Measurements in Cosolvent Mixtures

Poor water solubility is still a problem for all these methods, but it can be overcome to some extent by using a mixture of solvents, although the presence of organic modifiers causes the pH scale to shift and may cause the pKa to change. The 

Apparent pfCa values measured in the presence of cosolvents are therefore different from those measured in purely aqueous systems and will not give a true indication of the % ionized of the species. One-unit error in pKa calculation will carry through to a 1-unit error in log DpH if calculated from a measured log P value. 

---