Osmium concentration

To avoid the risk of contamination by seawater, either through direct contamination of the samples or contamination of the magma by assimilation of contaminated material, many authors disregard samples with very low osmium concentrations. Unfortunately, this approach does not remove the inherent ambiguity of interpretation, and it may simply bias the sampling. What is clearly needed are independent measures of very low levels of magma chamber and sample contamination. 

Since negative thermal ion mass spectrometry (N-TIMS) for rhenium/osmium dating (see O Sect. 17.18) was developed about 1990, not too much work has been published. This is probably due to the high rhenium blanks from the current generation of clean platinum filaments and to problems in achieving isotopic exchange and equilibrium between sample and spike for osmium. Another drawback is the non-homogeneity of samples. Because of this, rhenium and osmium concentrations may vary hy up to 40% in the same sample. 

Asymmetric dihydroxylation reactions using the commercial materials (AD-mix-a, Aldrich 39,275-8 AD-mix-p, Aldrich 39,276-6) result in typical reaction times of 16-24 h for most alkenes at 0°C. For 1,2-disubstituted, trisubstituted, and tetrasubstituted alkenes further promotion by addition of methane-sulfonamide is also required. The dihydroxylation rate is considerably improved by increasing the osmium concentration in the commercial product from 0.2 mol% to 1 mol% by adding 3.68 mg of finely ground K2l0s(=0)2 0H)4) per 1.4 g of AD-mix-a (or AD-mix-p) and mixing well. 

Concentrations in air as low as IO7 g/ms can cause lung congestion, skin damage, or eye damage. Exposure to osmium tetroxide should not exceed 0.0016 mg/ms (8-hour time weighted average - 40-hour work week). 

The PGM concentrate is attacked with aqua regia to dissolve gold, platinum, and palladium. The more insoluble metals, iridium, rhodium, mthenium, and osmium remain as a residue. Gold is recovered from the aqua regia solution either by reduction to the metallic form with ferrous salts or by solvent-extraction methods. The solution is then treated with ammonium chloride to produce a precipitate of ammonium hexachloroplatinate(IV),... 

Consequent potentiometric titration of osmium(IV) and laithenium (IV) in their mixtures has been canied out in broad range of concentrations from 1 mkg to 200 mkg in samples of 20 ml. It has been shown the possibility of amperemetric determination of osmium(VI) in binary and triple systems with silver(I), platinum(IV), palladium(II), gold(III), founded on formation of corresponding compounds with dimerkaptotiopiron, having a different solubility. The deteriuination of Os(VI) is possible under tenfold - hundredfold excess of above mentioned metals. 

The extraction of osmium(VI) and osmium(IV) compounds with dimerkaptotiopiron by polar solvents has been resear ched. 50 -100 - multiple concentrating has been carried out in acid solutions regardless from chemical form of Os(VI) and Os(IV) with the following determination of each of them from one sample by amperemetric titration of reextracts by means of DT. A methodic of atom absor ption determination of osmium has been suggested. 

As already noted (p. 1073), the platinum metals are all isolated from concentrates obtained as anode slimes or converter matte. In the classical process, after ruthenium and osmium have been removed, excess oxidants are removed by boiling, iridium is precipitated as (NH4)2lrCl6 and rhodium as [Rh(NH3)5Cl]Cl2. In alternative solvent extraction processes (p. 1147) [IrClg] " is extracted in organic amines leaving rhodium in the aqueous phase to be precipitated, again, as [Rh(NH3)5Cl]Cl2. In all cases ignition in H2... 

A solution of 1.0 g of A -3,11-diketo-20-cyano-21-acetoxy-pregnene in 10 cc of benzene is treated with 1.0 g of osmium tetroxide and 0.43 g of pyridine. After standing at room temperature for 18 hours, the resulting solution is treated successively with 50 cc of alcohol, and with 50 cc of water containing 2.5 g of sodium sulfite. The mixture is stirred for 30 hours, filtered, and the filtrate acidified with 0.5 cc of acetic acid and concentrated to small volume in vacuo. The aqueous suspension is then extracted four times with chloroform, the chloroform extracts are combined, washed with water and concentrated to dryness in vacuo. Recrystallization of the residue from acetone gives 3,11,20-triketo-17(a)-21-dihydroxy-pregnane MP 227° to 229°C. This compound is then treated with acetic anhydride and pyridine for 15 minutes at room temperature to produce 3,11,20-triketo-17(a)-hydroxy-21-acetoxy-pregnane or cortisone acetate. 

To a solution of 6.36 parts of 17(3-hydroxy-17a-methyl-5o -androst-Ten-3-one in 95 parts of acetic acid and 12 parts of water is added 40 parts of lead tetracetate and 0.6 part of osmium tetroxide. This mixture is stored at room temperature for about 24 hours, then is treated with 2 parts of lead tetracetate. Evaporation to dryness at reduced pressure affords a residue, which is extracted with benzene. The benzene extract is washed with water, and extracted with aqueous potassium bicarbonate. The aqueous extract is washed with ether, acidified with dilute sulfuric acid, then extracted with ethyl acetate-benzene. This organic extract is washed with water, dried over anhydrous sodium sulfate, and concentrated to dryness in vacuo. To a solution of the residual crude product in 20 parts of pyridine is added 10 parts of 20% aqueous sodium bisulfite and the mixture is stirred for about 20 minutes at room temperature. 

To a stirred solution of 4 mmol of the diamide in 40 mL of dioxane (distilled from LiAlH4) and 13 mL of water is added 1 mg of osmium tetroxide. When the solution turns brownish (after about 10 min) 2.06 g (9.2 mmol) of sodium metaperiodale are added at 25 26 "C. The progress of the reaction is monitored by TLC on silica gel coated plastic sheets with CHCI,/diethyl ether/methanol (3.3 0.1) as eluent. When the reaction is complete, the precipitated solid is filtered and the filtrate concentrated in vacuo at 1 Torr. The residue is dissolved in 50 mL of CHC13, dried over MgSO,. and evaporated in vacuo to leave a residue, which is crystallized from a suitable solvent. 

In the solvent-extraction process, the platinum metal concentrate is solubilized in acid using chlorine oxidant. Ruthenium and osmium are separated by turning them into the volatile tetroxides. 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

During the induced reduction of chlorate a considerable oxygen effect was observed. The air oxidation of arsenic(ril) is also an induced reaction, the extent of which decreases with increasing acid concentration and is increased by decreasing the rate of primary reaction. The induced oxidation caused by air also can be observed during the osmium tetroxide-catalyzed chlorate-arsenic(III) reaction. 

The ruthenium-copper and osmium-copper systems represent extreme cases in view of the very limited miscibility of either ruthenium or osmium with copper. It may also be noted that the crystal structure of ruthenium or osmium is different from that of copper, the former metals possessing the hep structure and the latter the fee structure. A system which is less extreme in these respects is the rhodium-copper system, since the components both possess the face centered cubic structure and also exhibit at least some miscibility at conditions of interest in catalysis. Recent EXAFS results from our group on rhodium-copper clusters (14) are similar to the earlier results on ruthenium-copper ( ) and osmium-copper (12) clusters, in that the rhodium atoms are coordinated predominantly to other rhodium atoms while the copper atoms are coordinated extensively to both copper and rhodium atoms. Also, we conclude that the copper concentrates in the surface of rhodium-copper clusters, as in the case of the ruthenium-copper and osmium-copper clusters. 

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

The magnitudes of the rate constants for the iridium catalyst were close to those obtained for rhodium 3 and osmium 5 based catalyst systems at similar conditions. However, the unusual dependence on catalyst concentration affects its general utility in comparison to other homogeneous catalysts for the hydrogenation of NBR. 

The formation of these compounds has been rationalized according to Scheme 6. The reaction of Os (E )-CH=C 11 Ph C1 (C())( P Pr3)2 with n-BuLi involves replacement of the chloride anion by a butyl group to afford the intermediate Os (/i> CH=CHPh ( -Bu)(CO)(P Pr3)2, which by subsequent hydrogen (3 elimination gives OsH ( >CI I=CHPh (CO)( P Pr3)2. The intramolecular reductive elimination of styrene from this compound followed by the C—H activation of the o-aryl proton leads to the hydride-aryl species via the styrene-osmium(O) intermediate Os r 2-CH2=CHPh (CO)(P Pr3)2. In spite of the fact that the hydride-aryl complex is the only species detected in solution, the formation of OsH ( )-CH=CHPh L(CO)(P Pr3)2 and 0s ( )-CH=CHPh (K2-02CH)(C0)(P,Pr3)2 suggests that in solution the hydride-aryl complex is in equilibrium with undetectable concentrations of OsH ( )-CH=CHPh (CO)(P,Pr3)2. This implies that the olehn-osmium(O) intermediate is easily accessible and can give rise to activation reactions at both the olefinic and the ortho phenyl C—H bonds of the... 

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