ISO 7097-1:2004

Overview

ISO 7097-1:2004 specifies a titrimetric analytical method for the determination of uranium in solutions, uranium hexafluoride and solids using an iron(II) reduction / potassium dichromate oxidation procedure. It covers assay of pure product materials from the nuclear fuel cycle - e.g., U metal, UO2, UO3, U3O8, uranyl nitrate hexahydrate and UF6 - and is suitable for nuclear materials accountability and quality control. The method uses mass titration to improve precision and a potentiometric end point for the dichromate titration.

Key topics and technical requirements

• Analytical principle: Reduce U(VI) to U(IV) with Fe(II) in concentrated phosphoric acid (with amidosulfuric acid to remove nitrite), oxidize excess Fe(II) with nitric acid catalysed by molybdenum, then titrate U(IV) with standardized potassium dichromate to a potentiometric end point. Vanadium(IV) is added to enhance kinetics and shift redox potential.
• Sample size and titration: Typical aliquot contains about 40–60 mg U in nitric-acid solution; method recommends mass titration for improved accuracy.
• Reagents and concentrations: Specifies analytical-grade reagents (HF, HNO3, H3PO4, FeSO4, amidosulfuric acid, vanadyl sulfate, and K2Cr2O7). Example: K2Cr2O7 solution ≈ 0.0085 mol·kg−1 and FeSO4 ≈ 1 mol·l−1; reagent stability and lot verification are required.
• Calibration & verification: Potassium dichromate solution must be standardized against internationally recognized uranium reference materials or NIST SRM 136 per ISO 10980.
• Interferences: Procedure is robust - common ions and many fission products (up to ~10% burn-up equivalent) do not significantly interfere. Specific interfering species (e.g., iodide/iodate, some platinum-group elements, As(III), Sb(III)) are identified and should be considered case-by-case.
• Quality considerations: Some steps are critical; automated devices can improve repeatability for routine analysis. Precision, repeatability and bias guidance are included.

Practical applications

• Routine uranium assay in nuclear fuel production and reprocessing facilities.
• Material accountancy and safeguards measurements for uranium product streams and fuel pellets.
• Quality control for uranium compounds (UO2 powder, U metal, UF6, uranyl nitrate) and validation of analytical laboratories performing redox titrations.
• Method selection where potentiometric end points and traceable calibration to certified reference materials are required.

Who uses this standard

• Nuclear fuel manufacturers and fabrication labs
• Radiochemical and analytical chemistry laboratories
• Regulatory and safeguards organizations responsible for nuclear materials accountancy
• Metrology institutes and laboratories validating uranium reference materials

Related standards

• ISO 7097-2 (iron(II) reduction / cerium(IV) oxidation titrimetric method)
• ISO 9894 (subsampling of UF6 in liquid phase)
• ISO 10980 (validation of strength of reference solutions)

Keywords: ISO 7097-1, uranium determination, potassium dichromate titration, iron(II) reduction, nuclear fuel technology, uranium assay, potentiometric titration, nuclear materials accountability.