Development of certified reference materials of medium crude oil for water content with enhanced stability

The accurate determination of water content in crude oil requires matrix-matched certified reference materials (CRMs) with reliable certified values. This work presents the development of six medium crude oil CRMs for water content (GBW 11221-11226), certified at 0.842, 2.02, 2.98, 5.02, 9.92, and 18.62 mg/g. The CRMs were prepared and certified gravimetrically using a mass-balance approach: the water content of the dehydrated crude oil was first determined by azeotropic distillation-Karl Fischer coulometric titration (AD-KFCT), a known volume of deionized water was then added and weighed, and the final value was calculated from the masses of the dehydrated oil, its initial water content, and the mass of added water. To ensure long-term stability, two strategies were implemented: the water activity of the matrix was measured (0.399-0.467), which was found to be close to the ambient humidity, thereby suppressing moisture exchange between the CRM and its environment, and the packaging of CRMs was evaluated by monitoring water uptake of vials containing ultradry Karl Fischer anolyte under 100% humidity. The stability of the CRMs was confirmed over a three-year period using thermometric titration. The evaluated expanded uncertainties (U, k = 2) range from 0.04 to 0.18 mg/g. Finally, this series of CRMs was applied to validate the accuracy of three analytical methods. Thermometric titration and AD-KFCT showed excellent recoveries (99.1-101.4% and 96.4-100.4%, respectively), whereas the distillation method exhibited significant underestimation (recoveries of 54.8-89.9%). These CRMs provide a reliable benchmark for method validation and standardization in crude oil analysis.