Spectrophotometric determination of tungsten in ores and steel by chloroform extraction of the tungsten-thiocyanate-diantipyrylmethane complex

A method for determining up to about 6% of tungsten in ores and mill products is described. It is based on the extraction of the yellow tungsten(V)-thiocyanate-diantipyrylmethane ion-association complex into chloroform from a 2.4M sulphuric acid-7.8M hydrochloric acid medium containing ammonium hydrogen fluoride as masking agent for niobium. The molar absorptivity of the complex is 1510 1. mole(-1).mm(-1) at 404 nm, the wavelength of maximum absorption. Moderate amounts of molybdenum and selenium may be present in the sample solution without causing appreciable error in the result. Interference from large amounts is avoided by separating these elements from tungsten by chloroform extraction of their xanthate complexes. Large amounts of copper interfere during the extraction of tungsten because of the precipitation of cuprous thiocyanate. Common ions, including uranium, vanadium, cobalt, titanium, arsenic and tellurium, do not interfere. The proposed method is also applicable to steel.     

Extractive separation of Tungsten as phosphotungstate

Tungsten can be extracted quantitatively as phosphotungstate in micro as well as milligram concentrations by extraction with MIBK from 0.1-1M hydrochloric acid if the w/w tungsten:phosphorus ratio = 7, and separated from Fe, Ni, Co, Cr, Mn, Cu, Ca, U, Th, As, Sb, Bi and Si, after reduction of Fe(III) and cr(VI) with thiosulphate, in natural and industrial samples. Mo and V suppress the extraction of tungsten and therefore require prior separation. The method takes 15 min for a single separation and gives highly satisfactory results with an overall error of about 0.1-0.2% over the range 10-100 mg tungsten in the sample.     

Extraction of tungsten with 8-hydroxyquinoline and some of its derivatives

The extraction of tungsten by chloroform solutions of 8-hydroxyquinoline(I), 2-methyl-8-hydroxyquinoline(II), 5,7-dibromo-8-hydroxyquinoline(III) and 8-mercaptoquinoline(IV), as a function of the concentration of tungsten and reagent and the acidity of the aqueous phase, has been studied. Evidence was obtained for the quantitative extraction of tungsten over a wide range of acidity. The degree of extraction of tungsten at 10(-5)M concentration with I,III and IV gives two maxima when plotted against acidity. The extraction maximum for the more acidic solutions lies in the region where the reagents exist in the protonated form and its position depends on the reagent used. It is suggested that different tungsten complexes are extracted, depending on the acidity of the aqueous phase.     

Extractive separation and spectrophotometric determination of tungsten as phosphotungsten blue

Phosphotungsten blue is produced by tin(II) reduction of tungstate solution complexed with phosphate at a w/w ratio of W/P = 5, in 4M hydrochloric acid medium, and extracted with isoamyl alcohol; thus tungsten is separated from Fe(III), Ni, Co, Cr(III), V(V), As(V), Sb(III), Bi, Si, U(VI), Ca and Cu(II). In presence of bismuth (0.5 mg/ml), 99.7% W is separated in a single extraction. After alkaline back-extraction, tungsten is determined spectrophotometrically as phosphotungsten blue; it is measured at 930 nm in aqueous solution or at 900-960 nm after isoamyl alcohol extraction, the Beer's law ranges being 0.08-0.6 and 0.16-0.72 mg/ml respectively. The methods are shown to give satisfactory results in the analysis of practical samples containing some milligrams of tungsten.     

Simultaneous spectrophotometric determination of Tungsten and Molybdenum with Thiocyanate after α-Benzoinoxime extraction

The extractability of tungsten α-benzoinoximate by chloroform as a function of the reagent concentration and acidity has been studied. In 0.5 M hydrochloric acid solution the extraction coefficient for tungsten (~ l p.p.m.) is given by the relation

An acidity range of 0.01–1 M provides favorable extraction coefficients. Tungsten can be separated by α-benzoinoxime extraction from much iron and most other metals. Molybdenum accompanies tungsten quantitatively and the two elements can be determined simultaneously by the familiar thiocyanate method if the absorbance of the isopropyl ether extract is measured at 405 mμ and 490 (or 475) mμ. As little as 1 μg W can thus be determined in the presence of 10 μg Mo without separation.     

Determination of cobalt and zinc in highpurity niobium, tantalum, molybdenum and tungsten metals by atomicabsorption spectrophotometry after separation by extraction

A method for determining 0.0005-0.05% of cobalt and zinc in high-purity niobium, tantalum, molybdenum and tungsten metals by atomic-absorption spectrophotometry is described. After sample dissolution, cobalt and zinc are separated simultaneously from the matrix materials by chloroform extraction of their thiocyanatediantipyrylmethane ion-association complexes, at pH 3.25, from a citric acid medium approximately 1.2M in sodium thiocyanate. Interference from copper is eliminated with thiourea. Large amounts of iron interfere under the recommended conditions, but moderate amounts may be present in the sample solution without causing appreciable error in the results. Phosphorus (as orthophosphate) interferes in the extraction of cobalt from tungsten solutions. Moderate amounts of other impurities do not interfere in the proposed method.     

Extractive separation and spectrophotometric determination of tungsten as ferrocyanide

Tungsten, in amounts ranging from micrograms to milligrams, can be extracted into isoamyl alcohol, as the tungsten(V) ferrocyanide complex obtained by reduction of tungsten(VI) with tin(II) in 4M hydrochloric acid containing ferrocyanide. It can thus be separated from iron, cobalt, chromium, manganese, arsenic, antimony, bismuth, silicon, calcium and copper, their precipitation being prevented by addition of glycerol and, in the case of iron, sulphosalicyclic acid. Molybdenum, vanadium and nickel are not separated from tungsten, however. Tungsten can also be determined spectrophotometrically as tungsten(V) ferrocyanide. The absorbance of the brown complex is measured in aqueous solution or preferably after extraction into isoamyl alcohol. As many alloying elements interfere, they should be separated by the ferrocyanide extraction or other suitable method. Both the separation and the determination methods give satisfactory results with an overall error of not more than 0.5% in the analysis of practical samples containing low or high percentages of tungsten.     

Determination of soluble/insoluble tungsten compounds as discrete entities in industrial hygiene samples

A method was developed for the determination of soluble and insoluble tungsten compounds collected simultaneously in industrial hygiene air samples. Soluble tungsten compounds are leached from the collection filter using deionized water. The residual tungsten material is dissolved by a $\text{HNO}{}_3\text{HF}$ digestion, after the removal of potential interfering metal ions by a hydrochloric acid extraction. Atomic absorption spectrometric determination of tungsten in a nitrous oxide—acetylene flame is feasible over the range 10—500 μg ml^-1 at the 255.1-nm line; the working range may be extended to 1000 mg ml^-1 without dilution of the sample by using the 400.8-nm line.     

Liquid-liquid extraction of 99Mo and 187W with trioctylamine

Molybdenum and tungsten can be separated from each other by extraction with trioctylamine in hydrochloric acid medium.     

Determination of molybdenum and tungsten at trace levels in rocks and minerals by solvent extraction and X-ray fluorescence spectrometry

Separation by solvent extraction followed by X-ray fluorescence spectrometry has been used for determination of molybdenum and tungsten in rocks and minerals. Samples are decomposed either by heating with a mixture of hydrofluoric acid and perchloric acid or by fusion with potassium pyrosulphate, followed by extraction of molybdenum and tungsten with N-benzoylphenylhydroxylamine in toluene from 4-5M sulphuric acid medium. The extract is collected on a mass of cellulose powder, which is dried in vacuum, mixed thoroughly and pressed into a disc for XRF measurements. The method is free from all matrix effects and needs no mathematical corrections for interelement effects. The method is suitable for determination of molybdenum and tungsten in geological materials down to ppm levels, with reasonable precision and accuracy.     

Liquid-liquid extraction of 99Mo and 187W with trioctylamine

Molybdenum and tungsten can be separated from each other by extraction with trioctylamine in hydrochloric acid medium.     

The use of long-chain alkylamines for preconcentration of traces of molybdenum, tungsten and rhenium in their determination by atomic-absorption spectroscopy-I General studies

Long-chain alkylamines are used for the preconcentration of traces of molybdenum, tungsten and rhenium as thiocyanate complexes, in their determination by atomic-absorption spectroscopy. General studies of factors, influencing the extraction show that the thiocyanate complexes can be extracted into chloroform containing a low concentration of Amberlite LA1. Detection limits are 0.02 ppm Mo, 0.75 ppm W and 0.34 ppm Re in the final MIBK solution and are improved by a factor of 5-10 over those obtained by using current extraction methods. Serious interelement effects are eliminated and a range of other cations and anions are shown to have little effect on the absorption.     

Spectrophotometric determination of molybdenum and tungsten in niobium with dithiol

A new procedure for the determination of molybdenum and tungsten in niobium has been developed. The method involves the formation of the intensely colored complex of molybdenum with toluene-3,4-dithiol in an aqueous medium and its extraction into carbon tetrachloride followed by the reduction of tungsten and the formation and extraction of its complex. The recommended reagent is stable for at least 90 days. Both the molybdenum and the tungsten dithiol complexes are formed quantitatively within 5 min. Interlaboratory evaluation of the method reveals within-laboratory and between-laboratory relative standard deviations of about 1.5% and 2.9% respectively.     

Separation of tungsten from molybdenum by liquid-liquid extraction and extraction chromatography using thiocyanate and a quaternary ammonium salt

Methods were developed for the separation of tungsten from molybdenum by liquid-liquid extraction and extraction chromatography using thiocyanate and a quaternary ammonium salt, Zephiramine. Tungsten was extracted into chloroform as an ion associate of tungsten(V)-thiocyanate complex and Zephiramine cation was retained on a column of Teflon powder coated with Zephiramine, but molybdenum(III) was neither extracted nor retained. The extraction chromatographic method was successfully applied to the determination of trace amounts of tungsten in molybdenum by neutron activation analysis.     

Determination of traces of carbon, nitrogen and oxygen in molybdenum and tungsten

The determination of carbon and nitrogen in molybdenum and of carbon, nitrogen and oxygen in tungsten, is described. The analytical techniques applied were charged-particle activation (carbon, nitrogen and oxygen), photon activation (carbon and oxygen), combustion (carbon) and vacuum-fusion extraction (nitrogen and oxygen). Chemical methods yielded upper limits in the 2-5 mug/g range. Activation analysis yielded 100 and 8 ng/g for carbon in molybdenum and tungsten respectively, 500 and 74 ng/g for nitrogen in molybdenum and tungsten respectively and 70 ng/g for oxygen in tungsten. The results obtained by charged-particle and photon activation agreed satisfactorily.     

Simultaneous determination of tungsten and molybdenum in sea water by catalytic current polarography after preconcentration on a resin column

The simultaneous determination of tungsten and molybdenum in sea water is based on preconcentration by column extraction with 7-(1-vinyl-3,3,5,5-tetramethylhexyl)-8-quinolinol (Kelex- 100) resin, and measurement of the polarographic catalytic currents obtained in a solution of chlorate, benzilic acid and 2-methyl-8-quinolinol. When the concentration factor is 50, the detection limits are 2.4 pM for tungsten and 17 pM for molybdenum (for a signal-to-noise ratio of 3). The precision of the determination is ca. 10% for 67 pM tungsten and ca. 5% for 106 nM molybdenum in sea water (n=4). Results for sea water and other natural waters are presented.     

Determination of tungsten in rocks and minerals by chelate extraction and atomic-absorption spectrometry

An atomic-absorption method for determination of tungsten in rocks and minerals is proposed. The method involves sample decomposition by acid digestion or by pyrosulphate fusion, followed by chelate extraction of tungsten by N-benzoylphenylhydroxylamine in toluene. Atomic-absorption measurements are made on the organic phase aspirated into a nitrous oxide-acetylene flame. Quantitative extraction with efficient separation from other elements is achieved in a single extraction from strong acid media. The method is rapid and reliable in terms of precision and accuracy and is applicable to rocks and minerals containing tungsten in the range from 100 ppm to 15%.     

The composition of tetraphenylarsonium thiocyanatotungstate in chloroform

The variation of extraction efficiency as a function of thiocyanate concentration and of tetraphenylarsonium concentration has established part of the composition of the extracted species which is involved in the determination of tungsten by extraction as the tetraphenylarsonium tungsten-thiocyanate ion-pair. The thiocyanate : tungsten ratio is 2:1, and the tetraphenylarsonium : tungsten ratio is 1:1, indicating an anionic charge of one for a mononuclear complex.     

The determination of tungsten by electron paramagnetic resonance spectrometry

Electron paramagnetic resonance is used to determine tungsten in the range 5–400 μg ml^-1. Tungstate is reduced to tungsten(V) in the presence of thiocyanate in acidic medium and detected as the tungsten(V)—thiocyanate complex in amyl acetate after extraction. Molybdenum does not interfere; vanadium (5 mg) interferes. The relative standard deviation for mid-range concentrations is about 3%.     

Estimation of thorium by organic reagents: recovery from worn-out gas mantles and tungsten filaments by 2 : 4-dichlorophenoxyacetic acid

2 : 4-Di- chlorophenoxyacetic acid can be used as a reagent for the determination and extraction of thorium present in industrial wastes like worn-out gas mantles and tungsten filaments. At a pH of 2.8 to 3 thorium gives a voluminous precipitate with 2 : 4-D, which filters, washes, and ignites readily. The voluminous nature of the precipitate is advantageous in the detection and determination of the very small quantity of thorium present in the filaments. The % recovery of thorium from gas mantles and tungsten filaments is 89.5 and 0.8 respectively. The process can only be utilised for the extraction of thorium from the filaments if a huge quantity of such materials can be procured at a time.