Speciation of carbon dioxide

What is the parent or form of CO2 detected in these studies Theoretical considerations suggest that, of the various carbonates and bicarbonates likely to occur in rocks and soils, NaHC03 releases the most CO2 at relatively low temperatures at I00 C it liberates 32% of its CO2 content, whereas metal carbonates, CaC03, K2CO3 and Na2C03 do not begin to break down until much higher temperatures are reached (Fig. 4-7). The thermal stability of these carbonates was confirmed by laboratory experiments in which they were boiled in distilled water and the CO2 released into solution was determined by titration (Table 4-1). 

Release of CO2 into solution from pure carbonates at 100 C 

Some CO2 could also be present in soils in an adsorbed form, having previously been transported in solution. This hypothesis was investigated using five soil samples, of which three were from a humid area and two were from as arid area (Table 4-11). A sample from a background site in the humid area was found to contain 84 ppm CO2 in the adsorbed form, compared with only 14% that was released by boiling thus 86% of the CO2 was in the adsorbed form. In anomalous samples from the same area, 34% and 52% of the CO2 was in the adsorbed form. In the arid area, however, the two anomalous samples contained only 26% and 27% of their CO2 in the adsorbed form. 

These experiments suggest that in arid areas, CO2 related to mineral deposits occurs in soils mainly as salts of bicarbonates (which break down upon gentle heating). In humid areas, by contrast, CO2 related to mineral deposits coexists as bicarbonates and adsorbed CO2. 

Form of CO2 Background Humid area Anomaly 1 Anomaly 2 Arid area Anomaly 1 Anomaly 2  

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Fine G. J. and Stolper E. M. (1985) The speciation of carbon dioxide in sodium aluminosilicate glasses. Contrib. Mineral. Petrol. 91, 105-121. 

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