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

A new grafted polymer electrode (GPE) (polystyrene as polymer) was grafted with acrylonitrile as a monomer using gamma irradiation to produce a new grafted polymer. The redox process of K₃Fe(CN)₆ during cyclic voltammetry was studied by the new GPE. The ratio of Ipc/Ipa >1 of GPE to GCE Ipc/Ipa = 1.7, indicating that this electrode is a reversible electrode and can be used in conductivity studies by voltammetric analysis. The physical properties of the new electrode GP have good hardness, insolubility, and stability at different high temperatures and at different pH. Also, the sensitivity under conditions of cyclic voltammetry is significantly dependent on pH, electrolyte, and scan rate. At different scan rates, two oxidation peaks and two reduction peaks of Fe(III) were observed in a reversible process: Fe(III) Fe(II), and Fe(II) Fe(0). Interestingly, the redox reaction of Fe(III) solution using GPE remained constant even after 15 cycles. It is therefore evident that the GPE possesses some degree of stability. The potential use of the grafted polymer as a useful electrode material is therefore clearly evident.     

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.     

Electrocatalysis of Hemoglobin in ZnO Nanoparticle/Ionic Liquid Composite Film Modified Glassy Carbon Electrode

A nanobiocompatible composite containing hemoglobin (Hb), ZnO nanoparticles (nano‐ZnO) and ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIMPF₆) was fabricated and further modified on the glassy carbon electrode (GCE). The electrochemical behaviours of Hb in the composite film were carefully studied and a pair of quasi‐reversible redox peaks appeared in pH 7.0 phosphate buffer solution, which was attributed to the electrode reaction of Hb heme Fe(III)/Fe(II) redox couple. The presences of nano‐ZnO and BMIMPF₆ in the film can retain the bioactivity of Hb and greatly enhance the direct electron transfer of Hb. The immobilized Hb showed high stability and good electrocatalytic ability to the reduction of hydrogen peroxide and O₂.     

Cyclic voltammetric study of the redox behavior of Fe(II)/Fe(III) systems forming during the oxidation of Fe(II) complexes with saccharin and with saccharin and 1,10-phenanthroline

The electrochemical redox behavior of Fe(II)/Fe(III) systems formed during the oxidation of complexes [Fe(C₇H₄NO₃S)₂(H₂O)₄] · 2H₂O (Fe-sac) and [Fe(C₇H₄NO₃S)₂(C₁₂H₈N₂] · 2H₂O (Fe-sac-phen) have been investigated using cyclic voltammetry in the aqueous medium. In the CVs one pair of well-defined cathodic and anodic peaks appear for the transfer of single electron in the Fe-sac complex. The peak potentials are much wider separated as compared with the free (uncoordinated) Fe(II)/Fe(III) system. The ΔE values demonstrate that the electrode process is irreversible. In the presence of secondary ligand, 1,10-phenanthroline (Fe-sac-phen complex), the redox behavior of iron complexes is quasireversible. The effect of pH on the redox behavior of iron system is studied in acetate buffer. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 12, pp. 1504–1509. The text was submitted by author in English     

Copper hexacyanoferrate multilayer films on glassy carbon electrode modified with 4-aminobenzoic acid in aqueous solution

4-Aminobenzoic acid (4-ABA) was covalently grafted on a glassy carbon electrode (GCE) by amine cation radical formation during the electrooxidation process in 0.1 M KCl aqueous solution. X-ray photoelectron spectroscopy (XPS) measurement proves the presence of 4-carboxylphenylamine on the GCE. Electron transfer processes of Fe(CN)₆³⁻ in solutions of various pHs at the modified electrode are studied by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Changing the solution pH would result in the variation of the terminal group's charge state, based on which the surface pK_a values were estimated. The copper hexacyanoferrate (CuHCF) multilayer films were formed on 4-ABA/GCE prepared in aqueous solution, and which exhibit good electrochemical behavior with high stability.     

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.     

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.     

Grafting of diaminoalkane on glassy carbon surface and its functionalization

Diaminoalkanes (NH₂(CH₂)_nNH₂, n=7,10,12) were grafted onto a glassy carbon electrode (GCE) surface by amino cation radical formed during electrooxidation of amino group. The presence of diamine grafted layer at the GCE is demonstrated by X-ray photoelectron spectroscopy. The effect of the grafted layer at the GCE surface on the redox responses of Ru(NH₃)₆³⁺ and Fe(CN)₆³⁻ redox probes has been investigated. Electrochemical impedance experiments indicate that the kinetics of electron transfer are slowed down when the scan rate taken to modify the GCE is low, and that diaminoalkane with longer alkyl-chain used has higher blocking characteristics. The amine-functionalized GCE is versatile not only to further covalently immobilize ferrocene acetic acid via carbodiimide coupling, but also as a charge-rich substrate to successfully adsorb heteropolyanion P₂W₁₈ in acidic solution by electrostatic interaction.     

Discrete Rh(III)/Fe(II) and Rh(III)/Fe(II)/Co(III) cyanide-bridged mixed valence compounds

The heterotrinuclear complexes trans- and cis-[{cis-VI-L(15)Rh(III)(μ-NC)}{trans-III-L(14S)Co(III)(μ-NC)}Fe(II)(CN)(4)](2+) are unprecedented examples of mixed valence complexes based on ferrocyanide bearing three different metal centers. These complexes have been assembled in a stepwise manner from their {trans-III-L(14S)Co(III)}, {cis-VI-L(15)Rh(III)}, and {Fe(II)(CN)(6)} building blocks. The preparative procedure follows that found for other known discrete assemblies of mixed valence dinuclear Cr(III)/Fe(II) and polynuclear Co(III)/Fe(II) complexes of the same family. A simple slow substitution process of [Fe(II)(CN)(6)](4-) on inert cis-VI-[Rh(III)L(15)(OH)](2+) leads to the preparation of the new dinuclear mixed valence complex [{cis-VI-L(15)Rh(III)(μ-NC)}Fe(II)(CN)(5)](-) with a redox reactivity that parallels that found for dinuclear complexes from the same family. The combination of this dinuclear precursor with mononuclear trans-III-[Co(III)L(14S)Cl](2+) enables a redox-assisted substitution on the transient {L(14S)Co(II)} unit to form [{cis-VI-L(15)Rh(III)(μ-NC)}{trans-III-L(14S)Co(III)(μ-NC)}Fe(II)(CN)(4)](2+). The structure of the final cis-[{cis-VI-L(15)Rh(III)(μ-NC)}{trans-III-L(14S)Co(III)(μ-NC)}Fe(II)(CN)(4)](2+) complex has been established via X-ray diffraction and fully agrees with its solution spectroscopy and electrochemistry data. The new species [{cis-VI-L(15)Rh(III)(μ-NC)}{trans-III-L(14S)Co(III)(μ-NC)}Fe(II)(CN)(4)](2+) and [{cis-VI-L(15)Rh(III)(μ-NC)}Fe(II)(CN)(5)](-) show the expected electronic spectra and electrochemical features typical of Class II mixed valence complexes. Interestingly, in the trinuclear complex, these features appear to be a simple addition of those for the Rh(III)/Fe(II) and Co(III)/Fe(II) moieties, despite the vast differences existent in the electronic spectra and electrochemical properties of the two isolated units.     

Study of the Electrostatic Interaction of Lysine Monolayer on Glassy Carbon Electrodes Using the Electrochemical Method

A covalent modified glassy carbon electrode (GCE) with Lysine (Lys) has been fabricated via an electrochemical oxidation procedure. The electrostatic interaction of the monolayer has been investigated by cyclic voltammetry (CV) with Fe(CN)₆^3? redox probe in different concentrations of protons and different charged cations, respectively. The electrochemical method can be a new feasible method for the study of electrostatic interaction of the monolayer.     

The electrochemical Peltier effect observed with electrode reactions of Fe(II)/Fe(III) redox couples at a gold electrode

The electrochemical Peltier effect was studied at a gold electrode in solutions containing some Fe(II)/Fe(III) redox couples by measuring the local temperature change in the electrode/solution interphase under controlled-potential and controlled-current polarization. Relative values of the electrochemical Peltier coefficient for the cathodic process at equilibrium potential, which is denoted by (Π_c)_I=0, were determined by analyzing the observed temperature change as a function of current. The values of (Π_c)_I=0 were found to be positive for the Fe(H₂O)₆²⁺/Fe(H₂O)₆³⁺ systems in HClO₄ (1 M), HNO₃ (1 M), H₂SO₄ (0.5 M), and HCl (1 M), their magnitudes being very similar in the first three acid solutions, but smaller in the HCl solution. On the other hand, a negative value of (Π_c)_I=0 was obtained in the case of a Fe(CN)₆^4?/Fe(CN)₆^3? couple in a H₂SO₄ (0.5 M) solution. Such a difference in the Peltier coefficient is considered to be due to the difference in the ionic species of iron involved in the electrode reaction.     

Electrochemical Characteristics of Poly(<Emphasis Type="Italic">o</Emphasis>-Aminobenzoic Acid) Modified Glassy-Carbon Electrode and Its Electrocatalytic Activity towards Oxidation of Epinephrine

Poly(o-aminobenzoic acid) (o-ABA) film is deposited on glassy-carbon electrode (GCE) by electropolymerization in pH 7.0 phosphate buffer solution (PBS). Electrochemical behavior of modified electrode is investigated by electrochemical impedance spectroscopy (EIS), different pulse voltammetry (DPV), and cyclic voltammetry (CV). The results indicate that there is a greater resistance during the electron transfer process in poly(o-ABA) film than in bare GCE for the redox of [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻. Further research indicates that epinephrine (EP) can be strongly absorbed on the surface of the poly(o-ABA) film-modified electrode. The modified electrode shows an excellent electrocatalytical activity on EP oxidation. The EP cathodic peak potential shifts negatively with a slope of −53.5 mV/pH, indicating that equal amounts of proton and electron are involved in the electrode reaction process. In pH 7.0 PBS, the peak current of EP and the concentration has a linear relationship from 0 to 65 μM by amperometric current-time curve. __________ From Elektrokhimiya, Vol. 41, No. 9, 2005, pp. 1059–1065. Original English Text Copyright ? 2005 by Cheng, Jin, Zhang. The text was submitted by the authors in English.     

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.     

Determination of experimental parameters in the Fe(III)/Fe(II) system in 1 M H2SO4, with a polycrystalline gold electrode, taking advantage of its non-linear behaviour

In this work, the values of the heterogeneous standard rate constant and the transfer coefficient of the electrochemical system Fe(III)/Fe(II) in 1 M H₂SO₄ at a polycrystalline gold electrode were determined. The response spectrum to an ac potential of such amplitude as to make the behaviour of the electrode process non-linear was analysed. The experimental study was complemented by a theoretical study of the Fe(III)/Fe(II) system using numerical methods. Comparison of the experimental and theoretical data enabled the kinetic parameters of this electrode process to be determined.     

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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Mit 9 Abbildungen

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Herrn Prof. Dr.H. Nowotny gewidmet.     

Voltammetry of the iron(II)-{3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine} complex using GCE in acetonitrile

Summary During cyclic voltammetry the ferrous complex Fe(PDT) ₃ ²⁺ is both reduced and oxidized on a GCE in acetonitrile solvent. The reduction occurs in three steps, while during its oxidation, the Fe(III) species is generated at a more positive potential than the standard potential of the simple Fe(III)/Fe(II) couple. The anodic peak potential promises to be useful as a reference for measurements in non-aqueous solvents. The diffusion coefficient of the species in acetonitrile is reported.     

Electrochemical Study of the Interactions of DNA with Redox‐Active Molecules Based on the Immobilization of dsDNA on the Sol‐Gel Derived Nano Porous Hydroxyapatite‐Polyvinyl Alcohol Hybrid Material Coating

A novel type of sol‐gel inorganic‐organic hybrid material coated on glassy carbon electrode used for immobilization of double‐stranded DNA (dsDNA) and study of dsDNA with redox‐active molecules was developed. The hybrid material coating was produced by sol‐gel method with nano hydroxyapatite (HAp)‐polyvinyl alcohol (PVA). The optimum composition of the hybrid material was first examined, and the morphology of the nano HAp‐PVA coatings was investigated with the help of Scanning Electron Microscope (SEM). DsDNA was immobilized in/on the nano HAp‐PVA hybrid coatings by adsorption and the characteristics of the dsDNA/HAp‐PVA/GCE were studied by cyclic voltammetry (CV) using the probes of Co(phen) and Fe(CN) . The results indicate that the dsDNA can be immobilized on the nano porous HAp‐PVA coating effectively and its stability can satisfy the necessity of study on the interactions of dsDNA with redox‐active molecules on the electrode surface. Co(bpy) and Co(phen) were used as the model molecule to study the interactions of dsDNA with redox‐active molecules. Information such as ratio (K_Ox/K_Red) of the binding constant for the oxidized and reduced forms of a bound species, interaction mode, including change in the mode of interaction, and “limiting” ratio K /K at zero ionic strength (μ) can be obtained using dsDNA/HAp‐PVA/GCE with about 2 μg of DNA samples.     

Solvent effects on the redox potentials of tetraethylammonium hexacyanomanganate(III) and hexacyanoferrate(III)

Tetraethylammonium hexacyanomanganate(III) was studied in formamide, N-methylformamide, methanol, propylenecarbonate, N,N-dimethylthioformamide, N-methylthiopyrrolidinone(2), butyrolactone, acetonitrile, dimethylsulfoxide, N,N-dimethylformamide, N-methylpyrrolidinone(2), nitromethane and tetramethylenesulfone employing polarographic and voltammetric techniques. Reversible or nearly reversible behaviour for the reaction Mn(CN)₆^3?/Mn(CN)₆^2? was observed in most solvents on the stationary platinum electrode. The reaction Mn(CN)₆^3?/Mn(CN)₆^4? was studied on both the dropping mercury electrode and the stationary platinum electrode. Besides the reaction Mn(CN)₆^3?/Mn(CN)₆^4? several anodic waves due to successive reactions of mercury with the cyano-ligand of the complex occurred at the dropping mercury electrode. No redox reaction for (et₄N)₃Mn(CN)₆ was found in nitromethane. The polarographic behaviour of tetraethylammonium hexacyanoferrate(III) was studied in formamide, N-methylformamide, N-methylpyrrolidinone(2) and butyrolactone. The variation of E_1/2 and 1/2 (E_pa+E_pc) values versus bisphenylchromium(I)/bisbiphenylchromium(0) as reference redox system of the processes Mn(CN)₆^3?/Mn(CN)₆^2?, Mn(CN)₆^3?/Mn(CN)₆^4? and Fe(CN)₆^3?/Fe(CN)₆^4? with the nature of the solvent is discussed within the donor-acceptor concept. Correlations between the E_1/2 and 1/2(E_pa+E_pc) values and the acceptor properties of the solvent have been observed. The preparation of tetraethyl- and tetrabutylammonium hexacyanomanganate(III) is described.     

Direct electrochemistry and enzymatic activity of hemoglobin immobilized in ordered mesoporous titanium oxide matrix

The novel highly ordered mesoporous titanium oxide (mesoTiO₂) materials, prepared by the “acid–base pairs” route, were firstly used for the immobilization of hemoglobin (Hb) and its bioelectrochemical properties were studied. FTIR and UV–vis spectroscopy demonstrated that Hb in the mesoTiO₂ matrix could retain its native secondary structure. The CV results of Hb/mesoTiO₂-modified electrode showed a pair of well-defined and quasi-reversible redox peaks centered at approximate −0.158 V (vs. SCE) in pH 6.0 phosphate buffer solution. It reflects the characteristic of Hb heme Fe (III)/Fe(II) redox couple with fast heterogeneous electron transfer rate. The immobilized Hb also displayed its good electrocatalytic activity for the reduction of hydrogen peroxide. The results demonstrate that the mesoTiO₂ matrix may improve the protein loading with the retention of bioactivity and greatly promote the direct electron transfer, which can be attributed to its high specific surface area, uniform three-dimensional well-ordered porous structure, suitable pore size and biocompatibility.     

Fe(III)/Fe(II) separation by solvent extraction for the determination of oxygen stoichiometry in yttrium barium copper oxide superconductor materials

Summary Oxygen stoichiometry is a critical parameter defining the T_c of cuprate superconductors (e.g. YBa₂Cu₃O₇). On dissolution excess or deficiency of oxygen can be converted into shifts of the Fe(II)/Fe(III) concentration ratio of an aqueous solution. For small samples a solvent extraction technique for the separation of Fe(III) from Fe(II) was developed, to make use of the superior sensitivity of atomic spectrometry (AAS, ICP-AES). The system n-benzoylphenylhydroxylamine (BPHA)/CHCl₃ was chosen because it is relatively inactive as a redox partner. Despite the catalytic effect of Cu(II) on the oxidation of Fe(II), oxidation blanks can be kept down at negligible levels. Less than 0.55% of residual Fe(II) is converted to Fe(III) during the extraction procedure (argon atmosphere). In the presence of air, oxidation levels are still practical (3%). Extraction is from 0.3 mol/l HBr providing excellent recovery of Fe(III) (e.g. 98.8%). All Fe(II), Y, Ba (including BaSO₄ precipitate) and 99.4% of the Cu remain in the aqueous phase. Fe(III) is rapidly back-extracted into an aqueous phase by 6 mol/l HCl for dilution and aspiration into the flame or ICP. Recovery of Fe(III) after the two extraction steps is still 98.3%.