ISO 11427 is the international standard, published by the International Organization for Standardization, that prescribes the method for the determination of silver content in jewellery silver alloys. The method is volumetric, using a precipitation titration with potassium bromide as the standardised titrant, and is the reference assay method for sterling silver, Britannia silver, and the broader range of jewellery silver alloys recognised in international hallmarking practice.

Method

The standard prescribes the dissolution of a known mass of silver alloy sample in nitric acid, conversion of the silver to soluble silver nitrate, and subsequent titration with a standardised potassium bromide solution. The endpoint is determined potentiometrically using a silver electrode, with the formation of insoluble silver bromide marking the endpoint when all silver has been precipitated. The silver content is calculated from the volume of titrant consumed.

The method is specified to a reproducibility tolerance sufficient to verify hallmarking compliance for the standard fineness levels of 800, 830, 925 (sterling), 958 (Britannia), and 999 parts per thousand. Sample preparation, titrant standardisation, electrode calibration, and the titration protocol are all set out in the standard text in detail.

Use in hallmarking

National hallmarking offices apply ISO 11427, or national assay methods harmonised with it, as the technical basis for verifying silver content in pieces submitted for hallmarking. The British Assay Offices in London, Birmingham, Sheffield, and Edinburgh use methods consistent with ISO 11427 in their assay laboratories, as do the corresponding offices in France, Italy, Spain, the Netherlands, Sweden, Finland, and Russia. ISO 17025 accreditation provides the third-party validation that the laboratory's procedure delivers the precision and reproducibility specified by the standard.

Related standards

ISO 11427 sits within a wider suite of ISO standards on precious metal assaying that includes ISO 11210 on platinum, ISO 11426 on gold by cupellation, ISO 15093 on gold by ICP-OES, and ISO 9202 on fineness designations for precious metal alloys. The standards are maintained by ISO Technical Committee 174 (Jewellery) and revised at intervals to reflect methodological advances and changes in the precious metal trade.

For the working trade, the practical consequence of ISO 11427 is that any silver piece bearing a national hallmark from an ISO 17025-accredited assay office has been verified to its stated fineness by a method whose precision and reproducibility are documented and internationally recognised. This is the basis on which sterling silver and Britannia silver carry their consumer-protection value across borders.