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Tham khảo tài liệu 'handbook of extractive metallurgy 3 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 1576 Handbook of Extractive Metallurgy 39.6 Quality Specifications Qualities ranging from 99.5 to 99.9999 tellurium are commercially available. Impurities may be metallic As Na Ca Pb Cu Ni Sb Sn Se Fe and nonmetallic S Cl O . Commercial grades and forms are 99.5-99.9 2N5-3N powder sticks or ingots 99.99 4N ingots or lumps 99.999 5N ingots and 99.9999 9 6N-7N ingots. Tellurium dioxide powder is available with purity 99. 5-99.9 . 39.7 Analysis Table 39.2 13 33 34 Emission spectrometry ES is frequently used for the analysis of standard grade materials up to a purity of 99.99-99.999 with preconcentration if necessary . Atomic absorption spectrometry AAS is used for analysis down to sub-ppm levels this method has however no multielement capability. AAS can be adapted with a graphite furnace GFAAS and coupled with a Zeeman background correction to allow detection limits at tyg kg levels. Techniques such as spark source mass spectrometry SSMS and glow discharge mass spectrometry GDMS offer panoramic elemental capability with low detection limits and maximum selectivity. Table 39.2 Detection limits obtained by GFAAS SSMS and ES. pg kg- Element GFAAS SSMS ES Ag 10 1.0 100 Al 50 0.8 300 Bi 50 2.0 100 Cu 50 a 200 Fe 50 1.0 200 Mg 20 19 100 Ni 20 1.6 300 Pb 50 1.0 200 Sb 50 1.0 300 Sn 50 4.0 100 1 Interference by doubly charged ions. 39.8 Compounds 5 11 35-37 Tellurium dioxide TeO2 is found in the minerals tellurite and paratellurite which have a rhombic and a tetragonal lattice structure respectively. It can be formed by burning tellurium in air or by treating tellurium powder with nitric acid followed by heating the nitrate 2Te 9HNO3 - 2TeO -HNO3 8NOj 4H O The melting point of TeO2 is 1005.8 K 732.6 C the clear dark red liquid oxide vaporizes between 790 and 940 . Tellurium dioxide is very soluble in water and is amphoteric the minimum solubility is at pH 3.8-4.2 the isoelectric point. Tellurium dioxide is an important catalyst in oxidation hydrogenation and dehydrogenation
