Tellurium tellurites

Tellurous acid (H2Te03) is a white solid that dehydrates readily to tellurium dioxide. Tellurous acid is a much weaker acid than selenious acid (H2Se03). The alkali and alkaline earth metal tellurites are water-soluble. When tellurous acid is heated to decomposition, it emits toxic fumes of tellurium. Tellurites cause nausea, somnolence, and loss of appetite. 

TEL, Et4pb. Lead tetraethyl, fellurates Formally salts containing oxy-anions of tellurium. Tellurales(VI) contain octahedral [TeO ] " species and derivatives of these. Tellurates(IV), tellurites, contain [TeOa] " species. 

Tellurite, see Tellurium dioxide Tenorite, see Copper(II) oxide Tephroite, see Manganese silicate(l—)... 

Separation of Se and Te can also be achieved by neutralizing the alkaline selenite and tellurite leach with H2SO4 this precipitates the tellurium as a hydrous dioxide and leaves the more acidic selenous acid, H2Se03, in solution from which 99.5% pure Se can be precipitated by S02 ... 

All the dialkali monosulfides are soluble in water and give alkaline solutions. The tellurides are instantly decomposed by air. They are soluble in water, but the solutions are easily oxidized to red polytellurides. The alkali metal tellurides are strong reducing agents which reduce tellurites to metallic tellurium. 

The irreversibility of Te electrodeposition is increased by changing the tellurite solution to pH 10, Figure 9C. At pH 10, the potential difference between deposition and stripping for the 1/1 couple is 600 mV. The origin of peak 0 is still unclear, but appears to be due to changes in an adsorbed layer of tellurium oxide. 

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... 

In experiments parallel to those of Cowie and Cohen (see above), a Te-resistant fungi grown in high tellurite amendments with little available sulfur produced telluromethionine, tellurocysteine, and tellurocystine.140,203 So like selenium, tellurium can apparently substitute for sulfur under certain... 

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... 

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). 

Crude tellurium dioxide is dissolved in a strong solution of caustic soda to form sodium tellurite. Electrolysis of sodium tellurite solution deposits tellurium metal on the stainless steel cathode. 

The higher sulfides of tellurium such as TeS2 and TeSs, are obtained from tellurite solutions by precipitation with hydrogen sulfide or sodium sulfide. Tellurium reacts with concentrated sulfuric acid to form red oxysulfide of the composition, TeSOs. With nitric acid, the metal is oxidized to dioxide, Te02. Oxidation of tellurium with chromic acid or potassium permanganate in nitric acid yields orthotelluric acid (HeTeOe). 

Tellurium dioxide in its orthorhombic form occurs in nature as mineral tellurite. It is mined from natural deposits. Also, tellurium dioxide is produced as an intermediate during recovery of tellurium metal from anode shmes of electrolytic copper refining (See Tellurium, Production). The dioxide also is prepared by treating tellurium metal with hot nitric acid to form 2Te02 HNO3. The product then is heated to drive off nitric acid. 

Although this procedure yields tellurium as the same compound found in the original feedstock, the copper telluride is recovered in a comparatively pure state which is readily amenable to processing to commercial elemental tellurium or tellurium dioxide. The upgraded copper telluride is leached with caustic soda and air to produce a sodium tellurite solution. The sodium tellurite solution can be used as the feed for the production of commercial grade tellurium metal or tellurium dioxide. 

If the final product desired is tellurium metal, excess free caustic soda is required in the sodium tellurite solution. The solution is electrolyzed in a cell using stainless steel anodes to produce tellurium metal (20). This technology is used at the CCR Division of Noranda Metalluigy Inc., Canada, and at Pacific Rare Metals Industries Inc., the Philippines. Typical electrolysis conditions are given in Table 2. 

Alternatively, if tellurium dioxide is the product desired, the sodium tellurite solution can be neutralized in a controlled fashion with sulfuric acid. As the pH is lowered, precipitates containing impurities such as lead and silica that form are filtered off. At pH 5.6 the solubility of tellurous acid reaches a minimum and essentially all of the tellurium precipitates (>98%). After filtration and drying, commercial tellurium dioxide is obtained. A diagram for the process of detellurizing of slimes and recovering tellurium products is shown in Figure 1. 

The oxidation of tellurium(IV) by permanganate as an analytical method has been studied in some detail (26). The sample is dissolved in 1 1 nitric-sulfuric acid mixture addition of potassium bisulfate and repeated fuming with sulfuric acid volatilizes the selenium. The tellurite is dissolved in 10 vol % sulfuric acid, followed by threefold dilution with water and titration with potassium permanganate ... 

Metal Coatings. Tellurium chlorides, as well as tellurium dioxide in hydrochloric acid solution, impart permanent and attractive black antique finish to silverware, aluminum, and brass. Anodized aluminum is colored dark gold by tellurium electro deposition. A solution containing sodium tellurate and copper ions forms a black or blue-black coating on ferrous and nonferrous metals and alloys. Addition of sodium tellurite improves the corrosion resistance of electroplated nickel. Tellurium diethyldithiocarbamate is an additive in bright copper electroplating (see Electroplating). 

Tellurium oxide, Te02, is found in rhombic crystals as the mineral tellurite, whilst ferrotellurite is probably an iron tellurate, emmonsite and dividensite being similar compounds. Other naturally occurring tellurates are montanite, a basic bismuth tellurate, and magmlite, a... 

Tellurium in a very finely divided condition may be obtained by mixing aqueous solutions of dextrose (30 per cent.) and sodium tellurite (5 per cent.) and heating to boiling for half an hour.11... 

Free tellurium and insoluble tellurium compounds resist bacterial action, but the soluble tellurites and tellurates are decomposed by... 

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