Landolt Reaction Research Articles

Modified Iodine Clock Reaction to Introduce the Concept of Activity

Modified Iodine Clock Reaction to Introduce the Concept of Activity

COMPASSO mission and its iodine clock: outline of the clock design

One of the limiting factors for GNSS geolocation capabilities is the clock technology deployed on the satellites and the knowledge of the satellite position. Consequently, there are numerous ongoing efforts to improve the stability of space-deployable clocks for next-generation GNSS. The COMPASSO mission is a German Aerospace Center (DLR) project to demonstrate high-performance quantum optical technologies in space with two laser-based absolute frequency references, a frequency comb and a laser communication and ranging terminal establishing a link with the ground station located in Oberpfaffenhofen, Germany. A successful mission will strongly improve the timing stability of space-deployable clocks, demonstrate time transfer between different clocks and allow for ranging in the mm-range. Thus, the technology is a strong candidate for future GNSS satellite clocks and offers possibilities for novel satellite system architectures and can improve the performance of scientific instruments as well. The COMPASSO payload will be delivered to the international space station in 2025 for a mission time of 2 years. In this article, we will highlight the key systems and functionalities of COMPASSO, with the focus set to the absolute frequency references.

Design and construction of a car automated by chemical processes

The need for affordable and efficient alternative energy sources is a defining issue of the twenty-first century that is receiving growing attention from both the scientific community and the public. While hydrocarbons have driven a majority of the world’s energy consumption for over a century, such sources are both unsustainable and environmentally detrimental. Hence, this research has been carried out to focus on the use of chemical processes to control a vehicle known as MODEL C, which was developed and modelled using a 3D computer graphics software called BLENDER, without the use of any form of internal combustion engine fuel in order to eradicate any form of hazardous emission. MODEL C is a chemical engineering car that is powered by chemical reactions. The car design is composed of separate processes for starting and stopping. The energy to move the car was taken from the sealed lead acid battery which consisted of two plates, viz. lead peroxide for the positive and sponge lead for the negative of which both plates were immersed in an electrolyte (sulphuric acid) for the conversion of chemical energy into electrical power. An iodine clock reaction was used in conjunction with a light dependent resistor (LDR) and a circuit board to stop the car after the reaction was going to completion. In this case, the iodine and the potassium iodide formed a complex with the starch solution that turned it black after a certain amount of time. This was based on the concentration of sodium thiosulphate that allowed the manipulation of how far the vehicle could travel at constant velocity.

Hemlata Agarwala.

"My favorite piece of research is anything with ruthenium in it … The most amusing chemistry adventure in my career was my students walking back into the lab after the practical course was over, wanting to re-perform the iodine clock experiment and record a video of it." Find out more about Hemlata Agarwala in her Introducing … Profile.

Hemlata Agarwala

Graphical Abstract „My favorite piece of research is anything with ruthenium in it … The most amusing chemistry adventure in my career was my students walking back into the lab after the practical course was over, wanting to re-perform the iodine clock experiment and record a video of it.“ Find out more about Hemlata Agarwala in her Introducing … Profile.

Influence of Oxygen on Chemoconvective Patterns in the Iodine Clock Reaction.

There is increasing interest in using chemical clock reactions to drive material formation; however, these reactions are often subject to chemoconvective effects, and control of such systems remains challenging. Here, we show how the transfer of oxygen at the air-water interface plays a crucial role in the spatiotemporal behavior of the iodine clock reaction with sulfite. A kinetic model was developed to demonstrate how the reaction of oxygen with sulfite can control a switch from a low-iodine to high-iodine state under well-stirred conditions and drive the formation of transient iodine gradients in unstirred solutions. In experiments in thin layers with optimal depths, the reaction couples with convective instability at the air-water interface forming an extended network-like structure of iodine at the surface that develops into a spotted pattern at the base of the layer. Thus, oxygen drives the spatial separation of iodine states essential for patterns in this system and may influence pattern selection in other clock reaction systems with sulfite.

Hands-on kinetic measurements and simulation for chemical process engineering students

Hands-on experience in the laboratory is essential in chemical engineering education to enhance the understanding of abstract theories and their effect on chemical processes. In this work, we describe a laboratory class, which combines some of the main engineering concepts into a set of hands-on experiments and simulations. Students are introduced to an iodine clock reaction performed in multiple different reactor types and are instructed to determine the reaction kinetics. Subsequent analysis of the experimental data in Python teaches basic programming skills and the concepts of numeric integration and optimization. Finally, a digital twin of one of the reactors is developed in COMSOL Multiphysics to give the students an application-focused introduction to more-dimensional multiphysics modeling. The students thereby get practical insights into the different methods and stages of reactor and reaction engineering. Based on the students’ assignments, we consistently see a deeper understanding of reaction kinetics and reactor engineering than in the accompanying traditional lecture.

Electrochemical Investigation of the Oxidation of Thiosulfate by 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane and Its Anion Radical

AbstractThe oxidation of thiosulfate has been of interest for more than a century, including the famous oscillating iodine clock reaction. From a thermodynamic perspective, calculations based on the reversible potentials reveal that both neutral TCNQF40 (2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane) or its radical anion (TCNQF41−) can oxidize thiosulfate to tetrathionate to form TCNQF42−. The reaction of S2O32− with TCNQF40 in a 2:1 or greater concentration ratio occurs in a step‐wise fashion to initially (and rapidly) form S4O62− and TCNQF41− (reaction 1), followed by a slow step involving the oxidation of residual S2O32− by TCNQF41− to give more S4O62− and TCNQF42− (reaction 2). Thus, this reaction occurs in two, temporally well‐resolved, steps with dramatically different kinetics. If S2O32− and TCNQF41− are mixed in a 1 : 1 ratio, then S4O62− and TCNQF42− are formed quantitatively on the same (hours) timescale, as for the second (slow) step described above. There is no evidence of further oxidation of S4O62− to SO42−. A slight increase in the rate of reaction 2 was observed in the presence of Bu4NPF6 and attributed to ion‐pairing effects. Interestingly, during voltammetric experiments, if the Pt counter electrode is not separated from the reaction solution via a salt bridge, reaction 2 is catalyzed. The rates of reaction 2 have been studied over a wide range of conditions. Intriguingly, the mechanisms are dependent on whether S2O32− or TCNQF41− (much faster) are in excess. With S2O32− in excess, the rate of reaction is first‐order in both S2O32− and TCNQF41−. With TCNQF41− in excess, the reaction is more complicated with indications of Li+ catalysis through electrostatic shielding of the TCNQF41− and/or S2O32− reactants. The large difference in rate of reaction of TCNQF40/S2O32− (rapid) compared with TCNQF41−/S2O32− (sluggish) is in part attributable to the much larger driving force in the former case. All experimental studies were undertaken in the mixed solvent system, acetonitrile (MeCN) containing 5 % water, v/v (95 % MeCN : 5 % water) where all the reactants and products are soluble and stable. Voltammetric analysis using steady‐state (microdisc) or transient cyclic voltammetry (macrodisc) with electrolyte (Bu4NPF6) and UV‐visible spectrophotometry with or without electrolyte were used to monitor the time dependence and establish the identity of the products.

Supramolecular gelation controlled by an iodine clock.

Programming supramolecular assembly in the time domain is a fundamental aspect of the design of biomimetic materials. We achieved the time-controlled sol-gel transition of a poly(vinyl alcohol)-iodine supramolecular complex by generating iodine in situ with a clock reaction. We demonstrate that both the gelation time and the mechanical properties of the resulting hydrogel can be tuned by properly selecting the clock parameters or through competitive iodine complexation.

Stable and transient self-propagating supramolecular gelation

Supramolecular sol–gel transition is time-programmed with an autocatalytic iodine clock, resulting in the autonomous generation of stable or transient gels depending on the chosen initial conditions.

Mix-Bricks and Flip-Lids: 3D Printed Devices for Simple, Simultaneous Mixing of Reactant Solutions.

Two types of 3D printed devices for simultaneous mixing of small volumes (e.g., 50-500 μL) of reactant solutions are described. One device (a "Flip-Lid") is a specially designed lid for commercially available 96-well plates, with which solutions in adjacent wells can be mixed by inversion. The other type ("Mix-Bricks") consists of two 3D printed parts: an interlocking "brick" that contains a selected number of wells of specified volume and a lid for mixing solutions in adjacent wells by inversion. In both cases, the lids contain transparent windows through which reactions can be visually monitored or recorded. The application of these devices to quantitative measurements was evaluated by use of an iodine clock reaction to quantify ascorbic acid in fruit juices and vitamin C tablets. A smartphone was used to record, via time-lapse video, the times at which color appeared as a function of ascorbic acid concentration, producing a linear calibration curve with time as the dependent variable. Up to 12 reactions, each involving four reactant solutions, were monitored simultaneously in a single device. Replicate measurements within a given device consistently yielded standard deviations of less than 5% RSD. Accuracy was evaluated by comparison of the vitamin C tablet results to those obtained by iodine titration and by HPLC-UV; all three methods were within 1.3% of the overall mean of 543 mg/tablet. These 3D printed devices thus show promise for simultaneous reaction monitoring in a wide variety of applications.

Exact Concentration Dependence of the Landolt Time in the Thiourea Dioxide-Bromate Substrate-Depletive Clock Reaction.

The thiourea dioxide (TDO)-bromate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid-dihydrogen-phosphate buffer within the pH range of 1.1-1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The title system shows a remarkable resemblance to the classical Landolt reaction, namely, the clock species (bromine) may only appear after the substrate TDO is completely consumed. Thus, the title system can be classified as substrate-depletive clock reaction. Despite the well-known slow rearrangement characteristic of TDO in acidic solution, it is surprisingly found that the Landolt time of the title reaction does not depend at all on the age of TDO solution applied. It is, however, shown experimentally that the inverse of Landolt time linearly depends on the initial bromate concentration as well as on the square of the hydrogen ion concentration. In addition to this, it is also noticed that dihydrogen phosphate markedly affects the Landolt time as well, and this feature may easily be taken into consideration by the H2PO4- dependence of the rate of bromate-bromide reaction quantitatively. Based on the experiments, a simple three-step kinetic model is proposed from which a complex formula is derived to indicate the exact concentration dependence of the Landolt time.

Conducting a Low-Waste Iodine Clock Experiment on Filter Paper To Discern the Rate Law

Herein we present a modified iodine clock experiment which replaces starch with cellulose paper. This provides the reaction with a white solid surface in which color change can be clearly observed and reduces reagent amounts required to 540 μL per group. After data acquisition, students are required to calculate reaction orders and the reaction constant k, culminating in their ability to discern the rate equation for this reaction. This experiment is ideal for teaching kinetics in high school and undergraduate settings, particularly if liquid waste disposal is not readily available or if high importance is placed upon minimizing their environmental impact.

Kinetic Determination of Thiocyanate by the Reaction of Bromate with Crystal Violet Immobilized in a Polymethacrylate Matrix

A procedure is proposed for the kinetic solid-phase spectrophotometric determination of thiocyanate using a polymethacrylate matrix. The procedure is based on the Landolt reaction between Crystal Violet immobilized in a polymethacrylate matrix and a bromate oxidizer, accompanied by the discoloration of the indicator in the matrix. During some induction period after the introduction of thiocyanate into the test solution, the dye in the matrix is not discolored. The duration of the induction period is proportional to the concentration of thiocyanate in the solution. The change in the color of the polymethacrylate matrix was recorded by measuring its absorbance at 600 nm. The developed procedure ensures the determination of thiocyanate in the concentration range 0.025–12 mg/L, depending on the Crystal Violet concentration in the matrix. The limit of detection calculated according to the 3s-test is 0.02 mg/L with the indicator concentration in the matrix of 0.06 mg/g. A possibility of using the proposed procedure for the determination of thiocyanate in near-wellbore water is shown.

Correction to 'A DFT investigation of the blue bottle experiment: E○half-cell analysis of autoxidation catalysed by redox indicators'.

[This corrects the article DOI: 10.1098/rsos.170708.].

A possible candidate to be classified as an autocatalysis-driven clock reaction: kinetics of the pentathionate-iodate reaction.

The pentathionate-iodate reaction has been investigated by spectrophotometrically monitoring the formation of the total amount of iodine at 468 nm in the presence of phosphoric acid/dihydrogen phosphate buffer. We noticed that iodine forms only after a fairly long time lag, and the inverse of time necessary to produce a certain amount of iodine is linearly proportional to the initial concentration of iodate ion and the square of the hydrogen ion concentration, while depending complexly on the concentration of substrate pentathionate. This reaction can therefore be treated as a clock reaction but differs from the original Landolt reaction in the sense that substrate pentathionate and the clock species iodine coexist for a relatively long time--due to their relatively slow direct reaction--depending on the experimental circumstances. Furthermore, we also provided experimental evidence that iodide ion acts as an autocatalyst of the system. A 14-step kinetic model is proposed in which the mechanisms of the pentathionate-iodine, bisulfite-iodate, and the well-known Dushman reactions are combined. A thorough analysis revealed that the direct pentathionate-iodate reaction plays a role only to produce iodide ions via a finite sequence of reactions, and once its concentration reaches a certain level, the reaction is almost exclusively governed by the pentathionate-iodine and the Dushman reactions. As expected, a strong catalytic effect of the buffer composition is also found that can readily be explained by its well-known catalytic influence on the original Dushman reaction.

Sustained large-amplitude chemomechanical oscillations induced by the Landolt clock reaction.

Synergetic chemomechanical oscillators represent a fundamentally new class of oscillators, where a clock reaction, owning no oscillatory chemical kinetics, generates shrinking-swelling cycles in a chemoresponsive gel under appropriate fixed nonequilibrium boundary conditions. Sufficiently large size-changes are a condition for continually switching between a reacted and an unreacted chemical state in the gel through sufficiently large differences in the diffusion time between the environment and the core of the gel. Two former experimental demonstrations with acid autocatalytic reactions were frustrated either by complex behaviors (chlorite-tetrathionate system) or by side reactions with the gel matrix (bromate-sulfite system). With the Landolt (iodate-sulfite) reaction, regular large-amplitude chemomechanical oscillations can be sustained for more than a week. This enabled a fine study of the temperature and stoichiometry range of operation. I have identified several key steps that are experimentally essential to the systematic design of further synergetic oscillators. The robust realization of this type of self-organization in artificial systems is currently unique.

Kinetics of the Rapid Reaction between Iodine and Ascorbic Acid in Aqueous Solution Using UV–Visible Absorbance and Titration by an Iodine Clock

An iodine clock is the basis for studying the kinetics of the fast reaction between iodine and ascorbic acid in aqueous solution. UV–visible absorbance, where the molar absorptivity of the triiodide ion is high, is equated with the total concentration of iodine, through the equilibrium, I2 + I– ⇌ I3–, established when the iodine clock is concluding its steady-state titration of ascorbic acid. Persulfate ion is the primary oxidant catalyzed by the iodide ion, which produces the iodine titrant and is recycled. A method testing the second-order rate equation, R1 = k[iodine][ascorbic acid], uses the coefficient of variation, CV. This locates equivalence, enabling [ascorbic acid] to be measured and contributes to a procedure where rate of reaction, R1, at different concentrations of iodine and ascorbic acid determines an average value of the overall rate constant, k. In combination with results from another source, the rate constants of the two individual forms of iodine reacting with ascorbic acid also are de...

Preparation and reaction dynamics on biodiesel made from Zanthoxylum bungeanum seed oil and methanol

Zanthoxylum bungeanum seed oil (ZSO) was the by-product of the zanthoxylum industry, and was a kind of cheap and abundant source in China; it can be of great potential to use as a feedstock for biodiesel production in terms of reducing the producing cost. Biodiesel preparation of ZSO methyl ester by transesterification using potassium hydroxide (KOH) as an alkaline catalyst was studied in this paper. First, the single-factor experiment was practiced, the influence on conversion rate of different factor was studied, such as methanol-to-oil molar ratio, content of catalyst, reaction temperature, and reaction time, etc. Second, the response surface methodology was used to optimize the conditions for ZSO biodiesel production using KOH as a catalyst. A quadratic polynomial equation was obtained for biodiesel conversion by multiple regression analysis and verification experiments confirmed the validity of the predicted model. The optimum combination for transesterification was methanol-to-oil molar ratio 8.5:1, catalyst amount 1.0%, reaction temperature 56 °C, and reaction time 67 min. At this optimum condition, the conversion to biodiesel reached above 98.4%. The relevant error was less than 5% between measurement value of the conversion to biodiesel and expected value. Finally, the reaction kinetics of transesterification was studied. The “Iodine Clock Reaction” principle was used for establishing dynamic model, and corresponding kinetic equation was established. The results showed that the progression of transesterification was grade 1.5, the reaction activation energy was 20.0434 kJ·mol−1, and the pre-exponential factor was 7.8963 dm3/(mol min).

A Kinetic–Equilibrium Study of a Triiodide Concentration Maximum Formed by the Persulfate–Iodide Reaction

The presence of triiodide, I3–, throughout the lifetime of a persulfate–iodide reaction with persulfate in excess, is controlled by an opposing effect of falling iodide concentration and rising iodine concentration. This brings triiodide to a concentration maximum, [I3–]max, which can be studied by UV–visible spectroscopy using absorbance measurements taken over an extended iodine clock time-scan. Equilibrium and conservation equations can be combined so that, together with [I3–]max, a value of the constant K for the equilibrium established between iodine, iodide, and the triiodide complex ion can be obtained.