Electroanalytical Chemistry of Sulfur Compounds For the New Coal Conversion Technologies

Abstract

Polarographic methods are described “tailor-made” for the speciation and determination of sulfur contaminants in synfuels and coal gasification/liquefaction process streams. In samples containing the anions, S^2? _x, sulfidic sulfur was quantitated by anodic depolarization of the dropping mercury electrode, while polysulfidic sulfur was determined with the aid of an electroreduction process implicating 2(x-1) electrons. Polythionates were electroreduced to thiosulfate, sulfite, sulfide and/or mixtures thereof, under judiciously controlled experimental conditions. Thiosulfate and sulfite were quantitated by differential pulse polarography at dropping mercury anodes via reactions involving formation of thiosulfato- and sulfitomercurates.     

Determination of cholride in natural and potable water samples byturbidimetric descrete-sample automatic analysis.

The reduction of the uranyl-mellitate complex at the dropping mercury electrode has been studied in aqueous and dimethyl sulfoxide solution. In aqueous solution, besides the reduction waves of the uranyl-mellitate complex, corresponding to the reduction of U(VI) to U(V), and of U(V) to U(III), an adsorption wave and a catalytic hydrogen wave were obtained; the species formed below pH 4.0 was UO₂(H₃A)^- and above pH 4.0 was UO₂(OH)(H₃A)^2-. In dimethyl sulfoxide solution, two well-defined waves were observed; the first wave is due to reduction of a uranyl-mellitate-DMSO complex, and the second to reduction of mellitic acid. The species involved are UO₂(DMSO)₆²⁺ below pH 2.2 and UO₂(H₃A)(DMSO)₅^-1 above pH 2.2. The activation energies of the reduction process were determined.     

The kinetics of reduction of palladium (II) complexes with β-alanine at dropping mercury electrode and the complexes’ stability constants

By using dc and ac polarography, the kinetics of electroreduction of the palladium (II) complexes with β-alanine at a dropping mercury electrode was studied in solutions with the palladium (II) concentration from 2 × 10^?5 to 2 × 10^?4 M and variable β-alanine and sodium perchlorate concentrations (pH 6–12). One polarographic wave was observed in solutions with pH 9 and 10 at the β-alanine overall concentration of c _βala = 1 × 10^?3 to 5 × 10^?2 M; two waves, at lower pH or higher c _βala. It was concluded on the formation of different forms of palladium (II) complexes in the studied solutions; the complexes contained two to four β-alanine coordinated anions. Using the limiting diffusion currents for the two waves at pH 9–11 and c _βala = 0.1 and 0.5 M, the stepwise stability constant for the Pd(βala) ₄ ^2? complex was calculated. Using two ac peaks observed at pH 7–8 and c _βala = 1 × 10^?2 to 0.1 M, the stepwise stability constant for the Pd(βala) ₃ ^? . was calculated. The perchlorate ions adsorbed at the dropping mercury electrode, as well as βala^? anions at their higher concentrations, hamper the electroreduction of the palladium (II) complexes with β-alanine.     

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.     

Influence of sulfide and cyanide ions on the electrochemical behavior of the Fe(II)/Fe(Hg) system in thiocyanate solutions at mercury electrodes

The reduction and reoxidation processes of the Fe(II)/Fe(Hg) system in thiocyanate solutions at stationary mercury electrodes have been investigated by cyclic voltammetric, anodic stripping and controlled potential electrolysis methods. In 0.1 M NaSCN and 0.4 M NaClO₄ solution containing 1×10^?3M Fe(II), the voltammogram on the first cycle at. 0.05 V s^?1 gives two consecutive cathodic peaks near ?1.2 and ?1.39 V with a hysteresis on the reversal, and an anodic wave with two large peaks near ?0.58 and ?0.05 V and two small peaks near ?0.52 and ?0.43 V, respectively. The multicyclic voltammogram under the same conditions in the potential region between 0.00 and ?1.50 V gives a cathodic wave with a principal peak near ?1.02 V and two small peaks near ?0.02 and ?0.53 V, respectively, and an anodic wave with a principal peak near ?0.72 V, three small peaks near ?0.64, ?0.52 and ?0.40 V, and with a shoulder near ?0.05 V, respectively. The variation of the shape of the voltammogram on the second and subsequent runs is due to the formation of S^2? and CN^? during the process of electroreduction of Fe(II). A mechanism is proposed which involves an initial reduction of Fe(II)?SCN^? produced in an activation step at a mercury electrode, followed by the chemical redox reaction of a part of Fe(0)?SCN^? in the species giving FeS and CN^?, and takes into account the influence of FeS and CN^? on the further reduction and reoxidation of iron. Both FeS and CN^? stimulate further reduction, and reoxidation of iron. The hysteresis of the cathodic wave on the first cycle arises from the fact that Fe(II) is reduced more easily at the mercury electrode covered with FeS than at a pure mercury electrode.     

The dependence of the kinetics of some simple outer-sphere electrode reactions on the nature of the electrode material

The kinetics of a number of simple inorganic electrode reactions that are known or expected to follow outer-sphere pathways have been examined at mercury, silver, platinum, and gold-aqueous interfaces in order to explore the effects of varying the electrode material on outer-sphere reactivity. The electroreduction kinetics of Co(III) ammine complexes exhibited only mild dependences on the nature of the electrode material which were compatible with the expected variations in double-layer effects. However, the electrooxidation of Cr²⁺ proceeded at strikingly higher overpotentials on the solid surfaces compared with mercury electrodes. Similar effects were also seen for the electrooxidation of V²⁺, Eu²⁺ and Ru²⁺ in the presence of Cr²⁺. Much larger rate constants were observed for these aquo reactions at solid surfaces in the absence of Cr²⁺, although Cr²⁺ had no influence on Co(NH₃)₆³⁺ electroreduction, or any reaction at mercury electrodes. It is speculated that the very large substrate effects upon the electrode kinetics of aquo couples arise from the influence of the inner-layer water structure on the reactant-solvent interactions experienced by these “structure-making” reactants at their plane of closest approach. The inhibiting influence of Cr²⁺ may be due to its ability to efficiently remove adsorbed catalytic contaminants by incorporation into a substitutionally inert Cr(III) electrooxidation product by means of a ligand-bridge mechanism.     

Effect of the Supporting-Electrolyte Cation on the Chloroplatinite Ion Reduction at a Dropping Mercury Electrode

The effect of nature and concentration of supporting-electrolyte cations (Li⁺, Na⁺, Cs⁺, (CH₃)₄N⁺, Ca²⁺, Ba²⁺) on electroreduction kinetics of PtCl^2- ₄at a dropping mercury electrode is studied. The electroreduction wave for PtCl^2- ₄is complicated by a polarographic maximum of first kind followed by a pronounced plateau of limiting current, after which the current passes through a minimum. The electroreduction occurs probably via two different mechanisms and presumably involves the same species, because the charge z _iof discharging species, determined by the Frumkin–Petrii method, remains virtually constant (z _i –1) on both descending (E= –0.6 to –1.0 V vs. SCE) and ascending (–1.3 to –1.6 V) branches of polarization curves and is independent of the nature of the supporting-electrolyte cation. The mechanism is presumably changed by a changed orientation of discharging species relative to the electrode surface.     

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.     

Determination of Hemin using a Parallel Catalytic Reduction Wave of its Protoporphyrin IX Ring

Abstract

Hemin, iron (III) protoporphyrin IX chloride, in NH₃‐NH₄Cl buffer solution of pH 9.5 produces an insensitive reduction wave at about ?0.70 V (vs. saturated calomel reference electrode, SCE) by using single sweep polarography. Adding oxidant K₂S₂O₈ to the solution, hemin reduction wave is catalyzed, yielding a parallel catalytic wave. The catalytic current is 20 times of hemin original reduction current. The derivative peak height is linearly proportional to the hemin concentration in the range of 7.5×10^?8 to 4.5×10^?6 mol/l, the detection limit is 5.0×10^?8 mol/l. Serum albumin, common amino acids, and metal ions have no interference with the hemin determination. The proposed method has been applied to the determination of hemin content in oral liquid samples with satisfactory results. The parallel catalytic wave is attributed to the catalytic reduction of porphyrin ring of hemin at the dropping mercury electrode. The new method could be useful in biochemical, clinical, and pharmaceutical analysis.     

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.     

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.     

Polarographic study of hydrogen peroxide anodic current and its application to antioxidant activity determination

Behavior of hydrogen peroxide in alkaline medium has been studied by direct current (DC) polarography with dropping mercury electrode (DME) aiming to apply it in antioxidant (AO) activity determination. Development of a peroxide anodic current having form of a peak, instead of common polarographic wave, has been investigated. As a base for this investigation the interaction of H₂O₂ with anodically dissolved mercury was followed. Formation of mercury complex [Hg(O₂H)(OH)] has been confirmed. The relevant experimental conditions, such as temperature, concentration and pH dependence, as well as time stability of hydrogen peroxide anodic current, have been assessed. Development of an AO assay based on decrease of anodic current of hydrogen peroxide in the presence of antioxidants (AOs) has been described. Under optimized working conditions, a series of benzoic acids along with corresponding cinnamate analogues have been tested for hydrogen peroxide scavenging activity. In addition, the assay versatility has been confirmed on various complex samples.     

Trace Analysis of Iron in Environmental Water and Snow Samples from Poland

Abstract

A voltammetric method for the determination of iron at detection limit of 4 μg/l is described, using the catalytic current of the reduction of the Fe(III)-triethanolamine (TEA) complex in the presence of bromate ions. the determination was performed at a mercury hanging drop electrode without preconcentration, using the TEA alkaline solution as a supporting electrolyte and the differential pulse technique. A peak current for the Fe(III)-TEA catalytic reduction was observed at a potential of-1.0 V (Ag/AgCl saturated electrode). the influence of TEA, BrO³ and NaOH concentrations on the peak height was studied. It was found that a 100-fold excess of Mn, a 50-fold excess of Cr(VI) and Zn did not interfere in the determination. This method was applied to the determination of iron in water, snow and waste water samples.     

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.     

The CuII−CuI−Cu0(Hg) system in the presence of benzotriazole: Part I. A polarographic investigation

A polarographic investigation of Cu^II electroreduction from solutions of benzotriazole (BTA) of pH from 1 to 3 at a dropping mercury electrode shows the presence of two successive adsorption waves of equal height (waves Ic and IIIc) due, respectively, to the formation and to the subsequent dissolution of an adsorbed film of a Cu^I compound. The dependence of the half-wave potential of wave IIIc, which is polarographically reversible, upon pH and BTA concentration indicates that the adsorbed film has the composition [Cu^I(BTA^?)], where BTA^? denotes the deprotonated anionic form of benzotriazole. Moreover, the maximum height of wave IIIc indicates that this film is one monolayer thick. A third cathodic wave (wave IIc), lying between waves Ic and IIIc, stems from the reduction to the metal state of the Cu^II ions diffusing from the bulk solution. Copper(0) oxidation at dropping amalgam electrodes in BTA solutions yields two successive adsorption waves (waves Ia and IIa). Wave Ia is due to the same electrode process, Cu⁰ (Hg)+BTA?Cu^I(BTA^?)+H⁺+e, responsible for the cathodic wave IIIc. On the other hand, wave IIa is due to the formation of roughly two adsorbed monolayers of [Cu^I(BTA^?)] upon that formed along wave Ia.     

Cadmium ion reduction at mercury electrode in aqueous KCl solution in presence of adsorbed polyvinyl alcohol

The influence of polyvinyl alcohol on the kinetics of cadmium ion reduction in 1M aqueous KCl solution at the dropping mercury electrode was investigated using the impedance method. It was found that small additions of PVA did not change the diffusion coefficient much, but affected only the rate of charge transfer process. The electroreduction of cadmium in the presence of PVA became less reversible. The standard rate constant dependence on coverage degree is described by the following equation: k_s=k_s0(1?ν)ⁿ, where n=2.     

The polarographic and voltammetric behaviour of acetylacetonato and hexafluoroacetylacetonato complexes in acetonitrile

The polarographic behaviour of Ce(acac)₄, Ce(acac)₃, Eu(acac)₃, Fe(acac)₃, Cr(acac)₃, Co(acac)₃, Mn(acac)₃, NaMn(acac)₃, Mn(acac)₂, Ni(acac)₂, Cu(acac)₂, VO(acac)₂, Fe(hfacac)₃, Cr(hfacac)₃ and Cu(hfacac)₂ has been studied in acetonitrile on the dropping mercury electrode. Half-wave potentials versus bisbiphenylchromium(I)/(0), the reversibility of the electrode reaction and the number of electrons participating in the electrode processes measured by coulometry are reported. Cyclovoltammetric measurements have been performed on the hanging mercury drop electrode and on the stationary platinum electrode, the data of these studies are given. quite different behaviour has been observed on the platinum electrode compared to the dropping mercury electrode. Large scale electrolysis was employed to obtain information on the reaction products. The influence of the electrode material and the reaction mechanisms are discussed.     

Study of the Complexation,Adsorption and Electrode Reaction Mechanisms of Chromium(VI) and (III) with DTPA Under Adsorptive Stripping Voltammetric Conditions

The complexation of Cr(III) and Cr(VI) with diethylenetriaminepentaacetic acid (DTPA), the redox behavior of these complexes and their adsorption on the mercury electrode surface were investigated by a combination of electrochemical techniques and UV/vis spectroscopy. A homogenous two‐step reaction was observed when mixing Cr(III), present as hexaquo complex, with DTPA. The first reaction product, the electroactive 1 : 1 complex, turns into an electroinactive form in the second step. The results indicate that the second reaction product is presumably a 1 : 2 Cr(III)/DTPA complex. The electroreduction of the DTPA‐Cr(III) complex to Cr(II) was found to be diffusion rather than adsorption controlled.The Cr(III) ion, generated in‐situ from Cr(VI) at the mercury electrode at about ?50 mV (vs. Ag|AgCl) (3 mol L^?1 KCl), was found to form instantly an electroactive and adsorbable complex with DTPA. By means of electrocapillary measurements its surface activity was shown to be 30 times higher than that of the complex built by homogenous reaction of DTPA with the hydrated Cr(III). Both components, DTPA and the in‐situ built complex Cr(III) ion were found to adsorb on the mercury electrode.The effect of nitrate, used as catalytic oxidant in the voltammetric determination method, on the complexation reaction and on the adsorption processes was found to be negligible.The proposed complex structures and an overall reaction scheme are shown.     

Voltammetric Characterisation of Anticancer Drug Irinotecan

Electrochemical reduction of irinotecan was investigated on a static mercury drop electrode using square‐wave voltammetry. The mechanism of irinotecan electroreduction is a complex, pH‐dependent, quasireversible process and includes the transfer of two electrons and two protons. In acidic medium, the first electron transfer reaction is followed by the chemical reaction, and the product of this chemical reaction undergoes further electrochemical reduction at more negative potentials. Both irinotecan and the product of its reduction adsorb on the mercury electrode surface. Based on the adsorptive character of irinotecan, a new adsorptive stripping square‐wave voltammetric method for its electroanalytical determination has been proposed. The voltammetric response could be used to determine irinotecan in the concentration range from 1×10⁻⁷ mol/L to 1.5×10⁻⁶ mol/L and from 5×10⁻⁹ mol/L to 1.2×10⁻⁷ mol/L, if the accumulation time is 20 s and 300 s, respectively. The calculated limit of detection for irinotecan was found to be 8.7×10⁻⁹ mol/L (if t_acc=300 s).     

Kinetics of nickel electroplating from methanesulfonate electrolyte

The overpotential of nickel ion electroreduction on the nickel and mercury electrodes is shown to increase in the following sequence of anions: ClO₄⁻,CH₃SO₃⁻, SO₄²⁻. On the nickel electrode, the overpotential of nickel evolution decreases as the pH^v increases from 1.5 to 4. This is associated with the increase in pH^s as the result of a parallel reaction of hydrogen evolution. It is shown that in contrast to mercury, the Tafel plots of the nickel electrode demonstrate a bend corresponding to the change in their slope from −0.044 to −0.132 V. This is accompanied by the lowering down of the reaction order in nickel ions from 2 to 1. A mechanism of nickel ion electroreduction that includes two parallel routes is proposed and substantiated by a model. In the low overpotential range, the predominant process is the electroreduction of nickel hydroxocomplexes, which is characterized by the strong dependence of the reaction rate on the potential and the concentration of electroactive species. For high overpotentials, the predominant process is the direct discharge of nickel aquacomplexes the rate of which depends weaker on the potential and the concentration of electroactive species.