Polarography of complex compounds in the absence of large excess of the complexing agent

A general expression for the current—voltage characteristic was derived which is valid for all degrees of reversibilities and ligand concentrations, taking into account quantitatively the effect of complex formation on the polarographic curves. When the reaction is reversible the current—voltage characteristic and half-wave potential are independent of the reaction mechanism. For quasi-reversible electron-transfer reactions both the reaction mechanism and the stability constants may be established. When the reaction is irreversible, quantitative investigation of the changes of (E_1/2^′)_irr with concentrations of complexing ligands leads to the composition of the activated complex. However, except in special cases independent information is necessary to determine stability constants. The theory of the polarographic reduction of metal ions in the absence of large excess of the complexing agent was tested experimentally. In the case of the Bi-NTA complex reduction, the electrode reaction was found to be quasi-reversible and the equations of the theoretical section to be applicable.     

Voltammetric study of humic and fulvic substances: Part II. Mechanism of reaction of the Pb-fulvic complexes on the mercury electrode

A model is presented for the electrode process corresponding to the reduction of Pb(II) complexed by fulvic substances. The various factors which may affect the polarographic results are discussed and a mathematical expression is given for the polarographic wave. The influence of various parameters was tested. It was found that the formation of complexes both present in solution and adsorbed at the surface of the electrode is the predominant factor which affects the potential of the wave. Moreover, the diffusion coefficient of the complexes is much lower than that of free lead. We discuss the nature of the complexes adsorbed at the electrode surface and the implications of the findings of this work when using the polarographic methods for the determination of the complexing ability of waters.     

镉离子在H2SO4溶液中极谱行为的研究

本文研究了在没有动物胶的0.5M H_2SO_4溶液中镉离子的极谱行为,得到了如下的结果:(1)在0.050～20.0mM CdSO_4 0.5M H_2SO_4的十种溶液中测得的电流-电位曲线都有良好的波形、恒定的极限扩散电流和易于确定的半波电位.这些曲线上都没有极谱极大出现;其极限扩散电流(波高)与镉离子的浓度成正比.在镉离子低浓度(0.050～0.20mM)时,半波电位保持不变,在镉离子高浓度(1.00～20.0mM)时,也仅有很小的变化.故镉离子在0.5MH_2SO_4,溶液中的电流-电位曲线可供定量和定性测定之用.(2)镉离子在低浓度时的极谱波是一种可逆波.(3)前人在H_2SO_4溶液中研究镉离子时之所以没有能得到令人满意的极谱行为的原因是他们在H_2SO4溶液中添加了动物胶的缘故.(4)作者从得到的波形良好的电流-电位曲线上,测定了25±0.2℃时镉离子在0.5M H_2SO_4溶液中的扩散电流常数、半波电位和电极反应中得失的电子数.结果如下: i_d/cm~(2/3)t~(1/6)=3.97μA/mM·mg~(2/3)·s~(-1/2) E_(1/2)=-1.011V(0.5M硫酸亚汞电极)=-0.559V(饱和甘汞电极) n=2 这些数据比Lingane的数据,扩散电流常数2.6μA/mM·mg~(2/3)·s~(-1/2)和半波电位-0.59V(饱和甘汞电极)],要合理些.     

Reduction of complexes in unbuffered solutions a unified treatment of the reduction of complexes at a stationary planar electrode or at an expanding mercury electrode

A general treatment is given of an electrode reaction connected with complexation at low ligand concentrations in unbuffered media. The reduction of the simple or complex metal ions occurs at an electrode expanding in accord with some power law; the stationary (solid or mercury) electrode and the dropping mercury electrode are special cases. The ligand is added to the solution as a j-protic acid. Linear semi-infinite diffusion is regarded as the sole mode of transport of all dissolved substances.The solution found, being of a very wide generality, was applied to the case of a potentiostatic regime of electrolysis on a dropping mercury electrode, as well as to the case of a galvanostatic regime on a stationary electrode. The voltammetric relationships obtained embrace all known equations of polarographic and chronopotentiometric reduction of simple and complex metal ions.The theoretical polarographic I—E curves in buffered and unbuffered solutions are presented graphically and compared. Apparently, the changes in the surface proton concentration cause a stretching of the wave.     

Electroanalytical Study of the Reduction of Sb(III) in Tartrate Medium

Abstract

A systematic study on the reduction of Sb(III) on a mercury electrode in aqueous solution of tartrate ion has been carried out. The nature of the electrode reactions has shown to be a function of the acidity of the solution. Results from polarographic, coulometric and voltammetric experiments allow us to outline a model of electrode reaction according to which different Sb-tartrate complex ions can be reduced. Sensitivity and detection limits for the polarographic determination of Sb(III) in tartrate medium are given using several polarographic waves obtained at different pH. In this way, the selection of the basic medium is vindicated.     

赖氨酸与香草醛反应产物的极谱波特征及其分析应用

研究了赖氨酸与香草醛反应产物的极谱波特征并建立了赖氨酸的分析方法。在0.1mol/L磷酸盐缓冲溶液中赖氨酸与香草醛反应的产物于－1.16V产生灵敏的极谱波,赖氨酸浓度在5×10     

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.     

Determination of subnanomolar concentrations of tungsten (VI) by catalytic adsorption polarography

A new differential pulse polarographic method for the determination of W(VI) using a catalytic adsorption wave is described. W(VI) is first chelated by 7-iodo-8-hydroxyquinoline-5-sulfonic acid at pH 0.5. The complex ion formed is strongly adsorbed on the surface of a dropping mercury electrode. At a potential of –0.95 V versus the Ag/AgCl (3M KCl) reference electrode the adsorbed complex is reduced by the polarographic current and oxidized very fast by hydrated hydrogen ions providing the oxidized form of the complex ion for repeated redox cycles. As the redox process taking place in the electric double layer, the diffusion of the complex does not limit the polarographic current. Therefore, high currents occur, and consequently, a very high sensitivity is obtained. The practical detection limit (PDL) is 3.7 ng W/kg solution corresponding to 2 × 10^–11 M. The standard deviation of single values is 1.2 ng/kg at the concentration of 91 ng/kg lying in the middle of the linear part of the calibration curve. Because Mo (VI) gives a very similar catalytic adsorption wave, serious mutual interferences occur in the analysis of mixtures of both species. An effective separation of Mo(VI) was worked out. Using 1% (w/v) solution of trioctylphosphinoxide in kerosene, Mo(VI) can almost completely be extracted from 1.8M HCl with a threefold extraction resulting in a separation factor of 40000.     

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.     

The dependence of the dropping mercury electrode potential at constant current on the concentration of an electroactive constituent involved in reversible or irreversible electrode reactions

A new method is described in which the potential of the d.m.e. is measured as a function of the concentration of an electroactive constituent when a constant current is maintained by adjusting the voltage applied to the polarographic cell. Equations are derived describing the expected potential change caused by changing the concentration of a constituent involved in a reversible or an irreversible electrode reaction. Graphical interpretations of the derived equation are made, indicating the most suitable conditions for performing potential measurements, yielding potential differences that greatly exceed those obtained by conventional potentiometry, especially when the method is applied to irreversible electrode reactions at the d.m.e. Experimental evidence is presented, which not only verifies all expectations, but also indicates that the method offers a new approach to the investigation of some fundamental problems.     

Single-sweep polarography of the copper(II)-3-hydroxy-1-p-sulphonatophenyl-3-phenyltriazene complex and its analytical applications

A polarographic investigation of the copper-3-hydroxy-1-p-sulphonatophenyl-3-phenyltriazene (HSPT) complex in 0.05 M sodium tetraborate medium is described and a simple and sensitive single-sweep polarographic method for the determination of trace amounts of copper in biological samples is proposed. The complex was shown to be Cu(HSPT)2 with log beta' = 11.38. The polarographic wave is caused by the reduction of copper(II) in the adsorbed complex to copper amalgam on the surface of a mercury electrode. The current peak is directly proportional to the concentration of copper in the range 8.0 x 10(-9)-4.0 x 10(-6) M and the detection limit is 5.0 x 10(-9) M.     

Theoretical analysis of catalytic current-potential curves: Part I. First-order regeneration mechanisms at different electrolysis regimes

The article describes a general treatment of the catalytic reaction mechanism at an electrode expanding in accord with any power law, the stationary electrode and the dropping mercury electrode being special cases. Reversible, irreversible and quasi-reversible electrode reactions are embraced by the treatment. A general expression for the current-voltage characteristic was derived, which was applied to different electrolysis regimes. The cases when the regeneration reaction will influence the catalytic wave characteristics are also discussed.     

铁(Ⅱ)-亚硝基R盐-双氧水体系极谱波研究及应用

在ｐＨ５．５ ＨＡｃ－ＮａＡｃ缓冲溶液中，以硫酸肼还原 Ｆｅ（Ⅲ）成 Ｆｅ（Ⅱ），Ｆｅ２＋与亚硝基 Ｒ盐生成络合物，在Ｈ２Ｏ２在下可于－０．３５ Ｖ（ｖｓ．ＣＥ）处产生一个灵敏的极谱波，二阶导数波高与铁浓度在０．５～２００μｇ／Ｌ范围内呈线性关系，检出限０．３μｇ／Ｌ。研究了极谱波性质和机理。该波为一带吸附性质的平行催化波，络合物中配位体亚硝基Ｒ盐在电极上还原，还原的中间产物被Ｈ２Ｏ２氧化成原来的状态。本法灵敏、操作简单，已用于水样中痕量铁的直接测定。     

A contribution to the theory of D.C. polarographic current-voltage curves: Electrode reactions with three parallel charge transfer processes

Equations of polarographic current-voltage curves are derived for electrode reactions with three parallel charge transfer processes coupled by two chemical reactions on the basis of the staeady-state assumption in the reaction layer. If the rate constants of the charge transfer processes are sufficiently different from one another, the current-voltage curve consists of three separated waves and the equation for each wave has the same form as for a simple electrode reaction with one pathway.     

微圆盘电极上耦合一级均相反应的稳态电流

在电极表面反应物浓度均匀的近似假设下, 得到了微圆盘电极上暂态可逆反应的一般解. 然后利用该一般解得到了微圆盘电极上耦合一级均相反应的可逆电极反应CE、EC、EC′和ECE 的稳态电流计算公式.     

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.     

Reversibility of electrode reactions involving organic compounds

A general empirical approach allowing one to describe the kinetics and evaluate the mechanism of the electrode electron transfer reactions is offered. The approach is based on the electrode potentials, the vertical ionization potentials (oxidation), and the affinity to electron (reduction). An equation linking kinetic and thermodynamic parameters is derived. Electrode reactions involving organic compounds are discussed in polarographic terms. The conclusion is drawn that most electron transfer reactions involving organic compounds are reversible, and that the irreversibility of the net electrode reaction is due to the irreversibility of subsequent chemical and electrochemical stages. An experimental observation of the slow electron transfer is possible in the cases of a substantial reorganization of molecules in the presence of fast subsequent chemical and electrochemical reactions.     

Electrochemical reduction of S-oxides of diphenyl disulfide: Part II. Polarography in aqueous ethanol

In 50% ethanol the polarographic reduction of the S-oxides of diphenyl disulfide results in a fission of the sulfur-sulfur bond. Diphenyl disulfone is reduced by 2 electrons per molecle with benzenesulfinate ion as reduction product and gives rise to one polarographic wave. In the polarograms of phenyl benzenethiolsulfonate as well as phenyl benzenethiolsul-finate several waves appear due to the intermediate formation of a mercury compound, which is strongly adsorbed at the mercury electrode. Under polarographic and coulometric conditions the thiolsulfonate is reduced by 2 electrons in all with benzenesulfinate ion and thiophenol as reduction products. The total limiting current of the thiolsulfinate corresponds to a reduction by 4 electrons whereas 3 electrons per molecule are exchanged in coulometric experiments at a Hg-pool with thiophenol as the main reduction product.     

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.     

Prediction and measurement of effect of chelating selectivity on precipitation reactions

A theoretical treatment of precipitation equilibrium in the presence of a chelating agent has been expanded to include the common ion effect on precipitation reactions. The extent of precipitation can be predicted over the full range of pH for any metal for which the solubility product of the precipitate and the stability constants for a complex are known. The expanded equation has been used in this investigation to predict the extent of precipitation for several metals in the presence of ethylenediaminetetra-acetic acid (EDTA) with common précipitants such as hydroxide, oxalate, sulphate, sulphide and 8-hydroxyquinoline, but may be applied to any system containing other chelating agents. The limits of precipitation in terms of pM', the negative logarithm of the concentration of unprecipitated metal ion, are presented graphically as a function of pH for a system containing axed amounts of precipitating and complexing agents. Measurements for observed reactions at two pH levels, 5 and 10, are compared with the calculated predictions of the equation. Thirty-five metals were investigated.