Carbonate equilibria in an open system

4 Carbonate equilibria in an open system. What is the pH of water in equilibrium with atmospheric C02 gas To answer such a question involves a knowledge of acid-base chemistry, the use of Henry s Law constant for the solubility of carbon dioxide and the use of the ENE to calculate the proton concentration of the equilibrium solution. The details of the equilibrium constants used are detailed below. 

Gaseous carbon dioxide is soluble in water according to the reaction 

The equilibrium partitioning between the gaseous and aqueous phase is given using the Henry s Law constant, fCH. Although C02(aq) is not intrinsically acidic it can react with water to produce a diprotic acid H2C03, as shown by the reaction below  

The total amount of aqueous C02 dissolved is represented by the hypothetical 

However, the amount of H2C03 formed is small (logK = —2.8), so the effective concentration of diprotic acid [H2C03] in the system is given by the approximation 

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