Overview
ISO 15096:2020 - "Jewellery and precious metals - Determination of high purity silver - Difference method using ICP‑OES" - defines a recommended analytical procedure for measuring the fineness of high‑purity silver (nominal content of 999 ‰ and above). The method uses inductively coupled plasma optical emission spectrometry (ICP‑OES) on two sample matrices (a nitric‑acid silver solution and an aqua regia digest of any residue). Total impurity contents are summed and subtracted from 1 000 ‰ to determine silver fineness; Annex B provides modifications for measurements at and above 999.9 ‰.
Key topics and technical requirements
- Analytical principle: Difference method - dissolve a weighed sample, separate any insoluble residue, analyse both nitric‑acid and aqua‑regia fractions by ICP‑OES, sum impurities and subtract from 1 000 ‰ to obtain silver fineness.
- Sample preparation: Typical sample portion ~500 mg; dissolution in nitric acid to prepare a ~10 g/l silver solution; residue treated with aqua regia and analysed separately. Suspension removal by centrifugation or microfiltration is specified.
- Reagents and standards: Use analytical‑grade acids (HCl, HNO3), freshly prepared aqua regia, and certified reference silver (≥ 999.9 ‰) for calibration and blank correction. Stock multi‑element solutions (A–E) are defined for calibration mixes.
- Instrument requirements: ICP‑OES with minimum optical resolution 0.02 nm, detection limit ≈ 0.02 mg/l, and background correction capability. Instrument stabilization and appropriate rinse/stabilization times are required.
- Calibration and calculation: Matrix‑matched calibrations for both nitric and aqua regia matrices; detection limit defined as three standard deviations of blank. Fineness is calculated by combining concentrations from both matrices and applying the specified mass‑ratio formulas.
- Quality criteria: Repeatability - duplicate determinations for the sum of impurities should agree within 10 %; otherwise repeat the assay. Sampling must follow ISO 11596.
Applications and users
ISO 15096:2020 is intended for:
- Assay and analytical laboratories performing fineness determinations on jewellery and precious metal articles
- Precious‑metal refiners and manufacturers requiring quality control of silver alloys
- Hallmarking and conformity assessment bodies verifying declared silver fineness
- R&D and method validation teams developing or comparing ICP‑OES procedures for high‑purity silver
The standard supports consistent, traceable fineness results used for product labelling, regulatory compliance, and commercial transactions.
Related standards
- ISO 11596 - Jewellery - Sampling of precious metal alloys (referenced for sampling procedure)
- ISO 15093 - (structural alignment referenced in the foreword) - consult for harmonized document structure and related procedural guidance
Keywords: ISO 15096:2020, high purity silver, ICP‑OES, silver fineness, jewellery, aqua regia, nitric acid, difference method, precious metals, analytical method.