The complexation of sodium fluorescein (NaFluor) with polyvinylpyrrolidone (PVP) was studied at four pH values using a method similar to that of Benesi and Hildebrand. It was found that fluorescein can exist in four forms, each of which is capable of complexing. The results tend to indicate that at low pH values PVP exists in more than one form and that not all forms of the PVP are capable of complexing with NaFluor. When the ratio of NaFluor to PVP is small, a 1:1 complex is favored; but as the ratio increases, higher complexes predominate. The stability of the 1:1 complex seems to increase as the negative charge on the fluorescein decreases. Copyright © 1968 Wiley‐Liss, Inc., A Wiley Company
