Molybdenum extractive concentration

The sample is wet digested, molybdenum is concentrated by chelation-solvent extraction, and determined by FAAS using a nitrous oxide-acetylene flame. 

Add 5 ml of 20 % sulphuric acid to the organic extract containing the molybdenum and concentrate by evaporation. Add a few drops of hydrogen peroxide (30 %) and heat over a sand bath until the organic matter is... 

Titanium is widely distributed throughout the universe. It has been discovered in the stars, in interstellar dust, in meteorites, and on the surface of the earth. Its concentration within the earth s crust of about 0.6% makes it the fourth most abundant of the structural metals (after aluminum, iron, and magnesium). It is 20 times more prevalent than chromium, 30 times more than nickel, 60 times more than copper, 100 times more than txmgsten, and 600 times more than molybdenum. This abimdance is to some extent illusory, however, in that titanium is not so frequently foimd in economically extractable concentrations. Concentrated sources of the metal are the minerals il-menite, titanomagnetite, rutile, anatase, and brookite. 

Discussion. Molybdenum(VI) in acid solution when treated with tin(II) chloride [best in the presence of a little iron(II) ion] is converted largely into molybdenum(V) this forms a complex with thiocyanate ion, probably largely Mo(SCN)5, which is red in colour. The latter may be extracted with solvents possessing donor oxygen atoms (3-methylbutanol is preferred). The colour depends upon the acid concentration (optimum concentration 1M) and the concentration of the thiocyanate ion (1 per cent, but colour intensity is constant in the range 2-10 per cent) it is little influenced by excess of tin(II) chloride. The molybdenum complex has maximum absorption at 465 nm. 

The reaction is quenched by the addition of 1.28 g (2.94 mmol) of molybdenum pentoxidc/pyridinc/UMPA, and the yellow slurry is stirred initially at OX (30 min), then for 45 min at 25 X. The mixture is added to 1 N sodium hydroxide and extracted with diethyl ether. The ethereal solution is washed with brine, dried over Na,S04 and concentrated in vacuo to give 0.705 g (100%) of an oily, light-yellow solid. Analysis of the crude aldol adduct by 1 C NMR and analytical HPLO (Waters, Radial Pak, 8 mm x 10 cm, silica gel, ethyl acetate/hexane, 15 85) indicates only one. un-diastereomer (2X3S ) accompanied by approximately 10% of the two ethyl acetate/hexane affords fine white needles yield 0.359 g (57%) mp 155.5 156.5X (a]u -92.5 (c = 0.0294, CHCfi). 

A circular TLC spectrophotometric method for the determination of lanthanum and yttrium at concentration level of 0.01 to 1.0% in molybdenum-based alloys has also been developed. It involves the separation of lanthanum and yttrium on cellulose layers impregnated with 0.2-Mtrioctylamine using aqueous HCl as developer, extraction from sorbent layer, and determination by spectrophotometry [69]. 

Isaeva [181] described a phosphomolybdate method for the determination of phosphate in turbid seawater. Molybdenum titration methods are subject to extensive interferences and are not considered to be reliable when compared with more recently developed methods based on solvent extraction [182-187], such as solvent-extraction spectrophotometric determination of phosphate using molybdate and malachite green [188]. In this method the ion pair formed between malachite green and phosphomolybdate is extracted from the seawater sample with an organic solvent. This extraction achieves a useful 20-fold increase in the concentration of the phosphate in the extract. The detection limit is about 0.1 ig/l, standard deviation 0.05 ng-1 (4.3 xg/l in tap water), and relative standard deviation 1.1%. Most cations and anions found in non-saline waters do not interfere, but arsenic (V) causes large positive errors. 

Korkisch and Koch [106,107] determined low concentrations of uranium in seawater by extraction and ion exchange in a solvent system containing trioctyl phosphine oxide. Uranium is extracted from the sample solution (adjusted to be 1 M in hydrochloric acid and to contain 0.5% of ascorbic acid) with 0.1 M trioctylphos-phine oxide in ethyl ether. The extract is treated with sufficient 2-methoxyethanol and 12 M hydrochloric acid to make the solvent composition 2-methoxyethanol-0.1 M ethereal trioctylphosphine acid-12 M hydrochloric acid (9 10 1) this solution is applied to a column of Dowex 1-X8 resin (Cl" form). Excess of trioctylphosphine oxide is removed by washing the column with the same solvent mixture. Molybdenum is removed by elution with 2-methoxyethanol-30% aqueous hydrogen peroxide-12 M hydrochloric... 

In the method for [17] inorganic arsenic the sample is treated with sodium borohydride added at a controlled rate (Fig. 10.1). The arsine evolved is absorbed in a solution of iodine and the resultant arsenate ion is determined photometrically by a molybdenum blue method. For seawater the range, standard deviation, and detection limit are 1—4 xg/l, 1.4%, and 0.14 pg/1, respectively for potable waters they are 0-800 pg/1, about 1% (at 2 pg/1 level), and 0.5 pg/1, respectively. Silver and copper cause serious interference at concentrations of a few tens of mg/1 however, these elements can be removed either by preliminary extraction with a solution of dithizone in chloroform or by ion exchange. 

Distillation methods using sulfuric acid are the most efficient for isolating technetium produced by neutron irradiation of kilogram amounts of molybdenum. Boyd et al. have used this method to separate technetium from pure molybdenum which had been irradiated for one year. In this case for each gram of molybdenum 6 ml of concentrated surfuric acid are added and about 75 % of technetium is passed into the distillate. When double the amount of acid is added, nearly 90 % of technetium are found in the distillate. More than 98 % of technetium are extracted after two distillations. 

Some typical and preferred heavy and trace element concentrations for soils and municipal composts are shown in Table 5.3. The levels in soils are typical for dilute aqueous extractants such as 0.05 M EDTA, 0.5 M acetic acid, hot water for boron, and, for molybdenum, Tamm s reagent (acid ammonium oxalate Reisenauer, 1965). Tables in the literature often give total values obtained spectrographically, by XRF, or by extraction with hot... 

Molybdenum may be identified at trace concentrations by flame atomic absorption spectrometry using nitrous oxide-acetylene flame. The metal is digested with nitric acid, diluted and analyzed. Aqueous solution of its compounds alternatively may be chelated with 8—hydroxyquinobne, extracted with methyl isobutyl ketone, and analyzed as above. The metal in solution may also be analyzed by ICP/AES at wavelengths 202.03 or 203.84 nm. Other instrumental techniques to measure molybdenum at trace concentrations include x-ray fluorescence, x-ray diffraction, neutron activation, and ICP-mass spectrometry, this last being most sensitive. 

Elemental composition Mo 59.94%, S 40.06%. The compound or mineral molybdenite may be identified nondestructively by x-ray and from physical properties. Molybdenum content of the material may be determined by various instrumental techniques after digestion of the solid in concentrated HNO3 or aqua regia followed by appropriate dilution of acid extract (See Molybdenum.)... 

Amine salts have been used to recover molybdenum from solutions arising from a variety of sources. Most of the western world s supply of this metal is derived from molybdenite (MoS2) concentrates obtained as a byproduct of copper production in the USA and Chile. Such concentrates are roasted to molybdenum(VI) oxide (volatile Re207 can often be recovered as a valuable byproduct from the roaster gases) and leached with dilute sulfuric acid to remove the copper from the crude M0O3 product. Some molybdenum also dissolves and can be recovered, for example, by the same technique as that practised at Kennecott s Utah Copper Division smelter,213 i.e. by extraction into a solution of a tertiary amine in kerosene at an aqueous pH value of about 1. 

Earlier atomic absorption methods [164-167] from the determination of molybdenum in soils employed a preliminary solvent extraction step to improve sensitivity in view of the low concentrations of molybdenum occurring in most soils. Baucells et al. [5] developed a graphite furnace atomic absorption procedure which was capable of determining down to 8.4 pg of molybdenum in a soil matrix solution with a precision of 4% for 100 pg/1 molybdenum. These workers showed that a char temperature of 1500 °C and an atomisation tem-... 

During the extraction method described by Baucells et al. [53] for the determination of molybdenum, the dry residue was solubilised with nitric acid. To observe the influence of nitric acid concentration on the absorbance signal of molybdenum, different acid concentrations were used. There was a decrease of 22.86% in the peak height when 10% nitric acid was present compared with no concentrated nitric acid. 

The plant sample is digested with 60% mlm perchloric acid 70% m/m nitric acid, 1 4 v/v. The acids are then removed by volatilisation and any silica present is dehydrated to render it insoluble. The concentration of molybdenum in this solution is then determined spectrophotometrically at 470 nm on a diisopropyl ether extract. 

Heanes [97] has described a method for determining copper, manganese and zinc in ashed plant extracts by flame AA spectrophotometry after cobalt and molybdenum have been assayed on separate aliquots of the same plant extracts by a spectrophotometric procedure [102]. (See Sect. 7.34.3). Ashing aids were necessary to maintain accuracy in the determinations. Concentrations of up to 3.5% m/m of silicon and calcium and 4% m/m of chlorine in the plants did not affect the determinations, but in some instances lower concentrations were determined in plant samples containing equal or higher levels of both added silica and calcium. 

The assessment of plant-available soil contents can frequently be achieved and validated by field experiments for nutritionally essential elements, and, for a few potentially toxic elements such as chromium, nickel and molybdenum, at the moderately elevated concentrations that can occur in agricultural situations. The validation of extraction methods, devised for agricultural and nutritional purposes, is much less easy to achieve when they are applied to heavy metals and other potentially toxic elements, especially at the higher concentrations obtained in industrially contaminated land. This is not surprising in view of the fact that for some heavy metals, for example lead, there is an effective root barrier, in many food crop plants, to their uptake and much of the metal enters plants not from the root but by deposition from the atmosphere on to leaves. In these circumstances little direct correlation would be expected between soil extractable contents and plant contents. For heavy metals and other potentially toxic elements, therefore, extraction methods are mainly of value for the assessment of the mobile and potentially mobile species rather than plant-available species. This assessment of mobile species contents may well, however, indicate the risk of plant availability in changing environmental conditions or changes in land use. 

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