Free Energies of Formation of Hydrous Oxides of Technetium in Its Lower Valencies

Abstract

It has been found possible to measure reversible electrode potentials for couples containing technetium and one or more of its hydrous oxides. The procedure disclosed the existence, in films, of technetium in 1+, 2+, and 3+ valence states, besides oxides Tc3O4 and Tc4O7 , and provided direct means for evaluating their free energies of formation from the metal. The previous calorimetrically based free energy of formation of normalTcfalse(OH)4 was confirmed to within 1%. It was found that the radioactivity of technetium is involved in certain processes on the electrode.

Evolution of Technetium Speciation in Reducing Grout

“…The hydrous Tc(IV) oxide, TcO 2 •xH 2 O, results from the reduction of TcO 4 -in the absence of other ligands both in solution and in grout samples. 14,26 In addition, sulfide, either added to the grout as Na 2 S or BFS, reduces TcO 4 -to a lower-valent technetium sulfide species thought to be similar to TcS 2 . 1 4 Interestingly, the reaction of sulfide with TcO 4 -in alkaline solution is a known route to Tc 2 S 7 , 2 7 which is generally believed to be the technetium species present in reducing CWFs.…”

Valence effects on solubility and sorption: the solubility of Tc(IV) oxides

“…The usual value quoted i n t h e 1 i t e r a t u r e f o r t h e standard p o t e n t i a l , 0.738 V, i s t h a t o f Cartledge (1955).…”

“…I n view o f t h e g r e a t i n t e r e s t i n technetium w i t h respect t o n u c l e a r waste, i t i s s u r p r i s i n g t h a t t h e r e have been sn few r e c e n t s t u d i e s on thP oxy-compounds o f technetium. Values f o r t h e standard p o t e n t i a l o f t h e important Tc(V1J)-Tc(JV) couple have been r e p o r t e d by a number o f i n v e s t i g a t o r s (COBBLE 1952, CARTLEDGE 1955, SPITSYN 1964, LIERXHEP 1975). There i s a d i f f e r e n c e o f about 70 mV between t h e h i g h e s t and lowest values.…”

“…Thus, a judgment must be made as t o which o f these values t o use f o r c a l c u l a t i o n of thermodynamic data. Values f o r t h e f r e e enerqies of formation o f a s e r i e s of lower oxides were given by Cartledge (1971) Because o f t h e s p a r c i t y o f data necessary f o r e s t i m a t i o n o f t h e s o l u b i l i t y o f Tc(1V) species; a c r i -t i c a l review of t h e references given above i s presented below from t h e e x p e r i m e n t a l i s t ' s p o i n t o f view. …”

Research Program to Investigate the Fundamental Chemistry of Technetium

“…-in the absence of other ligands both in solution and in grout samples. (Cartledge 1971;Allen, Siemering et al 1997) In addition, sulfide, either added to the grout as Na 2 S or BFS, reduces TcO 4 -to a lower-valent technetium sulfide species thought to be similar to TcS 2 . (Allen, Siemering et al 1997) Interestingly, the reaction of sulfide with TcO 4 -in alkaline solution is a known route to Tc 2 S 7 , (Lee and Bondietti 1983) which is generally believed to be the technetium species present in reducing CWFs.…”