Ag-AgCl Reference Electrode Research Articles

Design and analysis of a low-cost potentiostat for application with microbial electrochemical sensors

Potentiostats are often limited to relatively costly laboratory-based analysis, which limits their access and application by researchers and practitioners, including for mobile in-field systems. This paper presents an alternative low-cost potentiostat design, sufficiently accurate for application with a microbial electrochemical sensor. The design of the device is described, evaluated using simulation and applied to a microbial electrochemical sensor to measure acetate. Incorporated into the design are a micro-controller, local data storage, and a standard industrial communication protocol to provide flexibility and data management. The device was able to produce a potential at ±2.5 V relative to an Ag-AgCl reference electrode and to measure current at ±2 mA, which was comparable to the performance of laboratory potentiostats on the market. Simulation analysis using fundamental electronic principles, such as Ohm's Law, confirmed that operation of the system was valid and as expected. Further testing using a pseudo-cell showed the device was accurate across a relevant potential range. Results from the initial calibration of the device show less than 1.6 % standard error of the mean. The device was further applied to a microbial electrochemical cell to measure acetate, which confirmed the device was functional for this application and recorded values were consistent with expectations based on the relevant literature.

Development of an enhanced porosity AgAgCl reference electrode with improved stability

We have developed a new low-cost method for fabrication of an enhanced porosity ultra-low noise silver-silver chloride (Ag-AgCl) reference electrode with dramatically high stability, useful in precise electrochemical and biomedical measurements. Low noise level and high stability are the most important features when evaluating reference electrode for example when used as a half-cell at electrochemical cells. The noise was measured for bare Ag-AgCl electrode pairs in a saline electrolyte without any junction. The noise spectral density for such measurements increases at low frequencies due to flicker (1/f) noise. Thermal noise from the real part of the electrode impedance is always lower than 1/f noise and is shown as offset voltage in noise spectrum. Electrode noise is highly correlated to its offset voltage. Moreover, the variance of the potential difference in our measurements is correlated to the noise level of electrodes. In this paper, offset voltage spectrum and the variance of data while doing measurements are proposed for identification of noise in our electrodes. The results presented suggest a new low-cost process for fabrication of enhanced porosity reference electrodes with nanovolt noise level at 1 Hz, low enough for electrochemical and biomedical measurements.

Effect of the Composition of Co-W Oxide Conversion Layer on the Cr-Diffusion Barrier Property for SOFC Interconnect

Introduction. Solid-oxide fuel cells (SOFC) seems to the most reasonable solution for the next power generation because of high efficiency, low CO2 emission, and fuel flexibility. In the operation condition of SOFC, however, Cr in the stainless steel will react with O2 and H2O in the atmosphere to form CrO2(OH)2 or CrO3 gas which cause detrimental effects on cathode materials. Formation of Cr2O3 will also lead to high electrical resistance which decreases the cells efficiency. We found that a CoWO4 layer formed by the oxidation of Co-2.4 at% W electroplated stainless steel immediately in oxidation at 800 ˚C. This layer blocks the outward diffusion of Cr ion for it does not accommodate Cr3+ ion. As a result, no Cr was found at the oxide surface, and the thickness of inner Cr2O3 layer kept thin. However, we utilized only Co-2.4 at% W plating in the previous work, and the effect of plating composition was known. In this study, Co-W electrodeposition to obtain a wide range of W content was examined, then the high-temperature oxidation and area specific resistance (ASR) of the oxidized specimen was tested. Experimental. Type 430 (Fe-17 mass% Cr) and 445 (Fe-23 mass% Cr-1 mass% Mo) stainless steels were used as a substrate. The cathodic current density was 1-5 A dm-2 and plating time was adjusted to obtain a 10 µm of electroplating. Electroplating of Co-W alloy was carried out with sodium tungstate and citric acid solution at pH range of 2-10. Cathodic polarization and constant potential deposition were carried out with a Cu working electrode (cathode), Pt counter electrode (anode) and an Ag-AgCl reference electrode immersed in a saturated KCl solution. A thin Co layer was electroplated to enhance the adherence before Co-W electroplating at constant current density. After the electroplating at constant current densities, the samples were oxidized at 800 ºC in the air with 3 vol% water vapor with the flow rate of 100 cm3 min-1. X-ray diffraction (XRD) was used to identify crystal phase before and after the oxidation test. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) were used to observe and analyze morphology and distribution of the element to the depth direction of the oxide. ASR was measured at 800 ˚C for 100 h in air at the constant current of 0.5 A dm-2. Samples were pressurized at 30 kPa during the measurement. Results. (1) A uniformly distributed Co-W electroplating film with 2.4-30 at% W was deposited on Type 430 and 445 stainless steel. The alloy composition was determined only by the solution pH. Cathodic current density did not affect to the composition. (2) Co-W alloy plating with less than 5 at% W is enough to obtain Cr barrier layer. For the specimen with W content higher than 5 at% formed an W enriched layer within the substrate layer. Whether the formation of this layer is detrimental to oxidation and mechanical properties is not known, the thickness and the compactness of formed CoWO4 layer after oxidation are similar for the electroplating containing >5 at% W. (3) Both the increasing W content in deposits and decreasing Fe fraction in the substrate will lead to the reduction of Fe outward diffusion. Also, the increasing in W content lead to higher ASR.

Fluctuation Analysis of Redox Potential to Distinguish Microbial Fe(II) Oxidation.

We developed a novel method for distinguishing abiotic and biological iron oxidation in liquid media using oxidation-reduction (redox) potential time series data. The instrument and processing algorithm were tested by immersing the tip of a Pt electrode with an Ag-AgCl reference electrode into an active iron-oxidizing biofilm in a groundwater discharge zone, as well as in two abiotic systems: a killed sample and a chemical control from the same site. We used detrended fluctuation analysis to characterize average root mean square fluctuation behavior, which was distinct in the live system. The calculated α value scaling exponents determined by detrended fluctuation analysis were significantly different at p < 0.001. This indicates that time series of electrode response data may be used to distinguish live and abiotic chemical reaction pathways. Due to the simplicity, portability, and small size, it may be suitable for characterization of extraterrestrial environments where water has been observed, such as Mars and Europa. Key Words: Oxidation-reduction potential-Detrended fluctuation analysis-Iron-oxidizing bacteria. Astrobiology 16, 846-852.

A novel humid electronic nose combined with an electronic tongue for assessing deterioration of wine

We report herein the use of a combined system for the analysis of the spoilage of wine when in contact with air. The system consists of a potentiometric electronic tongue and a humid electronic nose. The potentiometric electronic tongue was built with thick-film serigraphic techniques using commercially available resistances and conductors for hybrid electronic circuits; i.e. Ag, Au, Cu, Ru, AgCl, and C. The humid electronic nose was designed in order to detect vapours that emanate from the wine and are apprehended by a moist environment. The humid nose was constructed using a piece of thin cloth sewn, damped with distilled water, forming five hollows of the right size to introduce the electrodes. In this particular case four electrodes were used for the humid electronic nose: a glass electrode, aluminium (Al), graphite and platinum (Pt) wires and an Ag–AgCl reference electrode. The humid electronic nose together with the potentiometric electronic tongue were used for the evaluation of the evolution in the course of time of wine samples. Additionally to the analysis performed by the tongue and nose, the spoilage of the wines was followed via a simple determination of the titratable (total) acidity.

Effect of the reference solution in the measurement of ion activity coefficients using cells with transference at T = 298.15 K

This work reports individual activity coefficients of ions at T = 298.15 K in aqueous solutions obtained from voltage values of the respective half-cell ion-selective-electrode and a single-junction Ag–AgCl reference electrode, filled with different reference solutions at different concentrations. For potassium and chloride ions in KCl aqueous solutions, reference solutions of KCl, NaCl, or CsCl were used. For sodium and chloride ions in aqueous NaCl solutions, reference solutions of CsCl were used. Experimental runs were performed at molalities (1, 2, and 3) m of the reference solution. The concentration of the sample solution was increased, starting from around 1 · 10 −3 m, up to the molality of the reference solution. The values of activity coefficients are calculated using the Henderson equation to estimate the liquid-junction potential. Results show that the ionic activity coefficients are independent of the nature and concentration of reference solution.

Electrochemical diamond sensors for TNT detection in water

An electrochemically stabilized boron doped diamond electrode prepared by chemical vapour deposition (CVD) is used for electrochemical TNT sensing in aqueous solutions. Square wave voltammograms (SWVs) exhibit three highly resolved peaks at −0.47, −0.62 and −0.76 V vs. Ag–AgCl reference electrode, respectively. The current vs. TNT concentration plot shows a linear relationship with a same slope for the two first TNT peaks at μg L −1 and mg L −1 concentration ranges. Detection and quantification limits of 10 and 25 μg L −1, respectively, were obtained without any preconcentration step. Relative standard deviation (RSD) of less than 1% measured over 10 runs has been found for the −0.47 V peak current showing the very high stability of the electrode without any significant fouling effect. An interference study with nitro aromatic compounds of the same family (nitro toluene and dinitrotoluene) has shown that the −0.47 V reduction peak enables TNT discrimination. Measurement of TNT in a natural medium (sea water without any purification step except filtering) has been also investigated.

Electrochemical Determination of Zn 2+ ion us- ing Diphenylamine/Single Walled Carbon Nanotube/Cetyltrimethylammonium Bromide Modified Glassy Carbon Electrode

Diphenylamine encapsulated inside a composite film of single walled carbon nano tube (SWCNT) and cationic cetyltrimethylammonium bromide (CTAB) on the surface of Glassy Carbon (GC) electrode can be converted into diphenylbenzidine on application of a potential of +0.600 V versus Ag-AgCl reference electrode for 60 seconds. This modified electrode, in tris buffer solution (pH 7.0), shows reversible cyclic voltammogram due to diphenylbenzidine / diphenylbenzidine violet redox couple. This cyclic voltammogram is observed in pure CTAB film but not observed inside the film of pure SWCNT, polyvinyl pyrollidone or TX-100. In the SWCNT + CTAB film the redox potential is ca. +0.430 V. The oxidation and reduction currents of the modified electrode decreases with the increase in the concentration of Zn 2+ ion in the electrolytic solution. The relative decrease is found to be 0.60 to 0.65. The ions such as Na + , K + , Ca 2+ and Mg 2+ , do not affect the redox currents. Diphenylbenzidine / SWCNT / CTAB modified GC electrode can determine Zn 2+ ion concentration in the range 20 × 10 -6 mol L -1 to 200 × 10 -6 mol L -1 while diphenylbenzidine/CTAB modified GC electrode can determine between 0.2 × 10 -6 mol L -1 to 2 × 10 -6 mol L -1 . A mechanism for the electrochemical inactivation of the electrode by Zn 2+ ions has been proposed.

Electrochemistry of (Protoporphyrinato IX) Iron(III) and its Imidazole Complexes in Liposomes of L-D DD DD -Phosphatidylcholine

The electrochemical behaviour of hemin ((3,7,12,17-tetramethyl-8,13-divinylporphyrin- 2,18-dipropanoate)iron(III)) and its complexes with imidazole and 1-methyl imidazole in aqueous liposomes of L-D -phosphatidylcholine (L-D -Lecithin) was studied by cyclic voltammetry (CV) and Square Wave Voltammetry (SWV) techniques. The redox potential versus Ag-AgCl reference electrode was found to be -339 mV, -390 mV and -355 mV for hemin complexes with ligand -OH, imidazole (ImH) and 1-methyl imidazole (1-MelmH), respectively. Compared to aqueous solutions where hemin exists as aggregated species, hemin encapsulated in liposomes shows a large positive shift of the redox potential of ca. 100 mV. The apolar nature of the local heme environment stabilizes the Fe(II) oxidation state relative to the Fe(III) state. Electron transfer at the heme is influenced by the uptake/release of protons with ca. -59 mV shift of potential per unit change of pH (' E/' pH).

High Mucosal Serotonin Availability in Neonatal Guinea Pig Ileum Is Associated With Low Serotonin Transporter Expression

5-Hydroxytryptamine (5-HT) is a neurotransmitter and paracrine signaling molecule in the gut. Paracrine signaling by enterochromaffin cells (EC), which release 5-HT, has not been studied in neonates. Our aim was to compare 5-HT disposition in the intestinal mucosa of neonatal and adult guinea pigs. 5-HT was locally measured in vitro from intestinal segments using a diamond microelectrode and continuous amperometry. The serotonin transporter (SERT) was measured using immunohistochemical and Western blot techniques. 5-HT intestinal content was measured using immunohistochemistry and high-performance liquid chromatography with electrochemical detection. An oxidation current, reflective of local 5-HT release, was recorded with the microelectrode near the mucosal surface, and this current was larger in neonatal than in adult tissues. Mechanically stimulating the mucosa with a fine glass probe evoked an additional current in adult but not neonatal tissues. Oxidation currents were reduced by tetrodotoxin and were blocked in calcium-free solutions. Fluoxetine (1 microM) potentiated oxidation currents in adult but not neonatal tissues. SERT levels were lower in neonatal vs adult tissues. There was no difference in 5-HT content between neonates and adults but 5-hydroxyindole acetic acid/5-HT ratios were higher in adults. EC cell counts showed no difference in cell number, but EC cells were found in the crypts in neonatal and along the villi in adult tissues. SERT expression is low in neonates, and this is associated with high levels of free mucosal 5-HT and reduced metabolism. Postnatal maturation of 5-HT signaling may be important for development of neurohumoral control of intestinal motor reflexes.

Measurement of the Interaction between Protein and Anesthetics Using Ion Selective Electrode

Anesthetics have in common the capacity to partition indiscriminately into all biological membranes and act as general perturbants of the membrane structure and function. However, anesthetic mechanisms have not yet been clarified. Some measurement techniques are used to investigate the interaction between anesthetic and biological materials. Thus, in this research we examined the utility of the electrode selectively sensitive to local anesthetics as a measurement technique of the interaction of the bovine serum albumin (BSA) with local anesthetics. The electromotive forces (EMF) of the local anesthetic dibucaine and tetracaine cation-selective electrodes were measured relative to an Ag-AgCl reference electrode in both the absence and presence of BSA at pH 5.5. In the absence of BSA, both the dibucaine and tetracaine electrodes showed a linear response with a Nernstian slope over the concentration of 10−5 mol dm−3. Most of local aneshetic molecules must exist as the protonated cation at pH 5.5, judging from the pKa values. In the presence of BSA, the response curve was shifted toward the higher concentration of anesthetics, which means a decrease of the concentration of free anesthetic cation due to binding to BSA. The binding isotherms of anesthetics to BSA were calculated from a pair of response curves. The slope of the binding isotherm increased steeply above a certain concentration of local anesthetics, which means that the binding form of local anesthetic into BSA is multiple-step.

Microfabricated needle-type sensors for pO/sub 2/, pCO/sub 2/, and pH

In order to conduct in vivo continuous monitoring of blood gas levels, very thin disposable pO/sub 2/, pCO/sub 2/, and pH sensors, which could be incorporated into a syringe needle or a catheter, were microfabricated. Electrode patterns were formed on a polyimide substrate and stacked with intervening polyimide insulating layers. An electrolyte layer was formed, which was covered with a silicone rubber gas-permeable membrane or a passivating layer. The most critical part of the sensor was a Ag-AgCl reference electrode with a pinhole structure. Because of the structure and novel operational mode of the reference electrode, its potential stabilized immediately, and reproducible responses were realized for the respective sensors. Performance characterization revealed clear responses and linear calibration plots in the ranges of partial pressures or pH of physiological levels. No significant damages or changes in the performance of the sensors were observed after ethylene oxide sterilization.

Determination of palladium in deoxo catalysts by differential pulse voltammetry

A differential pulse voltammetric (DPV) method is proposed for the determination of palladium in deoxo catalysts, which are used in recombination units of the cover gas system of nuclear reactors. The electrode system consists of hanging mercury drop electrode (HMDE)/ platinum auxillary/ Ag-AgCl reference electrode. The supporting electrolyte consists of 0.02 M diammonium tartrate + 0.08 M ammonia-ammonium chloride buffer at pH 9.0. Fe, Cu, Pb, Ni, Co and Zn did not interfere. The interference by Cd could be eliminated using 0.005 M EDTA. The relative standard deviation of the proposed method was about 4.0% at ∼ 0.5% palladium content in the catalyst. The method is simple, rapid and free from any possible interferences.

Nitric oxide release in penile corpora cavernosa in a rat model of erection.

1. Nitric oxide (NO) levels were measured in the corpus cavernosum of urethane-anaesthetized rats by using differential normal pulse voltammetry with carbon fibre microelectrodes coated with a polymeric porphyrin and a cation exchanger (Nafion). A NO oxidation peak could be recorded at 650 mV vs. a Ag-AgCl reference electrode every 100 s. 2. This NO signal was greatly decreased by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME), given by local and systemic routes, and enhanced by the NO precursor L-arginine. Treatment with L-arginine reversed the effect of L-NAME on the NO peak. 3. Both the NO signal and the intracavernosal pressure (ICP) were increased by electrical stimulation of cavernosal nerves (ESCN). However, the rise in the NO levels long outlived the rapid return to baseline of the ICP values at the end of nerve stimulation. 4. The ICP and the NO responses to ESCN were suppressed by local and systemic injections of L-NAME. Subsequent treatment with L-arginine of L-NAME-treated animals restored the NO signal to basal levels and the NO response to ESCN. The ICP response to ESCN was restored only in part by L-arginine. 5. The observed temporal dissociation between the NO and ICP responses could be accounted for by several factors, including the buffering of NO by the blood filling the cavernosal spaces during erection. 6. These findings indicate that an increased production of NO in the corpora cavernosa is necessary but not sufficient for maintaining penile erection and suggest a complex modulation of the NO-cGMP-cavernosal smooth muscle relaxation cascade.

Direct potentiometry and potentiotitrimetry of warfarin and ibuprofen in pharmaceutical preparations using PVC ferroin-based membrane sensors

Sensitive and fast-responding potentiometric sensors are described for the determination of warfarin and ibuprofen. They consist of PVC matrix membranes containing the drag-ferroin ion-association complexes as electroactive materials and dioctylphthalate as a solvent mediator. Linear dynamic response range between 1 × 10−2 and 2 × 10−5M with Nernstian slopes of 59–60 mV/decade concentration and a detection limit of 0.8–1.3 μg/ml are obtained. A wide range of organic anions and drag excipients do not interfere. Titration of the drugs with a standard ferroin solution using either a drag-ferroin or ferroin-TPB PVC sensor in conjunction with an Ag-AgCl reference electrode displaysS-shaped titration curves with sharp potential breaks at stoichiometric 1∶2 (ferroin:drug) reaction. Differential titration curves with well-defined peaks at the equivalence points are obtained using drug-ferroin/ferroin-TPB PVC membrane sensors. Direct potentiometry and potentiotitrimetry of warfarin and ibuprofen in various pharmaceutical preparations are presented and compared. Several advantages over the pharmacopoeial methods and other techniques in current use are offered by the proposed technique.

Study of the electro-adsorptive behaviour of the herbicide nitralin by means of voltammetric techniques

The electroanalytical behaviour of the herbicide Nitralin has been studied by means of different voltammetric techniques. Nitralin is adsorbed on the mercury electrode and gives 2 reduction waves at −0.6 and −0.7 V versus Ag AgCl reference electrode, for pH values > 10. The electrochemical process is irreversible and strongly influenced by pH solution. A systematic study of the several instrumental and accumulation variables affecting the stripping response was carried out using square wave (SWV; Osteryoung's method) and differential pulse voltammetry (DPV) as redissolution techniques. The limits of detection were 4.4 × 10 −1 mol 1 −1 (AdS-SWV) and 3.5 × 10 −11 mol 1 −1 (AdS-DPV), with variation coefficients of 4.17 and 3.90% respectively, at a concentration level of 8 × 10 −9 mol 1 −1 ( n = 10). A method, based on AdS-DPV, for the determination of Nitralin in ground-water is proposed. A detection limit of 8.7 × 10 −11 mol 1 −1 (AdS-DPV) was reached in real samples.

Voltammetric behaviour of the antimycotic terbinafine at the hanging mercury drop electrode

Terbinafine is adsorbed on a hanging mercury drop electrode at pH 6.0 and gives a single wave at −1.47 vs. Ag AgCl reference electrode, due to olefinic double bond reduction. The electrochemical process is irreversible and fundamentally controlled by adsorption. A systematic study of the several instrumental and accumulation variables affecting the adsorptive stripping (AdS) response was carried out using square wave voltammetry (SWV, Osteryoung's method) and differential pulse voltammetry (DPV) as redissolution techniques. The limits of detection were 1.7 × 10 −10 mol l −1 (AdS-SWV) and 6.3 × 10 −7 mol l −1 (AdS-DPV). The coefficients of variation were 2.71% (AdS-SWV) and 2.63% (AdS-DPV) at 4 × 10 −8 mol l −1 ( n = 10). For the determination of terbinafine in formulations and spiked human urine samples, a method based on a preseparation step at a solid phase C-18 cartridge and on the AdS-SWV procedure is proposed.

Mercury nucleation on glassy carbon electrodes

The amount of mercury, deposited on a glassy carbon electrode at various potentials during the first 200 ms of electrolysis, has been determined by constant current coulometric stripping potentiometry. Eight microliter samples, containing approximately 4 M chloride, were placed on top of a combined glassy carbon, Ag counter and Ag AgCl reference electrode positioned up-side-down under a stereo microscope. It was found that, if the electrode was conditioned for 200 ms at −1.80 V. mercury was reversibly deposited during the following electrolysis stripping cycle. If, however, the conditioning cycle was omitted overpotentials of the order of magnitudes 0.50 V were needed in order to initiate mercury nucleation. If the electrode was vigorously anodised at 1.80 V for one minute, 8–10 conditioning pulses were required to restore the electrode sensitivity. Under the microscope it was observed that, provided that the electrode was potentiostatically conditioned, mercury was deposited homogeneously on the glassy carbon surface as shown by a monochromatic sky blue colour of the electrode surface. If not conditioned, the electrode became multi-coloured indicating an inhomogeneous deposition of mercury droplets. Electrode conditioning was found to have a profound effect on the coulometric stripping potentiometry determination of copper(II). On a conditioned electrode, copper(II) could be determined in a five decades concentration range, 1–10000 μM, with a relative accuracy varying only between 96 and 103%. If not conditioned, the accuracy varied between 70–90%. Mechanisms for the deposition and stripping of mercury and copper on glassy carbon electrodes are proposed.

Immobilized enzyme electrode for the determination of l-lactate in food samples

A sensitive immobilized enzyme electrode for l-lactate is described. Toluidine blue O (TBO) and lactate dehydrogenase (LDH) were co-immobilized onto the surface of a graphite electrode by means of a dialysis membrane. l-lactate is oxidized to pyruvate by LDH in the presence of nicotinamide adenine dinucleotide NAD + which is then reduced to NADH. The NADH formed is electrocatalytically oxidized by TBO and the reduced mediator is detected at 0 V vs. a Ag AgCl reference electrode. Different dialysis membranes were tested in order to improve stability. The most stable enzyme electrode was fabricated with a benzoylated membrane, showing a half-life of eight days. This amperometric electrode responds fast and linearly to l-lactate in a concentration range 10 −6 − 6 × 10 −5 M. The limit of detection is 4 × 10 −7M. The enzyme electrode was applied to the determination of l-lactate in food samples. Results were compared to those obtained using a spectrophotometric method, showing a good agreement.

Electrochemical potential variations in a small 40 MW feedwater storage vessel—the influence of weldment surface condition

This paper describes a study performed to establish the real in situ electrochemical potential (ECP) prevalent in 40 MW deaerator feedwater storage vessel weldments. It was demonstrated that (a) the Ag AgCl reference electrode sensibly recorded the ECP over a period of 1000 h before failure, (b) transients (in the positive, more active direction) were the result of abrupt working unit initial load changes, (c) an initial ECP hysteresis lasting 400–500 h was recorded where the values remained much more positive than those normally associated with low dissolved oxygen levels and (d) a strong effect of weldment surface condition was evident inasmuch as the ECP values of the fresh bright weldment were consistently approximately 150 mV more positive than for the normal oxidised weldment.