Anion Exchange Method For The Determination Of Plutonium In Water

A single-laboratory evaluation and an interlaboratory collaborative study were made of a method for determining plutonium in water. The method was written for the analysis of one-liter samples and involves coprecipitation, acid dissolution, anion exchange, electrodeposition, and alpha pulse height analysis. After the single-laboratory evaluation of the selected method, four samples were prepared for the collaborative study: two river-water samples, a substitute ocean water sample, and sample containing sediment. These samples contained plutonium-239 and plutonium-238 at concentrations ranging from 0.42 to 28.9 dis/min/liter. In the collaborative study standard deviations of plutonium concentrations ranged from 5 to 13%. In three cases, standard deviations agreed with those expected from counting statistics. It is believed that hydrolysis occurred in the river water samples resulting in errors greater than those expected from counting statistics.

A procedure for the determination of plutonium 238 and plutonium 239 in water is described. The procedure consists of a coprecipitation, an anion exchange separation and electrodeposition, followed by alpha pulse height analysis. More specifically, the sample is acidified with nitric acid and plutonium-242 is added as a tracer before any chemical separations are performed. Iron is added to the water as iron (III) and the plutonium is coprecipitated with the iron as ferric hydroxide by adding ammonium hydroxide. After decantation and centrifugation, the ferric hydroxide precipitate containing the coprecipitated plutonium is dissolved and the solution is adjusted to 8M in HNO3 for anion exchange separation. When the sample fails to dissolve because of the presence of insoluble residue, the residue is treated by a rigorous acid dissolution using concentrated nitric acid and hydrofluoric acids. The sample is poured over an anion exchange column. The iron and most other elements that might be present pass through the column. Thorium is removed from the column with 12 M hydrochloric acid and then the plutonium is eluted by reducing it to plutonium (III) with the iodide ion. The plutonium is electrodeposited onto a stainless steel slide for counting by alpha pulse height analysis using a silicon surface barrier detector. From the recovery of the plutonium-242 tracer, the absolute amounts of plutonium-238 and plutonium-239 can be calculated, and from the volume of sample analyzed the concentrations of these two isotopes in the water sample can be calculated.