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AI2.2.2 Impact of CO2 on pH of water The dissolution of CO2 in water (this may be sea water, or the saline water in geological formations) involves a number of chemical reactions between gaseous and dissolved carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate ions (HCO3-) and carbonate ions (CO32-) which can be represented as follows:

Addition of CO2 to water initially leads to an increase in the amount of dissolved CO2. The dissolved CO2 reacts with water to form carbonic acid. Carbonic acid dissociates to form

Figure AI.7 Solubility of CO2 in brine relative to that in pure water, showing experimental points reported by Enick and Klara (1990) and correlation developed by those authors (TDS stands for total dissolved solids).

bicarbonate ions, which can further dissociate into carbonate ions. The net effect of dissolving anthropogenic CO2 in water is the removal of carbonate ions and production of bicarbonate ions, with a lowering in pH.

Figure AI.8 shows the dependence of pH on the extent to which CO2 dissolves in sea water at temperatures of 0oC and 25oC based on theoretical calculations (IEA Greenhouse Gas R&D Programme, 2000) by iterative solution of the relationships (Horne, 1969) for the carbonic acid/bicarbonate/ carbonate equilibria combined with activity coefficients for the bicarbonate and carbonate ions in sea water. The temperature dependence of the ionization of water and the bicarbonate equilibria were also included in this calculation. This gives values for the pH of typical sea water of 7.8-8.1 at 25oC and 8.1-8.4 at 0oC. These values, which are strongly dependent on carbonate/bicarbonate buffering, are in line with typical data for sea water (Figure AI.8 shows 2 experimental data points reported by Nishikawa et al., 1992).

Figure AI.8 also shows that there is a small effect of temperature on the reduction in pH that results from dissolution of CO2. A minor pressure dependence of water ionization is also reported (Handbook of Chemistry and Physics, 2000). The effect on water ionization of an increase in pressure from atmospheric to 250 bar (equivalent to 2500 m depth) is minor and about the same as would result from increasing temperature by about 2oC. The effect of pressure can therefore be ignored.

AI.2.3 Health and safety aspects of exposure to CO2

As a normal constituent of the atmosphere, where it is present in low concentrations (currently 370 ppmv), CO2 is considered harmless. CO2 is non-flammable.

As it is 1.5 times denser than air at normal temperature and pressure, there will be a tendency for any CO2 leaking from pipework or storage to collect in hollows and other low-lying confined spaces which could create hazardous situations. The hazardous nature of the release of CO2 is enhanced because the gas is colourless, tasteless and is generally considered odourless

Figure AI.7 Solubility of CO2 in brine relative to that in pure water, showing experimental points reported by Enick and Klara (1990) and correlation developed by those authors (TDS stands for total dissolved solids).

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