Protactinium purity

The efficiency of the van Arkel-De Boer process (Section II,D) for refining thorium and protactinium metals can be increased by repeating the process to achieve higher purity of product metal. 

Protactinium metal was first prepared in 1934 by thermal decomposition of a pentahalide on a hot filament 50). It has since been prepared from PaF4 by metallothermic reduction (Section II,A) with barium 26, 27, 34,102), lithium 40), and calcium 73, 74). However, the highest purity metal is achieved using the iodide transport (van Arkel-De Boer) process (Section II,D). 

Uranium-238 emits an alpha particle to become an isotope of thorium. This unstable element emits a beta particle to become the element now known as Protactinium (Pa), which then emits another beta particle to become an isotope of uranium. This chain proceeds through another isotope of thorium, through radium, radon, polonium, bismuth, thallium and lead. The final product is lead-206. The series that starts with thorium-232 ends with lead-208. Soddy was able to isolate the different lead isotopes in high enough purity to demonstrate using chemical techniques that the atomic weights of two samples of lead with identical chemical and spectroscopic properties had different atomic weights. The final picture of these elements reveals that there are several isotopes for each of them. 

Thorium phthalocyanine is prepared by heating the metal (previously etched with HC1) and o-phthalonitrile, 1 25, at 270 to 300°C for 5 hr. The dark blue product is cooled to room temperature, washed with benzene, and purified twice by sublimation at 520°C and 10 4Torr. The protactinium-233 produced by w-irradiation of pure thorium phthalocyanine is separated in high purity in the residue after repeated sublimation of the thorium phthalocyanine. The thorium-231 produced by (w,2w) reaction in the thorium phthalocyanine is found to be enriched in the residue after sublimation, indicating decomposition of the phthalocyanine by irradiation. Uranyl phthalocyanine is prepared by heating a mixture of uranyl acetate and phthalonitrile at 230 to 240°C. 

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