The competition method may be used when a stability constant value is too large to be determined by a direct method. It was first used by Schwarzenbach in the determination of the stability constants of complexes of EDTA with metal ions.
For simplicity consider the determination of the stability constant  of a binary complex, AB, of a reagent A with another reagent B.
where the [X] represents the concentration, at equilibrium, of a species X in a solution of given composition.
A ligand C is chosen which forms a weaker complex with A The stability constant, K, is small enough to be determined by a direct method. For example, in the case of EDTA complexes A is a  metal ion  and C may be a polyamine such as diethylenetriamine.
The stability constant, K for the competition reaction 
can be expressed as
It follows that
where K is the stability constant for the competition reaction. Thus, the value of the stability constant  may be derived from the experimentally determined values of K and  .