Electrochemical Equilibrium Research Articles

The impact of the glial spatial buffering on the K+ Nernst potential

Astrocytes play a critical role in CNS metabolism, regulation of volume and ion homeostasis of the interstitial space. Of special relevance is their clearance of K(+) that is released by active neurons into the extracellular space. Mathematical analysis of a modified Nernst equation for the electrochemical equilibrium of neuronal plasma membranes, suggests that K(+) uptake by glial cells is not only relevant during neuronal activity but also has a non-neglectable impact on the basic electrical membrane properties, specifically the resting membrane potential, of neurons and might be clinically valuable as a factor in the genetics and epigenetics of the epilepsy and tuberous sclerosis complex.

Thermodynamic Equilibrium Diagram of Oxygen-Chlorine-Titanium System

The chemical and electrochemical equilibrium of oxygen-chlorine-titanium system in the presence of gaseous phase were investigated. Many species, which consisted of oxygen, chlorine and titanium, were considered. Various thermodynamic equilibriums were calculated in the different pressures and temperatures. Calculation results were shown asE-Tdiagram. This diagram will be used as important tools for corrosion study and titanium production, and it is also used to thermodynamically determine the existence areas of various species and so on.

Two-photon microscopy and fluorescence lifetime imaging reveal stimulus-induced intracellular Na+ and Cl− changes in cockroach salivary acinar cells

The intracellular ion homeostasis in cockroach salivary acinar cells during salivation is not satisfactorily understood. This is mainly due to technical problems regarding strong tissue autofluorescence and ineffective ion concentration quantification. For minimizing these problems, we describe the successful application of two-photon (2P) microscopy partly in combination with fluorescence lifetime imaging microscopy (FLIM) to record intracellular Na(+) and Cl(-) concentrations ([Na(+)](i), [Cl(-)](i)) in cockroach salivary acinar cells. Quantitative 2P-FLIM Cl(-) measurements with the dye N-(ethoxycarbonylmethyl)-6-methoxy-quinolinium bromide indicate that the resting [Cl(-)](i) is 1.6 times above the Cl(-) electrochemical equilibrium but is not influenced by pharmacological inhibition of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC) and anion exchanger using bumetanide and 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid disodium salt. In contrast, rapid Cl(-) reuptake after extracellular Cl(-) removal is almost totally NKCC mediated both in the absence and presence of dopamine. However, in physiological saline [Cl(-)](i) does not change during dopamine stimulation although dopamine stimulates fluid secretion in these glands. On the other hand, dopamine causes a decrease in the sodium-binding benzofuran isophthalate tetra-ammonium salt (SBFI) fluorescence and an increase in the Sodium Green fluorescence after 2P excitation. This opposite behavior of both dyes suggests a dopamine-induced [Na(+)](i) rise in the acinar cells, which is supported by the determined 2P-action cross sections of SBFI. The [Na(+)](i) rise is Cl(-) dependent and inhibited by bumetanide. The Ca(2+)-ionophore ionomycin also causes a bumetanide-sensitive [Na(+)](i) rise. We propose that a Ca(2+)-mediated NKCC activity in acinar peripheral cells attributable to dopamine stimulation serves for basolateral Na(+) uptake during saliva secretion and that the concomitantly transported Cl(-) is recycled back to the bath.

A High Redox Potential Form of Cytochrome c550 in Photosystem II from Thermosynechococcus elongatus

Cytochrome c(550) (cyt c(550)) is a component of photosystem II (PSII) from cyanobacteria, red algae, and some other eukaryotic algae. Its physiological role remains unclear. In the present work, measurements of the midpoint redox potential (E(m)) were performed using intact PSII core complexes preparations from a histidine-tagged PSII mutant strain of the thermophilic cyanobacterium Thermosynechococcus (T.) elongatus. When redox titrations were done in the absence of redox mediators, an E(m) value of +200 mV was obtained for cyt c(550). This value is ∼300 mV more positive than that previously measured in the presence of mediators (E(m) = -80 mV). The shift from the high potential form (E(m) = +200 mV) to the low potential form (E(m) = -80 mV) of cyt c(550) is attributed to conformational changes, triggered by the reduction of a component of PSII that is sequestered and out of equilibrium with the medium, most likely the Mn(4)Ca cluster. This reduction can occur when reduced low potential redox mediators are present or under highly reducing conditions even in the absence of mediators. Based on these observations, it is suggested that the E(m) of +200 mV obtained without mediators could be the physiological redox potential of the cyt c(550) in PSII. This value opens the possibility of a redox function for cyt c(550) in PSII.

Mechanisms of chloride uptake in frog olfactory receptor neurons

Odorant stimulation of olfactory receptor neurons (ORNs) leads to the activation of a Ca(2+) permeable cyclic nucleotide-gated (CNG) channel followed by opening of an excitatory Ca(2+)-activated Cl(-) channel, which carries about 70% of the odorant-induced receptor current. This requires ORNs to have a [Cl(-)](i) above the electrochemical equilibrium to render this anionic current excitatory. In mammalian ORNs, the Na(+)-K(+)-2Cl(-) co-transporter 1 (NKCC1) has been characterized as the principal mechanism by which these neurons actively accumulate Cl(-). To determine if NKCC activity is needed in amphibian olfactory transduction, and to characterize its cellular location, we used the suction pipette technique to record from Rana pipiens ORNs. Application of bumetanide, an NKCC blocker, produced a 50% decrease of the odorant-induced current. Similar effects were observed when [Cl(-)](i) was decreased by bathing ORNs in low Cl(-) solution. Both manipulations reduced only the Cl(-) component of the current. Application of bumetanide only to the ORN cell body and not to the cilia decreased the current by again about 50%. The results show that NKCC is required for amphibian olfactory transduction, and suggest that the co-transporter is located basolaterally at the cell body although its presence at the cilia could not be discarded.

A mathematical model of blood-interstitial acid-base balance: application to dilution acidosis and acid-base status

We developed mathematical models that predict equilibrium distribution of water and electrolytes (proteins and simple ions), metabolites, and other species between plasma and erythrocyte fluids (blood) and interstitial fluid. The models use physicochemical principles of electroneutrality in a fluid compartment and osmotic equilibrium between compartments and transmembrane Donnan relationships for mobile species. Across the erythrocyte membrane, the significant mobile species Cl⁻ is assumed to reach electrochemical equilibrium, whereas Na(+) and K(+) distributions are away from equilibrium because of the Na(+)/K(+) pump, but movement from this steady state is restricted because of their effective short-term impermeability. Across the capillary membrane separating plasma and interstitial fluid, Na(+), K(+), Ca²(+), Mg²(+), Cl⁻, and H(+) are mobile and establish Donnan equilibrium distribution ratios. In each compartment, attainment of equilibrium by carbonates, phosphates, proteins, and metabolites is determined by their reactions with H(+). These relationships produce the recognized exchange of Cl(-) and bicarbonate across the erythrocyte membrane. The blood submodel was validated by its close predictions of in vitro experimental data, blood pH, pH-dependent ratio of H(+), Cl⁻, and HCO₃⁻ concentrations in erythrocytes to that in plasma, and blood hematocrit. The blood-interstitial model was validated against available in vivo laboratory data from humans with respiratory acid-base disorders. Model predictions were used to gain understanding of the important acid-base disorder caused by addition of saline solutions. Blood model results were used as a basis for estimating errors in base excess predictions in blood by the traditional approach of Siggaard-Andersen (acid-base status) and more recent approaches by others using measured blood pH and Pco₂ values. Blood-interstitial model predictions were also used as a basis for assessing prediction errors of extracellular acid-base status values, such as by the standard base excess approach. Hence, these new models can give considerable insight into the physicochemical mechanisms producing acid-base disorders and aid in their diagnoses.

Gibbs fluctuation theory in the context of electrochemical equilibrium noise

The electrochemical noise verification of the Gibbs fluctuation theory shows that the Gibbs ergodic idea works perfectly with respect to the pair correlations of the electrode charge thermal fluctuations. At the same time, the Gibbs formulae for the triple- and higher-order correlations of the electrode charge thermal fluctuations are outside of the ergodic hypothesis. This failure of the Gibbs ergodic idea suggests that the noise version of the electrochemical charge-transfer theory should be developed. In the context of nano-electrochemistry, the second- and higher-order correlations of the electrochemical noise processes may be considered as the quantities suitable for the nano-electrochemical characterization of both the electrode processes and electrochemical devices.

Non-equilibrium potentiometry—the very essence

In most interpretations of potentiometric ion sensor responses with glass, solid, or liquid/polymer membranes, a model assuming electrochemical equilibrium between the aqueous sample and the membrane is used. This model is often called a phase boundary model to emphasize the importance of ion-exchange processes at the interface. The essence of the phase boundary model is that it accepts electroneutrality and thermodynamic equilibrium, and thus ignores electrochemical migration and the time-dependent effects. For this reason, this model is in conflict with many experimental reports on ion sensors in which both kinetic (time-dependent) discrimination of ions to improve selectivity and non-equilibrium transmembrane ion transport for lowering the detection limits are deliberately used. To respond to the experimental challenges in the author’s groups, we elevated the potentiometric modeling by using the Nernst–Planck–Poisson (NPP) equations system to model the non-equilibrium response. In the NPP model, electroneutrality and steady-state/equilibrium assumptions are abandoned, and thus we access the space and time domain. This approach describes the concentration changes of ions participating in the ion-exchange and transport processes, as well as the electrical potential evolution over space and time, and allows in particular, the inspection of the equilibrium set by the phase boundary models as a special “stationary” case after infinite time. Additionally, directly predicting the selectivity and the low detection limit variability over time and the influence of other parameters, e.g., ion diffusibility, is possible. As a coherent and non-arbitral model, the NPP system facilitates solving the inverse problem, i.e., to optimize the sensor properties and measurement conditions in a customized way via desired target functions and hierarchical genetical strategy modeling. In this way the NPP allows setting the conditions under which the experimentally measured selectivity coefficients are true (unbiased) and the detection limits are optimized.

A novel non-platinum group electrocatalyst for PEM fuel cell application

Precious-metal catalysts (predominantly Pt or Pt-based alloys supported on carbon) have traditionally been used to catalyze the electrode reactions in polymer electrolyte membrane (PEM) fuel cells. However as PEM fuel systems begin to approach commercial reality, there is an impending need to replace Pt with a lower cost alternative. The present study investigates the performance of a carbon-supported tantalum oxide material as a potential oxygen reduction reaction (ORR) catalyst for use on the cathode side of the PEM fuel cell membrane electrode assembly. Although bulk tantalum oxide tends to exhibit poor electrochemical performance due to limited electrical conductivity, it displays a high oxygen reduction potential; one that is comparable to Pt. Analysis of the Pourbaix electrochemical equilibrium database also indicates that tantalum oxide (Ta 2O 5) is chemically stable under the pH and applied potential conditions to which the cathode catalyst is typically exposed during stack operation. Nanoscale tantalum oxide catalysts were fabricated using two approaches, by reactive oxidation sputtering and by direct chemical synthesis, each carried out on a carbon support material. Nanoscale tantalum oxide particles measuring approximately 6 nm in size that were sputtered onto carbon paper exhibited a mass-specific current density as high as one-third that of Pt when measured at 0.6 V vs. NHE. However, because of the two-dimensional nature of this particle-on-paper structure, which limits the overall length of the triple-phase boundary junctions where the oxide, carbon paper, and aqueous electrolyte meet, the corresponding area-specific current density was quite low. The second synthesis approach yielded a more extended, three-dimensional structure via chemical deposition of nanoscale tantalum oxide particles on carbon powder. These catalysts exhibited a high ORR onset potential, comparable to that of Pt, and displayed a significant improvement in the area-specific current density. Overall, the highest mass-specific current density of the carbon-powder supported catalyst was ˜9% of that of Pt.

An Acid-loading Chloride Transport Pathway in the Intraerythrocytic Malaria Parasite, Plasmodium falciparum

The intraerythrocytic malaria parasite exerts tight control over its ionic composition. In this study, a combination of fluorescent ion indicators and (36)Cl(-) flux measurements was used to investigate the transport of Cl(-) and the Cl(-)-dependent transport of "H(+)-equivalents" in mature (trophozoite stage) parasites, isolated from their host erythrocytes. Removal of extracellular Cl(-), resulting in an outward [Cl(-)] gradient, gave rise to a cytosolic alkalinization (i.e. a net efflux of H(+)-equivalents). This was reversed on restoration of extracellular Cl(-). The flux of H(+)-equivalents was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and, when measured in ATP-depleted parasites, showed a pronounced dependence on the pH of the parasite cytosol; the flux was low at cytosolic pH values < 7.2 but increased steeply with cytosolic pH at values > 7.2. (36)Cl(-) influx measurements revealed the presence of a Cl(-) uptake mechanism with characteristics similar to those of the Cl(-)-dependent H(+)-equivalent flux. The intracellular concentration of Cl(-) in the parasite was estimated to be approximately 48 mm in situ. The data are consistent with the intraerythrocytic parasite having in its plasma membrane a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive transporter that, under physiological conditions, imports Cl(-) together with H(+)-equivalents, resulting in an intracellular Cl(-) concentration well above that which would occur if Cl(-) ions were distributed passively in accordance with the parasite's large, inwardly negative membrane potential.

Electrodeposition of nanocrystalline Co-W coatings from citrate electrolytes under conditions of controlled hydrodynamic: II. The electrodeposition rate and composition of the coatings

The influence of the ionic mass-transfer effects on the deposition rate, the current efficiency, and the composition and morphology of the coatings has been studied using a rotating cylindrical electrode in a citrate electrolyte containing CoSO4 (0,2 mol/l) and Na2WO4 (0.2 mol/l) (pH = 6.8) at the electrodeposition temperature of 60°C. It has been found that the decrease of the electrodeposition potential and the tungsten concentration in the coating with the current efficiency increase upon the Re number growth (Re ≥ 200) occur only under galvanostatic conditions. At the potentiostatic mode, similar of influence fails to be observed. It is shown that the estimated effects take place due to the electrodeposition through the nonstoichiometric surface coating layer with electronic conductivity, the composition components of which are in electrochemical equilibrium with the components of the solution at the film-solution boundary.

Sensing and Tactile Artificial Muscles from Reactive Materials

Films of conducting polymers can be oxidized and reduced in a reversible way. Any intermediate oxidation state determines an electrochemical equilibrium. Chemical or physical variables acting on the film may modify the equilibrium potential, so that the film acts as a sensor of the variable. The working potential of polypyrrole/DBSA (Dodecylbenzenesulfonic acid) films, oxidized or reduced under constant currents, changes as a function of the working conditions: electrolyte concentration, temperature or mechanical stress. During oxidation, the reactive material is a sensor of the ambient, the consumed electrical energy being the sensing magnitude. Devices based on any of the electrochemical properties of conducting polymers must act simultaneously as sensors of the working conditions. Artificial muscles, as electrochemical actuators constituted by reactive materials, respond to the ambient conditions during actuation. In this way, they can be used as actuators, sensing the surrounding conditions during actuation. Actuating and sensing signals are simultaneously included by the same two connecting wires.

Semi-phenomenological thermodynamic description of chemical kinetics and mass transport

Conventional linear thermodynamic description of chemical mass transport is based on Onsager equations for flows proportional to thermodynamic forces, and analysis of expressions for entropy production. A recently developed mesoscopic thermodynamic description is able to explain also nonlinear chemical kinetics phenomena. An alternative unifying approach is summarized here and is essentially an extension of classical mechanics. It is based on the idea that the mass transfer flow is the product of Einstein's mobility, concentration, and a gradient of physicochemical potential, which is a more general than usual chemical potential. The approach easily leads to many classical equations for mass transport, including Fick's law and the Fokker–Einstein expression for diffusion coefficient, Smoluchowski and Fokker–Planck equations for simultaneous diffusion and force-driven transport, and Boltzmann distribution. Addition of a chemical reaction coordinate naturally leads to a general description of kinetics of elementary chemical reactions far fromequilibrium, which includes dimensions and dielectric properties of molecules but does not need reorganization energy. It easily leads to Arrhenius, transition state theory, and Kramers equations for the rate of elementary chemical reaction, Gibbs and Nernst equations for chemical and electrochemical equilibrium, Butler–Volmer and Tafel equations for kinetics of electrochemical reactions, the Schottky equation for semiconductor diodes, more general than Guldberg–Waage law for equilibrium, and so on. A new interpretation of classic Einstein's mobility in quantum mechanics terms is suggested. Using this approach it is possible to explain the effects of all possible driving factors on chemical reactions and even to give a general description of chemical kinetics, which uses physicochemical potential and does not include concentrations or activities.

Oxidation modes and thermodynamics of Fe II–III oxyhydroxycarbonate green rust: Dissolution–precipitation versus in situ deprotonation

Fe II–III hydroxycarbonate green rust GR(CO 3 2−), Fe II 4 Fe III 2 (OH) 12 CO 3·3H 2O, is oxidized in aqueous solutions with varying reaction kinetics. Rapid oxidation with either H 2O 2 or dissolved oxygen under neutral and alkaline conditions leads to the formation of ferric oxyhydroxycarbonate GR(CO 3 2−)∗, Fe III 6 O 12 H 8 CO 3·3H 2O, via a solid-state reaction. By decreasing the flow of oxygen bubbled in the solution, goethite α-FeOOH forms by dissolution–precipitation mechanism whereas a mixture of non-stoichiometric magnetite Fe (3− x )O 4 and goethite is observed for lower oxidation rates. The intermediate Fe II–III oxyhydroxycarbonate of formula Fe II 6(1− x ) Fe III 6 x O 12 H 2(7−3 x ) CO 3·3H 2O, i.e. GR( x)∗ for which x ϵ [1/3, 1], is the synthetic compound that is homologous to the fougerite mineral present in hydromorphic gleysol; in situ oxidation accounts for the variation of ferric molar fraction x = [Fe III]/{[Fe II]+[Fe III]} observed in the field as a function of depth and season but limited to the range [1/3, 2/3]. The domain of stability for partially oxidized green rust is observed in the E h-pH Pourbaix diagrams if thermodynamic properties of GR( x)∗ is compared with those of lepidocrocite, γ-FeOOH, and goethite, α-FeOOH. Electrochemical equilibrium between GR( x)∗ and Fe II in solution corresponds to E h-pH conditions close to those measured in the field. Therefore, the reductive dissolution of GR( x)∗ can explain the relatively large concentration of Fe II measured in aqueous medium of hydromorphic soils containing fougerite.

Diagrams of chemical and electrochemical equilibriums of nickel-molybdenum and nickel-chromium-molybdenum alloys

The phase diagram of the Ni-Mo-O system and the potential-pH diagram of the Ni-Mo-H2O system at 25°C are composed. The electrochemical equilibrium diagram of a NiCr15Mo15 alloy (70%Ni + 15%Mo + 15%Cr) at the standard temperature is refined. The beneficial effect of molybdenum on the stability of the alloys’ passive state is discussed in terms of thermodynamics.

A Small Molecule Screen Identifies Novel Inhibitors of the Neuronal K‐Cl cotransporter KCC2

KCC2, a neuronal‐specific K‐Cl cotransporter plays a major role in maintaining intracellular Cl− concentration in neurons below its electrochemical equilibrium potential, thus favoring robust GABA hyperpolarizing or inhibitory responses. The pharmacology of the K‐Cl cotransporter is dominated by the loop diuretics furosemide and bumetanide. These molecules are used in clinical medicine as they inhibit the loop of Henle Na‐K‐2Cl cotransporter with much higher affinity. In order to identify novel molecules that affect KCC2 activity, we developed a fluorescent‐based assay suitable for high throughput screening (HTS) and used the assay to screen a library of 234,000 small molecules. We identified a large number of molecules that either decrease or increase the activity of the cotransporter. We characterized a small number of inhibitory compounds, some of which inhibit KCC2 activity in the micromolar or sub‐micromolar range, without substantially affecting NKCC1 activity. We synthesized a number of chemical variants, tested their effect on KCC2 function, and provide a first analysis of structure/activity relationship. We also used one of the compounds to demonstrate competitive inhibition in regard to external [K+], and non‐competitive inhibition in respect to external [Cl−].

Small-molecule screen identifies inhibitors of the neuronal K-Cl cotransporter KCC2

KCC2, a neuronal-specific K-Cl cotransporter, plays a major role in maintaining intracellular Cl(-) concentration in neurons below its electrochemical equilibrium potential, thus favoring robust GABA hyperpolarizing or inhibitory responses. The pharmacology of the K-Cl cotransporter is dominated by loop diuretics such as furosemide and bumetanide, molecules used in clinical medicine because they inhibit the loop of Henle Na-K-2Cl cotransporter with much higher affinity. To identify molecules that affect KCC2 activity, we developed a fluorescence-based assay suitable for high-throughput screening (HTS) and used the assay to screen a library of 234,000 small molecules. We identified a large number of molecules that either decrease or increase the activity of the cotransporter. Here, we report the characterization of a small number of inhibitors, some of which inhibit KCC2 activity in the submicomolar range without substantially affecting NKCC1 activity. Using medicinal chemistry, we synthesized a number of variants, tested their effect on KCC2 function, and provide an analysis of structure/activity relationships. We also used one of the compounds to demonstrate competitive inhibition in regard to external [K(+)] versus noncompetitive inhibition in respect to external [Cl(-)].

Probing the electrochemical behaviour of SWCNT–cobalt nanoparticles and their electrocatalytic activities towards the detection of nitrite at acidic and physiological pH conditions

The electrochemical decoration of edge plane pyrolytic graphite electrode (EPPGE) with cobalt and cobalt oxide nanoparticles integrated with and without single-walled carbon nanotubes (SWCNTs) is described. Successful modification of the electrodes was confirmed by field emission scanning electron microscopy (FESEM), AFM and EDX techniques. The electron transfer behaviour of the modified electrodes was investigated in [Fe(CN)6] 3−/4− redox probe using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and discussed. The study showed that cobalt nanoparticles modified electrodes exhibit faster electron transfer behaviour than their oxides. The catalytic rate constant ( K) obtained at the EPPGE–SWCNT–Co for nitrite at pH 7.4 and 3.0 are approximately the same (∼3 × 10 4 cm 3 mol −1 s −1) while the limits of detection (LoD = 3.3 δ/ m) are in the μM order. From the adsorption stripping voltammetry, the electrochemical adsorption equilibrium constant β was estimated as (13.0 ± 0.1) × 10 3 M −1 at pH 7.4 and (56.7 ± 0.1) × 10 3 M −1 at pH 3.0 while the free energy change (Δ G°) due to the adsorption was estimated as −6.36 and −10.00 kJ mol −1 for nitrite at pH 7.4 and 3.0, respectively.

Copper Adsorption Kinetics onto Pseudomonas aeruginosa Immobilized Multiwalled Carbon Nanotubes in an Aqueous Solution

Pseudomonas aeruginosa immobilized multiwalled carbon nanotubes were used as a novel kind of solid-phase extraction adsorbents of heavy metals, and the concentration of copper ions in the aqueous solution was obtained with the use of linear sweep anodic stripping voltammetry (LSASV). The adsorption of copper ions was rapid, and equilibrium was attained within 30 min. The combination technique provides fast, real-time information for adsorption process. Based on the electrochemical results, the kinetics and equilibrium were systematically examined. The pseudo-second-order kinetic model was used to correlate the kinetic experimental data, and the kinetic parameters were evaluated. The Langmuir and Freundlich models were applied to describe the adsorption isotherms.

J0601-1-2 混合導電性多孔質空気極の3次元数値解析(SOFC内の電極近傍現象)

A numerical simulation of SOFC mixed conducting cathode polarization based on electrochemical local equilibrium is conducted. Gaseous, ionic and electronic transport equations, and the electrochemical reaction at mixed conductor surface are solved using the Lattice Boltzmann Method (LBM). In the LBM, the gas mole fraction, electronic and ionic electrochemical potential distributions are considered. Gas transport is modeled using the Dusty Gas Model (DGM). The numerical simulation is conducted under 0.01, 0.05, 0.1, 0.3 A/cm^2. This numerical method can be applied to porous cathode, and it will be a promising tool for predicting SOFC electrode polarization.