Potentiometric End-point Detection Research Articles

Characterisation of reduction state of cystine linkages on wool fibre surface under heterogeneous reaction conditions

As a natural biomaterial with outstanding properties, keratin fibres hold promising potential in many application fields. The reduction of disulphides in keratin to the corresponding thiols plays a central role e.g. in modification and functionalisation of wool, in hair cosmetics, as well as in advanced applications using keratin as biomaterial. To achieve controlled conditions, the need for a quantitative determination of thiols on the solid wool fibre is evident. This work presents an advanced method of heterogeneous potentiometric back titration method for thiol detection in intact keratin fibres. First, keratin is reductively treated with NaBH4, then the thiol content in keratin is determined using K3[Fe(CN)6] and K2CrO4 as reagents and potentiometric endpoint detection. Thiol concentration in the range between 6.83 and 26.17 (±2.13) μmol/g can be detected with the heterogeneous back titration. Thus, the method is simple in execution and reliable in performance and of significant potential, both as scientific technique to quantify thiols in insoluble keratin material, as well as for process and quality control in industry.

Determination of Metoclopramide Hydrochloride in Pharmaceuticals and Spiked Human Urine through Diazotization Reaction

Three simple, rapid, sensitive and cost-effective titrimetric and spectrophotometric methods were described for the determination of metoclopramide (MCP) in its pharmaceutical dosage forms and spiked human urine. The titrimetric methods were based on the diazotization reaction involving the primary amine group of MCP and NaNO2 in acid medium with visual (method A) and potentiometric (method B) end point detection. The spectrophotometric method (method C) depends upon the diazotization of MCP in acid medium followed by coupling with diphenylamine to give a red colored chromogen with a wavelength of maximum absorption at 530 nm. The experimental conditions were optimized. Both the titrimetric methods were applicable over the concentration range of 1.0-20.0 mg and the calculations were based on a 1 : 1 (MCP: NaNO2) reaction stoichiometry. In the spectrophotometric method, Beer's law was obeyed over the concentration range or 0.3-7.5 ug/mL with a molar absorptivity of 4.73 x 10^4 L/mol cm and Sandell's sensitivity being 0.007 μg/cm^2. The limit of detection and the limit of quantification were calculated to be 0.22 and 0.67 μg/mL, respectively. The proposed methods were applied to the determination of MCP in tablets, injection and also in spiked human urine.

A simple potentiometric titration method for estimation of maleic anhydride in high molecular weight styrene-maleic anhydride copolymer

A potentiometric titration method for estimation of maleic anhydride content in high molecular weight styrene maleic anhydride copolymer was developed. The method involves reaction of maleic anhydride with aniline to convert anhydride to monoacid, which was then titrated with an organic base namely alcoholic tetra-butylammonium hydroxide (TBAOH). The misperception over end-point detection by color indicators was circumvented in this method by potentiometric end-point detection. The method is accurate, repeatable and easy to implement.

Thermodynamic analysis of free cyanide determination by silver nitrate titration in copper bearing solutions

The present study is a thermodynamic approach to explain the titration of free cyanide by silver nitrate in cyanide solutions containing copper. When copper is present in cyanide solutions, the visual determination of the endpoint of the titration leads to an overestimation of the free cyanide concentration that can be up to 56% when the indicator is potassium iodide, or up to 121% for rhodanine. Four alternatives were evaluated to detect the endpoint of the titration: visual detection with potassium iodide, visual detection with rhodanine, potentiometric endpoint detection with silver electrode and derivative curve of the potentiometric titration. The last two methods showed the best results (the smallest overestimation, reaching no more than 8%). A comparison between the titration graphs and the equilibrium composition diagrams allows the understanding of the overestimation, and permits the qualitative and quantitative determination of all the copper cyanide species that are being titrated with silver nitrate.Cette étude est une approche thermodynamique ayant pour but d’expliquer le titrage au nitrate d’argent du cyanure libre de solutions de cyanure contenant du cuivre. Lorsque le cuivre est présent dans les solutions de cyanure, la détermination visuelle du point final du titrage mène à une surestimation de la concentration du cyanure libre, qui peut aller jusqu’à 56% lorsque l’indicateur est l’iodure de potassium, ou jusqu’à 121% pour la rhodanine. On a évalué quatre solutions possibles de détection du point final du titrage: détection visuelle avec l’iodure de potassium, détection visuelle avec la rhodanine, détection potentiométrique du point final avec électrode d’argent et courbe dérivée du titrage potentiométrique. Les deux dernières méthodes ont montré les meilleurs résultats (surestimation la plus petite, n’atteignant pas plus de 8%). Une comparaison entre les graphes de titrage et les diagrammes d’équilibre de la composition permet de comprendre la surestimation et permet la détermination qualitative et quantitative de toutes les espèces du cyanure de cuivre qui sont titrées avec le nitrate d’argent.

On determination of low oxygen concentrations with Winkler technique

The accuracy and detection limit of the oxygen technique is an actual problem in studying the processes that occur in the redox zone. The formal accuracy of the Winkler technique is 1 μM (0.02 mL L−1) and its detection limit is about 3.0 μM (0.06 mL L−1). These values are significantly higher than the similar characteristics (in molar concentrations) for such parameters as the hydrogen sulphide, nitrates, manganese, and others. In this work, we describe some recommendations for increasing the accuracy of the Winkler technique. The results of the application of these recommendations for the suboxic zone of the Black Sea are presented. During the 100th cruise of the R/V Professor Shtokman, argon-filled balloons were attached to the upper valves of Niskin bottles during the sampling, which allowed protecting the samples from contamination with atmospheric oxygen. The titration was performed with an automatic Metrohm Titrino burette with potentiometric end point detection. That allowed us to significantly increase the accuracy and decrease the detection limit compared with the visual techniques. The oxidized forms of metals present in the sea water (Mn(IV), Mn(III); Fe(III)) were measured, which allowed us to correct the Winkler technique calculations for the oxidizers. The studies performed during the 100th cruise of the R/V Professor Shtokman confirmed the absence of a layer of the coexistence of oxygen and hydrogen sulphide.

Microtitrimetric determination of a drug content of pharmaceuticals containing olanzapine in non-aqueous medium

Two simple, rapid, reliable and cost-effective methods based on titrimetry in non-aqueous medium are described for the determination of olanzapine in pharmaceuticals. In these methods, the drug dissolved in the glacial acetic acid was titrated with the acetous perchloric acid with visual and potentiometric end point detection, crystal violet being used as the indicator for visual titration. The methods are applicable over 1-15 mg range of olanzapine. The procedures were applied to determine olanzapine in pharmaceutical products and the results were found to be in a good agreement with those obtained by the reference method. Associated pharmaceutical materials did not interfere. The precision results, expressed by inter-day and intra-day relative standard deviation values, were satisfactory, higher than 2%. The accuracy was satisfactory as well. The methods proved to be suitable for the analysis of olanzapine in bulk drug and in tablets. The accuracy and reliability of the methods were further ascertained by recovery studies via a standard addition technique with percent recoveries in the range 97.51-103.7% with a standard deviation of less than 2%.

Potentiometric and Coulometric Determination of 5-Phenyl-1,3,4-Oxadiazole-2-Thiol

A method for the determination of 5-phenyl-1,3,4-oxadiazole-2-thiol involving its reaction with iodine in an alkaline medium has been presented. In volumetric titration with potentiometric endpoint detection, the determination range is 25–500 µmol. In coulometric titration using biamperometric endpoint detection, 0.5–5 µmol of 5-phenyl-1,3,4-oxadiazole-2-thiol were successfully determined. The errors and the relative standard deviations were below 1%. The shapes of the obtained potentiometric titration curves of the tested compound are unusual at higher concentrations of sodium hydroxide and depend on the material of the indicator electrode (platinum and gold). Addition of iodine resulted in a significant potential drop. The studied system did not exhibit Nernstian behavior.

Coulometric generation of H+ ions by the anodic oxidation of esters of gallic acid

The application of esters of gallic acid (propyl, butyl and dodecyl gallate) for the coulometric generation of H+ ions in aqueous media is described. The current-potential curves recorded for these depolarizers, as well as for tris(hydrosymethyl) aminomethane (?THAM?) and the employed solvent showed that the investigated depolarizes are oxidized at potentials lower than the oxidation potentials of the other components in the solution. The H+ ions formed by the oxidation of these depolarizers (in 1 mol/L sodium perchlorate in water) were used for the titration of THAM with potentiometric end-point detection using a hydrogen/palladium indicator electrode. The current efficiency was 100 %.

Coulometric generation of hydrogen ions by oxidation of mercury in γ-butyrolactone and propylene carbonate

The application of mercury for the coulometric generation of hydrogen ions in γ-butyrolactone and propylene carbonate is described. The current–potential curves recorded for this depolarizer, titrated bases, indicator, and the solvents used showed that the investigated depolarizer was oxidized at lower potentials than the oxidation potentials of other components in the solution. The hydrogen ions generated by the oxidation of mercury and the reaction of mercury ions with γ-butyrolactone or propylene carbonate, were used for the titration of organic bases (piperidine, quinoline-8-ol, N, N′-diphenylguanidine, triethylamine, morpholine, urea, butylamine, and α,α′-bipyridyl) with visual (Methyl Red as indicator) and potentiometric end-point detection. The relative error of the determination of the bases was about 1%.

Alkalimetry in alcohol–water mixtures with potentiometric end-point detection: Critical remarks on a newer method of European Pharmacopoeia

A new method of the European Pharmacopoeia (Ph. Eur.)—alkalimetry in alcohol–water mixture with potentiometric end-point detection—for the assay of halide salts of alkaloids and other organic N-bases has been investigated using 13 substances. The results were compared to those obtained by nonaqueous direct acidimetry. In general, our measurements show that the Ph. Eur. method may be regarded as an environment-friendly, precise, reproducible approach, with average S.D. ±0.33. However, in six cases out of 13, the method did not work. In these cases, the first inflexion point, which should appear due to the neutralization of a defined, small volume of HCl added to the solution of the sample before the titration, was missing on the potentiometric titration curves. This was observed for papaverine chloride, pilocarpine chloride, pyridoxine chloride, thiamine chloride, histamine dichloride and noscapine chloride; this missing inflexion point hampered the exact measurement of the alkaloid content. This study shows that the method, in the present official form, can be applied only for compounds with p K a values of seven or higher. For the salts of weaker bases, a modified approach (titration in 70% ethanol, without addition of HCl) is proposed.

Coulometric generation of hydrogen ions by oxidation of mercury in methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone

Mercury(II)-chloride reacts with anhydrous methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone in a precise stoichiometry ratio (1:2), and weakly ionized compounds of mercury with ketones are formed and equivalent quantity of HCl is released. The application of a mercury anode for the quantitative generation of H + ions in 0.25 M sodium perchlorate in anhydrous methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone has been investigated. Current/potentials curves for the solvents, titrated bases, indicator and mercury showed that in these solvents mercury is oxidized at potentials much more negative than those for the titrated bases and other components present in the solution. The protons generated in this way have been used for the titration of some organic bases, with either visual or potentiometric end-point detection. The oxidation of mercury in methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone and the reaction of mercury ions with these solvents have been found to proceed with 100% current efficiency.

Coulometric–potentiometric titration of bases and acids in γ-butyrolactone

The coulometric generation of hydrogen ions by the anodic oxidation of esters of gallic acid (methyl, ethyl, propyl and octyl gallate), hydroquinone, ascorbic acid, 2,3,4-trihydroxybenzoic acid, tiron and hydrogen dissolved in palladium in γ-butyrolactone is described. The current–potential curves recorded for these depolarizers, titrated bases, indicator and solvent used showed that the investigated depolarizers were oxidized at more negative potentials than the oxidation potential of the titrated base and other components in the solution. The generated hydrogen ions were used for the titration of organic bases (triethylamine, N, N′-diphenylguanidine, piperidine and quinoline) with visual and potentiometric end-point detection. The current efficiency was 100%. The coulometric generation of bases ( m-cresolate ions) by the cathode reduction of m-cresol in γ-butyrolactone (GBL) is described. The current efficiency was 100%.

Electrooxidation of iodide in the presence of 4-hydroxycoumarin: application to a simple coulometric titration of 4-hydroxycoumarin.

The electrochemical oxidation of iodide ion in the presence of 4-hydroxycoumarin (1) was studied using cyclic voltammetry and controlled-potential coulometry. The result indicates that the resulting iodine takes part in a halogenation reaction and reacts with 4-hydroxycoumarin (1). According to the obtained results, a new and simple coulometric titration method with potentiometric end-point detection for the determination of 4-hydroxycoumarin (1) is presented. In the presented method, 2-200 micromol of 4-hydroxycoumarin (1) was successfully determined.

Potentiometric Titration of Triazolethiols and Tetrazolethiols with Iodine in Alkaline Medium

The iodimetric determination of triazolethiols and tetrazolethiols in alkaline medium is presented. The volumetric titration with potentiometric end-point detection was applied. The range of determination, in which the error is lower than 1%, is 20-2 000 μmol for 1H-1,2,4-triazole-3-thiol (1), 25-1 000 μmol for 3-phenyl-1H-1,2,4-triazole-5-thiol (2), 25-500 μmol for 4-methyl-5-(trifluoromethyl)-4H-1,2,4-triazole-3-thiol (3), 50-500 μmol for 3-amino-1H-1,2,4-triazole-5-thiol (4), 10-1 000 μmol for sodium (5-mercapto-1H-tetrazol-1-yl)acetate (5), 125-500 μmol for 1-phenyl-1H-tetrazole-5-thiol (6) and 50-1 000 μmol for 1-(4-hydroxyphenyl)-1H-tetrazole-5-thiol (7). The relative standard deviation for all determinations was below 1%. The shape of potentiometric titration curve of 6 and 7 is noteworthy at higher concentrations of sodium hydroxide and depends on the type of the indicator electrode (platinum, gold). An introduction of iodine results in a strong potential drop. Some systems do not follow the Nernst equation.

Determination of stoichiometry of Li 1+ yCoO 2 materials by flame atomic absorption spectrometry and automated potentiometric titration

A rapid, accurate and precise analytical method to determine the stoichiometry of pure, excess lithium and magnesium-doped Li 1+ y CoO 2 materials is described. The method is based on the determination of lithium, cobalt and magnesium by flame atomic absorption spectrometry after dissolution of samples in dilute hydrochloric acid. Five-point calibration curves using aqueous standard solutions have been constructed for all the analytes. Relative standard deviation (R.S.D.) of five repeated measurements are better than 0.3% for all metals when the absorbance signal of analytes is set near the middle of the regression lines. The total oxygen content is indirectly determined by potentiometric titration of average oxidation state of cobalt. The Co 3+ present in the oxide powders is reduced to Co 2+ at room temperature with a known excess of 0.1 M Fe 2+ solution in 6 M sulphuric and phosphoric acid mixture. The samples completely dissolved in a few minutes developing a pink coloured and clear solution. The excess of Fe 2+ ions is back titrated with potassium dichromate standard solution using automated potentiometric end-point detection. The difference between the total cobalt content (found by AAS) and Co 3+ (found by potentiometric titration) gives the amount of Co 2+ present in the materials. The S.D. of the determined Co 3+ value has been estimated to be below 0.03 mg for samples of 10–20 mg. By the proposed method the LiCoO 2 is well characterised and can be applied as a standard reference material for use in lithium batteries technology.

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.

Electrochemical study of iodide in the presence of barbituric acid. Application to coulometric titration of barbituric acid

Electrochemical oxidation of iodide in the presence of barbituric acid has been studied using cyclic voltammetry and controlled-potential coulometry. The results indicate that the resulting iodine takes part in halogenation reaction and reacts with barbituric acid. According to obtained results, a new coulometric titration method with potentiometric end-point detection for the determination of barbituric acid is presented. In the presented method, 1–200 μmol of barbituric acid was successfully determined.

Titration of Chlorine: Amperometric versus Potentiometric

The ability to accurately measure chlorine or other disinfectants in water is important for control of microorganisms. Whereas amperometry is specified for chlorine in Standard Methods for the Examination of Water and Wastewater, manufacturers of automatic titrators are marketing their potentiometric devices for such titrations. In this article, the electrochemical basis for amperometric and potentiometric endpoint detection is examined, and two amperometric and two potentiometric methods are compared. Both amperometric methods are shown to be superior to potentiometric endpoint detection and amenable to automatic titration. Finally, fundamentals of amperometry are discussed, and the simple circuit that was used to perform amperometric titrations using a modified automated pH/potentiometric titrator is presented.

Coulometric titrations of bases in propylene carbonate and γ-butyrolactone using hydroquinone as the depolarizer and a quinhydrone indicator electrode

The application of hydroquinone for the coulometric generation of hydrogen ions in propylene carbonate (PC) and ?-butyrolactone (GBL) is described. The current-potential curves recorded for this depolarizer, titrated bases, indicator and the solvents used showed that the investigated depolarizer is oxidized at lower potentials than the oxidation potentials of other components in the solution. The hydrogen ions generated by the oxidation of hydroquinone were used for the titration of organic bases (triethylamine, n-butylamine, pyridine, quinoline, aniline, N,N'-diphenylguanidine, piperidine, and 2,2?-bi?pyridine) in PC and GBL with with visual (Crystal Violet as indicator) and potentiometric end-point detection using a quinhydrone electrode as the indicator electrode. The quinhydrone added to the to be analyzed solution served both as a source of hydrogen ions and, together with the immersed platinum electrode, as a quinhydrone electrode. The relative error of the determination of the bases was about 1%.

Potentiometric and Coulometric Determination of Carbimazole

ABSTRACT A method for the determination of carbimazole involving its reaction with iodine in an alkaline medium is presented. In volumetric titration with potentiometric end-point detection the range of determination is 30–500 μmol (5.6–93 mg). In coulometric titration using biamperometric end-point detection, 0.5–20 μmol (0.09–3.7 mg) of carbimazole was successfully determined. The coulometric titration was applied to the determination of carbimazole in tablets.