectrophotometric Determination of Bromide Based on the Diphenylcarbazide–Chromium(VI)–Iodate Reaction

A new catalytic spectrophotometric method for the determination of bromide is proposed. Diphenylcarbazide reacts with chromium(VI) ions in an acid medium to give a violet complex. The method is based on the bromide-catalyzed oxidation of this violet complex by iodate followed by measuring the color change at λ = 542 nm spectrophotometrically. The reaction starts after an induction period. Bromide ions catalyze this reaction, so that the induction period is decreased by the addition of Br–. The derivative of the absorbance with respect to time was calculated. The maximum absorbance difference (ΔAm) and thetime taken to reach the maximum difference (tm) were measured from the derivative curve. These values are used as the measured parameters for bromide determination. Under optimum conditions bromide can be determined in the range of 0.5–8 μg/mL. The detection limit is 0.25 μg/mL, and the relative standard deviation for ten replicate measurements of 1 μg/mL of bromide is 1.57%. The method was applied to the determination of bromide in the Persian Gulf water.

Influence of External Electric Fields on Reaction Fronts in the Iodate−Arsenous Acid System

The propagation of arsenous acid−iodate reaction fronts of different net stoichiometries in externally applied dc electric fields is studied for a range of both electric field intensities and initial compositions of the reacting mixture (represented by the stoichiometric factor S0). Regions of three different types of net stoichiometry in the parametric space ℰ/V vs S0, where ℰ is the intensity of the applied electric field and V the reaction front propagation velocity, are determined both experimentally and by analyzing a reaction−diffusion−migration model that includes a realistic kinetic scheme of the reaction studied. Both agreement with and discrepancies between the theoretical predictions and experimental findings are discussed.

Characterisation of micromixing efficiency by the iodide–iodate reaction system. Part II: kinetic study

In a previous paper (Part I), the operating conditions of the iodide–iodate reaction system used to quantify micromixing phenomena in chemical reactors and mixers has been presented. In order to determine the micromixing time through models, the complete kinetics of the reactions must be known. This paper presents the kinetics of the Dushman reaction obtained in operating conditions close to the micromixing test conditions. The results confirm the single fifth-order process widely reported in published work and establish the rate constant as a function of ionic strength ranging between 0 and 2 M.

Characterisation of micromixing efficiency by the iodide–iodate reaction system. Part I: experimental procedure

This paper describes in a very practicable way the operating conditions of the iodide–iodate reaction system used to quantify micromixing phenomena in chemical reactors and mixers. Several operating procedures are given for batch and continuous mixers. Recommendations to adapt these procedures to any specific mixers and complete analysis procedures are also provided. Finally, a simple micromixing model is used to calculate micromixing time t m . It is checked that t m is non-dependent on the reactant concentrations at constant hydrodynamic conditions. In a second following paper (Part II), the reactions kinetics, essential for a knowledge of micromixing time t m , are presented.

Stirring Effect on Bistability in a CSTR. 1. Experiments and Simulations for the AsO33-/IO3- Reaction

The effect of mixing-induced inhomogeneity on the bistability hysteresis of the arsenite−iodate reaction was studied experimentally in a CSTR, involving a single premixed feedstream, and by simulation using the coalescence−redispersion model. Conditions were chosen for which the system may be described by a single dynamical variable. It was found that reduced stirring causes the hysteresis loop to contract and to remain inside the high-stirring loop. The steady-state probability density functions were constructed from the fluctuating time series and were found to be gaussian. They represent the stochastic description of the macroscopic stirring effect. The agreement of experiments and simulations confirms the micromixing scenario and the appropriateness of the mixing model.

Pulse radiolysis of lodate in aqueous solution

The reactions of primary water radiolysis radicals with IO–3 have been re-investigated using electron pulse radiolysis and absorption spectroscopy. Rate constants for iodate reaction with e–(aq)[(5.45 ± 0.45)× 109 dm3 mol–1 s–1], O˙–[(1.82 ± 0.04)× 109 dm3 mol–1 s–1] and ˙OH (< 105 dm3 mol–1 s–1) have been measured and compared with previously determined values. The observed iodate concentration dependence of the two transient absorptions produced by its reaction with the hydrated electron and the oxide radical were found to be due only to spur scavenging effects, and thus the dimerization equilibria proposed by previous workers is not necessary. The previously reported rate constants for the reaction of iodate with hydroxyl radicals have been shown to be due to just iodate reaction with the small concentration of O˙– present in equilibrium with the hydroxyl radical.

Kinetics and mechanism of indigo carmine‐acidic iodate reaction. An indicator reaction for catalytic determination of Ru(III) ion

AbstractA kinetic study of uncatalyzed and Ru(III) catalyzed oxidation of indigo carmine(IC) (disodium 3,3′‐dioxobi‐indolin‐2,2′‐ylidene‐5,5′‐disulphonate) by iodate ion in aqueous sulphuric acid solution is reported. The uncatalyzed reaction order was found to be four; one each with respect to IC and iodate ion and second order with H+ ion. The Ru(III) catalyzed reaction was of fifth order, second order with respect to H+ and first order with respect to reductant, oxidant, and catalyst. Stoichiometric ratios of both reactions were the same with a 3:2 reductant‐oxidant ratio. In both uncatalyzed and catalyzed reactions isatin‐5‐monosulphonic acid (2,3‐dioxoindoline‐5‐sulphonic acid) was observed as the oxidation product. Rate constants for both the reactions are reported. Reaction mechanisms consistent with the experimental data are suggested.Further, a fixed time method is described for the determination of Ru(III), based on its ability to catalyze the oxidation of IC by acidic iodate. Using [H+] 2.25M, [iodate] 1.00 × 10−3M and [IC] 5.0 × 10−5M, in presence of Ru(III), the reaction followed first order kinetics with respect to IC. The interference of various cations, neutral salts, and potassium iodide on the determination of Ru(III) was studied using synthetic mixtures. The selectivity of the method and the recommended procedure are described.