Polarographic study of Mo(VI) in 0.1–5 M sulfuric acid solutions

Reduction mechanisms of polarographic reduction waves of Mo(VI) in 0.1–5 M sulfuric acid solutions are described. Three reduction waves are observed when the concentration of sulfuric acid is >3 M. From the results of coulometry and the catalytic behavior of Mo(V), it is concluded that three different reduction mechanisms of Mo(VI) to Mo(V) are present and that two separate reductions of Mo(VI) to Mo(V) and of Mo(V) to Mo(III) are involved at the potential of the third wave. The presence of three reduction mechanisms of Mo(VI) to Mo(V) in sulfuric acid α 3 M seems to indicate the existence of three different chemical species of Mo(VI). Two of these three species are different from the present in 0.1 M sulfuric acid.     

The polarographic determination of traces of titanium-(IV) in the presence of n-benzoyl-n-phenylhydroxylamine

The polarographic behavior of the titanium(IV)-N-benzoyl-N-phenyl-hydroxylamine (BPHA) system in acidic medium and in water-ethanol mixtures has been studied. In (1+3) water-ethanol containing 2 M sulfuric acid and 0.05 M BPHA, titanium(IV) gives a single kinetically controlled wave. Titanium(IV) can be determined at concentrations as low as 5·10^-6M, in the presence of Fe(III), Cu(II), V(V), etc., but Cd(II), Sn(II and IV), As(V), U(VI) and Mo(VI) interfere.     

Dimerization of electrogenerated Mo(V) in sulphuric acid solutions

The electrochemical behaviour of molybdenum(VI) in sulphuric acid solutions was investigated by cyclic voltammetry. In the reduction of Mo(VI) to Mo(III) a dimerization reaction of Mo(V) is involved; the rate constant for the reaction was estimated to be 2.79×10² M⁻¹ s⁻¹ and the activation energy was ca. 35 kJ mol⁻¹ in 0.1 M H₂O₄. Oxidation of the monomer and dimer Mo(V) species take place at −0.31 and +0.18 V (vs. SCE), respectively.     

Anodic stripping voltammetry of hexavalent molybdate

The anodic stripping voltammetry of hexavalent molybdate was investigated with a hanging mercury drop electrode in phosphate-borate buffer solution of pH 5.8–6.7. It was found that the deposition reaction was limited by the diffusion of the protonated molybdate ion, and the anodic stripping reaction was kinetically controlled by the dissolution of the deposited molybdate. A calibration curve for the anodic peak current versus Mo(VI) concentration was linear when Mo(VI) was within the range of 7×10^?6M and 2×10^?4M at pH 5.80. Various effects for the calibration curve were studied, and its application for the determination of Mo(VI) was discussed.     

Oxidation of organic substances with compounds of trivalent manganese: XXIII. Preparation,stability, and determination of the titer of standard solutions of the fluoride complex of trivalent manganese

A method of preparation of 10^?2 ?10^?3M standard solutions of the fluoride complex of manganese(III) by the reaction of manganese(II) with permanganate in a medium of potassium fluoride acidified with sulfuric acid has been developed. It has been found that in a medium of 1 M sulfuric acid, 0.5 M manganese(II) sulfate, and 0.1 M potassium fluoride these solutions are sufficiently stable for both direct and indirect titrimetric determinations. The titer was determined using potassium iodide as a primary standard and potentiometric, bipotentiometric, or biamperometric titration.     

Spectrophotometric study and extraction of niobium(V) complexes with catechol and its derivatives as ion-pairs with quaternary ammonium ions: Application to the separation of niobium from other refractory metal ions

The reactions of niobium(V) with catechol and several substituted derivatives were studied. Optimal conditions for colour development in the niobium(V)-catechol system are: pH 6.7–8.0, and heating at 85° for at least 6 min; the wavelength of maximal absorption is 365 nm, and the molar absorptivity is 1.02·10⁴. Catechol derivatives require lower temperatures and develop colours with niobium(V) in neutral or slightly acidic solutions. The best of the derivatives studied were 3-methyl-catechol and 4-chlorocatechol. Extraction of the ion-pairs formed between niobium-(V)-catechol and different quaternary ammonium ions, was studied with representative solvents. Similar studies were made on other refractory metal ions, i.e., Ti(IV), V(V), Mo(VI) and W(VI), under the optimal conditions for the reaction with niobium(V). Separations of binary (1:1) mixtures of niobium(V) with Ti(IV), V(V), Mo(VI) or W(VI) at the 5·10^-5M level were shown to be possible under the predicted conditions.     

The adsorption behaviour of99Mo(VI),238U(VI),95Zr(IV) and95Nb(V) on alumina

The distribution of Mo(VI) and the interfering radiocontaminants U(VI), Zr(IV) and Nb(V) have been investigated between chromatographic alumina and aqueous hydrochloric acid solutions of concentrations ranging from 0.5M to 11M. At low acidities (less than 1M HCl) the distribution coefficients increase with the decrease of acid concentration, while in the region of 2–4M they increase with the increase of the acid concentration. Above 4M HCl, the increase inK _D continues with the acid concentration for both Zr(IV) and Nb(V), but constant values are reached for U(VI) and Mo(VI).     

Infrared spectra of carbon monoxide adsorbed on a smooth gold electrode Part I. EMIRS spectra in acid and alkaline solutions

Electrochemically modulated infrared spectroscopy (EMIRS) was applied to the study of adsorption of CO on gold in 0.5 M sulfuric acid and 1 M sodium hydroxide solutions. A CO stretch band was observed with a peak intensity of ca. 0.2 % between 1850 and 2000 cm⁻¹ in 1 M NaOH, while a very weak band was detected between 1950 and 2050 cm⁻¹ in 0.5 M H₂SO₄. The bands were assigned to linear CO species adsorbed on the gold surface. In 1 M NaOH, electrooxidation of the strongly adsorbed CO species, which was detected by EMIRS, starts from ca. 0.5 V (RHE) with a sharp voltammetric current peak at 1.0 V at 50 mV/s, while electrooxidation of the bulk CO starts from ca. 0 V in the absence of the strongly adsorbed CO species on Au. The strongly adsorbed CO species acts as a poison for the electrooxidation of CO in the lower potential region.     

Structural investigation of tetraperoxo complexes of Mo(VI) and W(VI): X-ray and theoretical studies

The family of very unstable tetraperoxo compounds has been prepared from aqueous solutions containing H₂O₂ and salts of Mo(VI) or W(VI). The crystal structures of Na₂[Mo(O₂)₄]·4H₂O and Na₂[W(O₂)₄]·4H₂O have been determined from single-crystal data, while the crystal structures of Rb₂[Mo(O₂)₄], Cs₂[Mo(O₂)₄], Rb₂[W(O₂)₄] and Cs₂[W(O₂)₄] have been determined from powder X-ray diffraction data. The compounds were also characterised by IR spectroscopy and the number of peroxo groups was determined by titration methods. By means of the density functional theory (DFT) method, the geometry and stability of tetraperoxo complexes have been studied. Even though in all tetraperoxo complexes the central atom Mo(VI), W(VI) or V(V) is surrounded by four peroxo groups and the geometry of the [Me(O₂)₄]ⁿ⁻ anion is essentially the same, the investigated compounds differ in stability and colour and crystallise in different crystallographic systems.     

Single-step radiochemical separations by column procedures in activation analysis

Some single-step column procedures are described for both individual and group activity separations. Besides the usual ion-exchange techniques, other methods such as reverse-phase chromatography, isotopic exchange and the use of resins converted into special forms were used. Fast and simple selective separations from 2N hydrochloric acid are reported for Mo(VI), Cu(II), Sb(V), and for AsO₄^3- + PO₄^3- from both 2 N hydrochloric acid and I N sulfuric acid; for Cu, Sb and As + P, the selectivity can be greatly increased by using a guard bed of resin in normal form. By combining the different techniques a single-step separation scheme for 6 elements (Mo, Au, Zn, As, Cu, Sb) in 2 N hydrochloric acid was developed; this allows high chemical recoveries, high cross-decontamination and very large decontamination from ²⁴Na to be reached, so that application for biological sample analysis can be envisaged. Simplified two-stage column separations for Au + Sb and Cu and Fe + Sb and Zn from concentrated hydrochloric acid (cationic and anionic resin beds coupled) are also reported.     

Determination of subnanomolar concentrations of tungsten (VI) by catalytic adsorption polarography

A new differential pulse polarographic method for the determination of W(VI) using a catalytic adsorption wave is described. W(VI) is first chelated by 7-iodo-8-hydroxyquinoline-5-sulfonic acid at pH 0.5. The complex ion formed is strongly adsorbed on the surface of a dropping mercury electrode. At a potential of –0.95 V versus the Ag/AgCl (3M KCl) reference electrode the adsorbed complex is reduced by the polarographic current and oxidized very fast by hydrated hydrogen ions providing the oxidized form of the complex ion for repeated redox cycles. As the redox process taking place in the electric double layer, the diffusion of the complex does not limit the polarographic current. Therefore, high currents occur, and consequently, a very high sensitivity is obtained. The practical detection limit (PDL) is 3.7 ng W/kg solution corresponding to 2 × 10^–11 M. The standard deviation of single values is 1.2 ng/kg at the concentration of 91 ng/kg lying in the middle of the linear part of the calibration curve. Because Mo (VI) gives a very similar catalytic adsorption wave, serious mutual interferences occur in the analysis of mixtures of both species. An effective separation of Mo(VI) was worked out. Using 1% (w/v) solution of trioctylphosphinoxide in kerosene, Mo(VI) can almost completely be extracted from 1.8M HCl with a threefold extraction resulting in a separation factor of 40000.     

Voltammetric behavior of neptunium(vi) in glutamic acid media at the rotating glassy carbon electrode

The voltammetry of neptunium(VI) glutamate was investigated over the pH range 3.8–10.0. A reversible, one-electron wave was obtained for glutamate concentrations above 0.1 M in the pH range 3.8–6.1, or above 0.3 M in the pH range 6.1–10.0. At pH 3.8–6.1, the half-wave potential was independent of pH, but at pH 6.1–10.0, it was a function of pH. The metal-ligand ratio was found to be 1:2 by conductometric titration. The limiting current was proportional to the concentration of the neptunium(VI) from 7.83·10^-5 to 1.96·10^-3M. The diffusion coefficient was 0.35·10^-5 cm² sec^-1 at pH 4.5 and 0.30·10^-5 cm² sec^-1 at pH 9.4.     

The extraction of niobium (V) and Tantalum (V) by Octylarsinic acid in chloroform

Dioctylarsinic acid (HDOAA) in chloroform solution extracts Nb(V) and Ta(V) efficiently from solutions containing oxalate and oxalic acid at hydrochloric acid concentrations greater than 1M.The extraction coefficients are 92.5 at 7M hydrochloric acid and 251 at 6M hydrochloric acid for niobium and tantalum, respectively. These metals can be extracted even more efficiently from sulfuric acid solutions. The results of the reagent- and pH-dependence studies suggested that a trimeric, monobasic oxoacid of niobium, associated with ten HDOAA molecules, is extracted. Tantalum appears to be present in the organic phase as (H₂DOAA)⁺ [Ta(C₂O₄)₃ (HDOAA_n] (n=l or 2).     

An examination of chlorpromazine hydrochloride as indicator and spectrophotometric reagent for the determination of molybdenum(V)

Chlorpromazine hydrochloride (CPA) has been tested as indicator in the titration of molybdenurn(V) with cerium(IV)sulphate at room temperature in sulphuric acid medium. It gives a sharp, reversible colour change from colourless to dark-red at the equivalence point. Chlorpromazine hydrochloride also reacts in acidic media with thiocyanatomolybdenum(V) complexes forming an orange compound with the formula (CPA · H) [MoO(SCN)₄]. This reaction provides a sensitive method for spectrophotometric determination of molybdenum(V). Molybdenum(VI) is reduced with ascorbic acid in hydrochloric acid solution, and complexed with thiocyanate, and the complex formed is extracted with chlorpromazine hydrochloride in chloroform. The molar absorptivity is 1.6 × 10⁴ l mol^-1 cm^-1 at 465 nm. Beer's law is obeyed in the range 0.5–5.0 μg Mo ml^-1.     

Voltammetric,potentiometric and amperometric studies with a rotated aluminum wire electrode : Amperometric titration of fluoride with aluminum

The rotated aluminum electrode is a suitable indicator electrode in the amperometric titration of fluoride, Fluoride in concentrations varying between 1.10^-4 and 2.10^-3M was titrated with a standard aluminum nitrate solution in aqueous buffer solutions (pH between 3.6 and 46) Equilibrium was established within 15 mim. The fluoride-aluminum ratio at the (true) end-point was found to increase from 2 2 in 10^-4M fluoride to 28 in. 2.10^-3M fluoride. The titration calculated from the stability constants of the various aluminum-fluoride complexes were found to be in excellent agreement with the experimental curves.From a practical point of view titration in an acetate buffer in 50% alcohol in the presence of 0 5 M potassium or sodium nitrate is recommended. The fluoride-aluminum ratio at the end-point was found to be 5 9 ± 0 1 and independent of the fluoride concentration.     

Metal chelates in adsorption polarography I: Mo(VI) and derivatives of oxine

The formation of complexes between Mo(VI) and 8-hydroxy-quinoline (oxine) and four oxine derivatives were investigated by multiwavelength molecular absorption spectrometry, potentiometry, and polarography. The following pK_OH- and pK_NH- values of the ligands and logK ₂₁₁-values of the complexes MoO₂(OH)₂L ^x– (x=1 or 2) were obtained at 25° C and an ionic strength of 1M(NaClO₄): 5,7-dinitro8-hydroxyquinoline 4.59, <0, 14.50; 7-nitro-8-hydroxyquinoline-5-sulfonic acid 5.34, 0.41, 15.70; 7-iodo-8-hydroxyquinoline-5-sulfonic acid 6.98, 2.62, 17.65; 8-hydroxyquinoline-5-sulfonic acid 8.33, 4.13, 18.71; and 8-hydroxyquinoline 9.62, 5.28, 19.69. A good linearity was found between logK ₂₁₁ and the sum of the pK-values of the OH- and NH⁺-groups. The dependence of the peak current of Mo(VI)-determinations by adsorption polarography of the 7-nitro-8-hydroxyquinoline-5-sulfonate complex of Mo(VI) MoO(OH)₃L^– can quantitatively be described at pH 0.8–2 using the corresponding pK-values and the log K₃₁₁ of 18.54±0.03, determined by polarography.     

Extraction of metal ions with a primary amine

This paper describes some experimental results obtained at the extraction of sulfate solutions of U(VI), Mo(VI), V(V), Ce(IV), Zr(IV), Fe(III), Al(III) with a benzene solution of Primene JMT. The aqueous solutions consist of metal sulfates (or other metal salts) in the presence of sulfuric acid with a concentration range of 0–2.1 mol·dm^–3, the concentration of amine in the organic phase being 0.1–0.3 mol·dm^–3. The presence of various species of metal ions in the aqueous phase is considered and the equilibrium concentration of substances extracted in the organic phase is determined. On the basis of the results of chemical analysis (concentration of metals and sulfate ions) the composition of the prevailing complexes in the organic phase is proposed.     

Polarography of hexavalent molybdenum in hypophosphorous acid solutions

The polarographic behaviour and determination of Mo(VI) in hypophosphorous acid solutions of concentrations varying from 0.1 to 5.0 mol l^?1 andT=25±0.1 °C have been investigated. It was shown that reduction of MoO ₄ ^2? takes place along a single or two waves depending upon the acid concentration. Microcoulometric experiments have been performed at the limiting region of the different waves obtained at different acid concentrations. A scheme for the mechanism of reduction occuring at theDME has been deduced. A method for analytical determination of Mo(VI) on both the micro- and macro-scales in hypophosphorous acid solutions has been reported. Analysis of a binary mixture Mo(VI)/Cd(II) and a tertiary mixture Mo(VI)/Cd(II)/Zn(II) in mol l^?1 hypophosphorous acid has been investigated.     

Voltammetric Behavior of Uranium(VI) in Sulfuric Acid Solutions Containing Pyridine

It was shown that 8.8 × 10^–5 to 4.4 × 10^–3 M uranium(VI) in sulfuric acid solutions containing pyridine give a clearly defined cathodic peak with a height proportional to uranium(VI) concentration. Under the specified conditions, the electroreduction of uranium(VI) is irreversible and involves one electron, the limiting current of uranium(VI) is diffusion- and kinetically controlled. The kinetic parameters of the process under study were determined, and the effect of concomitant elements on the voltammetric behavior of uranium(VI) in pyridine-containing sulfuric acid supporting electrolytes was estimated.     

Mechanism of oxidation of cyclohexanehexol (inositol) by quinquevalent vanadium

The oxidation of inositol by quinquevalent vandadium in acid medium is a first-order reaction both in vanadium (V) and inositol. The stoichiometry of the reaction is consistent with the use of two equivalents of vanadium (V) per mole of inositol with the formation of one mole of inosose. The reaction is catalyzed both by sulfuric and perchloric acid, but the rate is faster in sulfuric acid than in perchloric acid. In 1M–6M perchloric acid solutions the reaction has shown a variable order in H⁺, but in solutions of 2M–5M sulfuric and perchloric acid of constant ionic strength, the rate has a linear dependence on [H⁺]². There is also a linear correlation between the rate and bisulfate ions in sulfuric acid at constant hydrogen ion concentration. The energy of activation is found to be 19 kcal/mole and a negative entropy value of ? 14 e.u. A suitable mechanism, consistent with the kinetics in 2M–5M acid solutions, is suggested and the values of various rate constants are evaluated.