POLAROGRAPHIC STUDIES OF BEHAVIOUR OF FLUORESCEIN IN SOLUTIONS OF VARYING PH AT DROPPING MERCURY ELECTRODE

The polarographic behaviour of fluorescein in the concentration range 0·03-0·5 mM at the DME has been investigated in solutions of varying pH. The results indicate that solutions of pH < 3 contain three species of fluorescein; namely noncharged molecules, associated and protonated forms. In solutions with pH 3 to 6 the dominant forms are non- and singly charged species in equilibrium, whereas in solutions of pH 6 to 9 singly and doubly charged anions can exist. Above pH 9, the only doubly charged species can be traced. The second dissociation constant of the fluorescein molecule has also been determined. The reduction waves are irreversible and essentially diffusion controlled; no indication can be obtained about adsorption of fluorescein at the drop surface. © 1969.