The two most important attributes of water that control the chemical states of dissolved foreign species are the pH and the equivalent oxygen pressure. The latter is the oxygen pressure that would be in equilibrium with the actual concentration of dissolved O2
where KH = 885 atm/M is the Henry's law constant at 298 K for the equilibrium:
Thoroughly-aerated water has an equivalent oxygen pressure of 0.21 atm. Water that has been standing in pipes may have a significantly-lower (pO2 )eq.
Another airborne species that affects water chemistry is carbon dioxide. Unlike oxygen, this gas reacts with water in a way that reduces the pH. The pertinent equilibria are:
CO2 (dissolved) + H2O = HCO- + H+ K = 4.4x10-7 M (10.38b)
The equilibrium constants for these reactions are for 298 K. In all, carbon dioxide is present in water in four forms: the dissolved gas CO2 (dissolved); carbonic acid H2CO3; the bicarbonate ion HCO 3- ; and the carbonate ion, CO2- . Air, which consists of 0.035% CO2, is the primary source of dissolved CO2. In the absence of a CO2 - bearing gas phase, soluble salts such as sodium bicarbonate (bicarbonate of soda) can provide all of the forms of dissolved CO2.
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