The qualification “primary” can only be allocated on the basis of metrological criteria

Abstract

The use of the qualifier ‘‘primary’’ is ubiquitous in the chemical literature: ‘‘primary reference measurement procedure’’ or ‘‘primary reference procedure’’ (entry 2.8 in [1]), or ‘‘primary measurement standard’’ (‘‘primary standard’’ for short—entry 5.4 in [1]) are considered to have a high scientific status. Thus, they are used frequently. The qualifier ‘‘primary’’ is thereby allocated easily, rather loosely, and not necessarily on the basis of metrological criteria. A measurement standard (of which a certified reference material is a typical example in chemistry) usually is a material carrying a property value or having a property value embodied in the material. It is that property value which will be used in, e.g., a calibration by means of this material, or which will be used for verifying the presence or absence of systematic effects in a measuring system (entry 3.2 in [1]). If there is a strong wish for such a reference material to be viewed as having a ‘‘higher status’’ for commercial and/or political reasons, it is often called ‘‘primary,’’ just to increase that status. In addition, a very small or ‘‘smallest possible’’ measurement uncertainty for the embodied property value is implied or declared, sometimes without being consistent with the VIM definition of measurement uncertainty (entry 2.26 in [1]). The knowledge of the property value of a chemical measurement standard usually is obtained by means of a reference measurement procedure (entry 2.7 in [1]) (a measurement procedure is a description of how to carry out a measurement—see entry 2.6 in [1]). Such a procedure usually consists of a series of steps or operations each of which—through its uncertainty—contributes to the uncertainty budget (entry 2.33 in [1]) of the measured property value. A case in point is the measurement of the mass fraction of Ag (the measurand) by constant current coulometry in an impure raw silver-containing material. First, a weighed portion of the raw silver material is dissolved in a weighed portion of HNO3 solution. Measuring a constant electric current passing through the solution, depositing the Ag on an electrode and measuring the time needed until depletion of Ag? ions, leads to the knowledge of the measurand (entry 2.3 in [1]) by a calculation of the number of Ag atoms or the mass of the Ag per g of the raw silver material. Note that the term ‘‘analyte’’ is frequently used— erroneously—to designate either the pure Ag or the raw silver material which is a recipe for confusion because an analyte is not a quantity and can therefore not be a measurand. It is also possible to arrive at the property value to be certified, by using a ‘‘preparation procedure’’ through which the final property value embodied in the standard is obtained. A simple example of that is the preparation of a glucose measurement standard obtained by mixing a known mass of glucose and a known mass of water, both of known (preferably high) purity. This enables to calculate a property value expressed as a measured mass of glucose per measured mass of water or per measured mass of solution, expressed in the (derived) unit g/g for mass fraction. The end product: a g/g value or number ratio for glucose in water for which an associated uncertainty must be evaluated. It is essential to indicate the measurand (e.g., mass fraction) as well as the unit used, i.e., g/g. Once the measurement standard has been established, one can say Disclaimer: The author is a member of the Joint Committee on Guides for Metrology (JCGM), Working Group 2 (VIM). The opinions expressed in this Column do not necessarily represent the view of the Working Group or of ACQUAL.