General Methodology of Determining Stability Constants

Before investigating the calculations that permit us to obtain the concentrations of the different species from the values of stability constants, it is interesting to briefly recall some general principles of their determination in solutions. Of course, the principles followed to determine the stability constants of complexes are the same as those followed for any other sort of equilibrium constants. 

From a purely analytical standpoint, the experimental determination of a (or several) concentration(s) (first stage) is achieved with instrumental methods of analysis such as UV-visible spectrophotometry, varied electrochemical methods, calorimetric methods, and so forth. For example, one electrode may be sensitive to the metal that is complexed. Hence, potentiometry is recommended. 

A distinction between direct and indirect methods has been made. In the first group, the concentration of one of the species participating in the equilibrium is determined. In the second group, the concentration of a species that does not participate in this equilibrium is determined. In this case, of course, the complex s formation reaction must induce a change in the followed species concentration. This may be, for example, a change in the pH value. In this case, using pH-metry is recommended. 

The introduction of activities induces serious difficulties in the previous methodologies. They are overcome by working at constant ionic strength, about mol/L, since, for these values, the formal constants do not differ greatly from the thermodynamic ones. Another possibility is to use Debye-HiickeTs equations provided the ionic strengths are not too high. 

The determination of kinetic constants proceeds from the same methodology. However, in some cases, the kinetics may be very fast and the instrumental methods of analysis evoked above would be no longer suitable. Then some special methods such as flux and relaxation techniques would be required. 

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