DISPROPORTIONATION OF TECHNETATE ION IN AQUEOUS ALKALINE MEDIA . AN ELECTROCHEMICAL STUDY

The objective of this paper is to elucidate the primary reduction process when pertechnetate ion is electrochemically reduced in alkaline media. Using the techniques of coulometry (at controlled potential), chronoamperometry, and double potential step chronoamperometry we have demonstrated that technetate ion is produced at the electrode surface when gelatin is present in solution and then undergoes rapid disproportionation TcO4- ⇄e- TcO42- 2TcO42- → TcO4- + TcO43- where k ≈ 1.5 × 105 l. mol-1 sec-1 (when ionic strength is 1). If this mechanism obtains, then the following thermodynamically favored reaction must be kinetically blocked TcO42- ⇄e- TcO43- In the technetium system gelatin seems to be the blocking agent; in the absence of gelatin, production of the technetate ion is bypassed and we observe TcO4- →2e- TcO43- The dependence of the disproportionation rate constant on ionic strength confirms that the reacting species is doubly charged. The small dependence on temperature and pH (in the alkaline region) suggests that the kinetics are proceeding near the diffusion-controlled limit. The rate constant k (1.5 × 105 l. mol-1 sec-1), though considerably lower than the commonly accepted limiting value of 1010 l. mol-1 sec-1, is not unreasonable when the charges of the reacting ions are considered.