Units and quantities for analytical chemistry (background paper)

It is universally accepted that a measurement of a quantity of interest should be presented as the product of a numerical value and a unit (we should also include an estimate of measurement uncertainty, but this is beyond the scope of this Technical Brief). The International System of Units (the SI) provides the world's only practical system of coherent units of measurement for this purpose. For ease of expression and understanding it is generally preferred that the numerical value presented is between 1 and 100. One might refer to these as 'human-scale' numbers that are easy to relate to, conceptualise and communicate. The SI uses 'SI prefixes' to achieve this aim, allowing us to write 2.3 km rather than 2300 m. Equally we could use scientific nomenclature to write, arguably more inelegantly, 2.3 × 103 m. Deviation from these agreed practices may risk confusion in the communication of measurement results. This Technical Brief outlines the SI units and quantities available for use in analytical chemistry and explains the care that must be taken if alternative approaches are followed. One principle remains essential to aid understanding: the description of the quantity being expressed should always be stated unambiguously in words as part of the presentation of a measurement result.