In general, potentiometric measurements provide the most powerful means for determining stability constants. The fundamental requirement is that a suitable ion-selective electrode (i.s.e.) should be available. Of course the most commonly used i.s.e. is the glass electrode which responds to hydrogen ion concentration(**), but other electrodes could be used.
If potentiometry is not possible then either UV/visible spectroscopy,fluorimetry or nmr spectroscopy may be used. (Other techniques are not, as yet, covered by the Hyperquad suite.) The choice of technique will depend on a number of factors.
This involves such things as
The main purpose of doing preliminary experiments is to establish the viability of the project
The necessary requirement for the determination of the stability constant of a "complex" is that concentration of the complex varies appreciably over the range of experimental conditions. For example, to determine the protonation constant of of a monobasic base measurements should be made in a pH range which is numerically close to the pK value; ideally the pH range should be pK ± 2. Although this appears to be a trivial example since measurements could be made anyway over a pH range of, say, 3 - 10, it also defines conditions where it is pointless making measurements.
In general one needs to estimate the values of the stability constants in order to find the right experimental conditions. Suitable experimental conditions can then be established by simulation.
If the stability constants cannot be estimated very reliably it may be necessary to cycle through the whole process once in order to obtain approximate values for the stability constants and so to be able to repeat the experiments under better conditions.