Technetium determination

This book presents a unified treatment of the chemistry of the elements. At present 112 elements are known, though not all occur in nature of the 92 elements from hydrogen to uranium all except technetium and promethium are found on earth and technetium has been detected in some stars. To these elements a further 20 have been added by artificial nuclear syntheses in the laboratory. Why are there only 90 elements in nature Why do they have their observed abundances and why do their individual isotopes occur with the particular relative abundances observed Indeed, we must also ask to what extent these isotopic abundances commonly vary in nature, thus causing variability in atomic weights and possibly jeopardizing the classical means of determining chemical composition and structure by chemical analysis. 

ICP-MS (inductively coupled plasma mass spectrometry) is frequently used for determining ultratrace amounts of technetium [9]. In spite of the high cost of the equipment, this detection method is far superior to other radiometric methods as regards sensitivity. When a double focussing high-resolution system is used (HR-ICP-MS) and an ultrasonic nebulizer is introduced [10], the detection limit is in the order 0.002 mBq. The ICP-MS method has been successfully applied to the determination of environmental "Tc as well as to other long-lived radionuclides of neptunium and plutonium in the environment. 

Technetium compounds with amine/thioether coordination are the cationic trans-dioxotechnetium(V) complexes [Tc02(N2S2)]+. The complex in which N2S2 is l,4-dithia-8,ll-diazacyclotetradecane was prepared via an exchange reaction of NBu4[TcOBr4] with the ligand and fully characterized by X-ray crystal structure determination [109,110]. The coordination around technetium... 

It can be assumed a priori [11] that the reason for the variable composition of the octachloroditechnetates may be either a variable oxidation state of technetium or substitution of some of the cations by H30+ ions. The first explanation was proposed [22], when the accuracy ( + 0.017 A) of the X-ray diffraction study by a photomethod [81] precluded the determination of the fine difference between the Tc-Tc distances in the [Tc2CI8]3- and [Tc2Cl8]2- anions, which is 0.03 A according to the latest X-ray diffraction data [11,35,60,63,82] (Table 1). An attempt was made [11] to determine the H30+ ions in potassium octachloroditechnetate, which led to their detection from the weak characteristic absorption lines in the infrared spectra (v OH) = 2965,2930, and 2865 cm-1 <5(HOH) = 1610 cm-1 [83,84]). The results [11] did not conflict with the revised X-ray diffraction data [82] according to which the K+ ions in the unit cell of potassium octachloroditechnetate (+ 2.5) are present in two non-equivalent positions sixfold (K ) and threefold (K") under these conditions, the sixfold positions were fully occupied by K+ ions, while the threefold positions were half-occupied. 

The most fundamental approach to assessing lability of complexes is by determination of the rate of isotopic exchange reactions. In the technetium-complex systems, no study of the exchange reaction on the central metal ion has been reported, but several reports have been published on isotopic exchange by ligand substitution. 

The rates of ligand exchange of 0.01 M hexahalo-technetium(IV) and hexa-halo-rhenium(IV) complexes were measured in the 8 M solutions of their corresponding acid at 60 °C. The overall exchange rates R were determined as follows ... 

Chen et al. [31] preconcentrated "technetium in seawater on an anion exchange column to determination in amounts down to 3mBq/m3. 

Ballestra et al. [32] described a radiochemical measurement for determination of "technetium in rain, river, and seawater, which involved reduction to technetium (IV), followed by iron hydroxide precipitation and oxidation to the heptavalent state. Technetium (VII) was extracted with xylene and electrode-posited in sodium hydroxide solution. The radiochemical yield was determined by gamma counting on an anticoincidence shield GM-gas flow counter. The radiochemical yield of 50 to 150 litre water samples was 20-60%. 

Technetium has been determined in seawater by inductively coupled plasma mass spectrometry after preconcentration by coprecipitation with iron hydroxide [33]. 

Keith-Roach et al. [114] has described a radiochemical separation and ICPS protocol for determining "technetium in seawater. 

A radiochemical procedure is proposed for the determination of technetium activities from mixed fission products of uranium and thorium. The chief decontamination step is the extraction of TcO into a tetrapropylammonium hydroxide-bromoform mixture from 4.0 M NaOH solutions. Decontamination factors of 10 with chemical yields of 50-70% have been obtained. 

Voltz and Holt determined the optimum conditions necessary for the electrodeposition of macroamounts of technetium from an aqueous bath of NH TcO. The results reveal that technetium can be deposited in macro-amounts as a bright metal from aqueous solutions. A bath containing 1 M (NH )2S04 and 0.006-0.024 M NH TcO with HjSO added to give a pH of about 1.0 can be electrolyzed at 1-2 A/dm. Thus, a metallic cathodic deposit of technetium with a current efficiency range of 18-30% is obtained. 

The most sensitive method for determining trace amounts of technetium is the neutron activation . The Tc sample is irradiated by slow neutrons. The radioactive isotope Tc with a half-life of 15.8 s is formed by the reaction Tcfn, y) Tc, the neutron capture cross section of which is comparatively large (20 bams), so that it is possible to determine amounts < 2x 10 " g of Tc. However, the method is not widely used since the half-life of Tc is very short. Moreover, this method is only convenient when a reactor or a neutron source is available. 

A technique for the determination of Tc amounts as little as 4 x 10 g by neutron activation analysis has been described by Foti et al. . Tc in triply distilled water is irradiated in a thermal neutron flux of 5 x 10 neutrons per cm and per second to produce °°Tc. Other radionuclides are removed by co-precipi-tation with Fe(OH)j. Then, °°Tc is co-precipitated twice with tetraphenylarsonium perrhenate which can be removed by sublimation. The chemical purification of °°Tc requires 40-45 s and the technetium yield is about 53%. 

Trace amounts of Tc are also determined in filter paper and vegetable samples by neutron activation analysis The procedure consists of the following major steps separation of technetium from the sample, thermal neutron irradiation of the Tc fraction to produce °°Tc, post-irradiation separation and purification of °°Tc from other activated nuclides, and counting of the 16 s Tc in a low-background P counter. The estimated detection limits for Tc in this procedure are 5 x 10 g in filter paper and 9 x 10 g in vegetable samples. 

A method has been developed for the determination of technetium-99 in mixed fission products by neutron activation analysis Tc is separated from most fission products by a cyclohexanone extraction from carbonate solution, the stripping into water by addition of CCI4 to the cylohexanone phase, and the adsorption on an anion exchange column. Induced Tc radioactivity is determined using X-ray spectrometry to measure the 540 and 591 keV lines. The sensitivity of the analysis under these conditions is approximately 5 ng. The method has been successfully applied to reactor fuel solutions. 

The optical emission spectrum of technetium is uniquely characteristic of the element " with a few strong lines relatively widely spaced as in the spectra of manganese, molybdenum and rhenium. Twenty-five lines are observed in the arc and spark spectra between 2200 and 9000 A. Many of these lines are free from ruthenium or rhenium interferences and are therefore useful analytically. Using the resonance lines of Tc-I at 4297.06, 4262.26, 4238.19, and 4031.63 A as little as 0.1 ng of technetium can be reliably determined. 

The atomic absorption characteristics of technetium have been investigated with a technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 /ig/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. Only calcium, strontium, and barium cause severe technetium absorption suppression. Cationic interferences are eliminated by adding aluminum to the test solutions. The atomic absorption spectroscopy can be applied to the determination of technetium in uranium and its alloys and also successfully to the analysis of multicomponent samples. 

An X-ray fluorescence method has been developed for the determination of technetium in solution . At concentrations of less than 1.0 mg Tc per ml there are not interelement effects. Therefore, it is possible to ascertain technetiiun in its compounds without previous decomposition, provided that the compounds are soluble in water or dioxane. The detection limit is about 4 Tc. 

Technetium isotopes formed upon irradiation of a molybdenum target by protons with an energy of 22 MeV have been ascertained by mass spectrometry. After separation of technetium by ion exchange, the isotopes Tc (0.5 %), Tc (56.0%), Tc (17.3%) and Tc (26.7%) are detected. The sensitivity of this method is very high 5 x 10 g of technetium can be detected. Mass spectrometric determination of technetium is also described by Kukavadze et al. . Pertech-netate is reduced to technetium metal and Tc ions are produced at 1600 to 1800 °C. 

An isotope dilution mass spectrometric method involves the addition of a known quantity of Tc followed by chemical separation, purification, and measurement of the Tc/ Tc isotopic ratio . An improved technique has been developed for the analysis of Tc in environmental samples. After spiking with Tc the isolated technetium is concentrated onto anion exchange beads. Determination of as little as 1 pg has been achieved through the enhanced ionization efficiency afforded by the resin bead source ... 

Several spectrophotometric methods for the determination of small amounts of technetium are available. 

X1X cm respectively . Beer s law is obeyed up to a concentration of about 10 mol X r less than 1 /ig of technetium can be determined spectrophoto-metrically. This method is particularly advantageous in the simultaneous determination of technetium and rhenium. 

The strong absorptions of the complex technetium (IV) hexahalides (Fig. 10) can also be utilized for spectrophotometric determinations. A sensitive method has been developed using hexachlorotechnetate (IV) When pertechnetate is heated for 50- 0 min in cone, hydrochloric acid, it is reduced to the complex [TcClgp . The absorption curve of [TcClgf in cone. HCl has a maximum at 338 nm where technetium can be determined in the presence of microgram amounts of rhenium or molybdemun. The molar extinction coefficient is said to be 32.000 (after Jorgensen and Schwochau it amounts to 10.600). About 0.1 fig Tc/ml can be determined. Rhenium present in quantities up to 30 ng/ml has almost no influence on the determination of technetium. The error in the determination of the latter in the presence of molybdenum at a weight ratio of 1 1 is 1-2%. 

This technique of Crouthamel has been improved by Howard and Weber and used for the determination of technetium in uranium materials. Technetium,... 

Foster et al. have developed a method for determining technetium in dissolved nuclear fuel solutions. Tetrapropylammonium pertechnetate is doubly extracted from a basic medium into chloroform and the colored technetium (V) thiocyanate complex is formed in the chloroform phase by the addition of sulfuric acid, potassium thiocyanate and tetrapropylammonium hydroxide. The colored complex absorbs at 513 nm, has a molar extinction coefficient of 46,000 and is stable for several hours. Of more than 50 metals studied, none impairs measurements at ratios less than 100 to 1 mol with respect to technetium. Most anions do not disturb the determination of technetiiun. The standard deviation for a single determination is 0.09 fig over the range of 1 to 20 fig of technetium. 

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 ,... 

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