Chronopotentiometric Measurements Research Articles

Sulfate as a ligand in ruthenium(II) and (III) ammines

The trans-[RuSO4(NH3)4(L)]Cl complexes, (L = nicotinamide (nia), L-histidine (L-hist), 4-picoline (4-pic), 4-chloropyridine (4-Clpy), isonicotinamide (isn), pyrazine (pyz), 4-aminopyridine (4-NH2py), 4-cyanopyridine (4-CNpy), pyridine (py), imidazole (Him), and water (H2O)), were characterized by elemental analysis cyclic voltammetry, UV-vis, IR, and electron paramagnetic resonance spectroscopies. From the four ν (SO42–) observed only ν3 and ν4 split in two bands each for the sulfate unidentate coordination. The values of Δ/ξ parameters, extracted from g values, allow us to write the following order of increasing π-donation ability: pyz < (py, 4-CNpy, 4-Clpy. 4-pic, isn, nia) < Him < L-hist < 4-NH2py. The intense absorption in the 317-347 nm (ε ~ 2.3-5.6 × 103 M-1 cm-1) region was tentatively assigned to sulfate-to-metal charge transfer (LMCT) and the absorption in the range of 230-270 nm (ε ~ 2 - 6 × 103 M-1 cm-1) assigned to an internal (IL) π-π* transition in the heterocyclic ligands. The rate for the sulfate aquation in trans-[RuSO4(NH3)4L]0 complexes was evaluated through chronopotentiometric measurements and are in the 2.6 s-1 (chloropyridine) to 20 s-1 (pyrazine) range. The SO42– is aquated in trans-[RuIIISO4(NH3)4(4-pic)]Cl at the specific rate constant of (1.4 ± 0.4) × 10-5 s-1, which is very much slower than in trans-[RuIISO4(NH3)4(4-pic)] (5.4 s-1). The X-ray crystal structure data show that the Ru-Cl (2.3444(9) Å) and Ru-NH3 (2.100(2) Å) mean distances in trans-[RuCl(NH3)4(4-pic)]Cl2·H2O are similar to the ones observed in other tetraammineruthenium(III) complexes.Key words: ruthenium, sulfate, ammines.

Solubility of Platinum and Rhodium in Lime–Alumina–Silica Melts at 1700 K

Three oxide melts of composition 10‐20‐70, 30‐13‐57, and 40‐20‐40 (wt%) in the ternary system CaO–Al2O3–SiO2 were equilibrated at 1700 K in platinum or platinum–rhodium boats, under oxygen partial pressures (PO2) ranging from 10−13 to 1 atm, and fixed by various gaseous mixtures (H2/H2O, CO/CO2, and Ar/O2). The melts were analyzed for platinum and rhodium content after quenching. The complex dependence of solubility of these elements with PO2 was explained by their speciation. Chronopotentiometric measurements confirmed the passivation of the metallic boats. For increased PO2, this speciation followed the series Pt, PtO, PtO2 or RhO for the solid phases covering the metallic boats and Pt0, Pt2+, Pt4+, Pt6+ or Rh0, Rh2+, Rh3+, Rh4+ in solution in the oxide melt. The predominance diagrams were determined for the three compositions in the ternary system.

Chronopotentiometry for the advanced current–voltage characterisation of bipolar membranes

Compared to steady-state current–voltage curves, chronopotentiometric measurements allow us to distinguish the contributions to the overall electric potential difference across a bipolar membrane. In this paper, the characteristic values of the electric potential difference across the bipolar membrane at different times are correlated to the corresponding concentration profiles in the bipolar membrane layers and the ion-transport processes are identified. For over-limiting current densities (i.e. current densities above the limiting current density), it is possible to distinguish the reversible and irreversible contributions to the steady-state electric potential difference. The irreversible contribution is attributed to the energy required to overcome the electric resistance whereas the reversible contribution corresponds to the electrochemical potential due to concentration gradients in the membrane layers. Further, the ohmic resistance of the membrane in equilibrium with the surrounding solution has been compared to the resistance in the transport state. For low current densities, the equilibrium resistance is lower than the transport resistance stemming from internal concentration polarisation. In contrast, the large numbers of hydroxide ions and protons produced at high current densities result in a reduced ohmic transport resistance due to their high ionic mobility. This reduced resistance is not enough to stop the increase of the irreversible contribution with higher current densities. With the possibility to split the steady-state potential into its contributions, bipolar membrane chronopotentiometry is a useful tool to identify transport limitations and to improve bipolar membranes for a reduced overall electric potential.

Excess Gibbs free energy and kinetics of grain growth of nanostructured silver

Samples of nanostructured silver were prepared by a mechanochemical technique and by electrodeposition. In the former, the microstructure was stabilized by intergranular cosegregation of Mg and O atoms. The excess Gibbs free energy of these samples was investigated by e.m.f. measurements at 453 K in a solid state electrochemical cell with AgI as a solid electrolyte. Electrodeposited pure silver samples had a labile microstructure even at 298 K. Their structural relaxation process was monitored in situ by chronopotentiometric measurements in an aqueous iodoargentate electrolyte. The time dependence of the e.m.f. could be quantitatively described by a parabolic law of grain growth without any adjustable parameter.

Changes of structural and dynamic properties of model lipid membranes induced by α-tocopherol: implication to the membrane stabilization under external electric field

The effects of α-tocopherol on electric properties of bilayer lipid membranes were investigated. Planar bilayer membranes formed by the Mueller-Rudin method were used. Voltammetric and chronopotentiometric measurements were performed using a four-electrode potentiostat-galvanostat. It was demonstrated that registration of membrane capacitance, resistance, and voltammetric characteristics provided information about the change in the structure and permeability of bilayer lipid membranes. The results suggested that incorporation of α-tocopherol into lipid membrane destabilized its structure and facilitated the electrogeneration of pores. The possible role of observed changes in physiological functions of α-tocopherol was discussed.

Sulfate as a ligand in ruthenium(II) and (III) ammines

The trans-[RuSO4(NH3)4(L)]Cl complexes, (L = nicotinamide (nia), L-histidine (L-hist), 4-picoline (4-pic), 4-chloropyridine (4-Clpy), isonicotinamide (isn), pyrazine (pyz), 4-aminopyridine (4-NH2py), 4-cyanopyridine (4-CNpy), pyridine (py), imidazole (Him), and water (H2O)), were characterized by elemental analysis cyclic voltammetry, UV-vis, IR, and electron paramagnetic resonance spectroscopies. From the four ν (SO42–) observed only ν3 and ν4 split in two bands each for the sulfate unidentate coordination. The values of Δ/ξ parameters, extracted from g values, allow us to write the following order of increasing π-donation ability: pyz < (py, 4-CNpy, 4-Clpy. 4-pic, isn, nia) < Him < L-hist < 4-NH2py. The intense absorption in the 317-347 nm (ε ~ 2.3-5.6 × 103 M-1 cm-1) region was tentatively assigned to sulfate-to-metal charge transfer (LMCT) and the absorption in the range of 230-270 nm (ε ~ 2 - 6 × 103 M-1 cm-1) assigned to an internal (IL) π-π* transition in the heterocyclic ligands. The rate for the sulfate aquation in trans-[RuSO4(NH3)4L]0 complexes was evaluated through chronopotentiometric measurements and are in the 2.6 s-1 (chloropyridine) to 20 s-1 (pyrazine) range. The SO42– is aquated in trans-[RuIIISO4(NH3)4(4-pic)]Cl at the specific rate constant of (1.4 ± 0.4) × 10-5 s-1, which is very much slower than in trans-[RuIISO4(NH3)4(4-pic)] (5.4 s-1). The X-ray crystal structure data show that the Ru-Cl (2.3444(9) Å) and Ru-NH3 (2.100(2) Å) mean distances in trans-[RuCl(NH3)4(4-pic)]Cl2·H2O are similar to the ones observed in other tetraammineruthenium(III) complexes.Key words: ruthenium, sulfate, ammines.

Gold electrowinning from thiourea solutions

The use of thiourea as an alternative non-polluting reagent for gold leaching is very well known. However, the processes of gold recovering from thiourea solutions have received little attention, particularly the electrowinning process. Gold electrowinning from thiourea solutions was studied by linear sweep voltammetry and chronopotentiometry. For potentials more cathodic than 0 mV vs. SCE gold reduction is mass transport controlled. The estimated diffusion Coefficient for the complex gold (I)-thiourea, at 25°C was 1.0 × 10 −5, cm 2s −1. The influence of gold, thiourea and formamidine disulfide concentrations on the cathodic behavior ofgold is described. A passivation due to thiourea adsorption on the electrode surface was observed at −300mV vs. SCE. Experiments shown that gold reduction is favored by low thiourea and formamidine disulfide concentrations. Chronopotentiometric measurements indicated the presence of an adsorption step before the reduction of the complex gold (I) -thiourea, although the reduction of the species in solution is also feasible. The influence of current density on the cathodic current efficiency and gold structure is presented.

Electrowinning of zinc

The use of thiourea as an alternative non-polluting reagent for gold leaching is very well known. However, the processes of gold recovering from thiourea solutions have received little attention, particularly the electrowinning process. Gold electrowinning from thiourea solutions was studied by linear sweep voltammetry and chronopotentiometry. For potentials more cathodic than 0 mV vs. SCE gold reduction is mass transport controlled. The estimated diffusion Coefficient for the complex gold (I)-thiourea, at 25°C was 1.0 × 10−5, cm2s−1. The influence of gold, thiourea and formamidine disulfide concentrations on the cathodic behavior ofgold is described. A passivation due to thiourea adsorption on the electrode surface was observed at −300mV vs. SCE. Experiments shown that gold reduction is favored by low thiourea and formamidine disulfide concentrations. Chronopotentiometric measurements indicated the presence of an adsorption step before the reduction of the complex gold (I) -thiourea, although the reduction of the species in solution is also feasible. The influence of current density on the cathodic current efficiency and gold structure is presented.

Study of cerium species in molten Li2CO3-Na2CO3 in the conditions used in molten carbonate fuel cells. Part II : Potentiometric and voltammetric behaviour

The electrochemical behaviour of metallic cerium and cerium dioxide is analysed in molten Li2CO3–Na2CO3 under the anodic and cathodic conditions used for the molten carbonate fuel cells (MCFC). Chronopotentiometric measurements on metallic cerium rods or foils show the influence of CeO2 growth and the deviation from its stoichiometric composition. A study of the voltammetric characteristics of dissolved cerium at a gold electrode, of metallic cerium and cerium oxide is carried out in both oxidising and reducing MCFC atmospheres. Experimental evidence is given for the existence of a solid–solid cerium system: Ce2O3(s)|CeO2(s) and a system relative to dissolved cerium species Ce2O3(l)|CeO2(l). These systems are in agreement with thermodynamic predictions. Other more complex phenomena involving interactions with alkali species are discussed.

Chronopotentiometry and overlimiting ion transport through monopolar ion exchange membranes

In this paper chronopotentiometric measurements are described to study the overlimiting ion transport through a Neosepta CMX cation and AMX anion exchange membrane. This technique is used to characterise the fluctuations in membrane voltage drop observed in the overlimiting region of current–voltage curves and to investigate the structural inhomogeneity of the aforementioned membranes. Above the limiting current the measurements show large voltage drop fluctuations in time indicating hydrodynamic instabilities. The amplitude of these fluctuations is increasing with increasing applied current density. The fluctuations also occur when a set-up is used where there is no forced convection and the depleted diffusion layer is stabilised by gravitation. Experimental transition times are found to be smaller than calculated for an ideally permselective membrane and indicate a reduced permeable membrane area. The results can be related to the theory of electroconvection due to an inhomogeneous membrane structure.

Chronopotentiometric and open circuit potential analisys of the redox process in poly(3-methylthiophene) (PMT)

Abstract Chronopotentiometric measurements for PMT samples show that, for film previously in the reduced state, a maximum in the potentialtime curves was observed. The maximum occurs for times between 1 and 70 ms, the exact value depends on the anodic current density applied. It is followed by a potential minimum between 100 and 1500 μCcm −2 and finally by smooth increase of the potential. For samples lightly oxidized, Q = 6.25 μCcm −2 , the maximum potential disappears. In the reduction process, the decrease in the potentials is always linear. The open circuit potential (Eoc) as function of the oxidation charge curves show a strong increase in the Eoc for oxidation charge up to 1000 μCcm −2 followed by a linear increase for higher charges. Otherwise, the reduction process presents only one linear region.

Ultratrace Measurements of Nucleic Acids by Baseline-Corrected Adsorptive Stripping Square-Wave Voltammetry

The direct and reliable electrochemical detection of DNA is of paramount importance to the development of modern DNA hybridization chips, for the detection of nucleic acids following their electrophoretic separations, or for the sensing of DNA damage and interactions. Such solid electrode voltammetric measurements of nucleic acids have been traditionally hampered by the large solvent decomposition background current that obscures the oxidation signals of the purine nucleobases. This paper reports on the use of adsorptive stripping square-wave voltammetry, in connection with the "moving average baseline correction" approach, for monitoring ultratrace levels of DNA and RNA. Compared to other baseline-fitted or background-subtraction protocols, the moving average baseline scheme is particularly effective in isolating the small purine nucleobase peaks, which appear as small shoulders on the steep background discharge contribution. The remarkably flat baseline thus obtained (up to extreme potentials) leads to a dramatic lowering of the detection limits to the femtomole level and to a performance that compares favorably with that of computerized chronopotentiometric measurements of nucleic acids. Combined with the speed of square-wave voltammetric measurements, such developments should expand the role of voltammetry in DNA diagnostics and nucleic acid research.

Inverse derivative chronopotentiometry/quartz crystal microgravimetry of solid conducting salt films

An instrument, built in-house, for the purpose of making derivative chronopotentiometric and quartz crystal microgravimetric measurements is described. The spreadsheet-based software allows for presentation of the raw or derivative data of both the chronopotentiometric and gravimetric measurements in either the time or potential domain. The system was used to study the nucleation-controlled growth of the phase transition associated with the TCNQ(0/−1) couple isolated from a thin film of the conducting salt 9-aminoacridinium tetracyanoquinodimethane. The d t/d E– E curves for a nucleation process are presented along with the Hillman–Bruckenstein analysis of the corresponding mass changes of the film.

Electrochemical Sodium Insertion into MnCo Oxide

Electrochemical sodium insertion in the low‐temperature spinel oxide is investigated. Chronopotentiometric measurements show one single‐reduction process located at with a faradaic yield of of oxide for a cut‐ off voltage. During cycling experiments performed at discharge‐charge rates of , a practical specific capacity of was obtained after the fifth cycle. ©1999 The Electrochemical Society

Stripping chronopotentiometry of immobilized microparticles

Stripping chronopotentiometric measurements following the mechanical transfer of insoluble solid reactants to the surface of a paraffin impregnated graphite electrode are described. By the new technique, the potentials can be determined at which the heterogeneous redox reactions between the immobilized microcrystals and dissolved oxidants, or reductants occur. The method is tested with lead and tin metal particles and lead(II)oxide and sulfide powders in the oxygen saturated electrolytes of various pH. The oxidation of lead is a fast process, but tin particles are highly passivated in the basic media. A difference between the responses of electrodeposited and mechanically transferred lead is reported.

Chronopotentiometric studies of electroporation of bilayer lipid membranes

The constant-intensity current chronopotentiometric measurements of egg yolk phosphatidylcholine bilayer membranes (BLM) are presented. It is demonstrated that a constant-intensity current flowing through the bilayer membranes generates the pores in their structures. For the current intensity from 0.1 to 2.0 nA, the generated pores open and close cyclically. The frequency of oscillations depends on the current intensity: the higher current intensity, the higher frequency of pore oscillations. It is suggested that the presented method may allow to create one pore in BLM and to observe its dynamical behaviour. Based on chronopotentiometric curves, a method of pore conductance calculations is presented. It is demonstrated that the value of obtained conductance can be applied for pore diameter estimation. The hypothetical application of constant-current method as a biotechnological tool for selective and controlled incorporation of molecules into microorganisms is discussed.

Electrochemical characterization of a lithiated mixed nickel-cobalt oxide (LiNi0.5Co0.5O2) prepared by sol-gel process

Lithiated transition metal oxides having a layered structure and general formula LiMO2, have been extensively studied as positive electrode active materials for lithium or lithium-ion batteries. In particular, lithium nickel dioxide (LiNiO2) and lithium cobalt dioxide (LiCoO2) present a layered structure with high diffusion coefficients for the lithium ion. This latter property is very important in order to realize practical devices having high discharge rates. LiNiO2, compared with LiCoO2, has the advantage to be a cheaper material with a higher specific capacity for lithium cycling, but its stability upon cycling can be greatly influenced by the displacement of Ni ions from the Ni layers to the Li planes as the content in lithium is reduced over a certain value. Recently, solid solutions such as LiNixCo1−xO2 have been proposed to offer a compromise between stability, cost and capacity. In this work we have studied LiNi0.5Co0.5O2 prepared by the Complex Sol-Gel Process (CSGP). The advantage of this procedure toward the solid-state process is the high homogeneity in composition and in particle dimension of the synthesized compounds. The samples have been characterized electrochemically using chronopotentiometric, voltammetric and impedance measurements in liquid electrolyte. The results indicates that CSGP-synthesized LiNi0.5Co0.5O2 shows good cyclability (after 1000 cycles about 2/3 of the initial capacity can still be cycled) only if the anodic potential is limited to about 4.2 V. The quite low values of the specific capacity (∼70 mAh/g at C/1 charge-discharge rate) can be justified by the non-complete calcination reaction, as suggested by X-ray measurements. Kinetic properties have been evaluated by Electrochemical Impedance Spectroscopy measurements, which have shown quite high values for the lithium chemical diffusion coefficient (10−7÷10−8 cm2s−1) and its unexpected decrease as deintercalation proceeds from x=0.5 in LiNi0.5Co0.5O2.

Electrochemical insertion of magnesium in a mixed manganese–cobalt oxide

The electrochemical insertion of Mg 2+ into the cation-deficient mixed oxide Mn 2.15 Co 0.37 O 4 is studied in regard to its possible use as a positive electrode in magnesium ion-transfer batteries. The cation vacancies of the mixed oxide offer a pathway for Mg 2+ insertion with a maximum uptake of 0.23 Mg per mole of oxide. Voltammetric and chronopotentiometric measurements indicate a single insertion step located near 2.9 V vs Li/Li + . The strong polarizing effect of Mg 2+ ions induces a more pronounced disordering process than for Li accommodation. This results in a limited stable specific capacity of 30 A h kg -1 when discharge–charge cycles are performed at a C/6 rate within the cycling limits 4.05–1.85 V vs. Li/Li + .

Electrostatic incorporation of anthraquinonesulfonate ions into a polypyrrole film containing pyridinium groups

Poly-[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium] chloride films on glassy carbon electrodes greatly increased the voltammetric peak currents of anthraquinone-2,6-disulfonate, anthraquinone-2-sulfonate, and 5,5′-indigodisulfonate ions. This electrocatalytic action of the polymer film is attributed to the effective extraction of the anions from very dilute solutions; a partition coefficient of ~2 × 10 4 in 0.1 M NaCl solutions was evaluated from the coulometric data for anthraquinonesulfonates bound within the film. The merit of the polycationic film is to some extent counteracted by poor electronic conduction of the polymer and slow ion transport through the film; the diffusion coefficients of the anions were found to be of the order of (3–8) × 10 −10 cm 2 s −1 by linear sweep voltammetric and chronopotentiometric measurements. A decrease in partition coefficient with increasing concentration of NaCl in solution indicates that an electrostatic attraction dominates the binding of anions to the polymer. However, the order of the partition coefficients for organic anions cannot be rationalized solely in terms of the magnitudes of their negative charges. A great influence of 1-propanol added to solutions on the partition coefficient suggests that an additional factor affecting the distribution equilibria seems likely to be hydrophobic interaction between the polymer film and incorporating ions.

Electrochemical properties of a new non-stoichiometric spinel oxide (Mn 2.5O 4) as Li insertion compound

Preliminary results on electrochemical and structural properties of the cation deficient oxide Mn 2.5O 4 system are discussed in terms of its viability as an electrode for rechargeable lithium batteries. Chronopotentiometric measurements show two well-defined steps in the potential range of 3.5 V-2.0 V and 2.0 V-1.5 V, involving faradaic yields of ≈ 0.6 and ≈ 1.0 F mol −1 respectively. A loss of structural integrity was observed for Li ion contents higher than 0.65 per mol. Galvanostatic cycling experiments were performed in the voltage range 4.0-2.0 V and a specific capacity of 43 mAh g −1 was obtained after the twentieth cycle.