ROLE OF OXYGEN IN THE DETERMINATION OF OXIDE FORMING ELEMENTS BY ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY .3. EFFECT OF OXYGEN ON THE REACTIONS OF TIN IN UNCOATED, PYROLYTICALLY COATED AND ZIRCONIUM CARBIDE COATED GRAPHITE TUBE ATOMIZERS

The chemical reactions of tin have been investigated in uncoated, pyro lytic graphite coated, oxygen treated and zirconium carbide coated gra phite tube atomizers. In all types of tubes SnO2 is reduced to the vol atile tin monoxide, which is stable up to 1500 K. Above this temperatu re it is further reduced to the metal by collision with the atomizer s urface. The rate of losses during thermal pretreatment is highest in c oated tubes. SnO is formed in uncoated tubes also, but at a lower rate . The pyrolysis curve exhibits a significant plateau between 1500 and 1800 K. It is proposed that tin atoms are intercalated into the graphi te lattice in that temperature range. Treatment of the tubes with oxyg en does not principally change the processes, but reduces the rate of losses. Repeated coating of the tubes with zirconium carbide shifts th e onset of losses to higher temperatures and reduces the rate of losse s further. In addition, no plateau is observed as in the case of uncoa ted tubes.