Inclusion complexes of atrazine with alpha-, beta- and gamma-cyclodextrins. Evidence by polarographic kinetic currents

The reduction of atrazine in acidic aqueous media on mercury electrodes proceeds only after the protonation reaction. In fact, the efficiency of the reduction process is very low at pi-Is greater than 4. However, the addition of cyclodextrins (CDs) to neutral aqueous solutions of atrazine yields a kinetically controlled polarographic reduction wave, whose limiting current depends on the size of the CD cavity, and increases with the concentration of the CD itself. In particular, the size of the increase follows the order beta-CD < gamma-CD < alpha-CD for the same CD concentration. The half-wave potential shifts toward more negative values when the concentration of CDs increases. These findings lead to the conclusion that atrazine and CDs form an inclusion complex, whose stability constant we have estimated, and also that atrazine undergoes protonation facilitated by complexation with CDs. The stability constants of 1:1 complexes evaluated from polarographic data in 0.1 M-KCl and at neutral pH for alpha-CD, beta-CD and gamma-CD are 4900, 1970 and 19000, respectively. The formation of the inclusion complex was indirectly confirmed by UV-Vis measurements in the presence of methylred and phenolphthalein, which both compete with atrazine in the formation of the corresponding inclusion compounds with CDs.