Sodium metabisulfite, Na2S2Os, was investigated as a part of a study of the role of sulfur dioxide in the effects of air pollution on health. Although the compound itself may not be directly involved as a major air pollutant, the chemistry associated with its thermal decomposition becomes important in understanding the possibilities of interactions between sulfur dioxide and aerosol particles. It is the bound form of SO2, then, that must be considered as an air pollutant in determining its adverse health effects or control methods. It also is important to determine whether bound sulfur dioxide reaches the respiratory tract in a more, or a less hazardous form, e.g. whether it enters the lungs in an oxidizable state or perhaps as a metal sulfate.
Erdey et al. 1 described the thermal decomposition of sodium metabisulfite (sodium pyrosulfite) as a loss of sulfur dioxide followed by an oxidation of sodium sulfite. Cleghorn and Davies 2 used i.r. spectral data to suggest that there is an interval in the decomposition process when the evolution of sulfur dioxide and the oxidation of sodium sulfite proceed simultaneously. Cotton and Wilkinson a referred to a possible equilibrium between metabisulfite and the hydrogen sulflte, NaHSOa. A possible decomposition intermediate, sodium peroxide, Na202, is oxidized to the superoxide, NaO2, under conditions of elevated pressures and temperatures 3. Thermogravimetric analysis (TGA) showed'*, on the other hand, that NaO2 forms Na202 in air within the temperature range 2000-340 Β° .
The formation and presence of one or more of these oxide components is thought to be involved in the decomposition process and, consequently, to affect the thermal analysis record.
Thermal decomposition of sodium metabisulfite is expected to yield sulfur dioxide with a corresponding loss in weight at ca. 150 Β°. Further decomposition might be expected at a much higher temperature. The rate and the temperature range of decomposition varies with the atmosphere and with the rate of heating imposed upon the compound. An oxygen-containing atmosphere, for example, would probably cause an increase in weight through oxidation of the sample.
Sodium hydrogen sulfite preparations might be considered to be chiefly metabisulfite, and for all practical purposes, to act like it 5. The preparation of the hydrogen sulfite involves an equilibrium between the hydrogen sulfite and the metabisulfite forms that results in a mixture ofthe two, that can be found even in the reagent grade 6. EXPERIMENTAL