Chemical Equilibrium Diagrams / Examples

Example 5. Calculated pH

Data for the chemical system H⁺, Ca²⁺ and CO₃²⁻ was extracted using DATABASE. Then it was modified in SPANA to exchange components for reactions, in this case CO₃²⁻ was exchanged for CO₂ and Ca²⁺ for CaCO₃(cr).

In the diagram the calculated pH is plotted against the amount of added CO₂ (from 10⁻⁴ to 0.1M) in a closed system that contains an excess (0.05M) of calcite (CaCO₃).

Important: the total concentration of H⁺ must be given, instead of pH, if you wish to plot the calculated pH.

The reaction is: CaCO₃(cr) + CO₂ + H₂O = Ca²⁺ + 2 HCO₃⁻; but when more CO₂ is added, and the pH decreases, most of the added CO₂ remains as un-dissociated carbonic acid:

Note that this example uses the same chemical system as in Example 8.

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