Tellurium determination

Andreae [564] coprecipitated tellurium (V) and tellurium (VI) from seawater and other natural waters with magnesium hydroxide. After dissolution of the precipitate with hydrochloric acid, the tellurium (IV) was reduced to tellurium hydride in 3 M hydrochloric acid. The hydride was trapped inside the graphite tube of a graphite furnace atomic absorption spectrometer, heated to 300 °C, and tellurium (IV) determined. Tellurium (VI) was reduced to tellurium (IV) by boiling with hydrochloric acid and total tellurium determined. Tellurium (VI) was then calculated. The limit of detection was 0.5 pmol per litre and precision 10-20%. 

Ceric ammonium sulfate, 5 674 Ceric fluoride, 5 674 Ceric hydroxide, 5 676 Ceric oxide, 5 670, 675 Ceric rare earths (RE), 74 631 Ceric sulfate, 5 674 Ceric sulfate method, for tellurium determination, 24 415 Cerium (Ce), 5 670-692 74 630, 63 It, 634t. See also Cerium compounds analysis, 5 679-680 color, 7 335... 

Gravimetric methods, tellurium determination by, 24 415 Gravimetric techniques. See also Acoustic wave gravimetric technique for anthropogenic silicas and silicates, 22 470... 

A review of spectrophotometric methods for tellurium determination has been presented [3]. The systems employing inorganic compounds forming coloured products, N-... 

S. K. Aggarwal, M. Kinter, and D. A. Herold, Tellurium Determination by GC-MS in Biological Samples", Proceedings of the 40th ASMS Conference on Mass Spectrometry and Allied Topics, Washington, DC, pp. 1555-1556 (1992). 

A widely used procedure for determining trace amounts of selenium involves separating selenium from solution by reduction to elemental selenium using tellurium (as a carrier) and hypophosphorous acid as reductant. The precipitated selenium, together with the carrier, are collected by filtration and the filtered soflds examined directly in the wavelength-dispersive x-ray fluorescence spectrometer (70). Numerous spectrophotometric and other methods have been pubHshed for the deterruination of trace amounts of selenium (71—88). 

Although gravimetric methods have been used traditionally for the determination of large amounts of tellurium, more accurate and convenient volumetric methods are favored. The oxidation of teUurium(IV) by ceric sulfate in hot sulfuric acid solution in the presence of chromic ion as catalyst affords a convenient volumetric method for the determination of tellurium (32). Selenium(IV) does not interfere if the sulfuric acid is less than 2 N in concentration. Excess ceric sulfate is added, the excess being titrated with ferrous ammonium sulfate using o-phenanthroline ferrous—sulfate as indicator. The ceric sulfate method is best appHed in tellurium-rich materials such as refined tellurium or tellurium compounds. 

From a toxicological and physiological point of view, the determination of very small amounts of tellurium is becoming increasingly important. Interest is environmental and human health has promoted development in analytical techniques and methods for the trace and ultra trace levels (see Trace AND RESIDUE ANALYSIS). 

Numerous methods have been pubUshed for the determination of trace amounts of tellurium (33—42). Instmmental analytical methods (qv) used to determine trace amounts of tellurium include atomic absorption spectrometry, flame, graphite furnace, and hydride generation inductively coupled argon plasma optical emission spectrometry inductively coupled plasma mass spectrometry neutron activation analysis and spectrophotometry (see Mass spectrometry Spectroscopy, optical). Other instmmental methods include polarography, potentiometry, emission spectroscopy, x-ray diffraction, and x-ray fluorescence. 

Spot tests for determining chlorosulfuric acid are based on the use of powdered tellurium, which gives a cherry-red color, and powdered selenium, which gives a moss-green color in the presence of the acid. 

The major product obtained from the reaction of TeCU with (McsSijsN is determined by the stoichiometry. When approximately equimolar amounts of the two reagents in THF are used the dimeric cluster TceNs (stabilized by coordination to four TeCU molecules) is obtained in high yields (Section 5.2.8). When the same reaction is carried out in acetonitrile with a molar ratio of 2 1, followed by treatment of the product with AsFs in SO2, [Tc4N2Cl8][AsF6]2 is obtained Section The dication [Tc4N2Cl8] in this salt is the dimer of the hypothetical tellurium(IV) imide [Cl3Te-N=TeCl]. ... 

Tellurium nitride was first obtained by the reaction of TeBt4 with liquid ammonia more than 100 years ago. The empirical formula TeN was assigned to this yellow, highly insoluble and explosive substance. However, subsequent analytical data indicated the composition is Tc3N4 which, in contrast to 5.6a and 5.6b, would involve tetravalent tellurium. This conclusion is supported by the recent preparation and structural determination of Te6N8(TeCl4)4 from tellurium tetrachloride and tris(trimethylsilyl)amine (Eq. 5.5). The TceNs molecule (5.12), which is a dimer of Tc3N4, forms a rhombic dodecahedron in which the... 

Determination of copper as copper(I) thiocyanate Discussion. This is an excellent method, since most thiocyanates of other metals are soluble. Separation may thus be effected from bismuth, cadmium, arsenic, antimony, tin, iron, nickel, cobalt, manganese, and zinc. The addition of 2-3 g of tartaric acid is desirable for the prevention of hydrolysis when bismuth, antimony, or tin is present. Excessive amounts of ammonium salts or of the thiocyanate precipitant should be absent, as should also oxidising agents the solution should only be slightly acidic, since the solubility of the precipitate increases with decreasing pH. Lead, mercury, the precious metals, selenium, and tellurium interfere and contaminate the precipitate. 

Discussion. This gravimetric determination depends upon the separation and weighing as elementary selenium or tellurium (or as tellurium dioxide). Alkali selenites and selenious acid are reduced in hydrochloric acid solution with sulphur dioxide, hydroxylammonium chloride, hydrazinium sulphate or hydrazine hydrate. Alkali selenates and selenic acid are not reduced by sulphur dioxide alone, but are readily reduced by a saturated solution of sulphur dioxide in concentrated hydrochloric acid. In working with selenium it must be remembered that appreciable amounts of the element may be lost on warming strong hydrochloric acid solutions of its compounds if dilute acid solutions (concentration <6M) are heated at temperatures below 100 °C the loss is negligible. 

A process for the gravimetric determination of mixtures of selenium and tellurium is also described. Selenium and tellurium occur in practice either as the impure elements or as selenides or tellurides. They may be brought into solution by mixing intimately with 2 parts of sodium carbonate and 1 part of potassium nitrate in a nickel crucible, covering with a layer of the mixture, and then heating gradually to fusion. The cold melt is extracted with water, and filtered. The elements are then determined in the filtrate. 

B. Determination of tellurium Procedure. The solution should contain not more than 0.2 g tellurium in 50 mL of 3M hydrochloric acid (ca 25 per cent by volume of hydrochloric acid). Heat to boiling, add 15 mL of a freshly prepared, saturated solution of sulphur dioxide, then 10 mL of a 15 per cent aqueous solution of hydrazinium chloride, and finally 25 mL more of the saturated solution of sulphur dioxide. Boil until the precipitate settles in an easily filterable form this should require not more than 5 minutes. Allow to settle, filter through a weighed filtering crucible (sintered-glass, or porcelain), and immediately wash with hot water until free from chloride. Finally wash with ethanol (to remove all water and prevent oxidation), and dry to constant weight at 105 °C. Weigh as Te. 

In the alternative method of reduction, which is particularly valuable for the determination of small amounts of tellurium, the procedure is as follows. Treat the solution containing, say, up to about 0.01 g Te in 90 mL with 10 mL of 1 3-sulphuric acid, then add 10 g sodium hypophosphite (phosphinate), and heat on a steam bath for 3 hours. Collect and weigh the precipitated tellurium as above. 

C. Determination of mixtures of selenium and tellurium Procedure. Dissolve the mixed oxides (not exceeding 0.25 g of each) in 100 mL of concentrated hydrochloric acid, and add with constant stirring 50 mL cool concentrated hydrochloric acid which has been saturated with sulphur dioxide at the ordinary temperature. Allow the solution to stand until the red selenium has settled, filter through a weighed filtering crucible (sintered-glass or porcelain) and complete... 

Lingane and Niedrach have claimed that the h-VI states of tellurium (or selenium) are not reduced at the dropping electrode under any of the conditions of then-investigation however, Norton et al. [42] showed that under a variety of conditions, samples of telluric acid prepared by several different procedures do exhibit well-defined (though irreversible) waves, suitable for the analytical determination of the element. The reduction of Te(H-VI) at the dropping electrode was found coulometri-cally to proceed to the -II state (whereas selenate, Se(-i-VI), was not reduced at the dropping electrode in any of the media reported). 

Important thermodynamic properties that relate to the structure and stability of the chalcogen ailotropes and their polyatomic cations are the formation enthalpies listed in Table 2. Only reliable experimentally or quantum chemically established numbers have been included. From Table 2 it is evident that tellurium is the least investigated with respect to the entries thus, there is clearly space for more thorough experimental or quantum chemical work in this direction. Therefore, we have assessed the missing Te data from the IP determination in ref. 12 (PE spectroscopy) and ref. 13 (quantum chemical calculations) and have put them in the table in parentheses, although it is clear that the associated error bars are relatively high. The data in ref. 14 were not considered. 

On the other hand, optically active telluroxides have not been isolated until recently, although it has been surmised that they are key intermediates in asymmetric synthesis.3,4 In 1997, optically active telluroxides 3, stabilized by bulky substituents toward racemization, were isolated for the first time by liquid chromatography on optically active columns.13,14 The stereochemistry was determined by comparing their chiroptical properties with those of chiral selenoxides with known absolute configurations. The stability of the chiral telluroxides toward racemization was found to be lower than that of the corresponding selenoxides, and the racemization mechanism that involved formation of the achiral hydrate by reaction of water was also clarified. Telluroxides 4 and 5, which were thermodynamically stabilized by nitrogen-tellurium interactions, were also optically resolved and their absolute configurations and stability were studied (Scheme 2).12,14... 

Unlike selenium there is no required biological role for tellurium in bacteria or plants that has been determined however, this may ultimately not be the whole story.111 Selenium was only viewed as a toxic metalloid with no necessary role for metabolism until at least the 1950s see above. While tellurite is less soluble than tellurate in aqueous solution, in general tellurite is probably more toxic to most organisms.190 The non Te-resistant wild type E. coli bacterium (Gramnegative) has MICs of 1 to 3 ppm for tellurite and tellurate.144,191,192 Tellurite is used to enrich and select for Staphylococcus aureus.169,193,194... 

The mechanism of tellurium resistance has been investigated using genetic manipulation similar to that of Se (see above) and cellular oxidant capacity apparently plays an important role.144,206 A few tellurite determinants - both chromosomal and plasmid encoded - have been identified in bacte-ria.113,147 192 207 208 Recent studies have focused on the role of methyltransf-erases in Te resistance. Liu et a/.111 determined that the E. coli gene tehB uses S-adenosyl methionine and a methyltransferase in tellurite detoxification, but while no methylated tellurium compounds (see below) were observed, a loss of tellurite was observed in tellurite-amended cultures and Te complexation was inferred.191... 

Like selenium, the process of reduction/oxidation cycling in biological systems is important and changes in the oxidation state are often an easy means of determining bioreduction for added tellurium oxyanions. The general order of... 

As with selenium analysis, HGAAS also finds a place in Te determination. Basnayake et al. amended cultures of P. fluorescens K27 with 0.1 mM tellurite and, after 92-h growth, determined that approximately 34% of added Te was present as either precipitated, elemental Te in/or on centrifuged cells the balance of added Te remained in solution.190 In this same study, GC/MS was used for determination of DMTe in the same facultative anaerobe amended with tellurate.190 Earlier, GC/MS was used to analyze the headspace of a tellurium-resistant fungus amended with tellurite.215 This last is one of the few reports of the detection of dimethyl ditelluride in microbial headspace (see below). 

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