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The separation of Pa(IV) and Pa(V) was investigated using various co-precipitants which were insoluble in acidic solutions. Among them, thorium fluoride was found to be the most useful co-precipitant. The conditions for the separation of both valency states of protactinium were also investigated in detail. This method was applicable to protactinium solutions in the concentration range down to tracer level. It was difficult to reduce Pa(V) completely on a tracer scale by means of a liquid zinc amalgam. When Cr(III) was added to the system, however, the complete reduction of 233Pa(V) was readily attained. The role of Cr(III) was considered to be that Cr(II) formed by liquid-zinc-amalgam reduction mediates the reduction of Pa(V) and protects Pa(IV) from re-oxidation. By this method tetravalent protactinium in a perchloric acid medium was also confirmed to be very stable.

The Chemistry of Protactinium. V. The Separation of Pa(IV) and Pa(V) by Co-precipitation with Thorium Fluoride