Oxidation states of technetium in diphosphonate complexes

Valence states of technetium in two most prominent bone imaging agents. i.e., their model complexes with⁹⁹Tc (⁹⁹Tc-MDP and⁹⁹Tc-DPD) were determined by redox potentiometric titration. Titrations of⁹⁹Tc-pertechnetate by Sn(II) solutions were performed in the presence of a ligand (MDP or DPD), as well as without them. Tc(III) was obtained in complexes with MDP and DPD in strongly alkaline medium (pH 12–13) while in the absence of the ligands Tc(IV) was formed. In strongly acidic medium (pH 2–3) the redox process resulted Tc(IV) in both complexes, while Tc(III) was formed in the absence of the ligands. At pH values near neutral or exactly neutral (important for radiopharmaceutical complexes^99mTc-MDP and^99mTc-DPD), it was possible to perform titrations in the ligand absence only at pH values from 7 to 11.8 (due to in hydrolysis) while the resulting oxidation state of Tc in the presence of ligand depended on the used ligand: Tc(IV)-MDP (pH 5.4–5.9 and pH 7.0–7.4), and Tc(III)-DPD (pH 8.0).