The New Mole in the New International System of Units (SI)

Brynn Hibbert, School of Chemistry, UNSW Sydney, Sydney, Australia (

Although the concept of discrete atoms can be traced back to the Ancient Greeks, and was enshrined in modern scientific thinking in the 18th and 19th centuries, bringing countable entities into the SI has proved remarkably difficult.

The mole arrived in 1971, but even Max McGlashan, one of the architects of the 1971 definition of the mole, wrote in 1996: “…if any change were needed then abandoning amount-of-substance altogether would be much less unattractive.”  

As part of the discussions at the CCQM in 2014 the author demonstrated that of 18 first year university text books published in English, none (!) gave a correct description of amount-of-substance or properly defined the mole. None even copied the definition from the SI Brochure found for free on the BIPM web site. 10 out of 18 described the Avogadro number (unit ‘1’) as “the chemists’ dozen”, even though the only officially-recognised constant (for now) is the Avogadro constant (unit mol-1).

After well-timed advice from IUPAC the concept of numerosity has been returned to amount-of-substance. In the New SI the numerical values of certain constants will be fixed in order to define their units. In particular it is proposed that the Avogadro Constant NA will take the exact value of 6.02214076×1023 mol−1. On 20th May 2019 the mole will become:

The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.02214076×1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol–1 and is called the Avogadro number.  The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.

Short Biography of Presenting Author

Brynn Hibbert occupied the Chair of Analytical Chemistry at the University of New South Wales since arriving from England in 1987 until his retirement in 2013.  

His research interests are in metrology and statistics in chemistry, ionic liquids and electroanalytical chemistry, but he also does a sideline in expert opinion, scientific fraud and presenting science to the public. Long a member of IUPAC (International Union of Pure and Applied Chemistry) he has helped name elements, revise the SI units and write the terminology of chemistry.

More recently he has become a go-to expert witness in matters of drugs (of abuse, and sports). He is the Past President of the Royal Society of New South Wales, and was made a member of the Order of Australia in 2018.

He has published around 270 papers, 5 books and 3 patents.


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