Determination of Riboflavin Using Flow Injection Inhibitory Chemiluminescence

ABSTRACT

A new flow injection method for the determination of riboflavin based on the inhibition of the intensity of chemiluminescence (CL) from the luminol-K₃Fe(CN)₆ system is described. While riboflavin mixed with K₃Fe(CN)₆, by the fast oxidation reaction between riboflavin and K₃Fe(CN)₆, K₄Fe(CN)₆ was generated, which then inhibited the CL reaction of K₃Fe(CN)₆ and luminol in alkaline aqueous solution. The CL emission was correlated with the riboflavin concentration in the range from 0.032 to 100 μg·ml?¹, and the detection limit was 0.01 μg·ml?¹ (3σ). A complete analysis could be performed in 2 min with a relative standard deviation of less than 2.2%. The influence of foreign species was studied and the method has been applied successfully to the determination of riboflavin in pharmaceutical samples, the recovery was from 98.0% to 102%.     

Flow injection analysis applied to a voltammetric electronic tongue

A measurement system, based on flow injection analysis (FIA) technique applied to a voltammetric electronic tongue is described. A reference solution was thus continuously pumped through a cell with a voltammetric electronic tongue, and test samples were injected into the flow stream. Responses were obtained by measuring the resulting pulse height. The FIA technique offered several advantages, since relative measurements are performed, the system is less influenced by sensor baseline drift, calibration samples and/or washing solutions can be injected within a measurement series, and the system is well adapted for automatization. The system was used to analyze standard solutions of H₂O₂, KCl, CuNO₃, K₄[Fe(CN)₆], K₃[Fe(CN)₆] and NaCl, and results obtained were treated with multivariate data analysis. Principal component analysis performed showed that electrode drift could be considerably decreased, and the set-up was also used for classification of different apple juices.     

Preparation of57Fe enriched K4[Fe(CN)6] and K3[Fe(CN)6]

A simple method to prepare⁵⁷Fe enriched K₄[Fe(CN)₆] and K₃[Fe(CN)₆] is described. The yields of the products are much better than those reported in the literature so far. The enrichment is essential for⁵⁷Fe Mössbauer investigation in a variety of Prussiate type complexes and other inorganic compounds which are conveniently prepared from K₄[Fe(CN)₆] and K₃[Fe(CN)₆]. K₄[Fe(CN)₆] was obtained by reacting freshly prepared Fe(OH)₃ with glacial acetic acid and treating with iron acetate in boiling aqueous solution of KCN. The novel feature of the procedure to obtain K₃[Fe(CN)₆] is that the oxidation of K₄[Fe(CN)₆] has been carried out in the solid state by passing chlorine gas over the powdered specimen. K₃[Fe(CN)₆] was crystallised from alkaline solution of this oxidised powder. The compounds were characterised by Mössbauer spectroscopy.     

Electrochemical Behavior of K3[Fe(CN)6] on the Electrode Coated with Modified Multi‐Walled Carbon Nanotubes

The electrochemical behavior of K₃[Fe(CN)₆] was studied on an ITO electrode that was coated with β‐cyclodextrin (CD) modified multi‐walled carbon nanotubes (MWNTs) and with carboxyl modified multi‐walled carbon nanotubes (MWNT‐COOHs). MWNT‐COOHs showed an excellent electrocatalytic effect on the redox of K₃[Fe(CN)₆] while MWNT‐CDs had a subdued effect on the electrochemical response of K₃[Fe(CN)₆]. It is probably due to mismatching between K₃[Fe(CN)₆] and cyclodextrin, which hampers the contact of K₃[Fe(CN)₆] with carbon nanotubes. Moreover, the electrochemical behavior of K₃[Fe(CN)₆] on the MWNT‐COOHs coated ITO electrode at various scan rates also was measured. The results indicated that both potential difference between redox peaks and peak current of K₃[Fe(CN)₆] increased with increasing scan rate. A good linearity of peak current versus scan rate was observed.     

A study of ion exchange selectivity coefficient by cyclic voltammetric measurement

An equation to express ion exchange selectivity coefficient was derived and used for calculating that of PPY film with the results obtained by cyclic voltammetric measurement. PPY film was synthesized by electrochemical method in aqueous solution using K₄Fe(CN)₆ as supporting electrolyte, and the anions were doped into the film. Ion exchange behaviour of doped Fe (CN)₆^3-/4- in the PPY film with Cl^?, NO₃^? or F^? ions in solution has been studied, and the corresponding ion exchange selectivity coefficients were determined.     

Untersuchung der Aussalz- bzw. Dehydratationswirkung des Kaliumhydroxids auf Kalium-eisen(II)-cyanidlösungen

Zusammenfassung Mittels physikalisch-chemischer Analyse wurde das System K₄Fe(CN)₆/KOH/H₂O untersucht. Es wurde gezeigt, daß bei 30° bzw. 40°C KOH stark aussalzend auf die Kaliumeisen(II)-cyanidlösungen wirkt, wobei die feste Gleichgewichtsphase nicht dehydratisiert wird, sondern als K₄Fe(CN)₆·3H₂O erhalten bleibt. In der Isotherme bei 60°C bleibt die feste Phase — bis zur KOH-Konzentration 15% — K₄Fe(CN)₆·3H₂O. Über diese Konzentration hinaus tritt Entwässerung ein und die feste Phase stellt K₄Fe(CN)₆ dar. Es wurde festgestellt, daß die Eisen(II)-cyanwasserstoffsäure (in kleinen Konzentrationen) die Löslichkeit des Kaliumeisen(II)-cyanids erhöht.

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Investigation of the salting out and dehydratation effect of potassium hydroxide upon solutions of K₄Fe(CN)₆

The system K₄Fe(CN)₆/KOH/H₂O was investigated by means of physico-chemical analysis. It was shown that at 30° and 40°C KOH has a strong salting out effect upon the solutions of K₄Fe(CN)₆, but the equilibrium solid phase is not dehydrated and remains K₄Fe(CN)₆·3H₂O. In the isotherm at 60°C the solid phase remains K₄Fe(CN)₆·3H₂O up to a concentration of 15% KOH. Beyond this concentration dehydration occurs and the solid phase represents K₄Fe(CN)₆. It was found that H₄Fe(CN)₆ acid (in small concentrations) increases the solubility of K₄Fe(CN)₆.

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Mit 4 Abbildungen     

Potentiometric method for determination of kinetic characteristics of radical reactions in aqueous media

Kinetic features of the reactions of K₄[Fe(CN)₆] with radicals initiated by water-soluble azo-initiator 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) at 37 °C were studied using the potentiometric method. Potassium ferrocyanide was shown to be a radical acceptor, whereas K₃[Fe(CN)₆] formed by the oxidation with the radicals in combination with K₄[Fe(CN)₆] is an electrochemical system, the study of which makes it possible to determine kinetic characteristics of radical reactions. The rate constants for the reactions of peroxide radicals RO₂ · with K₄[Fe(CN)₆] were calculated.     

A cyano-bridged hetero-tetranuclear [Sm2(o-phen)2(DMF)6(H2O)2(μ-CN)4Fe2(CN)8] · 5H2O · CH3OH: synthesis,structure, Mössbauer spectrum,and magnetism

Two new hetero-tetranuclear complexes, [Sm₂(o-phen)₂(DMF)₆(H₂O)₂(µ-CN)₄Fe₂(CN)₈]·;5H₂O·;CH₃OH (1) and [Sm₂(o-phen)₂(DMF)₆(H₂O)₂(µ-CN)₄Co₂(CN)₈]·;5H₂O (2), have been prepared from reaction of SmCl₃·;6H₂O, K₃[Fe(CN)₆]·;3H₂O or K₃[Co(CN)₆], and o-phen in methanol/DMF, and characterized. The structure of 1 consists of a cyano-bridged discrete cyclic tetranuclear complex in which the Sm(III) and Fe(III) centers are linked by four CN groups. Mössbauer spectrum of ⁵⁷Fe indicates that both Fe(III) atoms in 1 have the same low-spin (S?=?1/2) electronic ground state. From comparison of the magnetic data of 1 and 2, at low temperature for 1 indicates weak ferromagnetic coupling between Sm(III) and Fe(III).     

The Effect of Adsorption of Ions of the Hexacyanoferrate(II)/(III) Redox Pair on Self-Assembly of Octanethiol at Its Adsorption from Aqueous Solutions on Gold Electrode

The effect of components of the redox pair K₃[Fe(CN)₆]/K₄[Fe(CN)₆] on the dynamics of formation of octanethiol (OT) monolayers from aqueous thiol-containing solutions of 0.1 М NaClO₄ is studied by cyclic voltammetry (CVA). The formation of OT monolayers is shown to depend on the presence of ions of hexacyanoferrate(II)/(III) in solution. Being added to solution, the components of the [Fe(CN)₆]^3–/4– redox pair sharply increase the time of formation of the insulating monolayer OT films and make them less stable. The destabilizing and inhibiting action of [Fe(CN)₆]^3–/4– ions becomes stronger as their concentration in solution increases. The adsorption activity of individual components of the redox pair is assessed. The strong and approximately equal adsorption activity of ions [Fe(CN)₆]^3– and [Fe(CN)₆]^4– on gold in the presence of octanethiol is observed. At the same time, OT and the hexacyanoferrate(II)/(III) ions can be placed in the following row: OT > [Fe(CN)6]^3– ≈ [Fe(CN)₆]^4–. Recommendations are given on how to eliminate the interfering action of the K₃[Fe(CN)₆]/K₄[Fe(CN)₆] redox-pair ions when studying the insulating properties of thiol monolayers on gold.     

Mechanisms involved in chemical vapor generation by aqueous tetrahydroborate(III) derivatization : Role of hexacyanoferrate(III) in plumbane generation

The role played by K₃Fe(CN)₆ (0.08 or 1.5 g l^− 1) in producing strong enhancement factors in the generation efficiency of plumbane in the reaction of NaBH₄ (10 or 40 g l^− 1) with Pb(II) (50 μg l^− 1) in 0.1 M HCl solution, was investigated by using continuous flow chemical vapor generation coupled with atomic fluorescence spectrometry (CF-CVG-AFS). Different mixing sequences and reaction times of reagents were tested using different chemifold setups. Part of CF-CVG-AFS experiments were performed using the on-line, delayed addition of Pb(II) to a K₃Fe(CN)₆ + NaBH₄ reaction mixture. Kinetic calculations estimating the concentration of K₃Fe(CN)₆ remaining in the K₃Fe(CN)₆ + NaBH₄ reaction mixture before it merged with Pb(II) solution were also performed. Batch experiments measuring the amount of hydrogen evolved (pressure of H₂ vs time) and pH variation during K₃Fe(CN)₆ + NaBH₄ + HCl reaction were performed in order to have a correct estimation of the concentration of K₃Fe(CN)₆ remaining in the reaction system. The comparison of CF-CVG-AFS experiments with kinetic calculations indicates that strong enhancement factors of plumbane generation can be obtained without any interaction of K₃Fe(CN)₆ with Pb(II). The key role of K₃Fe(CN)₆ is recognized in its reaction with NaBH₄ to give “special” borane complex intermediates, which are highly effective in the generation of plumbane from Pb(II).     

Determination of the oxidant activity of chlorinated water by chronoamperometry

A new version of the chronoamperometric method, adapted to the determination of the oxidative (oxidant) activity of a medium on an example of chlorinated water, was proposed. The supporting solution contained K₄Fe(CN)₆. By its oxidation with oxidants present in the sample, K₃Fe(CN)₆ formed; its reduction current served as an analytical signal. The current of K₃Fe(CN)₆ reduction was recorded at a potential of 0 V, selected from the cyclic voltammogram of the K₃Fe(CN)₆/K₄Fe(CN)₆ system in the range where the oxidized form of the reagent was reduced and the reduced form was stable. A gold electrode was the measuring electrode. The possibility of using a stable solution of K₃Fe(CN)₆ as a component of a standard addition and as a solution for plotting a calibration curve was demonstrated. The limit of detection of the amount of dissolved chlorine and products of its interaction with water is 8 × 10^–6 M. The data obtained by the proposed procedure and by the iodometric methods correlate well with each other (r = 0.93–0.95). The proposed version of the chronoamperometric method can be used to monitor the amount of dissolved chlorine and products of its interaction with water in water supply systems, pools, and disinfecting solutions and to determine other water-soluble oxidants.     

Activity and Osmotic Coefficients from the Emf of Liquid Membrane Cells. VIII. K.3[Co(CN)6], Mg3[Co(CN)6]2, and Ca3[Co(CN)6]2

The activity coefficients of K₃[Co(CN)₆], Mg₃[Co(CN)₆]₂, and Ca₃[Co(CN)₆]₂,are examined. The results highlight close similarity with the correspondinghexacyanoferrate (III) salts. On dilution, K₃[Co(CN)₆], like K₃[Fe(CN)₆], approachesthe limiting law from the upper side, while Mg₃[Co(CN)₆]₂ and Ca₃[Co(CN)₆]₂tend to the limiting law from the opposite side, like Mg₃[Fe(CN)₆]₂,Ca₃[Fe(CN)₆]₂, Sr₃[Fe(CN)₆]₂, and Ba₃[Fe(CN)₆]₂. Both kinds of behavior agreewith theory for a model of hard spheres bearing electric charges +1 and –3 or+2 and –3, respectively. The paramater values of the Pitzer equation for activityand osmotic coefficients are reported.     

MÖSSBAUER- und Infrarotspektren von Addukten des K4[Fe(CN)6] mit Antimontrihalogeniden

Mössbauer- and IR Spectra from the Addition Compounds of K₄[Fe(CN)₆] with Antimony Trihalides By the reaction of K₄[Fe(CN)₆] with SbX₃ in the melt (X = Cl, Br) or in SO₂ solution (X = F) the addition compounds K₄[Fe(CNSbX₃)₆] (X = F, Cl) and K₄[Fe(CNSbX₃)₄(CN)₂] (X = Cl, Br) are formed. The IR spectra of these compounds have been interpreted. The MÖSSBAUER spectra are single lines which exhibit a slight broadening compared with K₄[Fe(CN)₆] · 3 H₂O. The values of the isomer shifts following the order of the LEWIS acid strengths of the SbX₃ molecules correspond to a small but significant increase of the s-electron density, which is explained by the increasing influence of the π-acceptor function of the ligands.     

Electrochemical characterization of the redox couple of Fe(III)/Fe(II) mediated by grafted polymer reference electrode (GPRE)

A grafted polymer reference electrode (GPRE) (polystyrene grafted with acrylonitrile as a monomer using gamma irradiation) was fabricated as a reference electrode using cyclic voltammetry (CV). The redox process of K₃Fe(CN)₆ during CV was studied. It was found that the redox current peaks of Fe(II)/Fe(III) in 0.1 M of KCl as supporting electrolyte is given the same oxidation–reduction current as in the Ag/AgCl reference electrode, indicating a good result of GPRE and, hence, it can be used for voltammetric analysis technique. The physical properties of GPRE include good hardness, insoluble in non-aqueous electrolytes (except dimethyl formamide and chloroform), and good stability at different solvents. In addition, the sensitivity under conditions of CV is significantly dependent on the scan rate (SR) and variation in concentration. At different SRs, redox peaks of K₃Fe(CN)₆ were observed in a reversible process: Fe(II)/Fe(III). Interestingly, the redox reaction of Fe(II)/Fe(III) solution using GCE versus GPRE remains constant even after 15 cyclings. It is therefore evident that the GPRE possesses some degree of stability. Also, the new reference electrode GPRE has improved the properties of electroanalysis of CV on the working electrode GCE in reliability with the relative standard deviation.     

Elektrochemisches Verhalten von Redox-Systemen in Lösungsmittel-Gemischen, 1. Mitt.: Das System K4[Fe(CN)6]—K3[Fe(CN)6] in Gemischen von Wasser mit organischen Lösungsmitteln

Zusammenfassung Das elektrochemische Verhalten des Redoxsystems K₄[Fe(CN)₆]/K₃[Fe(CN)₆] wurde in Methanol-Wasser-, Äthanol-Wasser-, Dioxan-Wasser-, Tetrahydrofuran-Wasser-und Aceton-Wasser-Gemischen in Abhängigkeit von der Zusammensetzung des Lösungsmittels untersucht.Die Veränderungen des Redoxpotentials, der Leitfähigkeit und der Dielektrizitätskonstante wurden studiert, die Absorptionsspektra sowohl der einzelnen Komponenten als auch des Redoxsystems in sichtbaren und UV-Gebiet aufgenommen und ihre zeitliche Stabilität auch in Methanolhaltigen Lösungen festgestellt.Es wurde gezeigt, daß die Veränderung des Redoxpotentials — vor allem — von den, die Solvatation beeinflussenden Koordinationseigenschafte der Lösungsmittel verursacht wird. In den Lösungsmittelgemischen verändern sich die Dissoziationsverhältnisse von K₄[Fe(CN)₆] bzw. K₃[Fe(CN)₆] hauptsächlich infolge der Veränderung der Dielektrizitätskonstanten. Dieser Umstand wirkt indirekt auf das Redoxpotential des Systems.Die verschiedenen Lösungsmittelgemische rufen aber auch unmittelbar durch ihre, die Elektronendichte beeinflussenden Donor- und Acceptor-Eigenschaften die Veränderung des Redoxpotentials hervor.

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The electrochemical behaviour of redox systems in mixed solvents, I: The K₄[Fe(CN)₆]—K₃[Fe(CN)₆] system in organic solvent-water mixtures

The electrochemical behaviour of the K₄[Fe(CN)₆]—K₃[Fe(CN)₆] system has been investigated in methanol-water, ethanol-water, dioxane-water, tetrahydrofuran-water and acetone-water mixtures, as a function of the composition of the solvents.Changes in redox potential, conductivity, and dielectric constants have been investigated. In addition to the above the absorption spectra of the individual components and redox systems have been examined in the ultraviolet and visible range. The stabilities of the spectra have been established as a function of time, also in solvents containing methanol.It has been proved that the change of redox potential is caused-first of all—by the coordination behaviour of the solvent, affecting solvation. In the mixtures of solvents the dissociation properties of K₄[Fe(CN)₆] and K₃[Fe(CN)₆] are changed in consequence of the change in dielectric constants. The redox potential of the system is indirectly affected by this phenomenon.The change of redox potential, however, amy also be directly caused by the different mixtures of solvents, owing to their donor-acceptor properties affecting electron density.

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Herrn Prof. Dr.H. Nowotny gewidmet.     

DNA‐Modified Screen‐Printed Electrodes with Nanostructured Films of Multiwall Carbon Nanotubes,Hydroxyapatite and Montmorillonite

Nanostructured films were deposited at the surface of working electrode of the screen‐printed assembly and utilized for the surface modification with double‐stranded DNA. The basic electrochemical properties of the sensors were investigated using voltammetric methods. Modified electrodes were also characterized by scanning electron microscopy and electrochemical impedance measurements. It was found that the electrode modification with DNA and nanomodifier leads to an enhanced sensitivity of the DNA voltammetric detection. New potentialities of the utilization of the K₃[Fe(CN)₆] cyclic voltammetric signal and electrochemical impedance spectroscopy were found. The DNA‐based biosensors showed good repeability and necessary stability within several days.     

Study on Effect and Catalytic Mechanism of the Catalysts for Coal Oxidation in Alkaline Medium

Coal electro‐oxidation in sodium hydroxide solution with catalysts, K₃Fe(CN)₆, sodium hypochlorite and supported FeS, were investigated, respectively. Gas produced from electro‐analysis of coal slurry was collected by drainage‐method and I‐t curves were recorded to testify the catalysis of each catalyst for coal oxidation. The results show that the three kinds of catalysts can obviously improve the coal oxidation current. Furthermore, K₃Fe(CN)₆ and sodium hypochlorite played an indirect oxidation role in the electrolysis process. Catalysts bridge the coal particles and the solid electrode surface, thus increase the coal oxidation rates. The changes of catalyst content during the electrolysis were further determined by quantitative titration to discuss the catalytic Mechanism. The dynamic transition of K₃Fe(CN)₆/K₄Fe(CN)₆ and ClO⁻/Cl⁻ are proposed by iodometric method.     

Novel Methods for the Synthesis of 4‐Arylisoquinolinium Perchlorates and 4‐Arylisoquinolin‐1‐ones

2‐Benzylamino‐1‐phenyl‐ethanones 1 were converted to the corresponding isoquinolinium perchlorates 2 in high yields using 70% HClO₄‐FeCl₃ mixture as a cyclization and oxidation reagent. A mild and high yielding method for the oxidation of perchlorates 2 to isoquinolin‐1‐ones 3 involving the treatment of 2 with KOH and K₃[Fe(CN)₆] in THF‐H₂O two‐phase system at room temperature was developed. Compounds 2a–g were shown to be disproportionate to 3 and the corresponding 1,2‐dihydroisoquinoline 4 in the presence of base, which in turn is oxidized by K₃[Fe(CN)₆] to 2.     

Voltammetric Characterization of Grafted Polymer Modified with ZnO Nanoparticles on Glassy Carbon Electrode

In this study, a grafted polymer (GP) with ZnO nanoparticles (GP/ZnO NPs) was attached on the surface of glassy carbon electrode (GCE), in order to produce a new modified electrode (GP/ZnO NPs-GCE). The gamma irradiation method was used to grafted polystyrene (polymer) with acrylonitrile (monomer), while slow evaporation process was used to prepare the new modified electrode. The cyclic voltammetry (CV) of K₄[Fe(CN)₆] was used to study the electrochemical properties GP/ZnO NPs-GCE. The peak separation (ΔE_pa-c) was 500 mV between the redox peaks of Fe(II)/Fe(III) in an aqueous solution of 1 M KCl and the current ratio of redox current peaks (I_pa/I_pc) was ≈ 1 for the modified electrode. This indicated that the modified electrode has s good reversibility and conductivity, wherefore; it was applied in the voltammetric filed. It was found that the modified electrode GP/ZnO NPs-GCE have a reasonable solubility and stability at various pH medium. Additionally, the sensitivity of the electrochemical analysis by cyclic voltammetric (CV) method is extensively subjected to the pH medium and the scan rate (SR). A couple of redox current peaks of K₄[Fe(CN)₆] in KCl solution was observed with a reversible process: Fe³⁺/Fe²⁺. Finally a good diffusion coefficient of electroactive species (D) for the new modified electrode was found in this study by chronoamperometry method using Cottrell equation.     

Thermo- and Sulfate-Controllable Bioelectrocatalysis of Glucose Based on Horseradish Peroxidase and Glucose Oxidase Embedded in Poly(N,N-diethylacrylamide) Hydrogel Films

Dual-responsive poly(N,N-diethylacrylamide) (PDEA) hydrogel films with entrapped horseradish peroxidase (HRP) and glucose oxidase (GOD) were successfully prepared on electrode surface with a simple one-step polymerization procedure under mild conditions, designated as PDEA-HRP-GOD. Cyclic voltammetric (CV) response of electroactive probe K₃Fe(CN)₆ at the film electrodes displayed reversible thermo- and sulfate-responsive switching behavior. For example, at 25 °C, the K₃Fe(CN)₆ demonstrated a well-defined CV peak pair with large peak currents for the films, showing the on state, while at 40 °C, the CV response was greatly suppressed and the system was at the off state. The influence of temperature and Na₂SO₄ concentration on the switching behavior of the film system was not independent or separated, but was synergetic. The responsive mechanism of the system was ascribed to the structure change of PDEA component in the films with temperature and sulfate concentration. This switching property of the PDEA-HRP-GOD films could be further used to realize dual-responsive catalytic oxidation of glucose sequentially by HRP and GOD entrapped in the films with Fe(CN)₆ ^3? as the mediator through changing the surrounding temperature and Na₂SO₄ concentration. This system may establish a foundation for fabricating a new type of multi-switchable electrochemical biosensors based on bienzyme electrocatalysis.