Direct and Indirect Determination of K values

Determination of equilibrium constants from tables of Gibbs free energy values as described above is, in a sense, an indirect method, because the Gibbs energy values are themselves determined from other kinds of measurements, often calorimetric (measurements of quantities of heat involved in carefully controlled experiments). For the calcite reaction we have just considered, three separate free energy values are involved, and an error in any of them will result in an error in the equilibrium constant. 

Nevertheless, accurate values of A fG° are inherently more useful than accurate values of log K, in the sense that log K refers to a single reaction, whereas a few values of A/ G° can be used in innumerable reactions, some of which may be difficult or impossible to measure directly. For the environmental modeler, this point may be a bit academic, because the modeler is responsible for ensuring the accuracy of the data used, in whatever form it occurs. In comparing data from various sources, a knowledge of the basic relationships described here is necessary to convert data from one form to the other. 

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