Voltammetric and XPS investigations of nickel hydroxide electrochemically dispersed on gold surface electrodes

A chemically modified electrode composed of mixed hydroxide and oxyhydroxide nickel film (6–8 nmol cm^?2) on the gold substrate (Au ∣ Ni) was characterized by cyclic voltammetry and XPS techniques. The gold substrate electrodes were firstly electrochemically conditioned in 0.2 M NaOH by cycling the potential between ?0.25 and 0.6 V versus SCE, then modified by cathodic electrodeposition of nickel hydroxide films. These nickel films were obtained either by voltage cycling (50 mV s^?1) between 0.0 and ?0.5 V (SCE) or at constant potential of ?0.3 or ?0.5 V using non-deaerated 50 mM Ni(NO₃)₂ solutions. X-ray photoelectron spectroscopy (XPS) characterisation and voltammetric behaviour of Au ∣ Ni electrodes in alkaline solutions are described. Continuous electrochemical cycling of the Au ∣ Ni electrodes induces significant changes of the nickel films in terms of crystallographic structures and chemical composition. Combination of XPS and electrochemical methodologies have demonstrated the ability to follow the morphological and chemical changes in alkaline solutions upon cycling potentials. Angular-dependent XPS measurements have demonstrated that electrochemical treatment induces the formation of a uniform film layer with the following chemical distribution: Au ∣ Ni(OH)₂ ∣ NiOOH. The electrocatalytic activity of the Au ∣ Ni electrodes is investigated in alkaline medium using glucose as a model compound. The favourable combination of active species such as gold and nickel leads to a sensing electrode with strong catalytic activity over a wide range of applied potentials.     

Electrochemical switching of fluorescence emission between red and blue from complexed europium ions in poly(ethylene oxide)

Europium nitrate hexahydrate (Eu(NO₃)₃ · 6H₂O) and heterocyclic ligands were dissolved to form complexes in poly(ethylene oxide) (PEO). The complexed Eu ion was electrochemically redox active, like that without ligands, in PEO oligomers. The fluorescence intensity from Eu³⁺ was enhanced 632-fold after complexation with 2,2′:6′,2″-terpyridine (Terpy). The improved red fluorescence based on Eu³⁺ decreased at a negative potential (?1.0 V vs Ag) of the working ITO glass electrode. A positive potential (+1.0 V) increased the intensity of red fluorescence, suggesting a chemically reversible redox process. It was confirmed that complexed Eu³⁺ was reduced in PEO by the cyclic voltammetry. However, the blue fluorescence based on Eu²⁺ was not observed, due to energy migration through the ligand. On the other hand, the blue fluorescent emission was induced by the complexation between Eu ions and 2,2′-bibenzoxazoline (BA). Cyclic voltammetry showed that europium ions complexed with BA were electrochemically redox active. The blue fluorescence emission intensity of these Eu ∣ BA complexes was controllable by changing the potential polarity. Mixed PEO solutions of Eu ∣ Terpy and Eu ∣ BA complexes showed the alternate emission color change between red and blue.     

The electrochemistry and thin-layer luminescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode

We report on the first direct electrochemistry and fluorescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode. The value of n determined in spectropotentiostatic experiments at 1.87×10^?6 mol l^?1 of rhodamine 6G in 0.20 mol l^?1 KCl solution is 1.15, and the experimental value obtained for E⁰′ is ?0.787 V versus Ag ∣ AgCl ∣ KCl_sat, which agrees very well with the value (E⁰′=?0.791 V) obtained using cyclic voltammetry at a modified gold electrode. The values of the diffusion coefficients D_O and D_R for the oxidized and reduced forms of rhodamine 6G calculated from results of potential step and in situ fluorescence measurement experiments are 4.0×10^?6 cm² s^?1 and 4.2×10^?6 cm² s^?1, respectively. Cyclic voltammograms of rhodamine 6G show that the peak current I_p is proportional to the square root of the potential scan rate v^1/2, the ratio of the reduction to the oxidation peak height is about unity, and the separation of both reduction and reoxidation peak potentials ΔE_P is essentially constant at 135 mV at low scan rates. These results indicate that electrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode is a quasi-reversible one-electron electrode process.     

Application of multi-walled carbon nanotubes functionalized with hemin for oxygen detection in neutral solution

Functionalization of multi-walled carbon nanotubes (MWNTs) is of paramount importance for developing new sensors. In this study, a hemin-modified MWNT electrode was successfully constructed. Upon saturation, the amount of adsorbed hemin is estimated as 2.7 × 10^?9 mol cm^?2, which is 39 times larger than the value of the hemin monolayer. The electrochemical behavior of the hemin-modified MWNT electrode has been characterized by cyclic voltammetry (CV). The electron-transfer coefficient (α) is found to be 0.38 with a heterogeneous electron transfer rate (k) of 2.9 s^?1 for the adsorbed hemin. Both MWNT and hemin-modified MWNT electrodes show ideal reversibility in 5 mM K₃[Fe(CN)₆] in the range of 0.02–1.00 V s^?1, indicating fast electron-transfer kinetics. CV of the hemin-modified MWNT electrode in pH 7.4 phosphate buffer solution (PBS) clearly shows the dioxygen reduction peaks close to 0 V (vs. Ag|AgCl). These results are useful in the development of a novel oxygen sensor for working at a relatively low potential.     

Electrochemical behavior of iodide at a nickel pentacyanonitrosylferrate film modified aluminum electrode prepared by an electroless procedure

The surface of an aluminum disk electrode was modified by a thin film of nickel pentacyanonitrosylferrate and used for electrocatalytic oxidation of iodide. The cyclic voltammogram of the modified Al electrode showed surface redox behavior due to the [Ni^IIFe^III/II(CN)₅NO]^0/1? redox couple. The modifying layer shows excellent catalytic activity toward the oxidation of iodide. Different supporting electrolytes containing different alkali metal cations affected the apparent formal potential of the redox films and thus, changed the thermodynamic tendency and kinetics of the modifying film toward the catalytic oxidation of iodide. This was explained by including the concept of a surface coverage normalized-catalytic current. The kinetics of the catalytic reaction were investigated by cyclic voltammetry and rotating disk electrode voltammetry in a suitable supporting electrolyte. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The heterogeneous rate constant for the catalytic reaction, k, diffusion coefficient of iodide in solution, D, and transfer coefficient, α, were found to be 5.8 × 10² M^?1 s^?1, 1.3 × 10^?5 cm² s^?1 and 0.66, respectively. In addition the effect of electrode surface coverage on the dynamic range of a calibration curve was investigated. Under optimum conditions a linear calibration graph was obtained over an iodide concentration range of 2–100 mM.     

Characterization and electrocatalytic properties of cobalt hexacyanoferrate films immobilized on Au-colloid modified gold electrodes

An electroactive cobalt hexacyanoferrate (CoHCF) film was electrodeposited from a solution containing Co²⁺ and Fe(CN)₆^3? ions on the bare gold or the Au-colloid modified electrode. The cation (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) and the anion (F^?, Cl^? and Br^?) effects on the redox peak of the CoHCF film were investigated in detail. On the other hand, the electrocatalytic oxidations of thiosulfate at the CoHCF/gold and CoHCF/Au-colloid/gold electrodes were compared. At the CoHCF/Au-colloid/gold electrode, we obtained a response current larger by a factor of 2 and a three times lower detection limit than those at a CoHCF/gold electrode. The linear ranges were 1.0 × 10^?4 to 2.8 × 10^?3 M for the CoHCF/gold electrode and 7.5 × 10^?5 to 4.8 × 10^?3 M for the CoHCF/Au-colloid/gold electrode. These results showed that the immobilized CoHCF at the Au-colloid modified electrode exhibited a higher catalytic activity and a wider linear range toward thiosulfate. Additionally, the effects of the applied potential and the solution pH were studied.     

Semiconductor and electrochromic properties of electrochemically deposited nickel oxide films

Nickel oxide films have been prepared by electrochemical deposition on a conducting substrate such as Au or ITO from a basic solution with nickel sulfate and an amino acid. The NiO films thus prepared exhibit typical p-type semiconductor and electrochromic properties. From a Mott–Schottky plot, the flatband potential, V_FB and the acceptor density, N_A are determined as 0.47 V versus Ag|AgCl (at pH 7.0) and 1.8 × 10²¹ cm^?3, respectively. The optical band gap, E_g obtained from measurement of the absorption coefficient of the NiO at a particular photon energy is estimated as 3.31 eV. The absorption spectrum of a NiO|ITO electrode exhibits a maximum at a wavelength of 460 nm in the potential region above +0.4 V. The colors of the nickel oxide are dark blue and transparent in the potential regions above and below this potential, respectively.     

Electrochemical investigation on the preparation of Lu–Co thin films in dimethylsulfoxide

Cyclic voltammetry and chronopotentiometric and chronoamperometric curves were used to study the electroreduction of Lu³⁺ in a LiCl + DMSO system. The electrode process of Lu³⁺ reduced on a Pt electrode occurs in one step: Lu³⁺ + 3e^? → Lu. The transfer coefficient and diffusion coefficient of Lu³⁺ in the 0.1 mol dm^?3 LuCl₃ + 0.1 mol dm^?3 LiCl + DMSO system were calculated as 0.141 and 2.86 × 10^?10 m² s^?1 at 305 K, respectively. The experimental results indicate that a Lu–Co thin film containing 5.65–56.30 wt% Lu was obtained by potentiostatic electrolysis in 0.1 mol dm^?3 LuCl₃ + 0.1 mol dm^?3 CoCl₃ + 0.1 mol dm^?3 LiCl + DMSO. The surface of the Lu–Co thin film observed by scanning electron microscopy (SEM) was uniform, adhesive and had a metallic luster. The Lu–Co thin films obtained were amorphous as proven by X-ray diffraction analysis (XRD).     

Electrocatalytic oxidation of nitrite on a carbon paste electrode modified with Co(II) porphyrin adsorbed on SiO2/SnO2/Phosphate prepared by the sol–gel method

This paper describes the immobilization of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphyrin ion on SiO₂/SnO₂/Phosphate obtained by the sol–gel processing method. The porphyrin was adsorbed on the surface of the modified material and furthermore metallized in situ with Co(II) ion. The porphyrin metallation process was followed using UV–Vis spectroscopy by inspecting the Q bands of the free and metallated porphyrin. A carbon paste electrode modified with material containing metallated porphyrin was used to study the electro-catalytic oxidation of nitrite ions by means of cyclic voltammetry, chronoamperometry and RDE voltammetry. The modified electrode was very stable and exhibited the electro-catalytic oxidation of nitrite ions at 0.72 V vs. SCE by a two electron mechanism producing nitrate ions at pH 5.4. The kinetic parameters of the electrode reaction process were calculated; (1 ? α)n_a was 0.479, D was (5.3 ± 0.11) × 10^?5 cm s^?1, and k⁰ could be determined as (5.4 ± 0.14) × 10^?3 cm s^?1.     

Catechin antioxidant action at various pH studied by cyclic voltammetry and PM3 semi-empirical calculations

The mechanism of catechin electro-oxidation at various pH was studied using cyclic voltammetry (CV) on the glassy carbon (GC) electrode and PM3 semi-empirical calculations. The influence of activation of the surface of the GC electrode on CV results has been discussed. Mixed adsorption–diffusion control has been observed by applying mechanistic criteria of CV to the results obtained at the activated electrode. The calculated catechin diffusion coefficient D = 2.78 × 10^?6 cm² s^?1. A linear increase of the current peak has been observed with the increase of substrate concentration up to 40 μmol dm^?3 (surface coverage Γ ～ 10^?11 mol cm^?2). In the whole investigated pH range, the dE/dpH value is very close to the anticipated Nernstian dependence of ?59 mV/pH indicating that the slope is not affected by the different sequences of e^? and H⁺ transfer. Molecular modeling results show a decrease of ≈5 kcal mol^?1 in ΔHoF (between radical and parent molecule) and a decrease of ≈6 eV in IP (of the parent molecule) when the parent molecule is changed from neutral to monoanionic form of catechin showing that both processes – hydrogen and electron abstraction are facilitated by deprotonation. Electrochemical oxidation of catechin is known to proceed as a two step one-electron oxidation of the B-ring of o-phenolic groups. Upon an increase in the pH, the mechanistic pathway of catechin electro-oxidation in both oxidation steps changes from an eH to the He process. In the reaction with a free radical, this may induce the change from hydrogen to electron donation.     

Surface structures at the initial stages in passive film formation on Ni(1 1 1) electrodes in acidic electrolytes

The adsorption of sulfate or OH species and subsequent Ni(OH)₂ film growth on Ni(1 1 1) single crystal electrodes has been investigated using in situ infrared reflection absorption spectroscopy (IRAS) as well as scanning tunneling microscopy (STM). In a pH 3 sulfuric acid solution, STM images show that a well-defined Ni(1 1 1) surface with a (1 × 1) lattice is exposed at ?300 mV, while hexagonal close-packed images with an atomic spacing of 0.32 nm are grown on this electrode at 300 mV. On the other hand, IRAS results in a sulfuric acid solution (pH 3) reveal that an absorption band at 1116 cm^?1, which can be ascribed to ν(S–O) symmetric stretching of sulfate anion on Ni(1 1 1) surface, starts to appear at ?400 mV and develops its intensity with an electrode potential increase, while an absorption band at 930 cm^?1 begins to develop at 0 mV on Ni(1 1 1), Ni(1 0 0) and Ni(1 1 0) electrodes, which can be assigned to an in-plane δ(Ni–OH) bending vibration in Ni(OH)₂ passive film.     

Vibrational study of adsorbed phosphate ions on rhodium single crystal electrodes

The adsorption of H₂PO₄^? ions was studied on low Miller index rhodium single crystal electrodes by in situ FTIR spectroscopy. It is found that for Rh(1 0 0) and Rh(1 1 0), H₂PO₄^? ions are the major species at low potentials, but at higher potentials, some of the H₂PO₄^? ions undergo a potential induced deprotonation and probably there is a mixture of H₂PO₄^? and HPO₄^2? ions. On Rh(1 1 1) the deprotonation starts at very low potentials and at higher potentials the H₂PO₄^? is fully converted to HPO₄^2?. The behavior of the band center and of the band intensity with the applied potential was also analyzed. It is found that the adsorption increases from 0.08 V vs. a Pd–H₂ electrode up to 0.5 V and then it decreases when the OH starts to be adsorbed.     

Surface reconstruction at metal|solid polymer electrolyte interfaces: Au(111)|Nafion? interface1

The structure of the Au(111) surface in contact with a recast Nafion® film has been studied as a function of potential by using grazing incident angle X-ray diffraction. The high penetration depth of X-ray makes it possible to probe the electrode surface covered by a 1 to 3 μm thick Nafion® film. The Nafion®-coated Au(111) electrode surface is not reconstructed at open circuit potential. However, the $\text{(23×}\text{3}\text{)}$ reconstruction can be generated at sufficiently negative potentials. In comparison with the Au(111) electrode in NaClO₄ solution, the lifting of reconstruction is shifted positively by about 0.2 V. This fact supports the view of the primary effect of surface charge on the Au(111) reconstruction lifting. Adding chloride to the solution shifts the phase transition potential negatively after several hours, indicating a moderate ion permselectivity of the Nafion® film in NaCl solution. These results demonstrate that the Nafion® coating is stable over a large potential range and is suitable for studying surface properties and the effects of ion distribution on surface electrochemical processes at single crystal surfaces. © 1997 Elsevier Science S.A.     

EIS study of the FeIII/FeII redox couple at a Nafion®-coated Pt electrode

The redox reaction Fe^III + e ? Fe^II is studied at a Nafion^®-filmed Pt electrode using both steady-state and EIS methods. It is shown that the fitting of the EIS experimental data can be greatly improved by using the new concept of a ‘theoretical–experimental hybrid model’, i.e., by introducing experimental data obtained at a bare electrode in the theoretical model for a filmed electrode. It is also shown that a combined steady-state and EIS study cannot lead to a separate determination of the three key parameters of the system: the diffusion coefficient in the polymer film (D_f), the thickness of the film (L), and the partition coefficient between the solution and the polymer (γ), only the two parameters γD_f/L and L²/D_f being obtained from experimental data by a curve fitting procedure. Finally, an exhaustive theoretical EIS study in quiescent solution is also proposed.     

Electrochemical quartz crystal microbalance study of mass transport during electrochemical oxidation of [Os(bpy)3]2+ in clay-modified electrodes

Mass transport in clay films containing [Os(bpy)₃]²⁺ cations was investigated by EQCM and crystal impedance spectroscopy. Admittance measurements on 10 μg clay films exchanged with [Os(bpy)₃]²⁺ show no change in the width or height of the conductance peaks before and after potential scans or potential steps. The [Os(bpy)₃]²⁺ exchanged films could be considered to be rigid with no change in their viscoelastic properties, and the shifts in resonant frequency in the EQCM measurements interpreted as mass changes. In an electrode coated with a 8.5 μg clay film, oxidation of the adsorbed cations resulted in a large increase in frequency, corresponding to a decrease in mass. The mass per mole of electron transferred (MPE) was ?205 g/mol, or about one third of the weight of one [Os(bpy)₃]²⁺ ion. This was consistent with the ejection of one [Os(bpy)₃]³⁺ from the film for each three [Os(bpy)₃]²⁺ cations oxidized. The ratio of cathodic charge to anodic charge of 0.7 was also consistent with the loss of one third of the oxidized [Os(bpy)₃]³⁺ ions. The mechanism of charge neutralization was dependent of the weight of the clay films. Oxidation of [Os(bpy)₃]²⁺ in a 33 μg clay film resulted in a small decrease in frequency, corresponding to an increase in mass. The MPE, +50 g/mol, was consistent with charge neutralization by adsorption of sulfate anions from the electrolyte.     

Rotating ring-disk studies of oxidized nickel hydrous oxide: oxygen evolution and pseudocapacitance

A rotating ring-disk electrode (RRDE) method is herein described for studies of O₂ evolution on nickel hydrous oxide films, NiO_x(hydr.), electrodeposited on the gold disk of a Au–Au RRDE in aqueous 1.0 M KOH. This technique relies on the quantitative detection of O₂ generated at the NiO_x(hydr.) ∣ Au disk electrode during a linear potential scan, by the concentric, bare Au-ring electrode, which can then be used to determine contributions to the disk current (I_disk) derived solely from O₂ evolution. Subtraction of such contributions from I_disk in the potential range positive to the trailing edge of the peak ascribed to the oxidation of the NiO_x(hydr.) film revealed a constant, positive current when the voltage was scanned in the positive direction, and a constant, negative current, albeit of smaller magnitude, in the subsequent scan in the negative direction. This observation suggests that once account is made for O₂ evolution, the NiO_x(hydr.) ∣ Au-electrolyte interface in that potential range (0.5–0.65 V vs. Hg ∣ HgO,OH^?), behaves as a (pseudo) capacitor, a model that was further confirmed by monitoring the current as a function of the scan rate. The actual values of this pseudocapacitance were found to be on the order of ca. 80 kF mol^?1 Ni sites in the film (or equivalently, ca. 0.1 e^? per Ni site within the potential range specified above) and, thus, very similar to those reported earlier based on current interruption-potential decay and impedance measurements for O₂ evolution on NiO_x(hydr.) grown on Ni electrodes. Implications of these results to the mechanism of self-discharge of NiO_x(hydr.) electrodes for alkaline secondary batteries are discussed.     

Improved platinization conditions produce a 60-fold increase in sensitivity of amperometric biosensors using glucose oxidase immobilized in poly-o-phenylenediamine

Platinization has been used to increase electrode surface area and therefore electrode sensitivity. However there are few studies on platinization conditions and their effects on the reaction diffusion of enzyme biosensors. We report the fabrication of sensitive (192 ± 48 μA cm^?2 mM^?1) amperometric glucose sensors. The repeatability (precision) was at worst 2.32% ± 1.22% of the measured value, the limit of detection was 0.94 mM and the detection was linear up to at least 25 mM. Sensor-to-sensor variability was 24%. The half-life of the sensors at 21 °C was 12 days. Shelf life at 4 °C was at least one month without a decrease in sensitivity. Under continuous operation, sensors performed at least 120 determinations without a large decrease in sensitivity; however; fluctuations in current response indicate that frequent calibration is required. Sensors were used for the determination of glucose in apple juice and white wine. High sensitivity was achieved by improving the conditions of platinization and entrapment of glucose oxidase in poly-o-phenylenediamine. Potentiostatic platinization at ?100 or ?50 mV vs. Ag∣AgCl resulted in mechanically stable deposits unlike those formed at the commonly used ?250 mV. Large concentrations of glucose oxidase did not inhibit the electropolymerization of o-phenylenediamine. H₂O₂ diffusion experiments using polished and platinized, bare and polymer-covered rotating disk electrodes suggest that H₂O₂ diffuses from the bulk of the solution through the polymer film to be oxidized at the platinized surface and the diffusion is apparently one-dimensional with platinization compensating for the coverage of catalytic sites by the polymer. Conversely, when, on a platinized electrode, glucose oxidase is immobilized in a thin poly-o-phenylenediamine film, substrate diffuses rapidly through the porous matrix and H₂O₂ is produced throughout the enzyme/polymer film. Under these conditions, diffusion is apparently multidirectional and platinization results in an effectively large increase in surface area that results in a high amperometric response. A mathematical model of the reaction–diffusion matrix of polished electrodes supports this interpretation of the increased response.     

Oxygen reduction mechanism on corroded zinc

The mechanism of oxygen reduction on the as-polished and corroded zinc specimens has been studied using a rotating ring disc electrode (RRDE) system. On the as-polished surface, oxygen was reduced into two distinct steps. In the first step, about 44% of O₂ was reduced to H₂O₂ in a 2-electron reaction with the rest being reduced to OH^? in a 4-electron reaction. On the other hand, in the second step, with the increase of overpotential O₂ was almost exclusively reduced to OH^? in a 4-electron reaction. The first step reduction occurred on an air-formed oxide-covered surface at more positive potential than ?1.2 V vs. Ag/AgCl and the second step reduction (E < ?1.2 V) took place on a semi-uniformly active surface. On the corroded surface, the second step was not distinctly observed on the polarization curve, because reduction of the zinc corrosion products simultaneously took place around ?1.2 V. The O₂ reduction in the first step was inhibited by deposition of the corrosion products, though the ratio of amount of O₂ reduced to OH^? in a 4-electron reaction was larger than that on the as-polished surface. The mechanism of oxygen reduction is discussed on the basis of results obtained from the RRDE experiment.     

Voltammetric trace metal determinations by cathodic and anodic stripping voltammetry in environmental matrices in the presence of mutual interference

Arsenic(III), selenium(IV), copper(II), lead(II), cadmium(II), zinc(II) and manganese(II) have been determined in environmental matrices by differential pulse cathodic (DPCSV) and anodic (DPASV) stripping voltammetry. The voltammetric measurements were carried out using, as the working electrode, a stationary mercury electrode, and a platinum electrode and a Ag ∣ AgCl ∣ KCl_(sat) electrode as the auxiliary and reference electrode, respectively. An ammonia+ammonium chloride buffer of pH 9.0 was employed as the supporting electrolyte. The analytical procedure was verified by the analysis of the standard reference materials: Sea Water BCR-CRM 403, Estuarine Sediment BCR-CRM 277, Ulva Lactuca BCR-CRM 279 and Mussel Tissue BCR-CRM 278. The precision and accuracy, expressed as relative standard deviation and relative error, respectively, were lower than 5% in all cases, while the detection limit for each element was around 10^?9 mol l^?1. In the presence of high reciprocal interference, the standard addition method considerably improved the resolution of the voltammetric technique. The analytical procedure was transferred and applied to samples drawn in the Goro Bay (Ferrara, Italy).     

Voltammetric determination of stability constants of iron(III)–glycine complexes in water solution

Determination of the stability constants of dissolved iron(III)–glycine system in water solution (I = 0.6 mol L^?1 in NaClO₄ at 25 ± 1 °C) using differential pulse cathodic voltammetry (DPCV) was performed on a static mercury drop electrode (SMDE). Iron(III) concentration of 2.5 × 10^?5 mol L^?1 and the pH range from 9.05 to 6.36 ensured the formation of enough concentration of iron(III)–glycine higher coordination complexes (1:2 and 1:3) to be measured by the applied method. The concentrations of total glycine varied from 0.1 to 0.5 mol L^?1. Cyclic voltammetry (CV) measurements were used to investigate reversibility of the iron(III)–glycine complexes which showed one-electron reversible character. The stability constants of iron(III) [Fe(Gly)₂]⁺ and Fe(Gly)₃ complexes, which had not been reported in the literature so far, were found to be log β₂ = 16.83 ± 0.47 and log β₃ = 18.64 ± 0.70, respectively. The model that best fitted the data gave two iron(II)–glycine stability constants for [FeGly]⁺ log K₁ = 3.69 ± 0.19 and for Fe(Gly)₂ log β₂ = 5.08 ± 0.60. According to the constants found, chemical distribution of iron(III) in glycine water solution, as a function of pH, was calculated and proposed.