Theory and practice of electrochemical titrations with dual microband electrodes

Dual parallel microband electrodes, operated as a generator–collector pair and made by a simple, inexpensive mass-production method, have been used to implement a ‘titration’ method. The solution contains the electro-inactive analyte and a reagent from which the titrant can be generated electrochemically. The galvanostatically generated titrant is detected at the collector amperometrically. The collection efficiency is affected by the reaction between the titrant and the analyte. Determination of ascorbic acid by titration against ferricyanide is given as an example. The measurement is performed as follows: After application of the collector potential, the boundary conditions between measurements are renewed by a quick pulsed motion of the electrode assembly. Then the generator current is applied. Following an initial delay, the collector current increases as t^1/2, with slope and collection efficiency dependent on analyte concentration. This results in a fast and effective method for implementing some standard titration methods without the need for accurate volume measurement and reagent preparation. The accuracy is determined by the reproducibility of electrodes. The present work shows concentration measurements in the mM scale to ≈±10%, in a time of a few seconds. The highly stable extrapolation method used for numerical simulation of the generator–collector experiment, which takes into account the non-uniform current distribution over a microband electrode with a galvanostatic boundary condition, is developed using a conformal map. A good agreement between simulations and experimental results was obtained in voltammetric, potential step and generator–collector measurements. It is shown that a useful approximate calculation can be made rather easily by representing the problem in terms of a reaction layer in the conformal space.     

The effect of the binder morphology on the cycling stability of Li–alloy composite electrodes

It is demonstrated that the design of the composite electrode, or more precisely the morphology and distribution of the binder poly(vinylidine fluoride) (PVdF) within the composite electrode, has a significant impact on the cycling performance of Li storage alloy (Sn/SnSb) electrodes. Different binder morphologies and distributions have been obtained by using different solvents for the slurry preparation, such as 1-methyl-2-pyrrolidinone (NMP), in which PVdF is dissolved, yielding electrodes with a homogeneously and finely distributed binder, or decane, in which PVdF is only dispersed, yielding electrodes in which the original particle morphology of the binder powder is preserved. In constant current cycling tests carried out in an excess of electrolyte, the electrodes with the ‘dispersed’ binder show far better cycling capacities and stabilities than those with the ‘dissolved’ binder. This is explained by the different binding strengths, swelling behaviour in the electrolyte, electrode porosities, and possible ‘buffer’ effects of the compact and the finely distributed binders.     

A cell for high-precision constant-current coulometry with external generation of titrant

A cell has been designed for the high-precision coulometric titration, with externally generated titrant, of materials which otherwise undergo undesirable reactions at the working electrodes. With this cell potassium dichromate has been titrated, via its hydrolysis reaction, with hydroxyl ion generated at the cathode, cathodic reduction of the chromium(VI) being circumvented. In this cell 99.9% of the titrant required is generated in one chamber and transferred to another for reaction; the titration is then completed with titrant generated at a second, drip-type electrode working at much lower current. By means of commercially available Leeds and Northrup coulometric titration electrical equipment, titration of NBS 136b Potassium Dichromate gave a purity of 99.976%, standard deviation 0.005%, and of NBS 84d Potassium Acid Phthalate (done as a check) 99.991%, standard deviation 0.005%, both values being in excellent agreement with other work.     

Metallic and non-metallic redox response of conducting polymers

The potentiometric response of electrodes coated with polypyrrole or poly(N-methylpyrrole) films with different doping anions was studied in solutions containing the redox couples: Fe(CN)₆^3−/4−, Ru(NH₃)₆^3+/2+ and Fe(Ill)/Fe(II). The stable potential measured with the electrodes was the potential of the redox couple. The response time was instant for polypyrrole doped with dodecylsulphate ions, PPy(DS) and slow for the polymers doped with mobile anions. On the basis of electrochemical measurements and chemical analysis by EDAX spectroscopy it was found that with the PPy(DS) electrode the potentiometric response was of the ‘metallic’ type, with no change in the oxidation state of the bulk polymer. With the other polymer systems studied reduction or oxidation of the polymer bulk took place when it was in contact with a redox couple in the solution.     

Activation of solid electrodes made of platinum,gold, and a graphite-epoxy resin composite by mechanically cutting their surfaces <Emphasis Type="Italic">In Situ</Emphasis> in the anodic voltammetry of ascorbic acid

The oxidation of ascorbic acid (AA) was studied at the electrodes made of platinum, gold, and a graphite-epoxy resin composite. Cutting the surface electrode layer in situ in solution before each polarization activated the electrode surface. This activation manifested itself in a more negative potential of the AA anodic peak, a change in the current peak shape, and an increase in the current density at the electrodes as compared to the polished electrodes. The in situ renewal of electrodes allowed us to develop a rapid voltammetric procedure for determining AA at an electrode made of a graphite-epoxy resin composite in different samples. The analytical range for AA was 0.1–200 mg/L, the determination limit for AA was 0.05 mg/L, and the RSD was 3–5%.Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 1, 2005, pp. 64–68.Original Russian Text Copyright © 2005 by Tarasova, Kiryushov.     

Chromium(III)-Selective Electrodes Based on Titanium Dichalcogenides Intercalated with Chromium

Solid-contact chromium-selective film electrodes were prepared on the basis of titanium diselenide and titanium ditelluride intercalated with chromium. It was found that the titanium diselenide–based electrode with the lowest concentration of the intercalant (Cr_0.1TiSe₂) exhibits the best properties and provides the determination of 5 × 10^–5–1 × 10⁰M chromium(III) in the pH range 2.75–4.7. The slope of the electrode function for all chromium-selective electrodes depends on the pH of the solution and is close to the theoretical slope for the singly charged Cr(OH)⁺ ₂ ion. It was demonstrated that chromium-selective electrodes can be used as indicator electrodes in determining chromium(III) by direct titration with the potentiometric detection of the titration end-point using EDTA and KMnO₄as titrants.     

PVC膜修饰电极上零电流示波电位滴定法测定硼的研究

以氯化十六烷基吡啶（ＣＰＣ）和氟硼酸钠形成的离子缔合物为活性物质，制成ＰＶＣ膜涂层石墨电极，提出了一个快速测定硼的零电流示波电位滴定法。以ＣＰＣ为滴定剂，用ＰＶＣ膜电极为指示电极，甘汞电极为参比电极，利用阴极射线示波器荧光屏上荧光点的突然位移来指示ＣＰＣ滴定硼的终点，具有灵敏、准确、快速等特点，用该法测定玻璃中的硼，结果良好。     

Sources of instability of ‘wired’ enzyme anodes in serum: urate and transition metal ions

The glucose electrooxidation currents of anodes with films of glucose oxidase and redox polymers electrically connecting (‘wiring’) its redox centers to electrodes decay much more rapidly in serum than in pH 7.2 saline phosphate buffer. Rapid decay is observed also in buffer in the presence of urate and transition metal ions, which are constituents of normal serum. Urate is electrooxidized to dimeric or trimeric products, which precipitate in the electrocatalytic film. Transition metal ions, like Zn²⁺ and Fe²⁺, coordinatively crosslink pyridine or imidazole functions of the redox polymers and also inhibit glucose oxidase.     

A copper ion-selective electrode with high selectivity prepared by sol-gel and coated wire techniques

A sol-gel electrode and a coated wire ion-selective poly(vinyl chloride) membrane, based on thiosemicarbazone as a neutral carrier, were successfully developed for the detection of Cu (II) in aqueous solutions. The sol-gel electrode and coated electrode exhibited linear response with Nernstian slopes of 29.2 and 28.1 mV per decade respectively, within the copper ion concentration ranges 1.0×10^–5–1.0×10^–1 M and 6.0×10^–6–1.0×10^–1 M for coated and sol-gel sensors. The coated and sol-gel electrodes show detection limits of 3.0×10^–6 and 6.0×10^–6 M respectively. The electrodes exhibited good selectivities for a number of alkali, alkaline earth, transition and heavy metal ions. The proposed electrodes have response times ranging from 10–50 s to achieve a 95% steady potential for Cu²⁺ concentration. The electrodes are suitable for use in aqueous solutions over a wide pH range (4–7.5). Applications of these electrodes for the determination of copper in real samples, and as an indicator electrode for potentiometric titration of Cu²⁺ ion using EDTA, are reported. The lifetimes of the electrodes were tested over a period of six months to investigate their stability. No significant change in the performance of the sol-gel electrode was observed over this period, but after two months the coated wire copper-selective electrode exhibited a gradual decrease in the slope. The selectivity of the sol-gel electrode was found to be better than that of the coated wire copper-selective electrode. Based on these results, a novel sol-gel copper-selective electrode is proposed for the determination of copper, and applied to real sample assays.     

Vacancy diffusion in Cu Σ = 9 [110] twist grain boundary

Both the formation energy and the diffusive activation energy of a single vacancy migrating in the first four atomic layers intra- and inter-layer near Cu Σ = 9 [110] twist GB have been investigated by using the MAEAM. The formation of the vacancy is favorable on the first layer (1L) near the GB plane and is spontaneous on sites ‘2’–‘5’ especially ‘2’ and ‘4’. The effects of the GB on the intra- as well as inter-layer migration are mainly for 1L–1L as well as for 1L–1L′, 2L–1L and 3L–1L related to the 1L, respectively. Furthermore, the vacancy in 1L is favorable to migrate in 1L (intra-layer) or through GB plane to 1L′, the one in either 2L or 3L is favorable to migrate to 1L. So the vacancy tends to converge to the first layer near the GB plane.     

A novel amperometric sensor and chromatographic detector for determination of parathion

A novel amperometric sensor and chromatographic detector for determination of parathion has been fabricated from a multi-wall carbon nano-tube (MWCNT)/Nafion film-modified glassy-carbon electrode (GCE). The electrochemical response to parathion at the MWCNT/Nafion film electrode was investigated by cyclic voltammetry and linear sweep voltammetry. The redox current of parathion at the MWCNT/Nafion film electrode was significantly higher than that at the bare GCE, the MWCNT-modified GCE, and the Nafion-modified GCE. The results indicated that the MWCNT/Nafion film had an efficient electrocatalytic effect on the electrochemical response to parathion. The peak current was proportional to the concentration of parathion in the range 5.0×10^–9–2.0×10^–5 mol L^–1. The detection limit was 1.0×10^–9 mol L^–1 (after 120 s accumulation). In high-performance liquid chromatography with electrochemical detection (HPLC–ED) a stable and sensitive current response was obtained for parathion at the MWCNT/Nafion film electrode. The linear range for parathion was over four orders of magnitude and the detection limit was 6.0×10^–9 mol L^–1. Application of the method for determination of parathion in rice was satisfactory.     

Potentiometric titrations using dual microband electrodes.

We present a novel method of analysis using potentiometric end-point detection and dual microband electrodes in generator-collector mode. The titrant is electrogenerated using either a constant current or a current that increases linearly with time, and the fluxes of reactive material rather than molar amounts are balanced. The advantage of the ramp current system over the constant current method is that all of the information needed for a full titration curve and a proper analysis can be obtained during a single scan. The method was applied to the determination of vitamin C with ferricyanide and to the determination of thiosulfate and sulfite with iodine using gold microband electrodes. As a new type of analysis, a potentiometric titration that uses dissolving silver microband electrodes in order to generate the titrant is demonstrated. The system was applied to the detection of chloride, iodide, thiosulfate and cyanide. The accuracy of the analysis is +/-10%, limited mainly by the present screen-printing process, and the method is well adapted for measurements on the millimolar scale.     

Bestimmung von Palladium in Katalysatoren durch potentiometrische Titration mit ionensensitiver Endpunktsanzeige

Zusammenfassung Zur Bestimmung von Milligramm-Mengen Palladium in Katalysatormassen wird die Fällungstitration mit Jodid herangezogen. Zur Indikation des Titrationsendpunktes wird eine Meßkette aus einer Silberjodid/Sulfidelektrode und einer Referenzelektrode mit Zwischenelektrolyten verwendet. Die potentio-metrische Indikation vereinfacht das Verfahren bezüglich der üblichen Trübungsmessung und bringt gegenüber der Redoxindikation eine weitgehende Unabhängigkeit von der Säurekonzentration. Palladiummengen von 1–20 mg können so mit einer relativen Standardabweichung von 0,4% bestimmt werden.

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Determination of palladium in catalysts by means of potentiometric titration with ion-sensitive endpoint indication

The determination of milligram amounts of palladium in catalysts is performed by iodide titration. The endpoint of the titration is indicated by a silver iodide — silver sulphide electrode together with a double junction reference electrode. Potentiometric indication facilitates the procedure considerably in comparison to the turbidimetric endpoint method. The indication is independent of the acid concentration of the solution, which gives also a major advantage over the redox indication technique. Palladium amounts of 1–20 mg can be determined with a relative standard deviation of 0.4 %.

     

Titrations using an apparatus for recording the antilogarithm of pH or pM

A titration apparatus plotting either the concentration of sample ion or the concentration of titrant ion has been tested. An antilog apparatus, converting measured e.m.f. values into concentrations is connected to ordinary titration equipment. The instrument has been tested by means of acid—base titrations (titrations of mixtures of weak acids and of a weak and a strong acid), precipitation titrations (determination of the chloride concentration in tap-water, titration of mixtures of halides), titrations with ion-selective electrodes (determination of the fluoride content of toothpaste) and complexometric titrations (determination of copper with EDTA, using mercuric ion as indicator ion and amalgamated silver rod as indicator electrode, or using a copper-selective indicator electrode). The method considerably simplifies the evaluation of the results as compared to conventional potentiometric titrations.     

Electrochemical titrations of thiosulfate, sulfite, dichromate and permanganate using dual microband electrodes

Applications of titration using dual gold microband electrodes in generator-collector mode are presented. The main advantage is that the method dispenses with the need for accurate volume measurements and reagent preparation: rather than balancing of molar amounts, the fluxes of analyte and titrant are balanced instead. The titrant is generated electrochemically and the end-point is detected amperometrically by the appearance of a current due to the presence of the titrant at the other band. The aim is to show the versatility and capability of the method that employs disposable mass-produced electrodes and uses pulsed motion of the electrode before a measurement to renew the boundary conditions. The titration of ascorbic acid (vitamin C) with ferricyanide, which has been a model system, was extended to the determination of thiosulfate and sulfite with iodine and the determination of dichromate and permanganate with iron(II). The accuracy, limited by the present fabrication reproducibility of the disposable electrodes, is +/- 10%. The results demonstrate that the method is effective even with a very simple set-up, and the actual time needed for common titrations is significantly decreased. These examples demonstrate a comprehensive basis for further development and applications, including standard titrations in industry.     

A periodic thermometric-titration calorimeter for researching reactions in solution

A titration calorimeter has been built with automatic recording and digital solution volume display; it combines high accuracy in measuring reaction heats with accurate titrant dispensing. The design allows one to replace the titrant during the experiment. The methods reduce the working time considerably by comparison with the usual calorimetric method.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 507–511, July–August, 1988.     

Polyelectrolyte-mediated non-micellar synthesis of monodispersed ‘aggregates’ of gold nanoparticles using a microwave approach

‘Aggregates’ of monodispersed gold nanoparticles were synthesized for the first time in large quantities. Those particles were made in presence or in absence of gold seed in a polyelectrolyte solution using microwave heating for about 30–60 s. The average diameters of the particles calculated from TEM and SEM analysis were 22 ± 2 nm. Our analysis found that the polyelectrolyte acted as a reducing as well as a stabilizing agent. It controlled the growth of particles by aligning them on the polymeric chain and the nanoparticles were subsequently self-assembled to form an ‘aggregate’ structure. The synthetic procedure is very fast and the particles are stable for at least 6 months under ambient conditions.     

Hemoglobin adsorption into TiO2 phytate multi-layer films: particle size and conductivity effects

Hemoglobin (molecular weight 64.5 kDa, isoelectric point 7.4) in 0.1 M phosphate buffer solution at pH 5.5 readily adsorbs onto mesoporous TiO₂ phytate films, which have been formed in a layer-by-layer deposition process from TiO₂ nanoparticles (ca. 6–10 nm diameter) and phytic acid at tin-doped indium oxide (ITO) electrodes. Quartz crystal microbalance data, voltammetry, and SEM evidence are consistent with hemoglobin adsorption only into the outer TiO₂ phytate surface layer. The size of the tetrametric hemoglobin protein (ca. 6 nm diameter) appears to be too big for a homogeneous film to form.The modified ITO electrode immersed in 0.1 M phosphate buffer solution at pH 5.5 allows reversible electron transfer for hemoglobin to be observed with a midpoint potential of 0.01 vs. SCE. Characteristic TiO₂ phytate film thickness and pH effects are observed with both thicker films and lower proton activity causing ‘decoupling’ of the protein redox chemistry due to a reduced electrical conductivity of the TiO₂ phytate film connecting hemoglobin with the electrode. This is the first example of a bi-layer nanofilm structure where the underlying TiO₂ phytate film controls the electrochemical properties of the hemoglobin modified top-layer.     

Potentiostatic voltammetry at spherical electrodes and microelectrodes in the presence of product

An explicit analytical solution for the complete anodic–cathodic I–E–t response of a reversible electrode process in presence/absence of amalgamation under potentiostatic conditions is presented. To obtain this solution we have taken into account the electrode curvature, using in the case of amalgamation, Koutecký’s approximation (i.e. the finite electrode volume has not been considered). Explicit expressions for the concentration profiles and the surface concentrations have been also deduced. All the results are applicable to electrodes of any radius including ultramicroelectrodes when both species are soluble in the electrolytic solution for any value of the ratio D_O/D_R. When amalgamation takes place, analytical results were compared with the numerical ones deduced by using the rigorous condition of null net flux in the electrode centre, pointing out that Koutecký’s approximation remains valid even for electrode radius and time values at which the diffusion layer reaches the electrode centre. For high electrode sphericities the I–t curves present a cross-linking for applied potential values higher than the equilibrium potential which is the cause of the appearance of a “peak” near to the anodic limit of the I–E curve.     

Differential electrolytic potentiometric titration method for the determination of ciprofloxacin in drug formulations

A simple and rapid differential electrolytic potentiometric titration method for the determination of ciprofloxacin was developed. The work is based on the fast complexation reaction between iron(III) and ciprofloxacin in a ratio of 1:3, respectively, in sulfuric acid media of 0.09 mol dm⁻³. Among the electrodes tested silver amalgam electrodes were found to be a suitable indicating system. By applying a current density of 0.5 μA cm⁻² to these electrodes and using iron(III) solution of 0.097 mol dm⁻³ as a titrant, normal titration curves were obtained. The method was successfully applied for the determination of ciprofloxacin in drug formulations as low as 4.0 ppm.