13.3. Measurement uncertainty estimation in coulometric Karl Fischer titration method

Active ingredients in pharmaceuticals, carbon-fiber composites, polymers, novel cellulose-based active paper, food powders, biomass – all of these and many other solid materials are highly affected by moisture when processing into various products. Errors and inconsistencies in moisture measurement and control in industrial processes lead to decreased process speed/throughput and increased wastage, shortened durability of biomaterials, increased energy consumption in drying and increased fine particle emissions in biomass combustion.

Coulometric Karl Fischer method is currently the most accurate method of moisture measurement. A Survey of the factors determining the uncertainty of coulometric Karl Fischer titration method has been carried out.

This survey gives an overview of the factors that determine the uncertainty of coulometric Karl Fischer (cKF) method for water determination. Distinction is made between uncertainty sources originating from the cKF method itself and uncertainty sources due to sample handling. The “compound” uncertainty sources – repeatability, reproducibility and bias that actually incorporate the contributions from these two classes of uncertainty sources – are also briefly discussed.

Based on the literature data the most influential uncertainty sources of coulometric KF titration method are possible chemical interferences, instrument instability and the accuracy of the end point determination. The uncertainty sources due to sample handling are more problematic with solid samples than with liquid samples. The most important sample preparation related uncertainty sources are change of water content in the sample before the measurement and incomplete transfer of water from the sample to the reaction vessel.

The general conclusion is that although the uncertainty sources of the cKF method are in general rather well known the discussion is almost always either qualitative or limited to the compound uncertainty sources and there is very limited quantitative information available on the contributions of the actual uncertainty sources.

Preparation of this report was supported by the European Metrology Research Programme (EMRP), project SIB64 METefnet “Metrology for Moisture in Materials”.