1.1 This International Standard describes a precise and accurate analytical method for determining 1 mg to 5 mg of plutonium per millilitre in nitric acid solutions. 1.2 The method is very selective for plutonium. It is suitable for the direct determination of plutonium in materials ranging from pure product solutions, to solutions of mixed nuclear materials with a uranium/plutonium ratio up to 20:1. However, potential application to the assay of plutonium in solutions of irradiated nuclear fuels and solutions of mixed nuclear materials with uranium/plutonium ratios of 20:1 to 33:1 has not yet been documented.1.3 The method recommends that the aliquot be weighed and that the titration burettes be calibrated gravimetrically in order to obtain adequate precision and accuracy. This does not preclude using any alternative technique which can be shown to give an equivalent accuracy.1.4 As the reproducibility of the reaction conditions is important to maintain good performance, extensive automatization of the procedure is beneficial.