Volatilisation studies of cadmium compounds by the combined quartz furnace and flame atomic absorption method: effects of magnesium chloride and ascorbic acid additives

Abstract

A flame atomic absorption spectrometer was coupled to a modified thermoanalytical quartz furnace for the element-specific detection of evolved decomposition products. Heating programmes with a constant heating rate (0.8 °C s^–1) and with an exponentially decreasing heating rate (typically 50–5 °C s^–1) were applied in the range 25–950 °C. Sample holders made of platinum, electrographite and solid pyrolytic graphite were used in furnace atmospheres of variable oxygen concentration. Decomposition and vaporisation of Cd(NO₃)₂, CdCl₂, MgCl₂, ascorbic acid and mixtures of these compounds were studied. The radiation scatter of the aggregates formed from the ascorbic acid by decomposition was detected in a flow cuvette. Because of the extremely long life of the atomic cadmium vapour, cold vapour detection of this species was also feasible in the combined system. This was used to supplement the flame atomic absorption measurements in elucidating vaporisation mechanisms. Samples in the form of aqueous solutions and occasionally as solids were applied to the furnace. It was found that the hydrolysis of CdCl₂ and MgCl₂ takes place to a lesser extent on increasing the sample mass and the heating rate and by using pyrolytic graphite and platinum sample holders. The enhanced rate of hydrolysis on the surface of porous graphite is explained by an adsorption effect (adsorption hydrolysis). The fraction of a hydrolysing chloride salt evolved in chloride form is also dependent on the pre-heating conditions. In the presence of ascorbic acid, Cd(l), CdO(s) and MgO(s) are formed from both chloride and nitrate salts of these elements in the first period of heating. On the surface of the pyrolytic sample holder, the formation of Cd(l) is enhanced in the presence of ascorbic acid and thus complete evaporation of cadmium can be achieved at low temperature (320–460 °C). The vaporisation of CdO(s) takes place by thermal dissociation from both platinum and graphite surfaces at a temperature (ranging from 580 to 800 °C) that depends on the effective oxygen concentration close to the sample. The decomposition products of the nitrate anion activate the graphite surface, which enhances the vaporisation rate of CdO(s) from such a surface.