The Bjerrum pH-Spectrophotometric Method

The heterocyclic ligand 1,10-phenanthroline forms an orange-red complex 

At a given pH the conditional constant P, so called because it is a constant valid for only a particular pH value, can be determined  

The pKa values of the ligand and the measured pH, can be used to calculate the ao factor (see Chapter 20) for the ligand where oq is defined as the fraction of 

Because of a change in spin state upon addition of the third ligand, the tris conaplex is much more stable than fne mono or bis complex. As a result, the reaction essentially yields only the tris complex. The concentration of the mono or the bis complex is close to zero. 

The equivalent equation using protonation constants instead of piCa values is given in equation 22-22. 

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