Carbonate Species Concentrations in Open and Closed Systems

Calculation of Carbonate Species Concentrations in Open and Closed Systems 

In working problems concerned with carbonate equilibria in waters, we can take one of two general approaches. The first method is to treat the problem rigorously. This procedure often leads to complex equations. The second method is to make simplifying assumptions to provide an easy but approximate solution. We intend to present both ways of attacking the problem because it is only by having a fundamental knowledge of the system that one can develop the ability to make rational simplifications. 

let us examine homogeneous (no precipitates), open systems. In these systems the carbonate species in solution are in equilibrium with the CO2 gas in the atmosphere above the solution. The normal atmosphere contains 10 - atm of CO2, and when this is in equilibrium with C02(a ,) from Eqs. 4-103 and 4-110, we obtain 

Let us now construct a pC-pH diagram for the carbonate system open to an atmosphere containing Pco = 10 - atm (Fig. 4-16). Our first step is to determine the concentration of each carbonate species as a function of pH. From Eq. 4-111, [HaCO ] =10 , 

Therefore, [H2CO3] versus pH plots as a horizontal straight line at -log C = 5 as shown in Fig. 4-16. 

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