Perrhenate, determination

Koide et al. [537] have described a graphite furnace atomic absorption method for the determination of rhenium at picomolar levels in seawater and parts-per-billion levels in marine sediments, based upon the isolation of heptavalent rhenium species upon anion exchange resins. All steps are followed with 186-rhenium as a yield tracer. A crucial part of the procedure is the separation of rhenium from molybdenum, which significantly interferes with the graphite furnace detection when the Mo Re ratio is 2 or greater. The separation is accomplished through an extraction of tetraphenylarsonium perrhenate into chloroform, in which the molybdenum remains in the aqueous phase. 

An ideal derivation would be via direct fluorination of Re and hydrolysis of ReF7 to perrhenic acid, especially since A/2f (Re207) has been determined by oxygen combustion of Re. However, ReF6 is the main fluorination product under normal conditions. It seems that further work on iodine fluorination to produce enhanced yields of the hepta-fluoride, and the hydrolysis of IF7 to periodic acid, is potentially the best route for improving the AHf(F( aq)) value. 

A technique for the determination of Tc amounts as little as 4 x 10 g by neutron activation analysis has been described by Foti et al. . Tc in triply distilled water is irradiated in a thermal neutron flux of 5 x 10 neutrons per cm and per second to produce °°Tc. Other radionuclides are removed by co-precipi-tation with Fe(OH)j. Then, °°Tc is co-precipitated twice with tetraphenylarsonium perrhenate which can be removed by sublimation. The chemical purification of °°Tc requires 40-45 s and the technetium yield is about 53%. 

Pertechnetate forms a blue complex and perrhenate a brownish-yellow complex with K4[Fe(CN) ] in presence of bismuth amalgam. This permits the spectrophotometric determination of both elements in the same solution . The adsorption maxima of the technetium and rhenium complexes are at 680 and 420 nm, respectively. The molar extinction coefficients are 10,800 for technetium and 4,000 for rhenium. Metals forming color or precipitates with K4[Fe(CN) ] must first be removed. 

Astheimer and Schwochau have applied the voltametry method to the determination of low technetium concentrations in the presence of molybdate and perrhenate ions. Using a Kemula electrode technetium is concentrated on a mercury drop from alkaline solution of 6.0 x 10 M KTcO by electrolysis at a potential of — 1.0 V vs. SCE. Anodic stripping in 1 M NaOH yields a characteristic stripping curve (Fig. 14). The height of the peak at —0.33 V is proportional to the concentration of technetium in the range of 10 to 3 x 10 M. Technetium can be detected with an accuracy of +4%. The determination of 0.5 ng of technetium in a 10 fold molar excess of ReO or MoO is possible. 

Nonahydridorhenate is conveniently oxidized to perrhenate by slow addition of the solid to 5 % hydrogen peroxide. Sodium and rhenium may then be determined on the solution by standard methods. Hydrogen is determined by ordinary combustion techniques. 

Many structural studies have been made on the species present in the vapors above oxysalts such as alkali metal nitrates, chlorates, perrhenates, and so on. In part, the interest in such work lies in determining the mode of coordination of the metal atom to the oxyanion in the monomeric vapor species. Both gas electron diffraction and matrix isolation have been employed to seek an answer to this question. 

Nitriles can be prepared by dehydration of primary amides (reaction 8) or of aldoximes (reaction 9), catalyzed by the Re trioxo compounds [ReOsX] (X = OSiMes, OReOs, OH) in azeotropic (e.g. toluene/water or mesitylene/water) reflux. Aqueous perrhenic acid (X = OH) is the most convenient catalyst in view of the moisture sensitivity of the others. The oxophilicity of the Re(VII) oxo-complexes is believed to play a determining role in the reactions, which are proposed to involve six-membered cyclic transition states formed upon preferential (9-coordination, relative to A-coordination, of the amide or oxime. ... 

Trimethylsilyl perrhenate forms colorless crystals, which are very sensitive to hydrolysis and which must be handled in a dry atmosphere. Under anhydrous conditions, the compound is stable to light and oxidation. Contaminated products undergo slow decomposition at room temperature, blue reduction products of Re(VII) being formed. In an excess of water trimethylsilyl perrhenate is hydrolyzed quickly and quantitatively. The perrhenic acid formed in this reaction together with trimethylsilanol can be determined by acidimetric titration or precipitation as nitron perrhenate. Trimethylsilyl perrhenate is soluble without decomposition in all common anhydrous and aprotic solvents (benzene, cyclohexane, ethyl ether, tetrahydro-furan, chloroform, etc.). 

The permanganate ion was labeled with " Mn. The separation of Mn04 for kinetic determinations was achieved by co-precipitation with tetraphenylarsonium perrhenate, and the activity was determined with a scintillation counter. The rate constant, Arexch = 710 M s at 273 K, was obtained by fitting of the data to Eq. 65... 

The salicylfluorone was used in determination of tungsten in geological samples [139], steel [185], alloys, alkali metal halides, and ammonium perrhenate [186]. 

Perrhenate is also leached with water from a sample melted with Na2C03 or NaOH. Rhenium heptoxide, Rc207, is volatile and can be separated by distillation at 260-280°C from a concentrated sulphuric acid medium [1,17,18]. It may be accompanied by As(III), Hg, Se, and to a lesser degree by Sb, Te and Mo. In the presence of hydrochloric acid, rhenium distils as the oxychloride at a lower temperature, but As, Ge, Hg, Sn, Se, Mo, Te, and T1 are wholly or partly co-distilled. In view of the partial co-distillation of Mo, Re cannot be directly determined in the distillate. In the distillation of Re, carbon dioxide or air is used as the carrier gas. 

Arsonium salts have found considerable use in analytical chemistry. One such use involves the extraction of a metal complex in aqueous solution with tetraphenylarsonium chloride in an organic solvent. Titanium (TV) thiocyanate [35787-79-2] (157) and copper(II) thiocyanate [15192-76-4] (158) in hydrochloric acid solution have been extracted using tetraphenylarsonium chloride in chloroform solution in this manner, and the Ti(IV) and Cu(II) thiocyanates determined spectrophotometrically. Cobalt, palladium, tungsten, niobium, and molybdenum have been determined in a similar manner. In addition to their use for the determination of metals, anions such as perchlorate and perrhenate have been determined as arsonium salts. Tetraphenylarsonium permanganate is the only known insoluble salt of this anion. 

In acetonitrile or dimethylsulphoxide the polarographic and controlled-potential coulometric reduction of the tetramethylammonium salts of permanganate, pertech-netate, and perrhenate yielded as the first reduction step a one-electron transfer the half-wave potentials vx SC E observed in solutions of acetonitrile arc T, i/2=-0.60 V, -1.74 V and -2.30 V, respectively (Fig. 6.1.A). The limiting ionic equivalent conductance of TcOj was determined in acetonitrile and dimethylsulphoxide to be P=127.5... 

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