Determination of the oxidation state of Tc in 99Tc(Sn)EHDP, 99mTc(Sn)EHDP, 99Tc(Sn)MDP and 99mTc(Sn)MDP complexes. Characterization of Tc(III)-, Tc(IV)- and Tc(V)EHDP complexes

Abstract

The oxidation state of complexed Tc was determined, by performing paper and thin-layer chromatography during potentiometric titrations. Both titrations of Sn(II) with TcO 4 − and of TcO 4 − with Sn(II) were performed at pH 2.5, 7.0 and 12.0. Differences were found in the number of electrons transferred to 1 mol of TcO 4 −, n, in the titration solutions at the end point of a direct and a reversed titration. The oxidation state of Tc changes during a titration with the titration degree. It was possible to obtain almost pure Tc(III)-, Tc(IV)- and Tc(V)EHDP complexes, depending on the pH, titration mode and titration degree. These TcEHDP complexes were analyzed by u.v./vis spectrometry, anion-exchange HPLC on Aminex A-28, gel chromatography on Biogel P 4 and extraction with oxine in chloroform. The spectra of the pure Tc(III), Tc(IV) and Tc(V) species were calculated. Adsorption of these complexes on calcium phosphate increased in the order Tc(V) < Tc(IV) < Tc(III). From the chromatographic characterizations it was concluded that in the 99mTc(Sn)EHDP bone scanning agent Tc is tetravalent. An analogous (though less elaborate) investigation was performed on the Tc(Sn)MDP complex.