Comparison of three electrochemical end-point detection methods to assay potassium dichromate by coulometric titration

Coulometric titrations with three electrochemical end-point detection methods were performed to assay potassium dichromate as a standard for oxidation–reduction titration. The assay as an oxidizing agent was carried out with ferrous ions produced by electrolytically reducing ferric ions. Three end-point detection methods were employed and compared with each other: constant potential amperometry, potentiometry, and constant voltage biamperometry (a dead-stop method). The last one was found to provide high accuracy in the coulometric titration of potassium dichromate. Solution form samples were also measured to confirm the possible existence of chromium(III) in potassium dichromate by both coulometric titration and ion-exchange chromatography with inductively coupled plasma time-of-flight mass spectrometry.     

Determination of chromate and cyanide by anion-exchange with lead iodate, and the analysis of mixtures of cyanide, thiocyanate and halides

Chromate and cyanide have been determined by their ability to displace iodate from sparingly soluble lead iodate. The released iodate is treated with acidified iodide to give iodine, which is determined either by titration with thiosulphate, or spectrophotometrically as its blue complex with starch. Chromium(III) has been determined as chromate after its oxidation with peroxydisulphate. Sulphate, iodide, bromide, chloride, fluoride, oxalate, tartrate, phosphate and thiocyanate do not interfere. Thiosulphate, sulphite, sulphide, hexacyanoferrate(II) and molybdate ions vitiate the results. Silver, mercury, barium and iron(III) should be masked. Mixtures of cyanide, thiocyanate and halides have been analysed by using complementary procedures that employ the iodates of lead and mercury, and bromine oxidimetry. It has been shown that cyanide or thiocyanate interferes in the determination of iodide by oxidation to iodic acid, because of formation of cyanogen bromide.     

A new oxidimetric reagent: potassium dichromate in a strong phosphoric acid medium-VI Potentiometric titration of vanadium(III) alone and in mixture with vanadium(IV)

Vanadium(III) can be titrated at room temperature with potassium dichromate in an 8-12M phosphoric acid medium. Two potential breaks are observed in 12M phosphoric add with 0.2N potassium dichromate, the first corresponding to the oxidation of vanadium(III) to vanadium(IV) and the second to the oxidation of vanadium(IV) to vanadium(V). In titrations with 0.05N dichromate only the first break in potential is clearly observed. The method has been extended to the titration of mixtures of vanadium(III) and vanadium(IV). Conditions have also been found for the visual titration of vanadium(III) using ferroln or barium diphenylamine sulphonate as indicator.     

Kinetics and mechanism of palladium (II) chloride catalyzed oxidation of allyl alcohol by potassium hexacyanoferrate (III)

A kinetic study of the oxidation of allyl alcohol by potassium hexacyanoferrate (III) in the presence of palladium (II) chloride is reported. The reaction was observed by measuring the disappearance of the potassium hexacyanoferrate (III) spectrophotometrically. The reaction is first order with respect to allyl alcohol and palladium (II) chloride, inverse second order with respect to [Cl^?], and zero order with respect to potassium hexacyanoferrate (III). The rate is found to increase linearly with hydroxyl ion concentration.     

Anwendung der ascorbinometrischen Bestimmung von Hexacyanoferrat(III) zu automatischen Mikrotitrationen

Zusammenfassung Nach theoretischen Erörterungen über die Redoxpotentiale des Ascorbinsäure-Dehydroascorbinsäure-Systems und Besprechung der möglichen Reduktionsreaktionen werden Vorschriften gegeben zur ascorbinometrischen Bestimmung von Hexacyanoferrat(III), Hexacyanoferrat(II), Dichromat, Permanganat, Jodat, Bromat und Chloramin T im Mikromaßstab mit Hilfe eines automatischen Mikrotitrators.

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Application of ascorbinometric determination of hexacyanoferrte(iii) to automatic micro-titrations

The reaction between hexacyanoferrate(III) and ascorbic acid has been applied to the ascorbinometric micro-titration of hexacyanoferrate(III), hexacyanoferrate(II), dichromate, permanganate, bromate and iodate ions as well as chloramine T. Titrations are carried out with an automatic titrimeter which records the potentiometric titration curve. Despite the small amounts of ions involved accuracy and precision of these methods are satisfactory.

     

Study and application of the complex of Sn(II) with glycerol in volumetric analysis

The preparation of standard Sn(II) solutions in glycerol and ethanol media was studied and the most suitable conditions were found for their standardization using dichromate and potassium hexacyanoferrate. Further, the long-term stability of standard Sn(II)-glyc solutions was studied and it was found that the titer of 0.1 N Sn(II)-glyc does not change in an inert atmosphere over a period of 4 months. The solution undergoes partial oxidation in the air (Fig. 1). It was demonstrated polarographically and voltammetrically that the studied redox system is irreversible. The formal redox potential value E_f⁰ = 0.80 V (SCE) 0.1 N Sn(II)-glyc can be back-titrated with 0.1 N K₂Cr₂O₇ in a sat. Na₂CO₃ solution under an inert atmosphere. Generally it is possible in sat. Na₂CO₃ medium to determine with great precision dichromate and potassium hexacyanoferrate, iodine, chloramine-T, chlorine lime, hydrogen peroxide, inorganic peroxides, and silver (Table 2). Equilibrium potentiometry, bipotentiometry, and biamperometry can be used for indication of the equivalence point.     

Methods for analysis of the mononuclear and binuclear manganese(III) cyano-complexes

Simple titrimetric methods are described for the analysis of potassium hexacyanomanganate(III) and of heptapotassium mu-oxo-bis[pentacyanomanganate(III)] cyanide. Mn(2)O(CN)(6-)(10) is determined by potentiometric titration with hexacyanoferrate(III). Cyanide is determined in both complexes by back-titration with Hg(II) and SCN(-), and manganese(III) is determined by back-titration with Fe(II) and MnO(-)(4). The absorption spectra of both cyano-complexes in cyanide and in acidic solutions are also described, and their molar absorptivities are reported.     

Reversed-phase ion-pair chromatographic method for the determination of nitroprusside in photolyzed solutions

A rapid reversed-phase ion-pair chromatographic method for disodium pentacyanonitrosylferrate(II) (nitroprusside) and its photodegradation products nitrite, nitrate, hexacyanoferrate(II) and hexacyanoferrate(III) is described. For chromatography, phenyl-bonded pellicular silica gel (10 μm) was used with a mobile phase consisting of water (0.005 M tetrabutylammonium phosphate, 0.0011 M n-octylamine, 0.01 M potassium dihydrogenphosphate, pH 7.0) and methanol (0.005 M tetrabutylammonium phosphate, 0.0011 M n-octylamine) (65:35); the detector was set at 220 nm. In 5% (w/v) dextrose solutions, the calibration graph for nitroprusside was linear over the concentration range 10–120 μg ml^?1. A qualitative explanation for the order of retention: hexacyanoferrate(II) < hexacyanoferrate(III) < nitroprusside is given. The method is suitable for the selective determination of nitroprusside in photolyzed infusion solutions (100 μg ml^?1 in aqueous 5% dextrose) and gives an impression of the decomposition products formed.     

Photochemical redox titrations with tungsten(V)

A titration method has been developed for the determination of micromolar quantities of dichromate, vanadate and hexacyanoferrate(III) with in situ photochemical generation of tungsten(V) as the titrant. Precision of 1–3% was obtained. Spectrophotometric end-points were utilized because of the intense blue color of the titrant (λ_max 770nm; _max7.10³). Because the titrant reacts with oxygen, a closed, argon-flushed, circulating photolysis apparatus was constructed. Optimal solution condi- tions were investigated ; tungsten reagent must be added as the solid just before titra-tion. An induced reaction between dichromate and glucose was found, but satisfactory linear calibration curves were obtained. With reduced photolytic intensity, 0.2 μeq of dichromate can be determined.     

Titration of cyanide and hexacyanoferrate(II) with silver nitrate-I Calculation of titration curves for potentiometric and conductometric titration

Stepwise potentiometric titration of cyanide and hexacyanoferrate(II) with silver nitrate is possible in the absence of potassium ions. At an initial concentration below 5.00 x 10(-4)M, cyanide can be titrated with silver nitrate (Ag:CN = 1:2) and the end-point indicated by precipitation of silver hexacyanoferrate(II); hexacyanoferrate(II) can be titrated with silver nitrate (Ag: Fe(CN)(6) = 4:1) and the end-point indicated by precipitation of silver dicyanoargentate. The hexacyanoferrate(II) reacts with silver to form two poorly soluble salts, Ag(4)Fe(CN)(6), KAg(3)Fe(CN)(6). The formation of these salts has been confirmed by conductometric titration of hexacyanoferrate(II) with silver nitrate in solutions containing varying concentrations of potassium nitrate.     

Electrocatalytic oxidation of nitric oxide at indium hexacyanoferrate film-modified electrodes

The electrocatalytic oxidation of nitric oxide (NO) by indium (III) hexacyanoferrate (III) films has been studied. These films are electrodeposited onto glassy carbon electrodes through potential cycling in acidic solutions containing a potassium electrolyte, indium (III), and potassium hexacyanoferrate. The resulting modified electrodes exhibit a reversible redox response ascribed to the oxidation/reduction of iron atoms presents in the electrodeposited film. The films have a potent and persistent electrocatalytic activity towards NO oxidation at neutral pH. The electrocatalytic oxidation of NO takes place at potentials around +0.75 V which represents a moderate diminution in the overpotential. In addition, interferences due to the presence of nitrate and nitrite have been significantly reduced. According to these results, the described modified electrodes have been used as sensors for the determination of NO generated by decomposition of a typical NO-donor, such as S-nitroso- N -acetyl- d, l -penicillamine (SNAP).     

Amperometric titrations of cobalt(II) with hexacyanoferrate(III) in ammonium citrate and glycine media

An amperometric titration of cobalt(II) with hexacyanoferrate(III) in aqueous ammonium citrate or aqueous glycine solution at pH 9.8 or pH 8.0 respectively, is reported. Cobalt concentrations of 2-30 mg/l were successfully determined. In citrate solutions cerium(III) and iron(III) interfered, and in glycine solutions, copper(II) and vanadium(V).     

Dichromate titration of thallium(I)

The formal redox potentials of the thallium(III)-thallium(I) couple in different acids of varying strengths are reported. The minimum concentration of hydrochloric acid required for a direct titration of thallium(I) with potassium dichromate is 5M. Thallium(I) can be titrated directly with the primary standard oxidant, potassium dichromate, at room temperature, with ferroin as indicator, in 6M hydrochloric acid. Atmospheric oxygen must be excluded.     

固相配位化学反应研究XXXIV. 室温下六氰合铁(II)及(III)酸钾 的固相酸碱反 应

本文研究了室温时K~3Fe(CN)~6,K~4Fe(CN)~6在酸碱条件下发生的固相配位化学反应。结果表明:K~3Fe(CN)~6与NaBH~4固相混合物室温下不反应,但加入固体氢氧化钠后,K~3Fe(CN)~6与NaBH~4的固相氧化还原反应在室温下很容易进行。K~4Fe(CN)~6与K~2S~2O~8的固相氧化还原反应在室温下能顺利进行,但当固体KOH存在时,反应明显受到抑制。K~3Fe(CN)~6与K~2C~2O~4.H~2O室温下无反应,但与H~2C~2O~4.2H~2O在室温时即发生固相取代反应。     

Electrochemical behavior of electrode modified with a cobalt hexacyanoferrate film: Effect of the supporting electrolyte cation

Cobalt hexacyanoferrate films are synthesized on a glassy carbon electrode with sodium, potassium, and ammonium salts in the supporting electrolyte. The electrochemical behavior of the modified electrode is studied in individual solutions of the salts and in their mixtures. The change in electrochemical properties of cobalt hexacyanoferrate agrees with an increase in the interaction of cations with the film in the series Li⁺, Na⁺, K⁺, NH ₄ ⁺ , Cs⁺. The effect of the energy of interaction between the modify ing-substance crystal lattice and counter-ions on the electrochemical processes is discussed     

Hexacyanoferrate(III) as a mediator in the determination of total iron in potable waters as iron(II)-1,10-phenanthroline at a single-use screen-printed carbon sensor device

Hexacyanoferrate(III) was used as a mediator in the determination of total iron, as iron(II)-1,10-phenanthroline, at a screen-printed carbon sensor device. Pre-reduction of iron(III) at −0.2 V versus Ag/AgCl (1 M KCl) in the presence of hexacyanoferrate(II) and 1,10-phenanthroline (pH 3.5-4.5), to iron(II)-1,10-phenanthroline, was complete at the unmodified carbon electrode surface. Total iron was then determined voltammetrically by oxidation of the iron(II)-1,10-phenanthroline at +0.82 V, with a detection limit of 10 μg l⁻¹.In potable waters, iron is present in hydrolysed form, and it was found necessary to change the pH to 2.5-2.7 in order to reduce the iron(III) within 30 s. A voltammetric response was not found at lower pH values owing to the non-formation of the iron(II)-1,10-phenanthroline complex below pH 2.5.Attempts to incorporate all the relevant reagents (1,10-phenanthroline, potassium hexacyanoferrate(III), potassium hydrogen sulphate, sodium acetate, and potassium chloride) into a modifying coated PVA film were partially successful. The coated electrode behaved very satisfactorily with freshly-prepared iron(II) and iron(III) solutions but with hydrolysed iron, the iron(III) signal was only 85% that of iron(II).     

Surfactant media for constant-current coulometry. Application for the determination of antioxidants in pharmaceuticals

Effect of surfactant presence on electrochemical generation of titrants has been evaluated and discussed for the first time. Cationic (1-dodecylpyridinium and cetylpyridinium bromide), anionic (sodium dodecyl sulfate) and nonionic (Triton X100 and Brij^® 35) surfactants as well as nonionic high molecular weight polymer (PEG 4000) do not react with the electrogenerated bromine, iodine and hexacyanoferrate(III) ions. The electrogenerated chlorine chemically interact with Triton X100 and Brij^® 35. The allowable range of surfactants concentrations providing 100% current yield has been found. Chain-breaking low molecular weight antioxidants (ascorbic acid, rutin, α-tocopherol and retinol) were determined by reaction with the electrogenerated titrants in surfactant media. Nonionic and cationic surfactants can be used for the determination of antioxidants by reaction with the electrogenerated halogens. On contrary, cationic surfactants gives significantly overstated results of antioxidants determination with electrogenerated hexacyanoferrate(III) ions. The use of surfactants in coulometry of α-tocopherol and retinol provides their solubilization and allows to perform titration in water media. Simple, express and reliable coulometric approach for determination of α-tocopherol, rutin and ascorbic acid in pharmaceuticals using surfactant media has been developed. The relative standard deviation of the measurements does not exceed of 5%.     

Selective cation exchange separation of bismuth on zirconium hexacyanoferrate(III) column

A new inorganic ion exchanger, zirconium hexacyanoferrate(III) prepared by adding potassium hexacyanoferrate(III) to zirconium oxychloride at 80°C has been found to be stable in acids and salt solutions. Distribution coefficient values determined for various metal ions show that the exchanger has a high affinity for bismuth. Binary separations of Bi(III) from a number of other metal ions at different concentrations were achieved on the column of the exchanger. Thus the exchanger has been found useful for the separation of bismuth from a large number of metals.     

Precise coulometric titration of sodium thiosulfate and development of potassium iodate as a redox standard

In this paper, we determine the effective purity of potassium iodate as a redox standard with a certified value linked to the international system of units (SI units). Concentration measurement of sodium thiosulfate solution was performed by precise coulometric titration with electrogenerated iodine, and an assay of potassium iodate was carried out by gravimetric titration based on the reductometric factor of sodium thiosulfate assigned by coulometry. The accuracy of the coulometric titration method was evaluated by examining the current efficiency of iodine electrogeneration, stability of sodium thiosulfate solutions and dependence on the amount of sodium thiosulfate solution used. The measurement procedure for gravimetric titration of potassium iodate with sodium thiosulfate was validated based on determination of a reference material of known purity (potassium dichromate determined by coulometry with electrogenerated ferrous ions) using the same gravimetric method. Solutions of 0.2 and 0.5 mol/L sodium thiosulfate were stable over 17 days without stabilizer. Investigation of the dependency of titration results on the amount of sodium thiosulfate solution used showed no significant effects, no evidence of diffusion of the sample, and no effect of contamination appearing during the experiment. Precise coulometric titration of sodium thiosulfate achieved a relative standard deviation of less than 0.005% under repeating conditions (six measurements). For gravimetric titration, the results obtained for the effective purity of potassium dichromate were sufficiently close to its certified value to allow confirmation of the validity of the gravimetric titration was confirmed. The relative standard deviation of gravimetric titration for potassium iodate was less than 0.011% (nine measurements), and a redox standard with a certified value linked to SI units was developed.     

A contribution to the use of tetravalent manganese in solutions of sulfuric acid as an oxidimetric reagent

The preparation of solutions of manganese(IV) sulfate in 9 M sulfuric acid as well as the stability of these solutions was studied for 0.00501 M concentration of manganese(IV) ions. Potentiometric titration of solutions of primary standard substances, potassium iodide and sodium hexacyanoferrate, was recommended for determining the titer of 0.005 M reagent solutions. It was verified that manganese(IV) sulfate can be used for determining low concentrations of organic substances by direct titration determination of hydroquinone, p-aminophenol, and metol and by indirect determination of oxalic acid.