Potentiometric End-point Research Articles

Development of a Method for the Quantitative Determination of Genisten for the Certification of a Certified Reference Material

Introduction. One of the priority directions in the development of modern pharmacy is the search for new highly effective pharmaceutical substances with a high spectrum of pharmacological action and low toxicity. As a result of studies conducted by a number of authors, positive results have been shown for the use of genistein for the prevention and treatment of cardiovascular diseases and osteoporosis in menopausal women. It exhibits hypocholesterolemic and antidiabetic effects, as well as radioprotective properties. A comparative evaluation of genistein from soybean seed cake [Glycine max (L.) Merr.] and synthesized by SPC "Pharmzashchita" and the department of chemical technology of medicinal substances, SPCPU was carried out. Studies of natural genistein by GC-MS showed the presence of an admixture of a related isoflavone, daidzein, in the sample. The synthesized genistein was assessed in terms of qualitative analysis.Aim. The aim of our study was to develop a method for the quantitative determination of genistein for its certification as a CRM.Materials and methods. The object of the study was 96 % genistein (abcr. Gute Chemie, Германия). Quantitative determination by non-aqueous titration was carried out on a laboratory pH meter F20 (METTLER TOLEDO, USA) in a reagent grade DMFA medium (JSC "EKOS-1", Russia). The HPLC study was carried out on an Agilent 1200 Series high-performance liquid chromatograph LC-20 Prominence (Shimadzu, Japan).Results and discussion. The method of non-aqueous titration in DMFA medium which recommended by the State Pharmacopoeia of the Russian Federation XIV edition, titrant 0.1 M sodium hydroxide solution, was used as the basis for the development of a method for the quantitative determination of genistein. The end point of the titration was set using thymol blue indicator. However, the difficulty in visual fixation by changing the color of the indicator was noted. It has been proposed to use a potentiometric end-point determination. For the quantitative determination of genistein in dietary supplements and drugs containing this substance a HPLC method was proposed.Conclusion. A method for non-aqueous titration of genistein with potentiometric end-point determination in DMFA medium has been developed. The integral curve is not very informative for a clear definition of the titration end point. It is recommended to use a differential curve. The validation evaluation of the obtained results showed that the method is trueness (RSD, % = 1.25), precision (RSD, % = 1.21) and intermediate precision (on the first day of RSD, % = 1.21, on the second day of RSD, % = 1.41).

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.

Colour maps of acid–base titrations with colour indicators: how to choose the appropriate indicator and how to estimate the systematic titration errors

To recognise the end point of acid–base titrations, colour indicators are still frequently used. Colour indicators can cause systematic errors, and the theoretical evaluation of these errors is given in many textbooks. Random errors caused by the transition range of an indicator can also be estimated with the help of pH-logc i diagrams. However, very often, undergraduate students have problems to correctly interpret the colours of the different species of an indicator (its acidic and alkaline forms) and colour changes during titration. Frequent questions are of the type: “Is it still blue, or is it already red?” The aim of the present text is to show how carefully a suitable colour indicator has to be chosen, and what colour changes happen at the inflection point of a titration. Typical acid–base titrations like the alkalimetry of hydrochloric acid, sulphuric acid, oxalic acid, and the acidimetry of borax at two concentration levels using eleven common colour indicators are compared with potentiometric end point determination. Photographs were taken of solutions containing the indicators during the course of titration, i.e., at different pH values. These photographs were used to prepare a colour map for each indicator. Such colour maps are very helpful to find the most suitable indicator for a certain reaction, and also to estimate the maximum accuracy.

Quantitative determination of fucoidan using polyion-sensitive membrane electrodes

The use of polyanion and polycation-sensitive membrane electrodes to detect five different preparations of fucoidan is described. Unlike linear polyanionic molecules previously measured with polymer membrane-based electrochemical sensors, fucoidans from marine brown algae are all highly branched, sulfated polysaccharides with varying charge densities and structures, depending on the species of seaweed, method of extraction used and extent of purification. When tridodecylmethylammonium (TDMA) was used as the ion-exchanger, a large, non-equilibrium EMF response was observed over a concentration range of 0.5–50μgmL−1 fucoidan. Fucoidan was also measured by titration with polycationic protamine, using a dinonylnaphthalene sulfonate (DNNS)-doped membrane electrode as the potentiometric endpoint detector. Potentiometric titration was used to determine the binding ratio between protamine and fucoidan at the neutralization endpoint for each fucoidan preparation. This binding ratio was then used to successfully determine the fucoidan content of commercially available nutritional supplements. Fucoidan was also measured in undiluted blood serum, demonstrating that this method may be applicable for measuring fucoidan for clinical applications.

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.

Cyanide measurement by silver nitrate titration: Comparison of rhodanine and potentiometric end-points

The measurement of cyanide using a silver nitrate titration with p-dimethylaminobenzylidene rhodanine indicator has been used for many years by the gold industry and has been adopted in many operations as the trusted free cyanide determination. However, this work shows that there are several interferences, particularly copper and thiosulfate, where the interference increase with increasing concentration of these species. In comparison, cyanide determination using a silver nitrate titration with potentiometric end-point has no interferences as long as the titration pH is adjusted above 12 to eliminate the interference of zinc. A potential issue with the identification of the potentiometric end-point at low copper and/or thiosulfate concentrations is highlighted. With the gold industry now treating more complex gold ores, this work clearly demonstrates that the rhodanine indicator determination for free cyanide in cyanidation leach solutions containing copper and/or thiosulfate will over-estimate the free cyanide concentration and cannot be compared directly against an online cyanide analyser using a potentiometric determination. As no simple direct correlation exists between the two methods for these complex cyanide solutions, it is recommended that gold operations treating complex gold ores and using online cyanide analysers adopt the potentiometric end-point method in the laboratory which allows for direct comparison.

Development of methodologies for dimethylaminoethanol glycolate assay in association with sunscreens in dermocosmetic formulation

DMAE glycolate (DG) and sunscreens have been used associated in anti-aging dermocosmetic formulations. Despite extensive use of these substances, methods for quantification of DG as raw material and in cosmetic formulations, especially when associated, are not described in the literature. RP-HPLC and non-aqueous titration methods, with determination potentiometric end-point (PT), were developed and validated for rapid assay of DG as raw material and in a topic emulsion in association with sunscreens. Both methods are simple, selective, linear, accurate and precise. The PT method was chosen for stability study of DG in the formulation developed. The proposed formulation presented good stability performance as regards aspect, pH, apparent viscosity, and SPF, with less than 5% of DG degradation compared to initial conditions.

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.

Development and validation of an alternative titration method for the determination of sulfate ion in indinavir sulfate

A simple and rapid precipitation titration method was developed and validated to determine sulfate ion content in indinavir sulfate raw material. 0.1 mol L-1 lead nitrate volumetric solution was used as titrant employing potentiometric endpoint determination using a lead-specific electrode. The United States Pharmacopoeia Forum indicates a potentiometric method for sulfate ion quantitation using 0.1 mol L-1 lead perchlorate as titrant. Both methods were validated concerning linearity, precision and accuracy, yielding good results. The sulfate ion content found by the two validated methods was compared by the statistical t-student test, indicating that there was no statistically significant difference between the methods.

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%.

Colloidal titration of aqueous zirconium solutions with poly(vinyl sulfate) by potentiometric endpoint detection using a toluidine blue selective electrode.

Zirconium oxy-salts were hydrolyzed to form positively charged polymer or cluster species in acidic solutions. The zirconium hydrolyzed polymer was found to react with a negatively charged polyelectrolyte, such as poly(vinyl sulfate), and to form a stoichiometric polyion complex. Thus, colloidal titration with poly(vinyl sulfate) was applied to measure the zirconium concentration in an acidic solution by using a Toluidine Blue selective plasticized poly(vinyl chloride) membrane electrode as a potentiometric end-point detecting device. The determination could be performed with 1% of the relative standard deviation. The colloidal titration stoichiometry at pH < or = 2 was one mol of zirconium per equivalent mol of poly(vinyl sulfate).

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.