ISO 80000-9:2009

Overview

ISO 80000-9:2009 - Quantities and units - Part 9: Physical chemistry and molecular physics (published by ISO in collaboration with IEC) standardizes the names, symbols and definitions of quantities and units used in physical chemistry and molecular physics. Where appropriate, it provides conversion factors and guidance on numerical notation and uncertainty. ISO 80000-9 replaces ISO 31-8:1992 and aligns chemical and molecular quantities with the International System of Quantities (ISQ) and the SI.

Key topics

• Core quantities and units for physical chemistry and molecular physics: amount of substance (mole), number of particles, Avogadro constant, molar mass, molar volume, relative atomic and molecular masses, pH, and related derived quantities.
• Unit presentation and ordering: coherent SI units are listed first; accepted non‑SI units and conversion factors are shown with clear separation.
• Definitions and symbols: recommended names and international symbols, including guidance on subscripts/superscripts to identify species, states (e.g., standard, pure), and reserved symbols.
• Numerical statements and uncertainties: conventions for exact (=), approximate (≈) and definition (:=) statements, and the standardized notation for measurement uncertainty (e.g., 2.34782(32) m).
• Dimensionless quantities: treatment of quantities of dimension one (unit 1), and guidance on use of radian (rad) and steradian (sr) for clarity.
• Annexes: normative lists such as atomic numbers, element symbols and names, nuclide symbols, and a revised annex on pH.

Requirements and conventions

• Prefer the coherent SI units and SI prefixes where applicable.
• Specify the elementary entity when using the mole (atoms, molecules, ions, electrons, etc.).
• Use the recommended symbols and names-variants are allowed but should not be given different meanings.
• Always report measurement uncertainty with experimentally determined values.

Practical applications and users

ISO 80000-9 is essential for professionals who require unambiguous chemical and molecular measurement communication, including:

• Analytical and physical chemists
• Molecular physicists and spectroscopists
• Metrologists and calibration laboratories
• Instrument and software manufacturers (data formats, GUIs)
• Standards developers, publishers, and educators
• Regulatory bodies and quality systems integrating SI-compliant reporting

Using ISO 80000-9 improves interoperability of data, clarity in publications and reports, and consistency across laboratories and instruments.

Related standards

• ISO 80000 series (general): Parts 1, 3, 4, 5, 6, 10, 11, 12
• SI Brochure (BIPM) - for SI unit definitions and conventions

Keywords: ISO 80000-9, quantities and units, physical chemistry, molecular physics, SI units, mole, Avogadro constant, molar mass, unit conversion, measurement uncertainty.