Low Conductivity Solutions Research Articles

Electrochemical investigations of aluminium alloys tribocorrosion

Electrochemical methods based on corrosion potential and polarization current measurements are widely used in tribocorrosion research. However, most equipment types allow recording of integrated electrochemical parameters of metals during tribocorrosion, but little attention is paid to local processes that occur on friction surfaces. It was proposed to use a capillary probe as a reference electrode, which reduces the effect of cathodic sites on sample tribopotential and allows measurements in low-conductivity solutions. The study was carried out on an example of the widely used AA2024 alloy, and may relate to structural aluminum alloys. It was shown that the difference between tribopotential values of the alloy measured by classic electrochemical technique and capillary probe may be up to 0.6 V in a study in glycerin. It was established that correlation exists between the current of alloy polarization at the corrosion potential in distilled water and the volume of worn material. The empirical relationship found for a ball-on-flat tribometer with reversible sliding provides a possibility to estimate the amount of worn material based on polarization currents. This method may also be promising in studies on the efficiency of corrosion inhibitors on mechanically activated surface of passive metal.

PH within pores in plant fiber cell walls assessed by Fluorescence Ratio Imaging

The pH within cell wall pores of filter paper fibers and hemp fibers was assessed by Fluorescence Ratio Imaging (FRIM). It was found that the Donnan effect affected the pH measured within the fibers. When the conductivity of the added liquid was low (0.7 mS), pH values were lower within the cell wall than in the bulk solution. This was not the case at high conductivity (22 mS). The occurrence of the Donnan effect allowed the pH values within pores in normal regions of the cell wall to be compared to the pH in regions with misaligned microfibrils (dislocations) when FRIM was carried out in a low conductivity solution. Surprisingly, no pH difference was observed between normal regions and dislocations, suggesting that pore sizes within the two different regions are approximately the same. In another experiment the Donnan effect was shown to have an effect on hydrolysis of hydrothermally pretreated wheat straw only when conducted in a low conductivity solution and only for xylanase, not cellulases. The hydrolysis experiments indicate that under typical conditions where conductivity is high, the Donnan effect does not lower the pH close to the substrate to an extent that affects enzymatic activity during hydrolysis of lignocellulose.

Certification of a low value electrolytic conductivity solution using traceable measurements

Reliable measurement results of electrolytic conductivity (EC), in particular for low values, must be metrologically traceable and be based on a realistic measurement uncertainty budget. The use of certified reference materials (CRMs) can help to achieve this goal. This paper presents results from all stages of the certification of an EC CRM with a conductivity of 1.5 μS cm−1, including the preparation of the batch solution and the evaluation of the homogeneity and stability of the bottled CRM. An uncertainty budget is presented for the CRM, including the main contributions from each of these sources. The CRM batch remained stable within its certified uncertainty for more than 1 year.

Effect of solution conductivity and electrode shape on the deposition of carbon nanotubes from solution using dielectrophoresis

Dielectrophoresis (DEP) is a popular technique for fabricating carbon nanotube (CNT) devices. The electric current passing through the solution during DEP creates a temperature gradient, which results in electrothermal fluid flow because of the presence of the electric field. CNT solutions prepared with various methods can have different conductivities and the motion of the solution because of the electrothermal phenomenon can affect the DEP deposition differently in each case. We investigated the effect of this movement in solutions with various levels of conductivity through experiments as well as numerical modeling. Our results show that electrothermal motion in the solution can alter the deposition pattern of the nanotubes drastically for high conductivity solutions, while DEP remains the dominant force when a low conductivity (surfactant-free) solution is used. The extent of effectiveness of each force is discussed in the various cases and the fluid movement model is investigated using two- and three-dimensional finite element simulations.

Differential pulse techniques in weakly supported media: Changes in the kinetics and thermodynamics of electrode processes resulting from the supporting electrolyte concentration

Square wave voltammetry (SWV) and differential multipulse voltammetry (DMPV) in weakly supported media are investigated. The numerical simulation procedures reported in literature (Streeter et al., J. Phys. Chem. C 112 (2008) 13716–13728; Limon-Petersen et al., J. Phys. Chem. C 114 (2010) 2227–2236) for electrochemical experiments in low conductivity solutions is applied with success. From this theory, the influence of the concentration of supporting electrolyte on the voltammograms is discussed for different redox couples and at electrodes of different size. The variation of the peak current and peak potential due to migration and ohmic drop effects are reported. The theory is applied to the experimental study of the one-electron reduction processes of cobaltocenium and cobalt (III) sepulchrate at mercury hemispherical electrodes of 25μm radius. The kinetic parameters and formal potential are obtained in a wide range of support ratio from the SWV and DMPV voltammograms. Possible changes of the kinetic and thermodynamic properties of the electrode reactions are analyzed as a function of the level of support.

Impedance spectra analysis to characterize interdigitated electrodes as electrochemical sensors

Interdigitated electrode (IDE) arrays have been used widely for electrochemical impedance spectroscopy (EIS). Here, we present an in-depth analysis of interdigitated electrode impedance spectra. Such an analysis is necessary for the identification of the contribution of each interdigitated electrode circuit element to the impedance change. We also discuss the importance of the solution conductivity in impedance spectroscopy showing that the use of low conductivity solutions is advantageous when inter-digit solution resistance and geometric capacitance need to be monitored.

Hydrodynamic and electrical considerations in the design of a four-electrode impedance-based microfluidic device

A four-electrode impedance-based microfluidic device has been designed with tunable sensitivity for future applications to the detection of pathogens and functionalized microparticles specifically bound to molecular recognition molecules on the surface of a microfluidic channel. In order to achieve tunable sensitivity, hydrodynamic focusing was employed to confine the electric current by simultaneous introduction of two fluids (high- and low-conductivity solutions) into a microchannel at variable flow-rate ratios. By increasing the volumetric flow rate of the low-conductivity solution (sheath fluid) relative to the high-conductivity solution (sample fluid), increased focusing of the high-conductivity solution over four coplanar electrodes was achieved, thereby confining the current during impedance interrogation. The hydrodynamic and electrical properties of the device were analyzed for optimization and to resolve issues that would impact sensitivity and reproducibility in subsequent biosensor applications. These include variability in the relative flow rates of the sheath and sample fluids, changes in microchannel dimensions, and ionic concentration of the sample fluid. A comparative analysis of impedance measurements using four-electrode versus two-electrode configurations for impedance measurements also highlighted the advantages of using four electrodes for portable sensor applications.

A study on electrodeposition of glucose oxidase from low conductivity solutions

In this paper, we have investigated the deposition of glucose oxidase from ultrapure water at relatively high applied potential of 4 V vs. Ag/AgCl, as a new deposition technique. Compared to GO x deposited from phosphate buffer, the enzyme electrode prepared from ultrapure water displayed a nearly 10 times higher amperometric response. EQCM measurements showed that the mass and thickness of the deposited enzyme layer from water is nearly 18 times higher than the enzyme deposited from buffer. This difference is also reflected in the impedance spectroscopy and AFM measurements. The electron transfer resistance is higher for the GO x film deposited from water than the GO x deposited from buffer and, AFM displayed a thick and rough film for the GO x deposited from water, while underlying and a smooth morphology were observed for GO x film deposited from buffer. The zeta potential of GO x in water is nearly 25 times higher than the zeta potential measured in buffer. These observations suggest that the GO x deposited from water is subject to both the electrophoretic migration of the enzyme from the bulk to the interface and precipitation at the electrode interface as a consequent of a local decrease in pH. By contrast, deposition of GO x from buffer is only due to a change in pH. The effects of deposition time, enzyme concentration and applied potential on the enzyme deposition from water were explored. The current response of the enzyme electrode as well as the uptake in mass onto the electrode measured by EQCM increase quasi-linearly with deposition time, enzyme concentration and applied potential up to 4 V.

Optically Modulated Electrokinetic Manipulation and Concentration of Colloidal Particles near an Electrode Surface

We study a recently demonstrated AC electrokinetic technique for manipulation and concentration of colloidal particles on an electrode surface. The technique uses indium tin oxide (ITO)-based parallel-plate electrodes on which highly localized infrared (1064 nm) laser illumination is shone. We show that the highly localized laser illumination leads to a highly nonuniform heating of the electrode substrate, which in turn drives an electrothermal microvortex resulting in a rapid transport of particles toward the illuminated site. Hundreds of polystyrene particles, with diameters ranging from 2.0 to 0.1 microm, suspended in a low conductivity solution (2.0 mS/m) could be aggregated at selected locations on the electrode by activating the laser illumination at suitable AC frequencies. Subsequent deactivation of the laser illumination causes the particles to scatter, and we explore this dynamical behavior for 1.0 microm particles using Delaunay tessellations and high-speed videography. We establish that drag from the electrothermal microvortex acts against a repulsive force, which decreases with increasing AC frequency, to create stable particle clusters. Moreover, experimentally we show that this particle capturing technique can be characterized by a critical frequency: a frequency at which the captured colloidal particle cluster becomes unstable and particles are carried away into the bulk by the electrothermal microvortex. This critical frequency increases with decreasing particle diameter for similar particles. For 0.1 microm particles, comparison of aggregation at different AC frequencies is achieved by the comparison of fluorescent intensity profiles of the aggregations.

Optically induced electrokinetic concentration and sorting of colloids

We demonstrate an optically induced ac electrokinetic technique that rapidly and continuously accumulates colloids on a parallel-plate electrode surface resulting in a crystalline-like aggregation. Electrothermal hydrodynamics produce a microfluidic vortex that carries suspended particles toward its center where they are trapped by local ac electrokinetic hydrodynamic forces. We characterize the rate of particle aggregation as a function of the applied ac voltage, ac frequency and illumination intensity. Hundreds of polystyrene particles (1.0 µm) suspended in a low conductivity solution (2.4 mS m−1) were captured at a range of voltages (5–20 Vpp) and frequencies (20–150 kHz) with an optical power of approximately 20 mW. This technique was not restricted to near infrared (1064 nm) illumination and was also demonstrated at 532 nm. The sorting capability of this technique was demonstrated with a solution containing 0.5 µm, 1.0 µm and 2.0 µm polystyrene particles. This dynamic optically induced technique rapidly concentrates, sorts and translates colloidal aggregates with a simple parallel-plate electrode configuration and can be used for a variety of lab-on-a-chip applications.

Alternating Encapsulation of Water-Soluble Components in a One-Dimensional Structure

A novel multi-compound electrospinning method is described, using high-conductivity aqueous solutions for the inner fluid and low-conductivity polymeric solutions for the outer fluids. The driving fluid among inner fluids at the equivalent conductivity is switched at a certain frequency. The switching of the Taylor cone results in the alternative embedding of inner components. Also, the number of inner capillaries is proportional to the encapsulation components. Therefore, our method might be useful to alternatively encapsulate a variety of water-soluble materials in fibers.

Glucose microbiosensor based on glucose oxidase immobilized by AC-EPD: Characteristics and performance in human serum and in blood of critically ill rabbits

A new glucose microbiosensor based on glucose oxidase immobilized on the transducer surface by alternating current electrophoretic deposition (AC-EPD) is described. Deposition is carried out by applying an asymmetrical alternating current (AC) polarization between two electrodes to an enzyme dissolved in a low conductivity solution. This process, based on enzyme electrophoresis, allows the rapid manufacturing of biosensors with a very good sensitivity. Current responses to glucose up to 430 nA/mM mm2 were obtained using a low-activity enzyme (5.6 U/mg), which is due to the preservation of the enzyme activity. The latter was confirmed by FT-IR. The thick enzymatic layer deposited on the transducer prevents a number of the interferences influencing the biosensor response, thus the need in an inner permselective membrane at formation of the biosensor recognition element is avoided. In order to optimize the linearity of the sensor response and provide a biocompatible interface in contact with biological samples, an outer layer of polyurethane (PU) was applied after the deposition of the enzyme. The resulting glucose sensor with PU outer layer had a sensitivity of at least 13 nA/mM mm2, a linear range up to 60 mM, low interference from endogenous species, short response time (<6 s) and was stable for at least 45 days. Contrary to the GOx electrode where errors in the measurements were observed due to the moderate linearity, the sensor with the PU outer layer is shown to be effective for the determination of glucose concentrations in critically sick rabbits where the concentration of glucose might be more than 30 mM.

Numerical Simulation of Coolant Electrolysis in Composite Plate Polymer Electrolyte Membrane Fuel Cell Stacks

Two- and three- dimensional numerical models of a PEM fuel cell have been developed to investigate the effects of shunt current within the coolant channel. The model accounts for the electrostatic equation solved for a ten cell fuel stack for idealized and realistic coolant channels including varying conductivities of the solution. The electrostatic equation was solved in the coolant channel for the fully insulated, non- insulated, and incremental lengths of insulation on the channels. The model results found that the length of the insulation on the coolant channel plays an important role in shunt current reduction and fuel cell performance. The case of insulating the coolant channels was found to show that current density propagated further into the channels as increased lengths of insulation were used. However, the strength of the current density from the ending point of the insulation was found to decrease with increasing insulation lengths. Furthermore, a simple experiment was conducted idealizing the fuel cell as batteries connected in series and then submerged in a low conductivity solution. A camera was used to capture the current potential in the different stacks and to validate the numerical model.

Bio-refractory organics degradation over semiconductor foam under a superimposed electric field

The production of oxidizing agents (mainly OH radicals) by heterogeneous catalytic reaction between a semiconductor and water under a superimposed electric field was assessed in a three-compartment electrochemical cell for the abatement of non-biodegradable toxic organic compounds. The performance of a batch-operated reactor was comparable with other advanced oxidation reactors, from both the abatement kinetics and energy consumption points of view, with no formation of chlorinated intermediates in the presence of chloride ions. The rate of radicals generation was found to depend on several factors: imposed current, electric field intensity and electrical resistivity of the phases involved. The innovative electro-oxidative treatment proposed seems to be particularly suitable for low conductivity solutions with low concentration of pollutants.

Separation of common organic and inorganic anions in atmospheric aerosols using a piperazine buffer and capillary electrophoresis

The ability to monitor and quantify anionic components of aerosols is important for developing a better fundamental understanding of temporal and spatial variations in aerosol composition. Of the many methods that can be used to detect anions, capillary electrophoresis is among the most attractive ones because of its high separation efficiency, high resolving power for ionic compounds, and ability to be miniaturized for in-field monitoring. Here we present a method to baseline resolve common aerosol components nitrate, sulfate, chloride, and over two dozen organic acids in a single separation. A capillary electrophoresis separation utilizing a pH 5.78 piperazine buffer with 1,5-naphthalenedisulfonic acid as a probe for indirect UV absorbance detection was developed for this analysis. Previously, two different buffers were required to adequately separate all of these compounds. Electrophoretic mobilities, limits of detection, and migration time reproducibilities were measured for 38 organic and 8 inorganic anions. For solutions of low conductivity, detection limits for electrokinetic injections were found to be up to two orders of magnitude lower (0.2–0.4 μM) than those for pressure injection (1–45 μM). This separation was optimized and used for routine analysis of aqueous extracts of ambient atmospheric aerosols, but may be extended to other samples containing similar mixtures of anions.

Changes of the solution pH due to exposure by high-voltage electric pulses

The change of the pH of a NaCl solution (139–149 mM NaCl) buffered with 5–15 mM sodium phosphates (pH 7.4) during electromanipulation was studied. It has been determined that an increase in the pH value of electroporation solution of a whole chamber volume, caused by the application of electric field pulses, commonly used in cell electromanipulation procedures, can exceed 1–2 pH units. Several materials for the cathode were tested. In all cases a stainless steel anode was utilized. The aluminum cathode gave a two-fold greater ΔpH in comparison with platinum, copper or stainless steel cathodes. In addition, a substantial release of aluminum (up to 1 mg/l) from the cathode was observed. It has also been found that the shift in pH depended on the medium conductivity: ΔpH of the solution, in which sucrose was substituted for NaCl, was about 5 times less. On the basis of the results obtained here, to avoid the plausible undesirable consequences of the cathodic electrolysis processes, in particular under the conditions of strong electric treatment, it could be recommended that chambers with aluminum electrodes not be utilized and one should use strongly buffered solutions of low conductivity and alternating current (sine or square wave) bipolar electric pulses.

Reductive Dehalogenation of Aqueous-Phase Chlorinated Hydrocarbons in an Electrochemical Reactor

This work describes the reductive dehalogenation of carbon tetrachloride (CT) in a novel liquid-phase electrochemical reactor. The reactor consists of a cylindrical porous copper cathode with a concentric carbon-cloth anode wrapped around the cathode. The results show that CT destruction can be achieved, even in low conductivity solutions (i.e., deionized water), reaching 80% conversion of CT with a residence time of 10 min when a cathode potential of −0.4 V (versus a standard hydrogen electrode) is used. A mathematical model was formulated to simulate reactor performance. The model accounted for CT reductive dechlorination, hydrogen evolution on the cathode surface, and CT mass-transfer limitations. The equilibrium potential for CT reduction on the cathode surface was the only adjustable parameter. The model adequately represented experimental data under high-conductivity (2.2 S/m) and low-conductivity (0.05 S/m) conditions. The model results and experimental observations suggest that the entire cathode ...

Negative ion mode evolution of potential buildup and mapping of potential gradients within the electrospray emitter

Differential electrospray emitter potential (DEEP) maps, displaying variations in potential in the electrospray (ES) capillary and in the Taylor cone, have been generated in the negative ion mode of ES operation. In all examples, measured potential was found to be the highest at the points furthest into the Taylor cone, and values descended to zero at distances beyond approximately 15 mm within the ES capillary. In agreement with results obtained previously in the positive ion mode, negative mode data show a strong influence of electrolyte concentration on measured potentials. Weakly conductive solutions exhibited the highest values, and the steepest gradients, at points furthest into the Taylor cone. However, these same low conductivity solutions did not yield nonzero measured potentials to as deep a distance into the ES capillary as was possible from their higher conductivity counterparts. Addition of a readily reducible compound lowered measured potentials at all points near the ES capillary exit, in accordance with the description of the ES device as a controlled-current electrolytic cell. The development of potential inside the ES capillary upon the onset of ES was also studied, and initial results are presented. Potential waves are observed that can require 15 min or longer, to stabilize. The slow drift to steady potentials is evidence of upstream movement of electrochemically-produced species and follow-up reaction products; low conductivity solutions require longer intervals to reach a steady state. Potentials measured along the central ES axis reflect those at the ES capillary surface, although equipotential lines can be considered to be more compressed at the latter surface.

Low-Voltage Conductivity Device: Editor's Note about Using Conductivity Devices in Nonaqueous Solutions

A conductivity demonstration device is described which operates on 12V and yet will illuminate a bulb brightly enough for use in a lecture hall, even when used with solutions of low conductivity.

Computational Electrochemistry: The Simulation of Voltammetry in Microchannels with Low Conductivity Solutions

A numerical approach to determining the secondary current−density distribution in a new class of microelectrochemical sensors with critical dimensions in the range 20−50 μm high by 100−500μm wide and 1−10 cm long is described. The finite element method is employed to solve the relevant fluid dynamic, mass transport, and potential distribution equations in three dimensions to reveal quantitative information on the current−voltage response. The Tafel slope of the device is calculated for a range of electrolyte conductivities from 0.001 to 0.01 Ω1-cm-1, and the distortion in the current−voltage curves from working with low conductivity solutions is determined. The effect on the current−voltage curve of using a range of reference electrode positions is investigated, and the optimal configuration is discovered. Finally, the current−voltage response of a multi-working electrode device is calculated using a finite element simulation for a cell of dimensions 400 μm high and assumed infinite width, for a range of ...