Determination of Water Content in Solid Matrices: How to Establish Metrological Traceability?

Michela Sega, Istituto Nazionale di Ricerca Metrologica, Torino, Italy
Vito Fernicola, Istituto Nazionale Di Ricerca Metrologica, Torino, Italy
Francesca Pennecchi, Istituto Nazionale Di Ricerca Metrologica, Torino, Italy
Enrica Pessana, Istituto Nazionale Di Ricerca Metrologica, Torino, Italy
Francesca Rolle, Istituto Nazionale Di Ricerca Metrologica, Torino, Italy

The accurate measurement of water content in solid materials has a widespread importance as it affects several physicochemical properties of materials. It has a strong economic impact as it influences, for example, the design of technological processes, the stability of industrial products, the preservation of foodstuff, the calorific value of solid fuels and biomasses, the biodegradability of bioplastics and other materials. The definition of the measurand is a crucial step: usually, “moisture content” and “water content” are improperly used as synonyms and moisture measurement in solids is strongly method dependent.

The current state of the art relies on moisture analyses by Loss-on-Drying (LoD), i.e. measuring by weighing the mass loss of a sample before and after a drying process. LoD techniques are prescribed by several standardised procedures (ISO, ASTM, CEN). However, oven drying is not water selective and it might also extract other volatiles or leave residual water in the sample. In addition, a rigorous measurement uncertainty analysis is unavailable in many cases. For these reasons, there is the need for implementing measurement methods which can assure a full metrological traceability to the SI.

This work presents an activity carried out at the Istituto Nazionale di Ricerca Metrologica within the European Joint Research Project EMRP SIB64-METefnet “Metrology for moisture in materials” (www.metef.net). Two electrochemical methods, namely coulometric Karl Fischer titration and Evolved Water Vapour analysis, were used to determine the water content in various solid samples, like simple sugars, wood pellets and polymers. A part of the work was devoted to establish the metrological traceability of measurement results by using suitable reference materials to calibrate the instruments and calibrated mass standards for the mass determination of samples. An appropriate uncertainty budget was assessed. The results were compared with those obtained by thermo-gravimetry, which is included among the LoD methods.