Determination of Ionol by Voltammetry and Coulometric Titration

Procedures were developed for determining ionol by voltammetry and by coulometric titration with electrogenerated chlorine using the amperometric indication of the titration end point. Possible mechanisms of ionol oxidation with electrogenerated chlorine and its electrochemical oxidation at a glassy carbon and a gold electrode were discussed. Procedures were developed for determining ionol in mineral oil in analytical ranges from 1.0 × 10^–4 to 1.0 × 10^–2 M (RSD = 9%) and from 3.0 × 10^–5 to 4.0 × 10^–3 M (RSD = 9%) using a glassy carbon and a gold electrode, respectively. The detection limits for ionol at the glassy carbon and gold electrode were 2.8 × 10^–4 and 1.0 × 10^–5 M, respectively. The detection limit in coulometric titration was 20 g/mL.     

Electrochemical Determination of Lipoic Acid

Procedures for determining lipoic acid by voltammetry and coulometric titration with electrogenerated halogens using the biamperometric indication of the titration end-point were developed. Possible mechanisms of lipoic acid oxidation with electrogenerated halogens are discussed. Microgram amounts of lipoic acid were determined in model solutions with an RSD of 1–2%. The analytical range of lipoic acid found by voltammetry at a glassy-carbon electrode lies between 1.15 × 10^–5 and 1.73 × 10^–4 M. The detection limit for lipoic acid is 5.75 × 10^–6 M.     

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.     

Schnelle coulometrische Mikrobestimmung einzelner Polythionate

Zusammenfassung Das neue Mikrobestimmungsverfahren beruht auf der coulometrischen Titration von S₂O₃ ^2– nach vorheriger Abbaureaktion der Polythionate mit Sulfit bzw. Cyanid. Es werden 10 ml Probelösung (S₄O₆ ^2–: 5 · 10^–5 bis 1 · 10^–3 M; S₅O₆ ^2–; 2,5 · 10^–5 bis 1· 10^–3 M; S₆O₆ ^2–: 1,66 · 10^–5 bis 1 · 10^–3 M) benötigt. Die Titrationskurve wird von einem Schreiber registriert. Die Reproduzierbarkeit der jeweiligen Einzelbestimmung liegt bei VK _p±0,1 bis ± 1,6%.

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Rapid coulometric microdetermination of individual polythionates

The method described is based on the coulometric titration of S₂O₃ ^2– after a preceding degradation of the polythionates with sulphite or cyanide. 10 ml of sample solution are required (S₄O₆ ^2–: 5× 10^–5 to 1×10^–3 M; S₅O₆ ^2– : 2.5×10^–5 to 1×10^–3 M; S₆O₆ ^2–: 1.66×10^–5 to 1×10^–3 M). The titration curve is recorded. The reproducibility of a single determination is VK _p±0.1 to ± 1.6%.

     

Voltammetric Determination of Rubeanic Acid

Procedures for the voltammetric determination of rubeanic acid (RA) at a mercury-film electrode were proposed. They are based on the oxidation of RA at –0.70 to –0.80 V or on the reduction of mercury sulfide, the product of RA oxidation, at –0.82 to –0.85 V (versus a saturated silver–silver chloride electrode) in a 1 M NaOH solution. The oxidation and reduction currents are linear functions of RA concentration in the ranges from 9 × 10^–6 to 3 × 10^–4 M and from 5 × 10^–6 to 3 × 10^–4 M, respectively.     

Voltammetric Determination of Sulfide Ions

Procedures were proposed for the voltammetric determination of S^2– at a mercury-film electrode. They are based on the oxidation of S^2– at –0.79 to –0.80 V and on the reduction of HgS, the product of the oxidation of S^2–, at –0.76 to –0.96 V in a 1 M NaOH solution. The anodic and cathodic currents are linear functions of S^2– concentration in the ranges from 1 × 10^–5 to 1 × 10^–4 M and from 2 × 10^–6 to 1 × 10^–4 M, respectively.     

Polarographic determination of uranium(VI) after salting-out extraction as 8-quinolinolate in acetonitrile

Summary Uranium(VI) can be extracted as its 8-quinolinolate into acetonitrile by means of salting-out with ammonium and sodium acetates, respectively; the metal oxinates extracted give a well-defined dc polarogram with E _1/2=–0.80V and a sharp square wave (sw) polarogram with E _p=–0.96V in the extract. The dc wave height and the sw peak current are directly proportional to the uranium(VI) concentration in the range of 6.0×10^–6 to 4.0×10^–4M at pH 6.7–10.0 and 8.0×10^–7 to 2.8×10^–5M at pH 10.5–11.0, respectively. A number of ions do not interfere in the presence of EDTA.

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Polarographische Bestimmung von Uran(VI) nach Aussalz-Extraktion als 8-Hydroxychinolat mit Acetonitril

Zusammenfassung Uran(VI) kann durch Aussalzen mit Ammonium- bzw. Natriumacetat als Oxinat mit Acetonitril extrahiert werden. Das extrahierte Oxinat ergibt ein gut ausgebildetes Gleichstrompolarogramm mit E _1/2=–0,80 V bzw. ein scharfes square-wave-Polarogramm mit E _p=–0,96 V. Die Gleichstrom-Stufenhöhe bzw. der square-wave-Peakstrom sind der U(VI)-Konzentration im Bereich 6,0·10^–6-4,0· 10^–4M (pH 6,7–10,0) bzw. 8,0·10^–7-2,8·10 ^–5M (pH 10,5–11,0) direkt proportional. Durch Zusatz von EDTA kann eine Reihe von Störungen ausgeschaltet werden.

     

Cobalt-Selective Electrode Based on Titanium Ditelluride Intercalated with Cobalt

A cobalt-selective solid-contact electrode was prepared on the basis of titanium ditelluride. The electrode exhibits the slope of the electrode function –(29 ± 1) mV/pC and provides the determination of 1 × 10^–5–1 × 10^–1 M cobalt(II) in the pH range 4.5–6.5. The electrode was used for the potentiometric indication of the titration end point in the determination of cobalt(II) in lanthanum–strontium cobaltite La_0.5Sr_0.5CoO₃. The intercalant concentration range providing optimal ion-selective properties was determined.     

Amperometric Sensor for Hydrazine Determination Based on Mechanically Immobilized Nickel Hexacyanoferrate Modified Electrode

An amperometric sensor constructed by mechanical immobilization of nickel hexacyanoferrate onto the paraffin impregnated graphite electrode is reported. The modified electrode exhibits a reversible redox peak, which corresponds to surface-confined Fe(CN)^4- ₆/Fe(CN)^3- ₆reaction. Electrocatalytic oxidation of hydrazine is effective on the modified electrode at a significantly reduced overpotential and in a broader pH range. Linear response for hydrazine is in the range 4.0 × 10^–4to 4.0 × 10^–3M with a correlation coefficient of 0.9963. The detection limit is 9.6 × 10^–5M (S/N = 3). The stability and reproducibility of the modified electrode for the determination of hydrazine is evaluated. The proposed method has been applied for the determination of hydrazine in drinking water.     

Electrochemical Study of Iodide in the Presence of Barbituric Acid: Application to the Catalytic Determination of Barbituric Acid

Electrochemical oxidation of iodide has been studied in the presence of barbituric acid using cyclic voltammetry and controlled-potential coulometry. The results indicate that the barbituric acid participating in the halogenation reaction converts to an iodo derivative of the parent compound. Moreover, the results are indicative of the suitability of iodide as a mediator for the determination of barbituric acid in aqueous solutions. The quasicatalytic peak current is linearly dependent on the barbituric acid concentration. The calibration graph is parabolic, with two linear sections of 6.0 × 10^–5–1.0 × 10^–3and 1.0 × 10^–3–1.0 × 10^–2M. The relative standard deviation for ten determinations of 2.0 × 10^–4M barbituric acid is 2.1%, and the detection limit of the method is 3.97 × 10^–5M.     

Flow injection analysis of gallic acid with inhibited electrochemiluminescence detection

A flow injection (FI)–electrochemiluminescent (ECL) method has been developed for the determination of gallic acid, based on an inhibition effect on the Ru(bpy)₃²⁺/tri-n-propylamine (TPrA) ECL system in pH 8.0 phosphate buffer solution. The method is simple and convenient with a determination limit of 9.0×10^–9 mol/L and a dynamic concentration range of 2×10^–8–2×10^–5 mol/L. The relative standard deviation (RSD) was 1.0% for 1.0×10^–6 mol/L gallic acid (n=11). It was successfully applied to the determination of gallic acid in Chinese proprietary medicine—Jianming Yanhou Pian. The inhibition mechanism proposed for the quenching effect of the gallic acid on the Ru(bpy)₃²⁺/TPrA ECL system was the interaction of electrogenerated Ru(bpy)₃^2+* and o-benzoquinone derivative at the electrode surface. The ECL emission spectra and UV-visible absorption spectra were applied to confirm the mechanism.     

Rapid determination of telmisartan in pharmaceuticals and serum by the parallel catalytic hydrogen wave method

The polarographic characteristics of telmisartan have been investigated in 0.8 mol L^–1 NH₃.H₂O–NH₄Cl (pH 8.9)–0.01 mol L^–1 H₂O₂ as supporting electrolyte. The results demonstrate that the polarographic reduction wave at ca. –1.30 V in the absence of H₂O₂ is a catalytic hydrogen wave, and the reduction wave enhanced by H₂O₂ is a so-called parallel catalytic hydrogen wave. The analytical sensitivity of the parallel catalytic hydrogen wave is ca. 60 times higher than that of the corresponding catalytic hydrogen wave. Based on the parallel catalytic hydrogen wave a novel method has been developed for determination of telmisartan by linear sweep polarography. The calibration curve is linear in the range 2.0×10^–8–2.0×10^–6 mol L^–1 and the detection limit is 1.0×10^–8 mol L^–1. The precision is excellent with relative standard deviations of 2.6% at a concentration of 1.0×10^–7 mol L^–1 telmisartan. The proposed method has been applied to the direct determination of the telmisartan in capsule forms and biological samples. The proposed method has been proved to be advantageous over existing CZE and MEKC methods in simplicity, rapidity, and reproducibility.     

Determination of Stability Constants of Copper(I) Chelates with 1,10-Phenanthroline by Thermal Lensing

Using investigations of the copper(I)–1,10-phenanthroline system as an example, it is shown that thermal lensing can be used for determining stability constants at a level of concentrations one–two orders of magnitude lower compared to conventional spectrophotometry, with better precision of measurements. The values of stability constants are log₂= 11.7 ± 0.7 without regard for stepwise chelation, and logK ₁= 5.9 ± 0.3, logK ₂= 5.4 ± 0.3, and log₂= 11.3 ± 0.6 taking into account stepwise chelation. It is shown that, when shifting from microgram to nanogram amounts of reactants in the determination of stability constants by thermal lensing, changes in the kinetic parameters of the reaction studied should be taken into account. The thermal-lens limit of detection of copper(I) is 2 × 10^–8M; the linear calibration range is 4 × 10^–8–2 × 10^–5M (488.0 nm, pump power 120 mW). The data obtained were used for determining copper(I) in the hydrogen sulfide layer of the Baltic Sea.     

Determination of Potassium and Sodium in Nonaqueous Solvents by Stripping Voltammetry in a Radio-Frequency Electromagnetic Field

The effect of a radio-frequency electromagnetic field on the parameters of the analytical signal of potassium and sodium in stripping voltammetry in nonaqueous solvents was studied. Sodium and potassium can be separately determined in a concentration range of 2 × 10^–5–2 × 10^–7 M (RSD = 10–17%) upon the irradiation of the solution with an electromagnetic field of a frequency of 55 MHz. The irradiation efficiency depends on the frequency and the nature of the solvent. The detection limits for potassium and sodium were 8.0 × 10^–8 and 1.0 × 10^–7 M, respectively, in the presence of 0.02 M [(C₄H₉)₄N]I in dimethylformamide at a field frequency of 55 MHz.     

Coulometric determination of organic substances using electrogenerated dipositive silver

Summary Conditions are described for the efficient generation of silver(II) and the titration of organic compounds by electrogenerated silver(II). An electrolyte of 0.1 M silver nitrate and 5 M nitric acid at –10°C is used, with a gold generating anode and a current density of 1.5–5 mA/cm². A potentiometric (platinum-S.C.E. pair) or biamperometric (platinum electrodes, 200 mV) end point detection is used. For potentiometric detection, the platinum electrode is preconditioned with AgO in nitric or by anodization. The compounds isoniazid, oxalyldihydrazide, hydroquinone, and resorcinol were titrated down to sub-milligram quantities with errors of 1–2%.

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Coulometrische Bestimmung von organischen Substanzen mit Hilfe von elektrolytisch erzeugtem zweiwertigem Silber

Zusammenfassung Der verwendete Elektrolyt besteht aus 0,1 M Silbernitrat in 5 M Salpetersäure bei –10°C. Die Generatoranode besteht aus Gold (Stromdichte 1,5–5 mA/cm²). Die Bestimmung des Endpunkts erfolgt potentiometrisch (Pt/SKE) oder biamperometrisch (Pt-Elektroden, 200 mV). Für die potentiometrische Bestimmung wird die Indicatorelektrode mit einer Lösung von AgO in Salpetersäure oder durch elektrochemische Anodisierung vorbehandelt. Folgende Substanzen wurden bis herab in den sub-Milligramm-Bereich mit Fehlern von 1–2% titriert: Isoniazid, Oxalyldihydrazid, Hydrochinon, Resorcin.

     

Determination of Selenium(IV) by Stripping Voltammetry at a Mercury-Film Electrode

A procedure was proposed for the determination of selenium(IV) by stripping voltammetry on a mercury-film electrode at an electrolysis potential of +0.4 V versus the saturated silver–silver chloride reference electrode in a 1 M H₂SO₄ solution. The current of the cathodic peak is a linear function of the selenium(IV) concentration in the range from 5 × 10^–3 to 3 × 10^–1 mg/L (6.3 × 10^–8 to 3.8 × 10^–6 M) at a time of electrolysis of 30 s (t _el). The detection limit for selenium is 1 × 10^–4 mg/L (1.3 × 10^–9 M) at t _el = 300 s. It was shown that selenium(IV) can be determined in the presence of 10 mg/L Zn(II), 1 mg/L Cd(II), 0.5 mg/L Pb(II), and 0.2 mg/L Cu(II). A procedure for the determination of selenium in natural, mineral, and potable water was proposed.     

Electrocatalytic Response of a Glassy-Carbon Electrode Modified with a Polyvinylpyridine Film with Electrodeposited Palladium in the Oxidation of Oxalic Acid

The electrochemical properties of a glassy-carbon electrode modified with a polyvinylpyridine film with electrodeposited palladium were studied. Conditions were selected for preparing a composite film on a glassy-carbon surface. It was found that palladium particles deposited on the polyvinylpyridine film exhibited electrocatalytic activity in the oxidation of H₂C₂O₄. Compared to an unmodified electrode, the oxidation potential of oxalic acid decreased and the current of its oxidation multiply increased. The catalytic current of oxalic acid oxidation was a linear function of its concentration in the range from 1 × 10^–2 to 1 × 10^–6 M.     

Determination of Teturam in Aqueous–Alcoholic Solutions by Cathodic Stripping Voltammetry at a Silver Electrode

The conditions for the determination of teturam in aqueous–alcoholic solutions by cathodic stripping voltammetry at a silver electrode were found. The detection limit for teturam electroaccumulated for 5 min in a 50% C₂H₅OH solution alkalized with NaOH to pH 12.7 was 5.6 × 10^–5 M (relative standard deviation of 5%) and the upper boundary of analytical range was 5 × 10^–4 M (relative standard deviation of 1.5%).     

Electrocatalytic Determination of Dithiocarbamate-Based Pesticides Using Electrodes Modified with Metal Phthalocyanines

An electrocatalytic method has been proposed for determining dithiocarbamate-based pesticides (carbathion, nabam, ferbam, thiram, and thiuram) using a carbon-paste electrode modified with iron(II) and cobalt(II) phthalocyanines. The first wave of carbathion oxidation in both aqueous and organic solutions does not change compared to an unmodified carbon-paste electrode; for the second stage of oxidation, a decrease by 100 mV in the overpotential and a significant increase in the current are observed. The electrochemical signal is observed in the range of potentials characteristic for the oxidation of metal phthalocyanines, and the metal phthalocyanine serves as the electron carrier. A similar effect is observed for the electrochemical oxidation of nabam. The catalytic activity of the metal phthalocyanines to ferbam, thiram, and thiuram in acetone has been determined. In the case of the electrochemical oxidation of ziram and zineb, the electrocatalytic effect is not observed. The dependence of the peak current on the pesticide concentration is linear within the range from 1 × 10^–3to 1 × 10^–7M. The adsorption preconcentration of pesticides decreases the limit of detection by an order of magnitude.     

Cobalt Hexacyanoferrate-Modified Electrode for Amperometric Assay of Hydrazine

A graphite electrode modified with cobalt hexacyanoferrate by mechanical immobilization was used for amperometric determination of hydrazine. The modified electrode exhibits good catalytic activity for the oxidation of hydrazine at a reduced overpotential with remarkable sensitivity. The modified electrode showed a linear response for hydrazine in the concentration range of 2.0 × 10^–5 to 2.8 × 10^–4 M. The detection limit was 9.8 × 10^–6 M (S/N = 3). The proposed modified electrode was simple, sensitive, rapid, stable and promising.