Determination of copper in sea water by cathodic stripping voltammetry of complexes with catechol

A reduction current is obtained when an aqueous solution of copper and catechol is subjected to differential-pulse cathodic stripping voltammetry (d.p.c.s.v.) because of the reduction of copper(II)—catechol complex ions which adsorb onto the hanging mercury drop electrode (HMDE). The most likely form of the adsorbed complex ions is CuL^2?₂ (L being catechol). A.c. polarographic measurements showed that these complex ions adsorb more strongly onto the drop than free catechol ions. Monolayer adsorption density is obtained at 2.1 × 10^?10 molecules/cm², equivalent to a surface area of 78 A² complex ion, which agrees well with the molecular surface area calculated from the bond lengths. Analytically useful currents are obtained at very low metal concentrations, such as in uncontaminated sea water. The possible interference by other trace metals, major cations, and organic complexing ligands is investigated. Competition for copper ions by natural organic complexing ligands is evident at low concentrations of catechol. Analysis of the dissolved copper concentration in sea water by d.p.c.s.v. at the HMDE (at neutral pH) compares favourably with the d.p.a.s.v. technique at a rotating disk electrode (at low pH) because of the shorter collection period and greater sensitivity.     

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.     

Unified treatment of the potentiostatic reduction on an expanding mercury electrode of complexes at any concentration ratio of reacting species

The article describes a general treatment of voltammetry of equilibrium complexation reactions at an electrode expanding in accord with any power law, the stationary electrode and the dropping mercury electrode being special cases. Reversible, irreversible and quasireversible electrode reactions are embraced by the treatment. A general expression for the current—voltage characteristic was derived, which was applied to the case of the polarographic reduction of complex ion, where either the simple or complex ion is electrochemically active. An equation accounting for the surface concentration of all reacting species is derived, the solution of which under appropriate conditions gives the equations of Lingane, Matsuda and Ayabe, etc. Correlations of potential, current and wave shape with complexing agent concentration are presented and discussed. A criterion for the splitting of the polarographic wave in the absence of excess of the complexing agent is proposed. Experimental results of several papers are discussed in the light of these criteria and a satisfactory agreement between the results and the predictions is established.     

Polarographic study of As(III)—Triethanolamine reaction

pH, conductometric titration and polarographic measurements revealed that the interaction of As(III) with triethanolamine is based on hydrogen ion liberation with the formation of only [As(OH)₂HX]^? species. The polarographic reduction waves for this reaction are irreversible and of diffusion controlled process. The conditional stability constant is calculated and found to be independent of both As(III) and TEA concentrations. The average log β value amounts to 14.33±0.11 and the average log γ value equals 5.63.     

Experimental evidence of consecutive electron transfers for the Sb(III)−Sb(Hg) couple in 4 M H(Cl,ClO4) at low chloride concentrations

A dc polarographic and cyclic voltammetric study has been made of the reduction of Sb(III) ions from 0.01 M HCl+3.99 M HClO₄ and 0.001 M HCl+3.999 M HClO₄ supporting electrolytes in which a quasi-reversible, respectively irreversible behaviour is observed. It is shown that the Sb(III) reduction can be explained on the assumption of a reaction mechanism that consists of three successive one-electron transfers. Along the reduction wave the Sb(III)→Sb(II) and Sb(II)→Sb(I) step are rate determining, respectively at more negative and more positive potentials. Kinetic parameters were determined and the rate constants are shown to increase with chloride ion concentration.     

Trapped interfacial redox introduces reversibility in the oxygen reduction reaction in a non-aqueous Ca2+ electrolyte

Electrochemical investigations of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have been conducted in a Ca²⁺-containing dimethyl sulfoxide electrolyte. While the ORR appears irreversible, the introduction of a tetrabutylammonium perchlorate (TBAClO₄) co-salt in excess concentrations results in the gradual appearance of a quasi-reversible OER process. Combining the results of systematic cyclic voltammetry investigations, the degree of reversibility depends on the ion pair competition between Ca²⁺ and TBA⁺ cations to interact with generated superoxide (O₂⁻). When TBA⁺ is in larger concentrations, and large reductive overpotentials are applied, a quasi-reversible OER peak emerges with repeated cycling (characteristic of formulations without Ca²⁺ cations). In situ Raman microscopy and rotating ring-disc electrode (RRDE) experiments revealed more about the nature of species formed at the electrode surface and indicated the progressive evolution of a charge storage mechanism based upon trapped interfacial redox. The first electrochemical step involves generation of O₂⁻, followed primarily by partial passivation of the surface by Ca_xO_y product formation (the dominant initial reaction). Once this product matrix develops, the subsequent formation of TBA⁺--O₂⁻ is contained within the Ca_xO_y product interlayer at the electrode surface and, consequently, undergoes a facile oxidation reaction to regenerate O₂.

An interlayer product of oxygen reduction with Ca²⁺/TBA⁺ yields a quasi-reversible oxygen evolution reaction by inducing a trapped interfacial redox process.     

Polarographic Studies of Dithiodiacetic Acid and the Uranyl-Dithiodiacetic Acid Complex

The polarographic behaviour of dithiodiacetic acid and that of U(VI) in a solution containing dithiodiacetic acid as complexing agent have been investigated. For the dithiodiacetic acid system, two waves appear over the pH range studied. The prewave is kinetic in nature and the mainwave is diffusion-controlled. However, as U(VI) is added into the dithiodiacetic acid system, the polarogram changes due to the existence of a complex. The current-potential curve of the first wave is not the normal S shape. This is due to the superposition of the first wave of the ligand and the wave due to the reduction of the U(VI) in the complex to U(V). The second wave is due to the reduction of the complex The first wave is an adsorption-controlled current and the second wave is partly diffusion-controlled and partly adsorption-controlled. We propose an electrode reaction mechanism for both systems and the complex species. The dissociation constant of the complex HASSAUO⁺₂ is found to be of the order of 10^?4.     

Electrode processes of the V(III)/V(II) and Eu(III)/Eu(II) systems in mixed water + acetone solvents

Rate constants of the electrode reaction V(III)+e⁻ → V(II) in water+acetone mixtures were determined. In the regions of irreversible and quasi-reversible behaviour we used polarographic and square-wave polarographic measurements, respectively. The values of the constant go through a minimum with increasing concentration of acetone. Following the published data for the Eu(III)/Eu(II) system (H. Elzanowska, Ph. D. Thesis, Warsaw, 1957), this behaviour was explained by the simultaneous reduction of differently solvated ions in the solution where, depending on the degree of electrode coverage, a partial resolvation at the electrode surface can occur. The calculated dependence of the rate constant on the solvent composition is in accord with experimental values.     

Mechanism of electroreduction of cobalt(II) in thiocyanate solutions at mercury electrodes

The process of electroreduction of cobalt(II) in thiocyanate solutions at mercury electrodes has been investigated by cyclic voltammetric, chronoamperometric and polarographic methods. The influences of pH, the concentrations of Co(II) and SCN^?, and the reduction products of SCN^?, CN^? and S^2? on the reduction waves are described. The polarographic pre-wave is an autocatalytic in nature. A mechanism involving an initial reduction of Co(II)—SCN^? at a mercury electrode followed by the chemical reduction of thiocyanate ion with the electroreduced metallic cobalt, and taking into account cyanide, sulfide, and hydroxide ions, the latter being produced by the hydrolysis of cyanide ion, is presented. Cobalt sulfide adsorbed at the electrode surface stimulates further reduction of Co(II)—CN^? and —SCN^? complexes, and depresses the interfering influence of Co(OH)₂, which is reductively desorbed from the electrode surface with giving rise to an additional peak near ?1.08 V vs. SCE.     

Electrochemical reduction of 5,5′-dichlorohydurilic acid. mechanism and analytical applications

The electrochemical reduction of 5,5′-dichlorohydurilic acid has been studied at the dropping mercury electrode (DME) and the pyrolytic graphite electrode (PGE). At the DME the single polarographic reduction wave observed at pH 6–11 involves a direct 4e—2H⁺ reduction of the carbon-halogen bond to give hydurilic acid and chloride. The state of hydration or ionization of the 5,5′-dichlorohydurilic acid has no effect on the electrochemical reaction. At the PGE, 5,5′-dichlorohydurilic acid shows two voltammetric peaks. Peak I_c, observed between pH 5 and 7, arises from an overall 4e—2H⁺ reduction of 5,5′-dichlorohydurilic acid via a mechanism that involves initial electron attack at a carbonyl group alpha to a carbon-halogen bond with simultaneous elimination of chloride ion. The peak II_c process involves an initial 2e—1H⁺ reduction of a partially hydrated form of 5,5′-dichlorohydurilic acid with only one unhydrated halocarbonyl moiety available for reaction. Attack is again via the carbonyl group with simultaneous elimination of chloride and formation of 5-chlorohydurilic acid. A chemical dehydration step then occurs with a rate constant of ca. 0.24 s^?1 at pH 8.2, with formation of a further reducible halocarbonyl group. This is again reduced in an overall 2e—2H⁺ reaction to give hydurilic acid and chloride ion. The peak II_c process hence proceeds via an ECE mechanism. The different mechanisms observed for reduction of 5,5′-dichlorohydurilic acid at mercury and pyrolytic graphite electrodes are unusual. Analytical methods have been developed for the polarographic determination of 5,5′-dichlorohydurilic acid via its reduction wave at the DME, and for the voltammetric determination of hydurilic acid via its first oxidation peak at the PGE.     

Determination of complexing capacities of ligands in natural waters and conditional stability constants of the copper complexes by means of manganese dioxide

A dispersion of manganese dioxide is used as a weak ion exchanger in the determination of the complexing capacities of natural waters for copper and in the estimation of the stability constants of the complexes formed, at the concentration and pH at which the ligands occur in natural waters. Experiments were done at 25°C and 0.01 M ionic strength. Stability constants varied from 10^7.8 to 10^8.8 at pH 7.6 for ligands in lakes and rivers. The constant for the copper—fulvic acid complex was estimated as 10^7.8 at pH 7.6. Only one complexing site of major importance for natural water environments was observed.     

A spectral and polarographic study of the acid--base and complexing behaviour of bromazepam.

A method for the microdetermination of periodate in the presence of iodate is presented. It is based on reduction of periodate to iodate by leucothionine generated in situ by photochemical reaction between EDTA and blue thionine. Biamperometric curves are used for evaluation, representing reduction time for the periodate versus the current generated in the oxidation of the T_red in excess after the end-point. A polarographic study of the process is presented. An applied potential of 100 mV is suitable for the biamperometric measurements with Pt—Pt electrodes. The method is applicable for 0.4–45 ppm periodate. Iodate does not interfere up to IO^-₃ : IO^-₄ ratios of 100:1.     

Theoretical analysis of catalytic current-potential curves: Part II. First-order regeneraration reactions coupled with dimeric product of the electrode reaction

Equations of the polarographic current-potential curves are derived for electrode reactions of the type 2A+ze^?k_s,α→B coupled by an irreversible chemical regeneration reaction B+C^k→2A+D. Analytical solutions based on a general treatment were derived, including reversible, irreversible and quasi-reversible electrode processes. The kinetic domain over which an irreversible following chemical reaction affects the half-wave potentials is defined.     

Numerical evaluation of complex stability constants from polarographic data for quasi-reversible processes

Stability constants of copper(II) butyrate complexes were determined by polarography. Numerical treatment of polarographic data for quasi-reversible electrode processes was developed and yielded good estimates of reversible E_{$\text{1}\text{2}$} values. An improved three-electrode polarographic apparatus was constructed based on operational amplifiers.     

The Effect of the Weak Completing Nature of NaF,NaN3, and NaC3H3O3 Supporting Electrolytes on the Electrocatalysis of Nickel(II) Reduction in the Presence of Organic Amines

Abstract

Contrary to the results obtained in essentially noncomplexing supporting electrolyte media (NaClo₄, NaN0₃, NaCl and NaBr), the kinetic limited currents of the catalytic prewaves obtained for the polarographic reduction of Ni(II) in the presence of organic amines (o-phenylenediamine, pyridine, etc.) employing weakly complexing supporting electrolytes (NaF, NaN₃, and NaC₂H₃O₂) do not correlate with ?₂ potentials as predicted by simple double layer theory. However, the limiting current does correlate with the ligand exchange rates for Ni(H₂O)₅X⁺ type complexes (where X = F^?, N₃ ^? and C₂H₃O₂ ^?) with an organic ligand. These results lead to a general mechanism for the electrocatalysis mechanism of Ni(II) reduction.     

Electrode kinetics of the metal complexes with aminopolycarboxylic acids: Part II. Cadmium-ethylenediamine-N,N,N′,N′-tetraacetate (EDTA) complexes

The d.c. polarographic current-potential curves of Cd(II)-EDTA complexes were examined in the pH range 0.5–10.0, to elucidate the mechanism of their electrode processes and to determine the relevant electrochemical kinetic parameters. It was shown that the first wave observed below pH 3 at ?0.58 to ?0.65 V vs. SCE is the reversible reduction wave of Cd(II) aquo-ion with kinetically-controlled limiting current, and the second wave observed above pH 1.5 at ?0.75 to ?1.21 V vs. SCE corresponds to the simultaneous irreversible reduction of four complex species, CdH₃L⁺, CdH₂L, CdHL^? and CdL^2?, where CdH_pL^(p?2)+ and L^4? denote the protonated complex species with p protons and the unprotonated EDTA ion, respectively. Analysis of the dependence of limiting current on the hydrogen ion concentration led to the conclusion that the preceding reaction determining the behaviour of limiting current is CdH₃L⁺?Cd²⁺+H₃L^? with k₃^d=6.3×10² s^?1 and k₃^f=3.3×10⁶ s^?1M^?1, where k₃^d and k₃^f are the dissociation and formation rate constants, respectively. On the other hand, from analysis of the dependence of half-wave potentials of the second wave on the hydrogen ion concentration, the kinetic parameters of the four complex species were evaluated, and are given in Table 1. Further, it was shown that the cathodic rate constants of these four charge transfer processes at some reference potential together with those of Cd(II)-HEDTA complexes fulfil the linear free energy relationship.     

Synthesis,stepwise instability constants and oxidation of bis-[bromazepan]CuI perchlorate by molecular oxygen in dimethyl sulphoxide

Summary The oxidation with molecular oxygen of the cuprous salt, Cu(MeCN)₄ClO₄, and of bis(bromazepan)Cu^I perchlorate complex in the presence of different concentrations of the complexing agent bromazepan in DMSO was studied and the formation constants of the complex were calculated from the kinetic data. For comparative purposes the formation constants were also determined spectrophotometrically.     

Polarographic studies on the electrode kinetics of the nickel(II) complexes with glycine and some of its derivatives: I. glycine complexes

D.C. polarograms of the Ni(II)-glycine complexes were measured with varying pH values at constant total glycine concentration (0.05 M) and conversely with varying total glycine concentration at constant pH values (3.5, 4.5, 7.5 and 9.5). The current—voltage curves obtained were analysed to determine the limiting currents, the transfer coefficients, and the half-wave potentials, by using the non-linear least squares method. From the dependence of the half-wave potentials on the free glycine concentration, the mechanism of the electrode processes was elucidated. It was shown that the first wave corresponded to the reduction of the Ni²⁺ aquo-complex, the second to the NiG⁺ complex, the third to the NiG₂ complex and the fourth to the NiG^?₃ complex. The kinetic parameters for the four charge transfer processes were determined.The second and third waves were kinetic in character and the rate constants of the dissociation and association reactions between NiG⁺ and NiG₂ and between NiG₂ and NiG^?₃ were determined by analysing the dependence of the kinetic limiting currents on the free glycine concentration.     

Neutron diffraction study of the electrochemical passivation of nickel powder

With the aid of the Yamaoka mechanism and a.c. polarographic observables, rate constants for the homogeneous reduction of Co(III)pentammine complexes by Eu(II) are measured. Where comparison is possible, rate parameters obtained by this electrochemical procedure are found to be in good agreement with previous measurements by stopped-flow and pulse radiolysis procedures, with one exception. The order of reactivity for the halopentammineCo(III) complexes is found to follow the sequence RF²⁺>RCl²⁺>RBr²⁺>RI²⁺, where R=Co(III)(NH₃)₅³⁺. This and a pH dependence noted for the RF²⁺ case are suggestive of a predominantly inner sphere reaction pathway.     

ECE mechanism as a tool for the investigation of unstable metal complexes: Part I. Electrochemical reduction of trans-[CoBr2(N)4]+-type complexes on the mercury electrode

A polarographic investigation of trans-[CoBr₂(N)₄]⁺-type complexe where (N)₄ represents (NH₃)₄, (ethylenediamine)₂ and (propylenediamine)₂, has been carried out in acetate buffer solutions. These complexes gave two polarographic waves; the first wave corresponds to the reduction of Co(III) to Co(II) and the second to the reduction of Co(II) to Co(0). The relation between current and time of the first wave in a positive potential is dependent on the dissolution wave due to bromide ions produced by acid hydrolysis and parallel ECE mechanism. Overall reaction is as follows: at the electrode surface, [Co(III)Br₂(N)₄]⁺+e^-→[Co(II)Br₂(N)₄] [Co(II)Br₂(N)₄]+6H₂O→[Co(II)(H₂O₆]²⁺+2Br^-+(N)₄ 2Br^-+2Hg→Hg₂Br₂+2e and in the solution, [Co(III)Br₂(N)₄]⁺+2H₂O→[Co(III)(H₂O₂(N)₄]³⁺+2Br^-The effects of the acid hydrolysis of a tervalent cobalt complex on the current—potential curve were simulated.