Application of an indicating system of bismuth electrodes for the determination of organic bases and acids in nonaqueous solutions

Summary Tertiary amines and salts of organic acids have been titrated by means of perchloric acid applying a biamperometric indicating system consisting of a polarised or unpolarised pair of bismuth electrodes. Also, mono- and polycarboxylic acids, alone and in mixtures, have been determined with a strong base using the same end-point detection technique. The results obtained are in good agreement with those of potentiometry or catalytic thermometry.

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Anwendung eines Indikationssystems aus Wismutelektroden zur Bestimmung von organischen Basen und Säuren in nichtwäßrigen Lösungen

Zusammenfassung Tertiäre Amine und Salze organischer Säuren werden durch Titration mit einer Lösung von Perchlorsäure quantitativ bestimmt. Das verwendete Indicatorsystem besteht aus einem Paar polarisierter oder nichtpolarisierter Wismutelektroden. Das gleiche System wurde auch angewandt auf die titrimetrische Bestimmung von Mono- und Polycarbonsäuren mit einer starken Base. Die Ergebnisse stimmen gut mit denen der potentiometrischen bzw. katalytisch-thermometrischen Methode überein.

     

Regioselectivity in deprotonation of ketone dimethylhydrazones with lithium amide bases

The regiochemistry of deprotonation of 3-pentanone dimethylhydrazone by lithium amide bases has been studied by ¹³C-NMR spectroscopy of ¹³C-enriched samples.     

The coulometric titration of acids and bases in dimethylsulfoxide media

The coulometric titration of 20–200 μeq of acids and bases in DMSO media is described. In the titration of bases, the electro-oxidation of hydrogen at a platinized platinum electrode is used as the source of protons. The conditions for 100 % current efficiency at this electrode are low current density to avoid passivity and regular treatment of the electrode with potassium dichromate—sulfuric acid to remove a poisoning sulfide layer. The accuracy of the titrations is better than ±1 %. Very weak acids like phenols (pK_a (DMSO) ≈16) can be titrated successfully. Tris(hydroxymethyl)aminomethane is the weakest base titrated.     

The electrooxidation of glucose on platinum electrodes in buffered media

Kinetic studies of glucose electrooxidation are of special interest in fundamental and applied electrochemistry. The electrochemical reaction on bright Pt electrodes is investigated through the application of combined potential/time perturbation programs and electrolytes of different compositions at temperatures ranging from 30 to 64°C. The electrooxidation of glucose should occur through the formation of several intermediate species and the overall reaction pathway should consist of different stages considering electroadsorption, chemical and electrochemical processes. The influence of potential perturbation conditions, anion adsorption, inhibition reactions of products and temperature are analysed through changes in U/I profiles.     

Anion exchange of uranium and thorium in ketone media

Summary In the present work the adsorption behaviour of uranium and thorium was investigated on the strongly basic anion exchange resin Dowex 1 in ketone media containing hydrochloric or nitric acid. As a result of these studies two methods for the separation of uranium from thorium in acetone solution were developed.

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Zusammenfassung In der vorliegenden Arbeit wurde das Adsorptionsverhalten von Uran und Thorium am stark basischen Anionenaustauscher Dowex 1 in salzs?ure- und salpeters?urehaltigen Ketonen untersucht. Es wurden zwei Methoden zur Trennung des Urans von Thorium in acetonhaltiger L?sung entwickelt.

     

PbO2-based graphite-epoxy electrode for potentiometric determination of acids and bases in aqueous and aqueous-ethanolic media

The construction and analytical evaluation of a PbO2-based graphite-epoxy electrode sensitive to H3O+, based on incorporation of lead(IV) oxide in a graphite-epoxy matrix, are described. The data obtained from a variety of acid-base titrations in aqueous and aqueous-ethanolic media were compared with those obtained by use of a glass electrode under the same conditions. The proposed electrode provides a linear response in the pH range from 1 to 11 with a slope of -58.7+/-0.3 mV pH(-1) and -60.8+/-0.2 mV pH(-1) in aqueous and ethanolic media, respectively. The response time was less than 15 s and the lifetime of the electrode was at least eight months (ca. 5000 determinations) and its performance is good in pH determination and end-point detection in potentiometric acid-base titrations in both aqueous and aqueous-ethanolic media.     

Titrations of some steroidal bases in non-aqueous media

Summary The potentiometric and catalytic thermometric titrations of some newly synthetised steroidal compounds containing the pyridine nucleus are performed with 0. 1 M perchloric acid in acetic acid. Amounts of 15.32–37.71 mg were determined with a relative standard deviation < 0.89 %. Results of separate determinations of the bases by catalytic thermometric and potentiometric titration are in good agreement.By means of the data of potentiometric titrations, pK-values of the investigated compounds were estimated and some possible explanations are provided for the noticeable differences in the two pairs of geometrical isomers. The obtained results show that these compounds behave as relatively stronger bases (pK 6.04–7.03) in acetic acid. It has also been found that the introduction of one vinyl-group into -position of the pyridine nucleus has no essential influence on the basicity, as confirmed by the obtained pK-values.Determination of the isomeric ketones, 3-acetoxy-17-picolinylidene-5-androstene-16-ones, in mixture was successfully performed by potentiometric titration in acetic anhydride only.

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Titrationen einiger Steroidbasen in nichtwäßrigen Medien

Zusammenfassung Potentiometrische und katalytische thermometrische Titrationen wurden für einige neusynthetisierte Androstenverbindungen mit pyridinischem Kern mittels 0,1 M Perchlorsäure in Essigsäure durchgeführt. Mengen von 15,32–37,31 mg wurden mit einer relativen Standardabweichung von <0,89% bestimmt. Die Ergebnisse der katalytischen thermometrischen Titrationen stimmen mit denen der potentiometrischen gut überein.Auf Grund der Daten der potentiometrischen Titrationen wurden die pK-Werte der untersuchten Verbindungen berechnet und mögliche Erklärungen für ihre geometrischen Isomeren gegeben. Die erhaltenen Ergebnisse zeigen, daß diese Verbindungen in Essigsäure starke Basen (pK 6,04–7,03) sind. Ebenso wurde festgestellt, daß die Einführung einer Vynilgruppe in -Stellung des pyridinischen Kerns sehr wenig die Basizität beeinflußt, was die erhaltenen pK-Werte bestätigen.Die Bestimmung von isomeren Ketonen, 3 -Acetoxy-17-picolinyliden-5-adrosten-16-onen, in Mischung wurde erfolgreich durch potentiometrische Titration nur in Essigsäureanhydrid ausgeführt.

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The authors thank the Scientific Fund of SAP Vojvodina for partial financial support of the present work     

The use of membrane electrodes in the determination of sulphides in sea water

The direct potentiometric determination of sulphides in natural sea water with a sulphide-selective membrane electrode is proposed. The experimental evaluation of the “apparent mixed” dissociation constants and the thermodynamic activity coefficient in spiked sea water samples, by means of the electrode, permits direct calibration in terms of activity. Alternatively, it is possible to establish, for natural sea water, an experimental equation for the correction of the electrode potentials in terms of pH; this allows direct calibration of the electrodes in terms of total sulphide concentration. This criterion can be applied to any aqueous solution.     

Potentiometric determination of pK(A) of organic bases in acetone by the application of coulometry

A coulometric-potentiometric method for the determination of pK(A) values of organic bases in anhydrous acetone is described. The bases were titrated with protons obtained by anodic oxidation of hydrogen dissolved in palladium, in the presence of sodium perchlorate as the supporting electrolyte. A pair of glass electrodes was used for measuring directly the difference between the half-neutralization potentials of the standard and the base being studied. The pK(A) values obtained were close to those reported in the literature. The effect of the supporting electrolyte concentration on the pK(A) values of some of the bases was also studied.     

The acidity constants of some pyrimidine bases in various water-organic solvent media

The acid ionization constants of some pyrimidine bases of nucleic acids were determined pH-metrically at 25 degrees C and at the constant ionic strength I = 0.10 mol l(-1) (KNO3) in pure water as well as in aqueous media containing variable mole percentages (5-30%) of organic solvents. The organic solvents used were methanol, ethanol, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetonitrile, acetone and dioxane. The results obtained indicated that the acidity constants are generally decreased as the content of an organic solvent in the medium is increased. It was deduced that the hydrogen bonding interactions and the solvent basicity in addition to the electrostatic effect are the major effects influencing significantly the acid ionization process of pyrimidine bases in the different water-organic solvent media. Some thermodynamic parameters (deltaH, deltaG degrees, deltaS degrees) of the ionization process over the temperature range 5-45 degrees C in pure water were also determined and discussed.     

Mechanism of chloramine-T oxidation of methyl vinyl ketone and isopropyl methyl ketone in aqueous alkaline media

The oxidation kinetics of methyl vinyl ketone and isopropyl ketone by chloramine-T in aqueous alkaline solutions show first-order dependence on chloramine-T, both substrates and alkali. No effect of p-toluenesulphonamide was evident. Observed stoichiometry, negligible effect of ionic strength and a positive dielectric effect point to a mechanism involving interaction of enolate anions with chloramine-T in the rate determining step. Activation parameters and the isolation of the product formaldehyde are in agreement with the proposed mechanism.     

Voltammetric determination of piroxicam in micellar media by using conventional and surfactant chemically modified carbon paste electrodes

A method for piroxicam determination based on adsorptive stripping voltammetric techniques, using conventional and chemically modified carbon paste electrodes in micellar media, is described. The employed surfactants were sodium dodecyl sulfate (SDS), Triton X-100, Triton X-405, Tween 80 and Brij 30. However, the purpose of this paper is, at present, to research the use of surfactants as carbon paste modifier because one of the mechanisms of hydrophobic drugs ad-accumulation on the carbon paste electrode is based on the chemical affinity. Besides, because of the water piroxicam insolubility, a special aqueous medium, such as a surfactant solution above its c.m.c. was used, this micellar media being very advantageous in relation to the use of organic or aqueous-organic media, in order to dissolve the studied drug and to remove the problems derived from the organic solvents use. In addition, a piroxicam ad-accumulation increase, on surfactant modified carbon paste electrode, with the surfactant mass incorporated into the electrode, was observed.     

Single-point titrations: Part-I. The determination of bases

Equations are derived for the calculation of acid mixtures, which upon titration with base show a linear relation between pH and the amount of base. Three to five weak acids were used and a linearity of better than ±0.02 pH units was obtained. The use of such mixtures for analysis of the base content of samples by means of a single pH measurement is described. A procedure for obtaining conditional pK_a values of the components of the acid mixture is also described. The single-point titration method is advocated for use when better accuracy than that of direct potentiometry is desired but less than that of an ordinary titration can be accepted. It is not necessary to know the pK_b or the number of weak bases.     

The application of boron-doped diamond electrodes in amperometric biosensors

Boron-doped diamond (BDD) electrodes outperform conventional electrodes in terms of high stability, chemical inertness, wide potential window and low background current. Combining the superior properties of BDD electrodes with the merits of biosensors, such as specificity, sensitivity, and fast response, amperometric biosensors based on BDD electrodes have attracted the interests of many researchers. In this review, the latest advances of BDD electrodes with different surfaces including hydrogen-terminated, oxygen-terminated, metal nanoparticles-modified, amine-terminated, and carboxyl-terminated thin films, and microelectrodes, for the construction of various biosensors or the direct detection of biomolecules were demonstrated. The future trends of BDD electrodes in biosensing were also discussed.     

Long-term stability of glass electrodes in aqueous media

The stability of the potential of glass electrodes has been studied. The potential changes only slightly during the hydration of freshly etched electrodes. With glass electrodes previously used in alkaline solutions, structural transformations within the gel-layer give rise to large potential drifts in neutral or acidic test solutions. In alkaline solutions all glass electrodes are attacked, especially the low-temperature type, and the potential changes with time. Drying hydrated electrodes affects the stability only slightly. Alternating transfers between acidic and basic solutions decrease the stability. Recommendations for precise measurements with glass electrodes are given.     

Membrane electrodes for the determination of glutathione

Four glutathione (GSH)-selective electrodes were developed with different techniques and in different polymeric matrices. Precipitation-based technique with bathophenanthroline-ferrous as cationic exchanger in polyvinyl chloride (PVC) matrix was used for sensor 1 fabrication. β-Cyclodextrin (β-CD)-based technique with either tetrakis(4-chlorophenyl)borate (TpClPB) or bathophenanthroline-ferrous as fixed anionic and cationic sites in PVC matrix was used for fabrication of sensors 2 and 3, respectively.β-CD-based technique with TpClPB as fixed anionic site in polyurethane (Tecoflex) matrix was used for sensor 4 fabrication. Linear responses of 1 × 10⁻⁵ to 1 × 10⁻⁴ M and 1 × 10⁻⁶ to 1 × 10⁻³ M with slopes of 37.5 and 32.0 mV/decade within pH 7-8 were obtained by using electrodes 1 and 3, respectively. On the other hand, linear responses of 1 × 10⁻⁵ to 1 × 10⁻² and 1 × 10⁻⁵ to 1 × 10⁻³ M with slopes of 47.9 and 54.3 mV/decade within pH 5-6 were obtained by using electrodes 2 and 4, respectively. The percentage recoveries for determination of GSH by the four proposed GSH-selective electrodes were 100 ± 1, 100.5 ± 0.7, 100 ± 1 and 99.0 ± 0.8% for sensors 1, 2, 3 and 4, respectively. Determination of GSH in capsules by the proposed electrodes revealed their applicability for determination of GSH in its pharmaceutical formulations. Also, they were used to determine GSH selectively in presence of its oxidized form (GSSG). Sensor 4 was successfully applied for determination of glutathione in plasma with average recovery of 100.4 ± 1.11%. The proposed method was compared with a reported one. No significant difference for both accuracy and precision was observed.     

胶束介质在速差动力学分析中的应用: 钴、镍的同时测定

研究溴化十六烷基三甲铵表面活性剂对钴(Ⅱ)、镍(Ⅱ)与二甲酚橙配位反应的影响, 测定了反应级数和表观活化能。由于胶束介质的存在, 钴(Ⅱ)、镍(Ⅱ)三元配合物的灵敏度比二元配合物分别提高了4.4和8.6倍, 并扩大了两组分的速率差别, 改善了反应的选择性。基于两组分反应速率的差别, 采用对数外推法计算机处理数据, 建立了速差动力学同时测定钴(Ⅱ)、镍(Ⅱ)的新方法。应用于钴精矿等几种实际样品中钴、镍的同时测定获得了满意结果, 其相对误差和标准偏差分别在4%和3%以内。     

Simultaneous determination of copper and lead in seawater using optimised thin-mercury film electrodes in situ plated in thiocyanate media

In the present work the anodic stripping voltammetric (ASV) methodology using a thin mercury film electrode in situ plated in thiocyanate media was re-assessed in order to allow the simultaneous determination of copper and lead in seawater. Under previously suggested conditions [6], i.e. using a concentration of thiocyanate of 5 mM, the ASV peaks of copper and lead overlapped due to the formation of a stable copper(I)-thiocyanate species, limiting the analytical determinations. Therefore, the best value for the thiocyanate concentration was re-evaluated: for 0.05 mM a trade-off between good resolution of the copper and lead peaks and high reproducibility of the mercury film formation/removing processes was achieved. In this media, the ASV peaks for Pb and Cu occurred, separated by 140 mV. Also, the in situ thin mercury film electrode was produced and removed with good repeatability, which was confirmed by the relative standard deviation values for the ASV determinations: 0.5% for Pb and 2.0% for Cu (10 replicate determinations in a solution with metal concentrations 1.5×10⁻⁸ M for lead and 2.2×10⁻⁸ M for copper). The optimised methodology was successfully applied to the determination of copper in the presence of lead, in certified seawater (NASS-5).