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A partially successful attempt was made in this investigation to eliminate molybdenum interference with the use of reagents that would strongly complex the molybdenum and move to its half-wave potential to more negative values, but have no effect on the Nb-EDTA complex. Reagents used were glycol dimercaptoacetate, 1,5-diphenyl-3-thiocarbohydrazide, quinoxaline-2,3dithiol, and mercaptoacetic acid. Of these, the glycol dimercaptoacetate was the most promising.The tantalum-EDTA complex was prepared (6) and found to have a halfwave potential more negative than the niobium-EDTA complex by about 0.5 volt. Nevertheless, tantalum interfered with the determination of niobium. The ratio of tantalum to niobium that may be tolerated in the polarographic determination of niobium was studied.The results are tabulated in Table IV.Each sample contained 20.00 yg. of Nb per ml. These solutions were 0.1M in EDTA and O.lili in sodium acetate, and had a pH of 4.06 ± 0.03.These results indicate that tantalum may be tolerated up to a ratio of 2:1, Ta:Nb with a deviation of 2.0% or less.

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