Some simple principles for basic measurement system construction

Abstract

The problems created for chemical measurement by the 1971 inclusion of the thermodynamic ‘mole’ in the Systeme International (SI) measurement units are a salutary lesson to all other measurement disciplines seeking inclusion in the SI. Do not do it! By all means make units useable with the SI but do not surrender basic principles. Chemical measurement has a serious semantic problem. Most analysts, not unreasonably, regard the SI as fundamentally irrelevant as far as the thermodynamic ‘mole’ is concerned and continue to use the earlier chemical mole, understood as an Avogadro number of specified things. However, the users of their measurements do not have this understanding. They examine the SI Brochure [1] to find to their surprise that a ‘mole’ is something quite different, having to do with a thermodynamic quantity puzzlingly named ‘amount of substance’. The next edition of the brochure [2] will make the difference even starker and more inexplicable. The longstanding ambiguity of two ‘moles’ cannot continue into the twenty-first century. Analysts need to consider their options. The term ‘mole’ is unsuitable for clear communication; it has two incompatible meanings. Chemists themselves need to clear up this confusion for it is clear that the SI will not. The solution is simple for metrology in chemistry. All we need is a new name for an Avogadro number of things that cannot be confused with the thermodynamic ‘mole’ and to make it crystal clear that the quantity we measure is a number of things and not the inexplicable thermodynamic construct termed ‘amount of substance’. The new name is obvious. It is an Avogadro number of things, although the obvious common abbreviation will be the avo. It’s equally obvious symbol is Av, following wellaccepted conventions for honouring appropriate scientists, in this case Amedeo Avogadro (1776–1856). The avo is a unit that can be used with the SI but it is not of the SI. It is SI compatible. It is not a base unit. The base unit for its corresponding quantity is one specified thing. Leonard calls it the entity (symbol ent) [3]. As Leonard shows, there are many ways in practice to construct a system of measurement units, with a variety of advantages, disadvantages, and risks. There are a few essential requirements of any practical system of measurement units. The first and highest priorities are that it be simple, concise, clear, and most essential of all, comprehensible to all users. This includes especially the audience to which the measurement results are communicated. A number of things is the simplest and most comprehensible quantity there is. That cannot be said of ‘amount of substance’. The next priority is that it be well anchored. Anchoring chemical measurements in an Avogadro number of things is simple in concept and straightforward, dependent vitally on the specific measurement problem at hand. Its ultimate anchor is the notion of one thing. After four decades, we still do not know how to anchor an ‘amount of substance’ for we still do not know in clear and concise terms what it is. The next priority, and one implied by the previous, is to strive for maximum independence of the base units and quantities. This may be a counsel of perfection, but it is something to strive for. However, there is at least one measurement unit that is independent of all others. It is one thing (or entity). One thing is what it is, quite irrespective of what other measurement units one may choose. If one G. Price (&) PO box 57, Menai, NSW 2234, Australia e-mail: gzprice@ozemail.com.au