Transport study of V(V), Zn(II) and Be(II) using supported liquid membranes

V(V), Zn(II) and Be(II) have been studied to test oxine and tri-n-butylphosphate (TBP) as carriers for transport through supported liquid membranes in polypropylene film. All the three types of ions can be passed through such membranes using oxine in case of V(V) and TBP in case of Zn(II) and Be(II). Maximum flux of metal ions has been observed from 0.01M H₂SO₄ for V(V) (3.22·10^–6 mol·m^–2·s^–1) and 2M HCl containing 3M CaCl₂ for Zn(II) solution (1.4·10^–6 mol·m^–2·s^–1). Low flux was observed in case of Be(II) since the membrane was affected by sulphocyanide group and did not remain hydrophobic. Mechanism of transport for these metal ions have been proposed separately. Distribution coefficient data for V(V) have also been evaluated to determine theoretical values of the permeability coefficient, and compared with experimental values.     

Differential pulse voltammetric determination of arsenic(III,V) and antimony (III,V) in glasses

Summary Analytical methods based on differential pulse voltammetry (DPV) have been described for the determination of total As, As(III), As(V), total Sb and Sb(III) as trace to minor constituents in complex glasses. For total As, the sample is decomposed with HF-H₂SO₄-KMnO₄. The As(V) is chemically reduced to As (III) by hypophosphite and a DPV scan is carried out at the dropping mercury electrode from –0.2 to –0.7 Vvs. SCE (E _p –0.41V). As(V) is determined by decomposing the sample in HF-H₂SO₄ and volatilizing the As(III) as AsF₃. The chemical reduction of As(V) and the DPV scan are then applied. If the glass can be decomposed with cold HF, the As(III) present in the glass can be determined by applying the DPV scan after cold sample-dissolution. For Sb(III), the sample is decomposed with HF-H₂SO₄, diluted, and adjusted to 1M in HCl. A DPV scan is conducted from –0.03 to –0.5 V (E _p –0.15 V). Sb(V) is not reduced in the 1M HCl supporting electrolyte. Total Sb is determined by using an aliquot of the sample solution adjusted to 6M in HCl. The DPV sweep is carried out from –0.5 to –0.1 V [E _p for Sb(V) and Sb(III) is –0.30 V]. The methods have been applied to a wide range of glass compositions and the results compared with values obtained by spectrophotometry and coulometric titration.

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Bestimmung von Arsen(III, V) und Antimon(III, V) in Gläsern mit Hilfe der Differential-Puls-Voltammetrie

Zusammenfassung Analytische Methoden auf der Grundlage der Differential-Puls-Voltammetrie (DPV) für die Bestimmung des gesamten Arsens, As(III), As(V), des gesamten Antimons und Sb(III) als Spuren in komplexen Gläsern wurden beschrieben. Zwecks Bestimmung des Gesamt-As wird die Probe mit Flußsäure +Schwefelsäure + Permanganat aufgeschlossen. As(V) wird mit Hypophosphit reduziert und die DPV wird an einer Quecksilber-Tropfelektrode zwischen –0,2 und –0,7V gegen eine ges. Kalomelelektrode (E _p =–0,41V) durchgeführt. Zur Bestimmung von As(V) wird die Probe mit HF-H₂SO₄ unter Verflüchtigung des As(III) als AsF₃ aufgeschlossen. Dann erfolgt die Reduktion des As(V) und die DPV. Wenn sich das Glas mit kalter HF lösen läßt, wird anwesendes As(III) mittels DPV in dieser Lösung bestimmt. Zur Bestimmung des Sb(III) wird die Probe mit HF-H₂SO₄ zersetzt, verdünnt und bis zur 1-Molarität mit HCl versetzt. Dann wird mit DPV zwischen –0,03 und –0,5V gemessen (E _p =–0,15V). Sb(V) wird in 1M salzsaurer Lösung nicht reduziert. Das Gesamt-Sb wird in einem Aliquot der Probelösung bestimmt, das dazu mit HCl bis zur 6fachen Molarität versetzt wird. Der DPV-Bereich wird von –0,5 bis –0,1 V ausgenützt (E _p f:ur Sb(V) und Sb(III) ist –0,30 V). Das Verfahren wurde für Gläser verschiedenster Zusammensetzung angewendet. Die Ergebnisse wurden mit den Resultaten der Spektrophotometrie und der coulometrischen Titration verglichen.

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Presented at the 8th International Microchemical Symposium, Graz, August 25–30, 1980.     

A study of the tautomerism of the N-butyl-4(2)-methoxycarbonyl-pyrrolid-3-ones and their hydrochlorides

The tautomerism of N-butyl-2-methoxycarbonyl-4-methylpyrrolid-3-one (I), N-butyl-4-methoxycarbonylpyrrolid-3-one (II), N-butyl-4-methoxycarbonyl-2-methylpyrrolid-3-one (III), N-butyl-4-methoxycarbonylpyrrolid-3-one hydrochloride (IV), and N-butyl-4-methoxy-carbonyl-2-methylpyrrolid-3-one hydrochloride (V) has been studied by UV and IR spectroscopy. It has been found that the esters I–V are highly ionized in aqueous and ethanolic solutions at concentrations of 10^–2–10^–3 M. On passing from methyl cyclopentran-1-one-2-carboxylate to the esters II and III the position of the equilibrium in heptane and CCl₄ shifts in the direction of the keto form. On passing from the esters II and III to the esters IV and V, the position of the equilibrium shifts in the direction of the enol. Meyer's relationship is not satisfied for the esters II and III, while it is satisfied for the esters IV and V. Hypotheses have been put forward on the causes of the phenomena mentioned.     

Electrochemical and spectroelectrochemical properties of manganese reconstituted myoglobin

Electrochemical and spectroelectrochemical properties of manganese(III) reconstituted myoglobin (Mn(III)–Mb) have been investigated. No redox wave of Mn(III)–Mb was observed at a highly hydrophilic indium oxide electrode on which rapid direct electron transfer of native myoglobin took place, suggesting the electron transfer reaction of Mn(III)–Mb at an indium oxide electrode is very slow. The rate constant of the chemical reduction of Mn(III)–Mb with dithionite was ca. 20 times smaller than that of native Mb. Using an optically transparent thin layer electrode (OTTLE) cell and Oxazine-170 perchlorate, 5,9-bis(diethylamino)-10-methyl-benzo[a]phenoxazonium perchlorate, as an electron transfer mediator, the redox potential for the Mn(III) Mn(II)–Mb couple was estimated to be −0.32 V versus Ag AgCl (sat. KCl) at 25°C.     

Modeling and analysis of vacuum membrane distillation for the recovery of volatile aroma compounds from black currant juice

A vacuum membrane distillation (VMD) model has been developed and validated with experimental data. The model consists of an extended transport model for the VMD process and is able to predict the effects of concentration and temperature polarization on the overall process performance. To validate the model, first it was tested with few experimental case studies from literature [S. Bandini, G.G. Sarti, Heat and mass transport resistances in vacuum membrane distillation per drop, AIChE J. 45 (7) (1999) 1422–1433; K.W. Lawson, D.R. Lloyd, Membrane distillation. I. Module design and performance evaluation using vacuum membrane distillation, J. Membr. Sci. 120 (1996) 111–121; A.M. Urtiaga, G. Ruiz, I. Ortiz, Kinetic analysis of the vacuum membrane distillation of chloroform from aqueous solutions, J. Membr. Sci. 165 (2000) 99–110]. Then the VMD model has been validated with experimental data collected from the recovery of aroma compounds from black currant [R.B. Jørgensen, A.S. Meyer, C. Varming, G. Jonsson, Recovery of volatile aroma compounds from black currant juice by vacuum membrane distillation, J. Food Eng. 64 (2004) 23–31]. In this work, recovery of 12 characteristic volatile aroma compounds from black currant juice has been studied. The simulated results from the VMD model, in terms of aroma concentration in the permeate have been compared with those obtained from laboratory experiments. The validated model has been used to study the effects of various process and membrane parameters on the concentration factor. The physical properties of various aroma compounds have been predicted using group contribution method as a function of temperature.     

Use of less polar organic solvents for the polarographic determination of metals after extraction of their diethyldithiocarbamates

Summary Physico-chemical parameters of various organic solvents, such as conductivity, viscosity, available potential range and phase-separation characteristics, have been evaluated for the use of direct polarographic analysis after liquid-liquid extracxtion. Due to its higher conductivity of 1.16 mS cm^–1 and a wide available potential range of 0.81 –1.72 V vs. Ag/AgCl, a dichloromethane solution containing 0.1 mol/l tetrabutylammonium perchlorate is chosen as the best organic solvent solution for the purpose of this study. Bismuth(III) is extracted into the organic phase and gives well-defined anodic and cathodic waves at –0.12 V and –0.51 V, respectively. A selective polarographic determination of the metal after the dichloromethane extraction has been developed.     

The catalytic adsorptive stripping voltammetric determination of chromium with TTHA and nitrate

A sensitive voltammetric method is presented for the determination of trace amounts of total chromium (Cr(III) and Cr(VI)) in natural waters, employing the square wave mode. The method is based on the preconcentration of the Cr(III)-TTHA complex by adsorption at the HMDE at the potential of –1.0 V vs. Ag/AgCl. The adsorbed complex is then reduced producing a response with a peak potential of –1.29 V and the peak height of the Cr(III) reduction is measured. The catalytic action of the nitrate ions on the Cr(III)-TTHA reduction has been elucidated using cyclic voltammetry. The adsorption of chromium complexes at the HMDE was investigated using out-of-phase a.c. voltammetry and the potential range of adsorption was determined.Based on these investigations optimal conditions for the determination of the total chromium concentration in the range 155–2000 ng 1^–1 have been established. The determination limit is 15 ng 1^–1 and the RSD is 3.5% for chromium concentrations 200 ng 1^–1.The usefulness and wide scope of this method for reliable and highly sensitive chromium analysis down to the ultra trace levels existing in various types of natural waters is demonstrated by determinations of the total chromium content in lake, sea and rain water.     

Polarographie in 1,2-Propandiolcarbonat, 3. Mitt.: Verbindungen von Titan,Vanadin und Chrom

Zusammenfassung Titan(III)chlorid, Titan (IV)oxyperchlorat, Vanadin(III)chlorid, Vanadin(IV)oxyperchlorat, Chrom(III)chlorid und Chrom(III)perchlorat wurden in 1,2-Propandiolcarbonat polarographisch untersucht; die Art der Grenzströme, die Diffusionsstromkonstanten, die Diffusionskoeffizienten, die Art der Abscheidungsvorgänge und die Lage der Halbwellenpotentiale, bez. auf die gesätt. wäßr. Kalomelelektrode, wurden bei 25° in 0,1M-Lösungen von Tetraäthylammoniumperchlorat bestimmt. Es wurden die Reduktionsvorgänge Ti(IV)–Ti(III), V(III)–V(II), V(IV)–V(II), Cr(III)–Cr(II) und Cr(II)–Cr(0) aufgefunden, wobei nur V(III)–V(II) reversibel verläuft.

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Polarographic investigation have been carried out on titanium(III)chloride, titanium(IV)oxoperchlorate, vanadium(III)chloride, vanadium(IV)oxoperchlorate, chromium(III)chloride and chromium(III)perchlorate in 1,2-propanediol-carbonate; the nature of the limiting currents, the diffusion current constants, the diffusion coefficients, the reversibility or irreversibility of the electrode process and the half-wave potentials vs. aqueous saturated calomel electrode have been determined in 0,1M solutions of tetraethylammonium perchlorate at 25°. Reduction waves were found corresponding to Ti(IV)–Ti(III), V(III)–V(II), V(IV)–V(II), Cr(III)–Cr(II) and Cr(II)–Cr(0)

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Mit 2 Abbildungen

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1. Mitt.:V. Gutmann, M. Kogelnig undM. Michlmayr, Mh. Chem.99, 693 (1968).

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2. Mitt.:V. Gutmann, M. Kogelnig undM. Michlmayr, Mh. Chem.99, 699 (1968).     

Electrodeposition of Lu on W and Al electrodes: Electrochemical formation of Lu–Al alloys and oxoacidity reactions of Lu(III) in the eutectic LiCl–KCl

The electrodeposition of lutetium on inert electrodes and the formation of lutetium–aluminium alloys were investigated in the eutectic LiCl–KCl in the temperature range 673–823 K. On a tungsten electrode, the electroreduction of Lu(III) proceeds in a single step and electrocrystalization plays an important role. Experimental current–time transients are in good agreement with theoretical models based on either instantaneous or progressive nucleation with three dimensional growth of the nuclei, depending on the working temperature. The diffusion coefficient of Lu(III) was determined by chronopotentiometry by applying the Sand equation. The activation energy for diffusion was found to be 31.5 ± 1.3 kJ mol⁻¹. Al₃Lu and mixtures of Al₃Lu and Al₂Lu, characterized by XRD analysis and SEM, were obtained from the LiCl–KCl melt containing Lu(III) by potentiostatic electrolysis using an Al electrode. The activity of Lu and the standard Gibbs energies of formation for Al₃Lu were estimated from open-circuit chronopotentiometric measurements. The E–pO²⁻(potential–oxoacidity) diagram for Lu–O stable compounds in LiCl–KCl at 723 K has been constructed by combining theoretical and experimental data. In this way, the apparent standard potential for the Lu(III)/Lu system has been determined by potentiometry. Potentiometric titrations of Lu(III) solutions with oxide donors, using a yttria stabilized zirconia membrane electrode “YSZME” as a pO²⁻ indicator electrode, have shown the stability of LuOCl and Lu₂O₃ in the melt and their solubility products have been determined at 723 K.     

Application of europium(III) extraction with organophosphinic acid to liquid membrane transport

The extraction behavior of Eu(III) has been studied using di(2,4,4-trimethylpentyl)phosphinic acid (DTMPPA, HA) in kerosene. Europium was extracted as Eu(HA₂)₃ with the extraction constant of 2.0·10^–3. This extraction system was applied to the transport of Eu(III) across a DTMPPA liquid membrane supported on porous polytetrafluoroethylene. Europium was quantitatively moved through the liquid membrane containing 0.1M (HA)₂ as a mobile carrier from the feed solution of pH above 3 into the product solution of 0.1M HNO₃, yielding a concentration factor of ten. The transport rate increased with increasing pH and DTMPPA concentration.     

Electrocatalytic oxidation of thiosulfate on a modified nickel hexacyanoferrate film electrode

Summary A modified nickel hexacyanoferrate film glassy carbon electrode is prepared by the electrochemical deposition technique. The film is very stable upon voltammetric scanning in the potential range of 1.0 to –0.5 V (vs. SCE) and an oxidation peak occurs at 0.35 V (vs. SCE) (1 mol/l NaNO₃). The effects of electrolyte, solvent, coexisting ions and other variables on the voltammetric behaviour of the modified film have been studied. The thickness of the resulting film can be controlled by changing the number of voltammetric cycles and the concentrations of nickel(II) and hexacyanoferrate(III) ions. The film shows catalytic activity towards electrooxidation of thiosulfate with a peak potential +0.5 V (K^–-containing media). This oxidation potential of thiosulfate on the modified electrode is shifted negatively by about 550 mV as compared to the naked glassy carbon electrode. For practical application, the modified electrode can be used for the determination of thiosulfate in concentrations from 5.0×10^–5 to 1.0×10^–1 mol/l. This method has been successfully applied to the determination of thiosulfate in photographic waste effluents.     

Volumetric titration of iron (III)-salts with disodium EDTA, using FeII-cacotheline as indicator

Summary A volumetric method has been developed for the determination of iron (III) with disodium EDTA, using a mixture of cacotheline and iron(II) as indicator. The titration of the iron (III) salt is carried out in a buffered solution ofph 4–5 in carbon dioxide atmosphere with magnetic stirring, until a pink color appears. The pink color is due to the reduction of cacotheline by ferrous-EDTA. The reduction occurs only when all the iron (III) is complexed by EDTA. The end point is sharp and the method has been found to give results accurate to±0.3 to±0.5 percent.     

Sb(III) and Sb(V) separation and analytical speciation by a continuous tandem on-line separation device in connection with inductively coupled plasma atomic emission spectrometry

A sensitive, precise and automated non-chromatographic method for Sb(III) and Sb(V) analytical speciation based on a continuous tandem on-line separation device in connection with inductively coupled plasma-atomic emission (ICP-AES) detection is proposed. Two on-line successive separation steps are included into this method: a continuous liquid-liquid extraction of Sb(III) with ammonium pyrrolidine dithiocarbamate (APDC) into methylisobuthylketone (MIBK), followed by direct stibine generation from the organic phase. Both separation steps are carried out in a continuous mode and on-line with the ICP-AES detector. Optimization of experimental conditions for the tandem separation and ICP-AES detection are investigated in detail. Detection limits for Sb(III) were 3 ng.mL^–1 and for Sb(V) 8 ng.mL^–1. Precisions observed are in the range ± 5%. The proposed methodology has been applied to Sb(III) and Sb(V) speciation in sea-water samples.     

Speciation of Cr(VI)/Cr(III) by electrothermal atomisation AAS after electrodeposition on a L'vov platform

Summary The pyrolysed graphite L'vov platform of a tube furnace is considered as an electrode for the electrodeposition and speciation of chromium by electrothermal atomisation atomic absorption spectrometry (ETA-AAS). Firstly, a preliminary study of the Cr(VI)/Cr(III) voltammetric behavior at pH 4.70 on a glassy-carbon electrode is carried out. Secondly, the L'vov platform is used as a cathodic macro-electrode for the selective preconcentration of Cr(VI)/Cr(III) on a mercury film. Speciation of Cr(VI)/Cr(III) is carried out on the basis of the electrolysis potential (E_e): at pH 4.70 and E_e=–0.30 V, only Cr(VI) is reduced to Cr(III) and accumulated as Cr(OH)₃ by adsorption on a mercury film; at E_e=–1.80 V both Cr(VI) and Cr(III) are accumulated forming an amalgam with added mercury(II) ions. Once the film has been formed, the platform is transferred to a graphite tube to atomise the element. The reliability of the method was tested for the speciation of chromium in natural waters and it proves to be highly sensitive thanks to the electroanalytical step. In all samples, the Cr(VI) concentration was less than the detection limit (0.15 ng ml^–1), and the concentration of Cr(III) agrees with those of total chromium. The analytical recovery of Cr(VI) added to water samples [3.50 ng ml^–1 of Cr(VI)] was 105±6.2%.     

Effect of using mixed ion-pairs on PVC-membrane electrodes for ephedrine

Mixed ion-pairs based on the use of ephedrinium (EPH)-TPB plus EPH-reineckate (II) and phenylephrine-TPB plus EPH-reineckate (III) were tried for use in plastic membranes. The results were compared to those of an EPH-reineckate (I) single ion-pair electrode. The Nernstian slopes were 50, 49 and 55 mV decade^–1 for membranes I, II and III, respectively. The linear concentration ranges were 10^–5–10^–1, 4.0 × 10^–5–10^–1 and 6.3 ×^–5–10^–1 M ephedrine. The detection limits were 4 ×^–6,10^–5 and 1.2 × 10^–5 M ephedrine for membranes I, II and III, respectively. The pH ranges were 4–9, 3–9 and 2–8 for I, II and III-membranes, respectively. Selectivity coefficient values for membrane II were better than those for membranes I and III. The effects of increasing KC1 concentration and temperature changes were explained for the three electrodes. The isothermal temperature coefficients were 0.00145, 0.0007 and 0.00055 V/ °C for electrodes I, II and III. Electrode III was applied for the determination of ephedrine in its pharmeaceutical preparations with an overall relative standard deviation range of 1.3–2.4% and an overall mean recovery value of 98.1%.     

Effect of the nature of the iron salt on the hydrothermal modification of ferrosilica gel

The effect of iron(III) salts (chloride or nitrate) on the hydrothermal modification of the texture and phase composition of ferrosilica gel has been studied. Special features in the crystallization of prepared specimens as a function of the nature of the anion of the initial iron salt have been revealed by x-ray phase analysis and nuclear gamma-resonance spectroscopy.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 2, pp. 251–253, March–April 1988.     

Sorption–Desorption Photometric Determination of Trace Iron in Sodium, Potassium, and Ammonium Thiocyanates Using Polyurethane Foams

A reagent-free sorption photometric method for determining trace iron(III) in alkali-metal and ammonium thiocyanates was developed. The method consists in the adsorption of iron(III) thiocyanate complexes on polyurethane foams at pH 2–3 followed by the desorption of complexes with acetone and a measurement of the absorbance of solutions. The analytical range for iron was 0.5–20 g in a 0.2–2.5-g portion of salt. The determination limit and relative standard deviation were 2 × 10^–5% and 20–30%, respectively (n= 3; P= 0.95). The specific feature of the determination is that an additional photometric reagent is not introduced in the solution of the salt to be analyzed, because thiocyanate, that is, the matrix component of the test sample, acts as such a reagent.     

Phosphoryl Containing Podands as Carriers in Plasticized Membrane Electrodes Selective to Quaternary Ammonium Surfactants

Open chain polyethers with phosphoryl-containing terminal groups have been studied as carriers in plasticized membrane electrodes that are selective towards cationic surfactants. Host–guest complexation has also been studied by means of batch extraction and bulk membrane transport experiments. For the various guests, the bulk membrane (chloroform) transport rate decreases in the series tetraalkyl ammonium > alkyl pyridinium > alkyl ammonium, while the series of solvent extraction efficiency is exactly opposite; an explanation for this behaviour is proposed. The performance of electrode membranes follows the series for bulk membrane transport. Among the ISE membranes of various composition, one containing the longest (seven oxygen atoms) podand, o-nitrophenyl octyl ether (plasticizer), and sodium tetraphenyl borate (anionic additive) performs the best. The slope of the electrode function equals 58.5 mV/decade, the detection limit for dodecyltrimethylammonium is 3.2 × 10^-6 M. ISE response time is 5–10 s, the working pH range is 2–11 and lifetime is at least 6 months. The electrode selectivity is significantly better than that of conventional ion-association and crown ether based electrodes.     

Fabrication and characterization of γ-Al2O3–clay composite ultrafiltration membrane for the separation of electrolytes from its aqueous solution

In this paper, we have reported the preparation of low cost γ-Al₂O₃ membrane on a macroporous clay support by dip-coating method. For the synthesis of γ-Al₂O₃ top layer on the support, a stable boehmite sol is prepared using aluminium chloride salt as a starting material by sol–gel route. The structural properties of the composite membrane as well as γ-Al₂O₃ powder is carried out using scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption–desorption isotherm data, Fourier transform infrared analysis (FTIR) and dynamic light scattering (DLS) analysis. The mean particle size of the boehmite sol used for coating is found to be 30.9 nm. The pore size distribution of the γ-Al₂O₃–clay composite membrane is found to be in the range of 5.4–13.6 nm. Separation performance of the membrane in terms of flux and rejection of single salts solution such as MgCl₂ and AlCl₃ as a function of pH, salt concentration and applied pressure is also studied. The rejection and flux behavior are found to be strongly dependent on electrostatic interaction between the charged molecules and γ-Al₂O₃–clay composite membrane. The intrinsic rejection has been determined by calculating the concentration at membrane surface (C_m) using Speigler–Kedem model. It is found that the observed rejection shows anomalous trend with increase in applied pressure and the intrinsic rejection increases with increase in applied pressure, a trend typical of the separation of electrolyte through charged membranes. At acidic pH, both the salt solution shows higher rejection. With increase in the salt concentration, observed rejection of salt decreases due to the enhanced concentration polarization. The maximum rejection of MgCl₂ and AlCl₃ is found to be 72% and 88%, respectively for salt concentration of 3000 ppm.     

Polarographische Bestimmung von Molybd?n neben anderen Metallen

Zusammenfassung Es wird eine polarographische Bestimmungsmethode für Molybdän in Gegenwart anderer Metalle beschrieben. Molybdän(VI) gibt in Anwesenheit einer 0,25 M Ammoniumtartratlösung eine gut reproduzierbare Reduktionsstufe bei E _1/2= – 1,27 V (GKE). Die Methode wurde mit guten Ergebnissen bei der Molybdänbestimmung in Legierungen (Fehler ±1%) und mineralischem Gestein (Fehler –2%) angewandt. Der störende Einfluß von größeren Mengen Chrom(VI) wird durch ein beschriebenes säulen-chromatographisches Trennverfahren mit Hilfe des Anionenaustauschers Wofatit SBW beseitigt. Ein zehnfacher Überschuß an W(VI), Mn(II), Ni(II), Cu(II) und Tl(I) sowie gleiche Mengen an Fe(II), Fe(III), Cd(II), Co(II), V(V) und Bi(III) stören nicht.

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Summary A polarographic method for the determination of molybdenum in presence of other metals is described. Molybdenum (VI) yields a well-defined cathodic wave in ammonium tartrate medium (0.25 M) at E _1/2= –1.27 V vs. S.C.E. The results obtained have been applied to the determination of molybdenum in ores (error ±1%) and alloys (error –2%). The interfering influence of greater amounts of chromium(VI) is eliminated by ion-exchange with the strongly basic anion-exchanger Wofatit SBW. A ten-fold excess of W(VI), Mn(II), Ni(II), Cu(II) and Tl(I) as well as equal amounts of Fe(II), Fe(III), Cd(II), Co(II), V(V) and Bi(III) do not interfere.