Determination of gold(III) alone or in some mixtures and alloys by potentiometric titration of iodide

The present work is an application of iodide to the reduction of gold(III), in an attempt to develop new method for gold(III) based on potentiometric back-titration of the excess of iodide with mercury(II). Although it was proved by calculation that the reduction of tetrachloroaurate to the metal should proceed quantitatively to completion, yet our experiments showed that the reduction under ordinary conditions, gave Au(I) in the form of a white precipitate of AuI. We succeeded to push the reduction with iodide to the metal by an excess of ethanol-ether catalyzed iodide at somewhat elevated temperature.With the experimental conditions established a reliable procedure have been developed involving a potentiometric finish which enabled accurate determination of gold(III), either alone or in some of its alloys.     

Potentiometric redox determination of gold(III) and its application to alloys

The simple potentiometric method proposed for the indirect determination of 1–10 mg of gold(III) is based on reduction to the metal with excess of cobalt(II) in the presence of 1,10-phenanthroline or 2,2'-bipyridine at pH 3 and 50°C, and titration of the unused cobalt(II) complex with iron(III) chloride solution. Many metal ions can be tolerated; Ag(I) and Pd(II) are eliminated by precipitation with sodium chloride and 1,10-phenanthroline or 2,2'-bipyridine, respectively, but Hg(II), Fe(III) and Pt(IV) interfere. The method is applied to the determination of gold in alloys.     

Redox properties of cobalt(III) mixed ligand complexes. II. Redox titration of some organic and inorganic substances potentiometrically

A stable solution of (cobalt(III)(salen))₂O₂ was prepared under optimum conditions for its use for the potentiometric determination of xylenols, hydroquinone, sodium thiosulfate, and sodium metabisulfite, by iodide reduction of unreacted cobalt(III) complex and back titrating excess iodide with mercury(II) using silver amalgum and SCE combination. The possible schemes for the course of the redox reactions were suggested. The procedure was also evaluated in comparison with those mentioned in the literature.     

Applications,involving the iodide ion: VIII. Direct and indirect determination of mercury(I) and analysis of mixtures. Analysis of chromium(VI)-chromium (III) mixtures. Determination of hypochlorite

Mercury(I), down to 3 ppm, has been accurately determined by direct titration with iodide or by back-titrating excess of iodide with mercury (II) using silver amalgam as the indicator electrode. The direct method and additional volumetric ones were applied to the rapid analysis of various mixtures involving mercury(I) with fair accuracy and precision. Analysis of Cr(VI)-Cr(III) mixtures involved potentiometric back-titration of excess iodide and of excess EDTA separately with mercury(II). Back-titration of excess iodide was successfully applied to the determination of hypochlorite.     

Flow potentiometric stripping analysis for mercury(II)

The optimum experimental conditions, with respect to sample and stripping solution composition, in computerised flow potentiometric stripping analysis for mercury(II) with a gold working electrode are described. When pre-electrolysis -was done in a sample to which ammonia and iodide had been added and stripping was done in an acidified bromide solution containing chromium(VI), a detection limit of 2 nM (0.4 μg kg^-1) was obtained after 90 s of pre-electrolysis, the dynamic range being almost three decades. Copper(II) interfered when present in a 1000-fold excess and silver(I) when present in a 5-fold excess over mercury(II).     

Potentiometric titration of gold in ores with potassium iodide

Summary A direct potentiometric titration method for the determination of gold in ores and alloys is described. It is based on the reduction of Au(III) with iodide ions yielding accurate and reproducible results. Detection limit and sensitivity were 0.06 and 0.032 mg/l Au, respectively. The linear response range was between 0.1 and 120 mg/l Au. The method can be used for the routine assay of gold in different kind of samples.     

Applications involving oxidation with potassium bromate : II. Potentiometric titration of chromium alone or in mixtures

A rapid and reliable determination of chromium was developed based on bromate oxidation of chromium(III) to chromium(VI). The reaction is complete under weakly acidic conditions and with cobalt(II) present as a catalyst. Unreacted bromate and chromium(VI) are then reduced with sulfite to bromide and chromium(III). The bromide is titrated potentiometrically with mercury(I) using a silver amalgam indicator electrode. Iron(III) if present is reduced by sulfite to iron(II) and does not interfere. Some binary and ternary metal mixtures containing chromium can be resolved by the determination of chromium, alone or with another metal, by the above procedure coupled with procedures for further sample portions involving the potentiometric titration of unreacted CyDTA or iodide, or both, with mercury(II).     

Iodine trichloride as an analytical reagent for determination of some organic compounds

Procedures are described for the determination of organic compounds with iodine trichloride under Andrews's titration conditions. Samples are directly titrated with iodine trichloride or first reacted with an excess of iodine monochloride, with subsequent titration of the iodine formed. The direct titration is done initially in feebly acid medium, then the acidity is raised (biotin, methionine, cystine and thiomersal). Pre-oxidation with iodine monochloride is used if the organic compound reacts slowly [tryptophan and arsenic(III) compounds] or is determined in bicarbonate medium (hydroxylamine and thiosemicarbazide). The ferrocyanide formed by the reduction of ferricyanide (by thiourea and allylthiourea) can also be titrated. Arsenic(V) compounds are determined after reduction to arsenic(III), and iodine in organic compounds is converted into iodide by alkaline fusion into iodide and the iodide titrated.     

Sequential potentiometric complexometric redox determination of iron(III) and cobalt(II) with application to alloys

A simple potentiometric method is presented for successive determination of iron(III) and cobalt(II) by complexometric titration of the iron(III) with EDTA at pH 2 and 40 degrees , followed by redox titration of the cobalt(II) complex with 1,10-phenanthroline or 2,2'-bipyridyl at pH 4-5 and 40 degrees , with gold(III). There is no interference in either determination from common metal ions other than copper(II), which severely affects the cobalt determination but can be removed by electrolysis. The method has been successfully applied to determination of iron and cobalt in Kovar and Alnico magnet alloys.     

Mechanism of trivalent gold reduction and reactivity of transient divalent and monovalent gold ions studied by gamma and pulse radiolysis

The detailed kinetics of the multistep mechanism of the Au(III) ion reduction into gold clusters have been investigated by radiation chemistry methods in 2-propanol. In particular, a discussion on the steady state radiolysis dose-dependence of the yields concludes to a comproportionation reaction of nascent gold atoms Au(0) with excess Au(III) ions into Au(II) and Au(I). This reaction should be achieved through Au(III) consumption before the coalescence of atoms Au(0) into gold clusters may occur. Then gold clusters catalyze the reduction of Au(I) by 2-propanol. It was also found that a long-lived Au(II) dimer, (Au(II))(2), was transiently formed according to the quantitative analysis of time-resolved absorbance signals obtained by pulse radiolysis. Then the disproportionation of Au(II) is intramolecular in the dimer instead of intermolecular, as usually reported. The yields, reaction rate constants, time-resolved spectra, and molar extinction coefficients are reported for the successive one-electron reduction steps, involving especially the transient species, such as Au(II), (Au(II))(2), and Au(I). The processes are discussed in comparison with other solvents and other metal ions.     

Determination of antimony in ores and related materials by continuous hydride-generation atomic-absorption spectrometry after separation by xanthate extraction

A continuous hydride-generation atomic-absorption spectrometric method for determining approximately 0.02 mug/g or more of antimony in ores, concentrates, rocks, soils and sediments is described. The method involves the reduction of antimony(V) to antimony(III) by heating with hypophosphorous acid in a 4.5M hydrochloric acid-tartaric acid medium and its separation by filtration, if necessary, from any elemental arsenic, selenium and tellurium produced during the reduction step. Antimony is subsequently separated from iron, lead, zinc, tin and various other elements by a single cyclohexane extraction of its xanthate complex from approximately 4.5M hydrochloric acid/0.2M sulphuric acid in the presence of ascorbic acid as a reluctant for iron(III). After the extract is washed, if necessary, with 10% hydrochloric acid-2% thiourea solution to remove co-extracted copper, followed by 4.5M hydrochloric acid to remove residual iron and other elements, antimony(III) in the extract is oxidized to antimony(V) with bromine solution in carbon tetrachloride and stripped into dilute sulphuric acid containing tartaric acid. Following the removal of bromine by evaporation of the solution, antimony(V) is reduced to antimony(III) with potassium iodide in approximately 3M hydrochloric acid and finally determined by hydride-generation atomic-absorption spectrometry at 217.8 nm with sodium borohydride as reluctant. Interference from platinum and palladium, which are partly co-extracted as xanthates under the proposed conditions, is eliminated by complexing them with thiosemicarbazide during the iodide reduction step. Interference from gold is avoided by using a 3M hydrochloric acid medium for the hydride-generation step. Under these conditions gold forms a stable iodide complex.     

Kinetic and mechanistic investigations of the light induced formation of gold nanoparticles on the surface of TiO2

The kinetics of the formation of gold nanoparticles on the surface of pre-illuminated TiO(2) have been investigated using stopped-flow technique and steady state UV/Vis spectroscopy. Excess electrons were loaded on the employed nanosized titanium dioxide particles by UV-A photolysis in the presence of methanol serving as hole scavenger, stored on them in the absence of oxygen and subsequently used for the reduction of Au(III) ions. The formation of gold nanoparticles with an average diameter of 5 nm was confirmed after mixing of the TiO(2) nanoparticles loaded with electrons with aqueous solution of tetrachloroaureate (HAuCl(4)) by their surface plasmon absorbance band at 530 nm, as well as by XRD and HRTEM measurements. The rate of formation of the gold nanoparticles was found to be a function of the concentration of the gold ions and the concentration of the stored electrons, respectively. The effect of PVA as a stabilizer of the gold nanoclusters was also studied. The observed kinetic behavior suggests that the formation of the gold nanoparticles on the TiO(2) surface is an autocatalytic process comprising of two main steps: 1) Reduction of the gold ions by the stored electrons on TiO(2) forming gold atoms that turn into gold nuclei. 2) Growth of the metal nuclei on the surface of TiO(2) forming the gold particles. Interestingly, at higher TiO(2) electron loading the excess electrons are subsequently transferred to the deposited gold metal particles resulting in "bleaching" of their surface plasmon band. This bleaching in the surface plasmon band is explained by the Fermi level equilibration of the Au/TiO(2) nanocomposites. Finally, the reduction of water resulting in the evolution of molecular hydrogen initiated by the excess electrons that have been transferred to the previously formed gold particles has also been observed. The mechanism of the underlying multistep electron-transfer process has been discussed in detail.     

Indirect cerimetric determination of gold at the milligram level

Summary A rapid volumetric method has been worked out for the indirect determination of 0.25–2.5 mg of gold in presence of many common ions. It is based on the reduction of gold(III) to metal with excess of cobalt(II) in the presence of 1,10-phenanthroline at pH 3 and 50°, and estimation of the unreacted cobalt(II) in the filtrate by visual, potentiometric or biamperometric titration with standardized cerium(IV) sulphate solution. It has been found that there is no interference from Ni(II), Pb(II), Zn(II), Cd(II), Mn(II), Mg(II), Ca(II), Al(III), Cr(III), Ti(IV), V(V) and W(VI). Interference due to Pd(II) and Ag(I) can be eliminated. Fe(III), Cu(II), Mo(VI), Hg(II) and Pt(IV) interfere, even present in small amounts.

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Zusammenfassung Ein schnelles maßanalytisches Verfahren zur indirekten Bestimmung von 0,25–2,5 mg Gold in Gegenwart vieler Ionen wurde ausgearbeitet. Es beruht auf der Reduktion zu metallischem Gold mit überschüssigem Kobalt(II) in Anwesenheit von 1,10-Phenanthrolin bei pH 3 und 50°. Die Rückbestimmung des unverbrauchten Kobalts im Filtrat erfolgt durch potentiometrische oder biamperometrische Titration mit Cer(IV)sulfat. Ni(II), Pb(II), Zn(II), Cd(II), Mn(II), Mg(II), Ca(II), Al(II), Cr(III), Ti(IV), V(V) und W(VI) stören nicht. Eine Störung durch Pd(II) oder Ag(I) kann man ausschalten. Fe(III), Cu(II), Mo(VI), Hg(II) und Pt(IV) stören auch in geringen Mengen.

     

Oxidation potentials of platinum(II) bromide and iodide complexes

The reactions of platinum(II) bromide and iodide complexes with potassium permanganate have been studied by potentiometric titration. The main regularities of the effect of complexation on the reduction properties of the central metal atom and ligands have been found.     

A kinetic-catalytic method with repeated addition of one reactant — the determination of manganese, iodide, urease and cadmium

The use of a new kinetic-catalytic method with repeated addition of one reactant in the same system is described. The determination of manganese (periodate—malachite green; photometric observation), iodide (cerium(IV)—arsenic(III); potentiometric observation) and urease (hydrolytic splitting of urea; potentiometric observation) serves to illustrate the procedure.     

Titrimetric determination of gold by precipitation with hydroquinone

A rapid method for the volumetric determination of gold is described. Gold(III) is reduced to metal with excess of hydroquinone at room temperature and the excess is titrated with ceric sulphate. The effect of chloride ion on the reducibility of gold(III) to the metal is discussed. This method can be successfully employed for the determination of gold in presence of certain base and noble metals. The method was successfully adopted for the determination of gold in a commercial sample.     

Binuclear Schiff base complex of manganese(III) as a neutral carrier for a highly selective iodide electrode.

A new highly selective iodide electrode incorporating a binuclear manganese(III) complex, bis(salicylaldehyde-aminopropanol)dichloroaceticdimanganese(III) [Mn(III)(2)-BSAPDCA], as a neutral carrier is described. The electrode displays an anti-Hofmeister selectivity sequence: iodide > perchlorate > salicylate > thiocyanate > nitrate > bromide > nitrite > chloride > sulfate. The excellent selectivity for iodide is related to a direct interaction between the central Mn(III) atom and iodide and a steric effect associated with the structure of the carrier, which is supported by UV spectroscopy and AC impedance techniques. The electrode exhibits a near-Nernstian potentiometric linear response range to iodide from 1.0 x 10(-1) to 2.0 x 10(-5) mol/L with a detection limit of 8.0 x 10(-6) mol/L and a slope of -60.3 mV/decade in pH 3.0 of phosphate buffer solutions at 20 degrees C. From a comparison of the potentiometric response characteristics between a binuclear manganese(III) complex, Mn(III)(2)-BSAPDCA, and a mononuclear manganese(III) complex, Mn(III)-BSAPB, an enhanced response towards iodide from a binuclear metallic complex-based electrode was observed. The electrode, based on binuclear manganese(III) complex, was successfully applied to the determination of inorganic total iodine in iodized table salt with satisfactory results.     

Extractive separation and spectrophotometric determination of palladium and platinum with dithizone in the presence of stannous chloride

The conditions [acid used, presence of chloride and tin(II)] for the extractive separation and spectrophotometric determination of palladium and platinum as the dithizonates Pd(HDz)(2) and Pt(HDz)(2) have been examined. In the absence of stannous chloride platinum does not undergo extraction. Conditions for the separation and determination of these metals in the presence of mercury, gold and copper, which are also extracted with dithizone into carbon tetrachloride or chloroform under the conditions suitable for palladium (1M sulphuric acid/0.1M hydrochloric acid), have been defined. The mercury and gold dithizonates are formed quickly and can be removed before the palladium and platinum compounds have had time to form. They can be decomposed with iodide. Copper dithizonate is decomposed by reduction with tin(II). The proposed procedure has been applied to the determination of palladium in technical platinum metal.     

A Comparative Study on Spectrophotometric,Conductometric and Potentiometric Determination of Ascorbic Acid

Abstract

A comparison has been made on the selectivity, accuracy and precision of spectrophotometric, conductometric and potentiometric methods for the determination of ascorbic acid in water which is based on the reduction of gold(III) ion by ascorbic acid. Gold(III) ion forms a complex with gelatin in alkaline medium which on reduction with ascorbic acid produces coloured gold sol. The sol shows an absorption maximum at 540 nm with molar absorptivity of 2.3×10^?3litre mol^?1 cm^?1 and the Sandell's sensitivity of 7.6 × 10^?2 μgcm^?2. The relative standard deviation is 0.22% and the confidence limit (20 determinations, 95%) is 2.0 ± 0.009%. To examine the extent of sensitivity of the spectrophotometric method the proposed method is compared with the sensitivities of the conductometric and potentiometric methods.     

Electrometric end points in the oxidation of thiocyanate by permanganate. Determination of cerium

Summary Oxidation of thiocyanate by standard permanganate using electrometric end points is shown to be quantitative in 1.5–2.5N HCl medium and in the presence of ICl as catalyst. A method of estimating cerium(IV), based upon its reduction to cerium(III) by excess KSCN and potentiometric titration of the excess against KMnO₄, is also described.Sincere thanks of the author are due to Prof. S. S. Joshi for research facilities.