Pertechnetate reduction

Momoshima et al. [19,20] developed an analytical procedure to determine "Tc in sea water by ICP-MS. "Tc was concentrated from the original sea water by the steps of filtration, reduction of pertechnetate with K2S2O5, co-precipta-... 

In a recent study of the complexation of technetium with humic acid (HA) Sekine et al. [34,35] obtained interesting results which show competition between Tc,v-0(0H) i precipitate formation and Tcin-HA precipitate formation during a reduction process of pertechnetate with Sn2+. A weighable amount of... 

Further reduction to the Tc(IV) species, [TcX6]2, is slow compared with their rapid formation from pertechnetate. This fact, as well as trapping by precipitation with large organic cations, allows isolation of the thermodynamically less favoured TcOX4 anions. 

A general mode of access to polyhydric complexes of Tc(V) is reduction of pertechnetate with two equivalents of stannous chloride in aqueous solution of the excess O-donor ligand, e.g. ... 

Complexation studies with bidentate phosphine ligands showed that stable cationic complexes of Tc(V), Tc(III), and Tc(I) are easily accessible. The influence of reaction conditions on reaction route and products is well demonstrated by the reaction of pertechnetate with the prototype 1,2-bis(dimethylphosphino)-ethane (dmpe) (Fig. 16). Careful control of reduction conditions allows the synthesis of [Tc02(dmpe)2]+, [TCl2(dmpe)2]+, and [Tc(dmpe)3]+, with the metal in the oxidation states V, III, and I [120,121]. This series illustrates the variety of oxidation states available to technetium and their successive generation by the action of a 2-electron reducing agent. 

In complexing acid media (e.g. hydrohalogenic acids HX) the reduction of pertechnetate ions proreeds in a different way. In the works [11,13,15] it was shown that such a reduction occurred according to the scheme (1). 

Technetium is usually supplied in the form of heptavalent pertechnetate. Consequently, the syntheses of technetium complexes is necessarily accompanied by the reduction of pertechnetate. When concentrated hydrochloric acid is employed as a reductant, tetrachlorooxotechnetate(V) complexes can easily be obtained. A further reduction procedure is required to obtain hexachlorotech-netate(IV). Using these complexes, a number of technetium complexes have been synthesized by ligand substitution. The importance of preparative substitution reactions also increases in the light of the design and preparation of radiopharmaceuticals labelled with 99mTc and 188Re. 

The technetium(III) complexes are synthesized by the direct reduction of pertechnetate with an appropriate reductant in the presence of the desired ligand. However, when sodium dithionite is used as a reductant, the oxidation state of the synthesized complex varies from III to V, depending significantly on the nature of the coexisting ligand. 

The reduction of pertechnetate with concentrated hydrochloric acid finally yields the tetravalent state, and no further reduction to the tervalent state takes place. Therefore, the tervalent technetium complex has usually been synthesized by the reduction of pertechnetate with an appropriate reductant in the presence of the desired ligand. Recently, the synthesis of tervalent technetium complexes with a new starting complex, hexakis(thiourea)technetium(III) chloride or chloropentakis(thiourea)technetium(III) chloride, has been developed. Thus, tris(P-diketonato)technetium(III) complexes (P-diketone acetylacetone, benzoyl-acetone, and 2-thenoyltrifluoroacetone) were synthesized by the ligand substitution reaction on refluxing [TcCl(tu)5]Cl2 with the desired P-diketone in methanol [28]. 

Crystals of [Tc(tu)6]Cl3 or [TcCl(tu)5]Cl2 are often employed for the synthesis of technetium(III) complexes. However, since the direct reduction of pertechnetate with excess thiourea in a hydrochloric acid solution yields [Tc(tu)6]3+ in high yield [37], direct use of the aqueous solution of the thiourea complex would be preferable for the synthesis of the technetium(III) complex without isolation of the crystals of the thiourea complex. In fact, technetium could be extracted from the aqueous solution of the Tc-thiourea complex with acetylacetone-benzene solution in two steps [38]. More than 95% extraction of technetium was attained using the following procedure [39] First a pertechnetate solution was added to a 0.5 M thiourea solution in 1 M hydrochloric acid. The solution turned red-orange as the Tc(III)-thiourea complex formed. Next, a benzene solution containing a suitable concentration of acetylacetone was added. After the mixture was shaken for a sufficient time (preliminary extraction), the pH of the aqueous phase was adjusted to 4.3 and the aqueous solution was shaken with a freshly prepared acetylacetonebenzene solution (main extraction). The extraction behavior of the technetium complex is shown in Fig. 6. The chemical species extracted into the organic phase seemed to differ from tris(acetylacetonato)technetium(III). Kinetic analysis of the two step extraction mechanism showed that the formation of 4,6-dimethylpyrimidine-... 

Another attempted synthesis of Tc(III)-EDTA and Tc(III)-HEDTA complexes (EDTA ethylenediaminetetraacetic acid HEDTA A -(2-hydroxy-methyl)ethylenediamine-N,AT, iV -triacetic acid) was carried out using [Tc(tu)6]3+ as the starting complex [40]. Technetium-EDTA complexes have been synthesized by the direct reduction of pertechnetate with a suitable reduc-tant in the presence of excess EDTA [41-43]. On addition of EDTA to the Tc(tu) + solution, the intensity of the absorption spectrum decreased with time and the solution color changed from reddish orange to light brown. An electrophoretic analysis for the Tc(III)-EDTA complex showed that more than 70%... 

Another distillation method involves reduction of pertechnetate by hydroxyl-amine . Rhenium is distilled from sulfuric acid. This method can be used for the separation of about 10 mg of rhenium from microamounts of technetium. 

Since the extraction behavior of pertechnetate is nearly equal to that of perrhenate, the separation of both elements is difficult and often involves reduction of Tc (VII) to lower oxidation states. 

An efficient method has been developed by Pozdnyakov and Spivakov . In alkaline solution pertechnetate, in contrast to perrhenate, is reduced by hydrazine sulfate. After reduction technetiiun is no more extracted by methyl ethyl ketone. The distribution coefficient of technetium is by a factor up to 2500 smaller than that of rhenium. 

Atteberry and Boyd have separated TcO and ReO on Dowex-2 resin using a mixture of 0.1 M (NH j SO and 0.1 M (NH j SCN as eluent. The two elution peaks are partially resolved, but the cross contamination is rather high. Much better results are obtained if perchlorate is used as the elutriant . Because of some peculiarities of commercial resins, the highest practically attainable separation factors of TcO and ReO are l(f -10 It has been found that the volumes of the eluents consumed in the elution of pertechnetate and perrhenate are inversely proportional to the perchlorate concentration. The separation on Dowex 1-X4 is described in Fig. 6. Partial reduction of TcO or ReO reduces the efficiency of the chromatographic separation since the elution peaks frequently become blurred . 

Co-precipitation of Re S with platinum sulfide from cone, hydrochloric acid solutions of microamounts of technetium and rhenium is suitable for the separation of technetium from rhenium , since technetium is only slightly co-precipitat-ed under these conditions (Fig. 7). At concentrations of 9 M HCl and above, virtually no technetium is co-precipitated with platinum sulfide at 90 °C, whereas rhenium is removed quantitatively even up to 10 M HCl. The reduction of pertechnetate at high chloride concentration may be the reason for this different behavior, because complete co-precipitation of technetiiun from sulfuric acid solutions up to 12 M has been observed. However, the separation of weighable amounts of technetium from rhenium by precipitation with hydrogen sulfide in a medium of 9-10 M HCl is not quantitative, since several percent of technetiiun coprecipitate with rhenium and measurable amounts of rhenium remain in solu-tion . Multiple reprecipitation of Re S is therefore necessary. 

Miller and Zittef have used 1,5-diphenylcarbazide (0.25% solution in acetone) for the spectrophotometric determination of technetiiun. 1 to 15 /ig of technetium in 10 ml solution can be ascertained by measuring the extinction at 520 nm of the Tc (IV) complex in 1.5 M sulfuric acid. The development of the most intense color takes about 35 min the reduction of pertechnetate to Tc (IV) is effected by the reagent itself before complexation occurs. The molar extinction coefficient of the complex at 520 nm is 48,600. The relative standard deviation is 2%. Fe ", Ce ", and CrOj" clearly disturb measurements, VO , MoOj ,... 

The existence of various oxidation states of technetium indicates the possibility of using polarography for its quantitative determination. Polarographic reduction of the pertechnetate ion at a dropping mercury electrode has been studied in different supporting electrolytes . 

Pertechnetate in 4 M hydrochloric acid has been found to undergo reduction to the oxidation state +4 a double wave was obtained corresponding to a one-and a two-electron transfer. The waves are rather poorly defined by half-wave potentials of —0.52 V and —0.68 V vs. SCE. Both waves are irreversible. No reduction occurs in 4 M perchloric acid. 

Technetium(V) complexes with /u,v(ohydroxyphenyl)phenylphosphine and (o-hydroxyphenyl)diphenylphosphine ligands were prepared by metathesis reactions with the appropriate Tc(V) precursor and/or by reduction/ligand-exchange reactions with ammonium pertechnetate [530]. It was expected that the combination of one soft phosphine P-donor and two hard phenolate O-donors in the chelate should stabilize Tc centers in intermediate oxidation states. 

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