A new volumetric method for the determination of uranium(VI)

Summary A new volumetric method for the estimation of uranium(VI) salt based on its photoreduction in the presence of diethyl ether has been developed. The recommended procedure consists of exposing uranium(VI) solution in about 1 N sulphuric acid solution with an excess of saturated aqueous ether solution in a glass vessel to the light from a Phillips repro lamp or sun light for 1 hour. The uranium(IV) salt formed is estimated by titration with a standard solution of sodium vanadate.The reduction does not proceed to uranium(III) stage under any conditions of exposure. Fluoride, phosphate, arsenate and perchlorate are not found to interfere either with the photochemical reduction or with the subsequent oxidimetric titration. But chloride and bromide ions markedly inhibit the photochemical reaction.     

Spectrophotometric determination of molybdenum(VI)

Molybdenum(IV) gives a red colour with ammonium thiocyanate in 5-8M hydrochloric acid medium, the Sandell sensitivity index being 0.018 ppm Mo(VI)/cm(2). Molybdenum(VI) in 4-7M hydrochloric acid medium forms a red complex with ethyl xanthate and ammonium thiocyanate and this can be extracted into acetophenone. Beer's law is obeyed over the range of 1.2-13.8 ppm, and the Sandell indices at 370 and 470 nm are 0.0016 and 0.0068 ppm/cm(2) respectively. The colour is stable for 40 hr. Most cations do not interfere.     

Complexometric determination of molybdenum(VI)

In acidic medium molybdenum(VI) forms a stable complex on boiling with excess of DCTA and hydroxylamine hydrochloride. Molybdenum can then be determined by back-titration of the excess of DCTA either with zinc chloride at pH 5-5.5 or with thorium nitrate at pH 3-4.5, Xylenol Orange being used as indicator in both cases. A simple method for the determination of molybdenum in the presence of moderate amounts of tungsten is also described.     

A new method for the voltammetric determination of molybdenum(VI) using carbon paste electrodes modified in situ with cetyltrimethylammonium bromide

Molybdenum(VI) is determined by anodic stripping voltammetry using a carbon paste electrode modified in situ with cetyltrimethylammonium bromide (CTAB). The preconcentration of molybdenum is performed by adsorption and reduction of ion-pairs of cetyltrimethylammonium and molybdenum(VI) oxalate at a potential of −0.4 V vs. the saturated calomel electrode (SCE). The supporting electrolyte contains 0.01 M oxalic acid and 0.075 mM CTAB. Differential pulse anodic stripping voltammetry exploiting the reoxidation signal is used for the determination of trace levels of molybdenum(VI). Linearity between current and concentration exists for a range of 0.5–500 μg 1⁻¹ Mo with proper preconcentration times; the limit of detection (calculated as 3σ) is 0.04 μg 1⁻¹ with an accumulation period of 10 min.     

Pyrogallol red and bromopyrogallol red in new optical methods for the determination of molybdenum(VI) and tungsten(VI)

The complexation of molybdenum(VI) and tungsten(VI) with pyrogallol red (PR) and bromopyrogallol red (BPR) in the presence of a cationic surfactant, cetylpyridinium bromide was studied. Conditions of the preconcentration of molybdenum(VI) and tungsten(VI) as complexes with PR and BPR on Silochrom S-120 were found. The concentration coefficients were no lower than 67 for a volume of the aqueous phase of 20 mL and a mass of the sorbent of 0.3 g. Chromaticity characteristics of the complexes in solutions and on the sorbent were determined. It was demonstrated that the complex of molybdenum(VI) with BPR in the presence of cetylpyridinium bromide should be used in the analysis of materials with low concentrations of molybdenum.     

Kinetic-spectrophotometric determination of molybdenum (VI) and tungsten (VI) in mixtures

A simple kinetic-spectrophotometric method is described for the determination of molybdenum(VI) and tungsten(VI) in mixtures, without prior separation. The method is based on the catalytic effect of molybdenum(VI) and tungsten(VI) on the oxidation of 2,4-diaminophenol dihydrochloride (DAP) by hydrogen peroxide in acidic medium. The reaction was followed spectrophotometrically by measuring the rate of change in absorbance with time at 500 nm. A partial inhibition in the catalytic activity of each catalyst, when the other one is present, at all ratios of Mo(VI) W(VI) mixtures studied was observed. On the other hand, the catalytic activity of tungsten(VI) dropped to zero whilst that of molybdenum(VI) decreased slightly, in the presence of citrate ions. Two sets of experiments were carried out, the first in the absence and the other in the presence of citrate, and the resolution of Mo(VI)/W(VI) mixtures was achieved by solving two simultaneous equations. Various molar ratios of Mo(VI) W(VI), at the 10^–6 M level, from 0.2 1 to 5 1 can be determined with satisfactory precision and accuracy. The selectivity of the method was investigated and the method was applied successfully to the determination of molybdenum and tungsten in each other's presence in steel.     

Azo compounds as reagents for the voltammetric determination of molybdenum(VI)

Linear sweep voltammograms of Lumogallion IREA (pH 2), Magneson IREA (pH 2), 4-(2-pyridylazo)resorcinol (pH 4.8), and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (pH 4.8) in the presence of molybdenum(VI) exhibit peaks at potentials more negative than the potentials of reduction peaks of the reagents by approximately 0.1 V. In all of the above cases, the heights of these peaks linearly increased with an increase in the concentration of molybdenum(VI) in the range from 5 x 10^-7 to 2.5 x 10^-6 M; thus, these peaks can be used for the determination of molybdenum. The simultaneous proportional decrease in the heights of the cathodic peaks of the reagents can be used for indirect determination of molybdenum(VI). The limits of detection without preliminary accumulation at a dropping mercury electrode with a drop time of 5 s are (1.5-3.9) x 10^-7 M, depending on the nature of the reagent and the technique used for determining the concentration.     

Polarographic determination of nanomolar concentrations of molybdenum(VI)

Summary Polarographic Determination of Nanomolar Concentrations of Molybdenum (VI) A new differential pulse polarographic method for the determination of Mo(VI) is described. Mo(VI) is first chelated by 7-nitro-8-hydroxychinoline-5-sulfonic acid at pH 1. The complex ion MoO₂L₂ ^2– formed is strongly adsorbed on the surface of a dropping mercury electrode. At a potential difference of 0.95 V the complex ion is reduced to a Mo(V)-complex, which is oxidized very fast by H_aq ⁺ providing the starting complex ion for repeated redox cycles. The net process consists in the catalytic reduction of H_aq ⁺ to 1/2 H₂ in the double layer. H₂ was detected by inductively coupled plasma atomic emission spectrometry. A modified preparation method for the chelating agent and its characterization are also described. The method was used for the determination of Mo(VI) in the surface water of Lake Zürich. An average value of 0.463±0.007 ng/g (4.83±0.07 nM) was calculated from 39 single values. The errors are the confidence intervals of the corresponding means at the 99% confidence level. The standard deviation and the practical detection limit were 0.016 and 0.031 ng/g, respectively, for single determinations on the average.     

A volumetric method for the determination of sulphoxides

Summary A volumetric method for the quantitative determination of Sulphoxides has been developed, based on their oxidation to sulphones by dichromate in presence of 5 M sulphuric acid at 105–110°C. Excess dichromate is titrated by ferrous ammonium sulphate solution. The method is also applicable to the determination of Sulphoxides in metal sulphoxide complexes.

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Zusammenfassung Zur Bestimmung von Sulfoxiden wurde ein volumetrisehes Verfahren ausgearbeitet, das auf der Oxydation zu Sulfonen mit Hilfe von Dichromat in Gegenwart von 5 M Schwefelsäure (105–110°C) beruht. Der Überschuß an Dichromat wird mit Eisen(II)-ammoniumsulfatlösung zurücktitriert. Der Verfahren eignet sich auch zur Sulfoxidbestimmung in Metallsulfoxidkomplexen.

     

Catalytic spectrophotometric determination of molybdenum (VI) with the hydrazine-metanil yellow system

A new catalytic Spectrophotometric method for the determination of trace amounts of molybdenum (VI) has been proposed. The method is based on the catalytic effect of Mo(VI) on the reduction of metanil yellow by hydrazine dihydrochloride. Under experiment condition, the linear range of determination is 20–160 ng/mL for molybdenum and the detection limit is 11.2ng/mL. The method has been used to determine trace molybdenum in bean samples with the recovery of 96.0–99.0%, with relative standard deviations of 1.50–2.53%.     

Comparison of different flow-through adsorptive stripping voltammetric methods for the determination of molybdenum(VI)

Four different complexing reagents namely chloranilic acid, oxine, tropolone, and cupferron were applied for AdSV determinations of molybdenum. The parameters for the determination using a flow-through cell with a hanging mercury drop (HMDE) as working electrode were examined systematically for all four systems and evaluated. Cyclic voltammograms were recorded to examine the electrode reaction, alternating current (AC) voltammetry was used to determine adsorption processes. The comparison includes sensitivity, detection limit, linear concentration range, the susceptibility to interference by organic compounds or foreign ions, and the applicability to sea and tap water samples. The interpretation of the electrode reaction mechanism for the reduction of the Mo cupferron complex published from Jiao et al. [10] was improved. Received: 12 February 1999 / Revised: 10 May 1999 / Accepted: 14 May 1999     

Comparison of different flow-through adsorptive stripping voltammetric methods for the determination of molybdenum(VI)

Four different complexing reagents namely chloranilic acid, oxine, tropolone, and cupferron were applied for AdSV determinations of molybdenum. The parameters for the determination using a flow-through cell with a hanging mercury drop (HMDE) as working electrode were examined systematically for all four systems and evaluated. Cyclic voltammograms were recorded to examine the electrode reaction, alternating current (AC) voltammetry was used to determine adsorption processes. The comparison includes sensitivity, detection limit, linear concentration range, the susceptibility to interference by organic compounds or foreign ions, and the applicability to sea and tap water samples. The interpretation of the electrode reaction mechanism for the reduction of the Mo cupferron complex published from Jiao et al. [10] was improved. Received: 12 February 1999 / Revised: 10 May 1999 / Accepted: 14 May 1999     

Anderson-type heteropolyanions of molybdenum(VI) and tungsten(VI)

The previously reported preparation of some Anderson-type molybdopolyanions containing divalent metal ions (Zn, Cu, Co or Mn) as a heteroatom has been reinvestigated. The molybdopolyanions of Zn(II) and Cu(II) were confirmed, although the Cu(II) polyanion was not stable and could not be recrystallized. On the other hand, the polyanions of Co(II) and Mn(II) could not be reproduced. Another type of heteropoly compound, [X(H₂O)_6-x(Mo₇O₂₄)]⁴⁻ [X = Cu(II), Co(II) or Mn(II)], was isolated as solids, which are not stable thermally. The mixed-type Anderson polyanions, [Ni(II)Mo_6-xW_x,O₂₄H₆]⁴⁻, which have been questioned as mixtures of species with different x values, were also reinvestigated using IR, UV absorption and MCD spectra. They are single species, but not mixtures, although some positional isomers may be present for the compounds where x = 2-4. The possibility of oxidation of the heteroatom with the Anderson structure maintained was examined. The oxidation of [Ni(II)Mo₆O₂₄H₆]⁴⁻ by the S₂O²⁻₈ ion in aqueous solution gave the Waugh-type [Ni(IV)Mo₉O₃₂]⁶⁻ polyanion, whereas the oxidation of [Ni(II)W₆O₂₄H₆]⁴⁻ gave no heteropoly compound.     

Simultaneous determination of molybdenum(VI) and tungsten(VI) and its application in elemental analysis of polyoxometalates

Spectrophotometric determination of molybdenum(VI) and tungsten(VI) with application of Artificial Neural Networks is proposed and it was applied for elemental analysis of solid polyoxometalates. Better results in comparison with previously those achieved by previous published method were demonstrated. MALDI-TOF Mass Spectrometry was tested for possible determination of molecular weight of polyoxometalates utilizing different matrices. Phenomena observed during desorption-ionisation processes are discussed. LDI-TOF MS was found to be suitable for the determination of Mo:W ratio in polyoxometalates as a rapid screening method to follow synthetic procedure.     

Adsorptive stripping voltammetry determination of molybdenum(VI) in water and soil

A differential pulse stripping voltammetry method for the trace determination of molybdenum(VI) in water and soil has been developed. In 0.048M oxalic acid and 6 x 10(-5)M Toluidine Blue (pH 1.8) solution, Mo(V), the reduction product of Mo(VI) in the sample solution, can form a ternary complex, which can be concentrated by adsorption on a static mercury drop electrode at -0.1 V (vs. Ag/AgCl). The adsorbed complex gives a well-defined cathodic stripping current peak at -0.30 V, which can be used for determining Mo(VI) in the range 5 x 10(-10)-7 x 10(-9)M, with a detection limit of 1 x 10(-10)M (4 min accumulation). The method is also selective. Most of the common ions do not interfere but Sn(IV) and large amounts of Cu(2+), Ag(+) and Au(3+) affect the determination.