- Email: [email protected]
Analysis for Neptunium by Controlled-Potential Coulometry*†
Talanta. 1960. Vol. 6, p. 197. Pergamon Press Ltd. Printed in Northern Ireland
ANALYSIS FOR NEPTUNIUM BY CONTROLLEDPOTENTIAL COULOMETRY*t R. W.
STROMATT
and F. A.
SCOTT
General Electric Company, Hanford Atomic Products Operation, Richland, Washington 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 neptuniumv-neptuniumVI couple is titrated. A 1.0N sulfuric acid solution is used as a titration medium. The precision of the method ranges from I per cent relative standard deviation for 20 f-lg 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 f-lg of neptunium can be detected and 7 f-lg 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 neptuniumVI with ceriumIV, then electrolytically reduce the neptuniumVI and excess ceriumIV to neptuniumv and ceriumlII and, finally, coulometrically oxidize the neptuniumV to neptuniumVI. 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 neptuniumVI is determined by a coulometric reduction to neptuniumv . The neptunium v is then oxidized coulometrically; the difference between these two titrations being the concentration of the neptuniumv . The total neptunium concentration is determined by the method described in the preceding paragraph, and the neptuniumIV is then calculated from the difference between the concentrations of neptuniumV plus neptuniumVI and the total neptunium. This analysis depends upon the fact that neptuniumIv is not oxidized at a significant rate during the coulometric oxidation of the neptuniumv. 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 controlledpotential 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.
* AEC Reports HW-58212 and HW-59447. Work performed under contract with the Atomic Energy Commission. t The full text of this paper is not included in the Proceedings because the information contained therein is available in documents numbered HW-58212 and HW-59447.
16
197
ANALYSIS FOR NEPTUNIUM BY CONTROLLEDPOTENTIAL COULOMETRY*t R. W.
STROMATT
and F. A.
SCOTT
General Electric Company, Hanford Atomic Products Operation, Richland, Washington 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 neptuniumv-neptuniumVI couple is titrated. A 1.0N sulfuric acid solution is used as a titration medium. The precision of the method ranges from I per cent relative standard deviation for 20 f-lg 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 f-lg of neptunium can be detected and 7 f-lg 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 neptuniumVI with ceriumIV, then electrolytically reduce the neptuniumVI and excess ceriumIV to neptuniumv and ceriumlII and, finally, coulometrically oxidize the neptuniumV to neptuniumVI. 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 neptuniumVI is determined by a coulometric reduction to neptuniumv . The neptunium v is then oxidized coulometrically; the difference between these two titrations being the concentration of the neptuniumv . The total neptunium concentration is determined by the method described in the preceding paragraph, and the neptuniumIV is then calculated from the difference between the concentrations of neptuniumV plus neptuniumVI and the total neptunium. This analysis depends upon the fact that neptuniumIv is not oxidized at a significant rate during the coulometric oxidation of the neptuniumv. 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 controlledpotential 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.
* AEC Reports HW-58212 and HW-59447. Work performed under contract with the Atomic Energy Commission. t The full text of this paper is not included in the Proceedings because the information contained therein is available in documents numbered HW-58212 and HW-59447.
16
197