Determination of plutonium oxidation state in large volumes of natural water

A method was developed to determine the relative concentrations of the lower and upper oxidation states of plutonium in large volumes of natural water. It is based on the classical lanthanum fluoride procedure in which Pu(III) and Pu(IV) are separated from Pu(VI) and most of the othe sample constituents by sequential precipitations of lanthanum fluoride. The first precipitation is carried out in the presence of Cr(VI), which oxidizes Pu(III) to Pu(IV) and Pu(V) to Pu(VI). Pu(IV) is carried on the first precipitate; Pu(VI) is reduced to Pu(III) by Fe(II) and is hence carried on the second precipitate. The plutonium fractions are separated from lanthanum by dissolving the precipitates in an aluminum nitrate-nitric acid solution, passing this solution through a Dowex-1 anion exchange column, washing the column with strong nitric acid, and eluting the plutonium from the column with a dilute hydrochloric acid-hydrofluoric acid solution. The fractions are electrodeposited onto planchets, and the depositions are assayed alpha spectrometrically. The isotopic diluents, /sup 242/Pu in the IV state and /sup 236/Pu in the VI state, as well as nitric and dichromate are added at the time the samples are taken. All the other operations can be performed in the laboratory at a later date. The period from the time a sample is taken to the time the laboratory work is performed may be as long as 30 days. The values obtained for the Pu(VI)-to-Pu(IV) concentration ratio when Lake Michigan water was analyzed by the lanthanum fluoride method agreed with those obtained by an independent method.