Analysis for Neptunium by Controlled-Potential Coulometry

Abstract

Summary An accurate and precise method for the determination of neptunium has been developed, based on a controlled-potential coulometric titration. The electrolysis cell contains a platinum working electrode at which the neptunium V -neptunium VI couple is titrated. A 1.0 N sulfuric acid solution is used as a titration medium. The precision of the method ranges from 1 per cent relative standard deviation for 20 μ g samples to 0.5 per cent relative standard deviation for samples of neptunium approximately 1 mg or larger in size. With care, as little as 2 μ g of neptunium can be detected and 7 μ g can be titrated with a relative standard deviation of 6 per cent. Under ordinary conditions, large excesses of plutonium, iron and uranium can be tolerated or the interference calculated. The general procedure is to first oxidize the neptunium to neptunium VI with cerium IV , then electrolytically reduce the neptunium VI and excess cerium IV to neptunium V and cerium III and, finally, coulometrically oxidize the neptunium V to neptunium VI . The integrated current in the last step is used to calculate the concentration of the neptunium. The solutions can be analyzed for the concentrations of neptunium in different oxidation states with the coulometer, using a platinum electrode cell. The neptunium VI is determined by a coulometric reduction to neptunium V . The neptunium V is then oxidized coulometrically; the difference between these two titrations being the concentration of the neptunium V . The total neptunium concentration is determined by the method described in the preceding paragraph, and the neptunium IV is then calculated from the difference between the concentrations of neptunium V plus neptunium VI and the total neptunium. This analysis depends upon the fact that neptunium IV is not oxidized at a significant rate during the coulometric oxidation of the neptunium V . All substances which undergo oxidation or reduction at the applied potentials interfere and must be removed or masked in order to carry out this determination. The accuracy and precision depend, of course, on the total neptunium concentration and the concentrations of neptunium in the various oxidation states. A simple and rapid method to determine oxidation-reduction potentials, using the controlled-potential coulometer, is described. In this method, the same electrode system is used to measure potentials and to adjust the concentrations of the element in various valence states by coulometric titrations. Some formal oxidation potentials are given for iron, neptunium and plutonium couples.