Electroreduction of Ammonia and Hydroxyammonia Complexes of Divalent Metals: Effect of the Ligand Concentration and the EDL Structure

Kinetics and mechanism of electroreduction of complexes Pd(NH₃)₄ ²⁺ on a dropping mercury electrode (DME) and a Pd electrode, as well as ammonia complexes of Co(II), Ni(II), and Zn(II) and hydroxyammonia complexes of Zn(II) on DME at different concentrations of ammonia and supporting electrolytes and different pH values are discussed. The half-wave potentials of electroreduction of ammonia complexes of Pd(II) and Ni(II) on DME in the absence of a polarographic maximum obey an equation that takes into account the effect the EDL structure has on the rate of a slow outer-sphere electrochemical stage. As opposed to Pd(II) complexes, the reduction of the other complexes involves preceding reversible chemical stages, which yield diammonia complexes undergoing a direct reduction on DME. The reasons for the emergence of a polarographic maximum upon an increase in the concentration of reduced complexes and the time of recording an instant current are discussed.     

Kinetics and Mechanism of Electroreduction of Ammonia Complexes of Cobalt(II) on a Dropping Mercury Electrode

Effects of concentrations of ammonia (0.3–5.8 M) and supporting electrolytes (NaF, NaClO₄; 0.1–0.5 M) on the kinetics of electroreduction of ammonia complexes of cobalt(II) at a dropping mercury electrode are studied. Most experiments are performed with low concentrations of cobalt(II) complexes (1 × 10^–5 to 2 × 10^–5 M) in the absence of a polarographic maximum. The dependence of the half-wave potential of the reversible cathodic wave pertaining to the reduction of ammonia complexes of cobalt(II) on the concentration of ammonia molecules is obtained. It is found from the dependence that, at ammonia concentrations of 0.5–2.6 M, the slow electrochemical stage involves predominantly complexes Co(NH₃)₂ ²⁺. At higher ammonia concentrations, the stage involves complexes Co(NH₃) _k ²⁺ (k > 2), which form in preceding chemical stages from complexes Co(NH₃) _i ²⁺ (i = 3–6) that are predominant in solution. Values of the diffusion coefficients for complexes Co(NH₃) _i ²⁺, apparent transfer coefficients, and rate constant of the process of electroreduction of ammonia complexes of cobalt(II) are determined. The reasons for the complicating effect the insoluble products of reduction of cobalt(II) complexes have on the shape of polarographic waves are discussed.     

Kinetics of Reduction of Glycinate and Alaninate Complexes of Palladium(II) on a Dropping Mercury Electrode at Different Temperatures

Kinetics of electroreduction of glycinate and alaninate complexes of Pd(II) is studied polarographically in a test mode at 15–50°C in solutions of pH 3, 5, 10, and 12 containing free ligands and NaF, Na₂SO₄, or NaClO₄ supporting electrolytes. Diffusion coefficients for Pd(II) complexes are calculated from values of limiting currents. Specifically adsorbed Pd(II) complexes with a composition identical to that in the bulk solution take part in the slow electrochemical stage. The mechanism of reduction of the complexes is temperature-independent and identical to that established earlier at 25°C. The study confirms the earlier assumption as to the mechanism of hindering action of the perchlorate and alaninate ions specifically adsorbed on the positively-charged surface of a mercury electrode on the reduction of these complexes.     

Kinetics and mechanism of electroreduction of ethylenediamine and hydroxyethylenediamine complexes of zinc(II) on a dropping-mercury electrode

The kinetics of electroreduction of ethylenediamine and hydroxyethylenediamine complexes of zinc(II) on a dropping-mercury electrode (DME) in 1 M NaClO₄ solutions of pH 9–11.5 is studied at different ethylenediamine concentrations at 25, 35, and 50°C. One wave with a diffusion limiting current is observed at an overall concentration of zinc(II) complexes of 2 × 10^–5 M and current recording times t ₁ = 0.3–4 s. The polarographic peak that distorts the wave at t ₁ 0.5 s, pH 11.5, and 25°C is due to the accumulation of insoluble reduction products on the electrode surface. The slow electrochemical step on DME involves complexes Znen²⁺, which form in preceding reversible chemical steps from complexes present in solution.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 397–405.Original Russian Text Copyright © 2005 by Kurtova, Kravtsov, Tsventarnyi.     

Electrochemical reduction of zinc complexes from glycinate solutions

Kinetics of electrochemical reduction of zinc(II) complexes is studied in a wide range of solution pH using the polarization measurements and chronopotentiometry. Adsorption of glycinate ions on zinc electrode and complexing of zinc(II) with glycine are studied using the ¹³C NMR method. Based on the experimental data, the electrochemical behavior of zinc(II)–glycine–water system is considered.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 228–231.Original Russian Text Copyright © 2005 by Berezin, Sagdeev, Gudin, Roev, Mezhevich.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Reduction mechanism of cobalt(II)-ammonia complexes on a dropping mercury electrode

The mechanism of the polarographic reduction of cobalt(II) complexes with ammonia at a dropping mercury electrode over a wide ligand concentration range was investigated. It was shown that the Co(II) aquo ion and the Co(NH₃)²⁺ and Co(NH₃²⁺₂ complexes participate in the electrode process. Transfer coefficients, α, for these species and the electrode reaction rates were evaluated. Stability constants of Co(II) complexes with ammonia in 0.5 M ammonium perchlorate were determined on the basis of the polarographic wave equation of totally irreversible reduction of complex specie.     

Ethylenediamine and Hydroxyethylenediamine Complexes of Zinc(II): A Potentiometric Study of Composition and Stability

The equilibrium potential of saturated zinc amalgam is studied as a function of concentration of free ethylenediamine molecules, [en], in the region [en] 0.001–1 M in solutions of pH 9.5, 10.5, and 11.5. At the concentration of zinc(II) ions 2 × 10^–3 M and [en] = 1 M only simple trisethylenediamine complexes of zinc(II) form in all the solutions. At smaller [en] and pH 9.5 and 10.5, complexes Zn(en)₂ ²⁺ and Zn(en)₂OH⁺ are also present; these are complemented at pH 11.5 by Zn(en)₂(OH)₂ at [en] 0.005–0.1 M. Stability constants for these complexes are calculated.     

ESR and TPD Study of the Interaction of Nitromethane and Ammonia with HZSM-5 and CuZSM-5 Zeolites

The properties of complexes formed on HZSM-5 and CuZSM-5 zeolites in the course of ammonia and nitromethane adsorption are studied. Ammonia adsorbs on CuZSM-5 and forms two species, which decompose at different temperatures T _dec. One is due to the formation of the Cu²⁺(NH₃)₄ complex (T _dec = 450 K), and the other is assigned to ammonia adsorbed on copper(II) compounds, Cu²⁺O^– and Cu²⁺–O^2––Cu²⁺, or CuO clusters (T _dec = 650–750 K). Ammonia adsorption on Cu⁺ and Cu⁰ is negligible compared with that on the Brönsted acid sites and copper(II). Nitromethane adsorbed on HZSM-5 and CuZSM-5 at 400–500 K transforms into a series of products including ammonia. Ammonia also forms complexes with the Brönsted acid sites and copper(II) similar to those formed in the course of adsorption from the gas phase, but the Cu²⁺(NH₃)₄ complexes on CuZSM-5 are not observed. Possible structures of ammonia and nitromethane complexes on Brönsted acid sites and the Cu²⁺ cations in zeolite channels are discussed. The role of these complexes in selective NO _x reduction by hydrocarbons over the zeolites is considered in connection with their thermal stability.     

Voltammetric study of the interaction of lomefloxacin (LMF)–Mg(II) complex with DNA and its analytical application

The voltammetric behavior of the LMF-Mg(II) complex with DNA at a mercury electrode is reported for the first time. In NH₃–NH₄Cl buffer (pH=9.10), the adsorption phenomena of the LMF–Mg(II) complex were observed by linear sweep voltammetry. The mechanism of the electrode reaction was found to be a reduction of LMF in the complex, and the composition of the LMF–Mg(II) complex is 2:1. In the presence of calf thymus DNA (ctDNA), the peak current of LMF–Mg(II) complex decreased considerably, and a new well-defined adsorptive reduction peak appeared at −1.63 V (vs. SCE). The electrochemical kinetic parameters and the binding number of LMF–Mg(II) with ctDNA were also obtained. Moreover, the new peak currents of LMF–Mg(II)–DNA system increased linearly correlated to the concentration of DNA in the 4.00×10⁻⁷–2.60×10⁻⁶ g ml⁻¹ range when the concentrations of LMF–Mg(II) complex was fixed at 5.00×10⁻⁶ mol l⁻¹, with the detection limits of 2.33×10⁻⁷ g ml⁻¹. An electrostatic interaction was suggested by electrochemical method.     

Kinetics and Mechanism of Electroreduction of Palladium(II) Bisethylenediamine Complexes on a Dropping Mercury Electrode

Kinetics of electroreduction of complexes Pd(en)²⁺ ₂(2 × 10^–5M) on a dropping mercury electrode is studied in solutions with various concentrations of ethylenediamine and supporting electrolytes (NaF, NaClO₄) at pH 4.5–11 and different instantaneous current recording times. Diffusion coefficients for reducing complexes and kinetic parameters of the slow electrochemical stage are determined. The concentration of supporting electrolytes is found to affect the half-wave potential, which points to an inner-sphere mechanism of electrochemical stage at supporting electrolyte concentrations of 0.005 to 0.03 M and to a predominantly outer-sphere mechanism at higher concentrations.     

Ammine Complexes of Na‐, Ag‐, Mn‐, and Zn‐Azides

The synthesis of ammine metal azides is achieved through a simple route in liquid ammonia solution. Azides of silver, sodium, manganese, and zinc were obtained as ammine complexes from liquid ammonia and the solid compounds were characterized. The NH₃ solutions were investigated by means of NMR spectroscopy as well. The triamminesilver(I) azide as well as the unusual tetraamminesilver(I) and pentaamminesodium(I) azides are presented. The structures of the hexaamminemanganese(II) and hexaamminezinc(II) azides, which crystallize with four ammonia molecules of solvation, are elucidated. From these compounds the binary azides Mn(N₃)₂ and Zn(N₃)₂ were obtained by (careful) heating.     

Effect of ammonia on the Zn(Hg)/Zn(II) electrode reactions in aqueous chloride solutions

The title subject has been studied using single pulse and chronopotentiometric polarization measurements on the Zn(Hg)/Zn(II) electrode and equilibrium measurements on the same and the Zn/Zn(II) electrode, mainly in 2 M NH₄Cl with 0–0.3 M NH₃. At low ammonia concentrations, the Zn(Hg)/Zn(II) reactions are found to occur in two consecutive charge-transfer steps with Zn(I) as intermediate, and with little or no participation of ammonia. At higher ammonia concentrations, however, nearly symmetric transfer by divalent zinc ion (α=0.5 and n=2) to and from diammine species appears to be the predominant charge-transfer step.     

Kinetics and Mechanism of Reduction of Nickel(II) Ethylenediamine Complexes at a Dropping Mercury Electrode

Effect of concentration of ethylenediamine molecules and supporting electrolytes (NaF, NaClO₄) on the kinetics of electroreduction of nickel(II) ethylenediamine complexes at dropping and stationary mercury electrodes is studied. The limiting current on the dropping electrode is found to have diffusion–kinetic nature at free ethylenediamine molecule concentrations of 0.5 mM to 0.05 M. The slow electrochemical stage is presumably preceded by a slow chemical stage and a reversible chemical stage. In the former, one chelate cycle in the source complex Ni(en)²⁺ ₃ opens; and in the latter, a monodentate-coordinated ethylenediamine molecule is abstracted. The conclusion is drawn about an inner-sphere mechanism of the electrochemical stage which involves Ni(en)²⁺ ₂ complexes specifically adsorbed on mercury.     

Test Determination of Lead and Zinc in Water with the Use of Xylenol Orange Immobilized on Silica

Xylenol Orange immobilized on silica as a complex of iron(III) was used for the test determination of lead(II) and zinc(II) in drinking water over concentration ranges of 10–100 and 13–130 g/L, respectively. The maximum distribution coefficients were found to be 7.50 × 10³ mL/g for Pb and 3.75 × 10³ mL/g for Zn. The macro main trace components of water at a level of their maximum permissible concentrations caused no interference. Al(III), Fe(III), and Zn(II) in the presence of NH₄F did not interfere with the determination of Pb(II), whereas lead in the presence of acetate caused no interference with the determination of Zn(II).     

A novel electrochemical method to detect mercury (II) ions

A novel and sensitive electrochemical method for determination of mercury (II) ions (Hg²⁺) based on the formation of thymine–Hg²⁺–thymine complexes and gold nanoparticle-mediated signal amplification is reported. Two 5′ end thiolated complementary oligonucleotides containing six strategically placed thymine–thymine mistakes were introduced in this work. One of the two oligonucleotides was immobilized on a gold electrode and the other one on gold nanoparticles (AuNPs). Due to six thymine–thymine mistakes the two oligonucleotides were not able to be hybridized, so AuNPs could not be immobilized onto the electrode surface after the electrode was immersed in the DNA–AuNPs solution. However, if Hg²⁺ existed, T–Hg²⁺–T complexes could be formed and AuNPs could be immobilized onto the electrode surface. Meanwhile, large numbers of [Ru(NH₃)₆]³⁺ molecules as electrochemical species could be localized onto the electrode surface. The Hg²⁺ detection limit of this assay could be as low as 10 nM, which is the US Environmental Protection Agency (EPA) limit of Hg²⁺ for drinkable water. This method is proven to be simple, convenient, high sensitive and selective.     

Copper(II)–Ammonia Complexation Equilibria in Aqueous Solutions at Temperatures from 30 to 250°C by Visible Spectroscopy

The spectra of copper(II)–ammonia solutions in 2 mol-kg^–1 NH₄NO₃(aq) were recorded as a function of pH with a new UV–visible flow cell, capable of operating at conditions up to 325°C and 300 bars. Equilibrium constants for the formation of copper(II)–ammonia complexes Cu(NH₃)_n ²⁺, 1 n 4, from 30 to 150°C were determined by evolving factor analysis and nonlinear least-squares regression. Measurements at higher temperatures were limited by thermal decomposition of NH₄NO₃(aq). The formation constants of Cu(NH₃)_n ²⁺ decrease with temperature, consistent with extrapolations of literature data from measurements below 100°C. Measurements above 150°C were carried out in 0.5 mol-kg^–1 CF₃SO₃H (aq), at the very high ammonia concentrations required to avoid the precipitation of CuO(s). The spectra are consistent with Cu(NH₃)₄ ²⁺ as the predominant species, based on extrapolations of peak maxima and molar absorptivities from lower temperatures. Shifts in the spectra of Cu²⁺ and the Cu(NH₃)_n ²⁺ species to higher wavelength and increases in molar absorbance with increasing temperature are discussed in terms of the structure of the complexes.     

A Simple and Reliable Spectrophotometric Method for the Determination of Ascorbic Acid in Pharmaceutical Preparations

In this study, a simple spectrophotometric method based on the reaction between ascorbic acid and the copper(II)–ammonia complex is presented for the determination of the vitamin C content of pharmaceutical preparations. During this reaction, ascorbic acid is oxidized and the copper(II)–ammonia complex is reduced to the copper(I)–ammonia complex, and the absorbance decrease at 600 nm (_max for the copper(II)–NH₃ complex) is measured. Stirring the final solution in the presence of air leads to the primary absorbance again being obtained, which indicates that the copper(I)–NH₃ complex is quantitatively oxidized to the copper(II)–NH₃ complex by O₂. The linear dynamic range of the calibration curve is 0.8–6 mmol with a detection limit of 0.26 mmol. The relative standard deviation for eight repeated experiments is 2.4%, which shows that the proposed method has a good repeatability. Finally, this method was used in the analysis of the vitamin C content of different pharmaceutical preparations, such as multivitamin tablets and syrups, vitamin-C tablets and powders, and effervescent tablets. The obtained results are in good agreement with iodimetric data.     

Kinetics and mechanism of electroreduction of Pd(II) complexes

The kinetics and mechanism of processes of reduction of Pd(II) complexes with a number of inorganic (NH₃ , Cl^– , etc.) and organic (ethylenediamine, glycine, -alanine, etc.) ligands on a dropping-mercury electrode and a Pd electrode in solutions with various concentrations of ligands, hydrogen ions, and supporting electrolytes are reviewed. The nature of electrochemical and chemical steps of processes of reduction of various complexes of Pd(II) is discussed.Translated from Elektrokhimiya, Vol. 40, No. 12, 2004, pp. 1494–1502.Original Russian Text Copyright © 2004 by Kravtsov.     

Selective complexometric determination of mercury using sodium nitrite as masking agent

A complexo-titrimetric method for the determination of mercury(II) in the presence of other metal ions is described based on the selective masking ability of sodium nitrite. Mercury and other ions are initially complexed with an excess of EDTA and the surplus EDTA is titrated with Pb(NO₃)₂ solution at pH 5.0–6.0 using xylenol orange as indicator. An excess of solid NaNO₂ is then added to decompose the Hg(II)-EDTA complex and the released EDTA is titrated with standard Pb(NO₃)₂ solution. Accurate results were obtained for 10–65 mg of mercury with relative errors <0.3% and standard deviations < 0.03 mg. Sn(IV) and Pd(II) interfere but can be easily masked. The method is applied for the determination of Hg(II) in its alloy compositions and complexes.     

TRANSITION METAL COMPLEXES OF AMINOPHOSPHONIC ACID ANALOGUES OF METHIONINE AND ALANINE IN AQUEOUS SOLUTION

Abstract

The stoichiometrics and stability constants of the nickel(II), copper(II) and zinc(II) complexes of l-amino-3-methylthiopropanephosphonic acid (MetP) and 1-amino-ethanephosphonic acid (a-AlaP) have been determined pH-metrically at 25°C at an ionic strength of 0.2 mol dm^?3 (KC1). From the stability data and the absorption spectra of the complexes it has been established that simple aminophosphonic acids coordinate to the nickel(II) and copper(II) ions forming chelate complexes in which the metal binding mode is bidentate with the {NH₂, PO₃ ^2-} donor set. ³¹P and ¹H NMR measurements showed that MetP and α-AlaP exhibit similar properties in the presence of zinc(II) ions, but the ligand reacts to form a cyclic phosphonoamidate in neutral and slightly alkaline solution in the Zn(II)-α-AlaP system and at slightly acidic conditions in the Zn(II)-MetP system. This difference reveals that the latter ligand at pH > 7 prefers Zn(II) coordination involving all possible (amino, phosphonate and thioether sulfur) donor groups.