Development of Four Certified Reference Materials for Water Content in Solids

Abstract

Water content in solid materials is usually measured by loss on drying and Karl Fischer titration (KFT) methods. The validation of KFT methods requires the certified reference materials (CRMs) for the water content in solids, especially those with lower water content. In the present study, two mixtures of sodium tartrate dihydrate and 4-methoxybenzoic acid were prepared using the gravimetric method and then used as candidates of CRMs with water content of 10.0 and 1.0 mg·g⁻¹, respectively. Furthermore, the CRM of lactose monohydrate and that of 4-methoxybenzoic acid were also developed. The stability against the humidity of the raw materials was characterized using water sorption isotherms. The thermal stability of the raw materials was characterized by the thermogravimetric analyzer. These four CRMs for water content were certified using Karl Fischer coulometric and volumetric titration. To make the water content traceable, both KFT methods were calibrated using a home-made water content standard prepared by the gravimetric method. The average value of the two methods was used as the certified value. The certified water contents and their expanded uncertainties (U, k = 2) of four CRMs were (50.7 ± 0.6) mg·g⁻¹, (9.90 ± 0.2) mg·g⁻¹, (0.878 ± 0.044) mg·g⁻¹ and (0.142 ± 0.013) mg·g⁻¹, respectively. This series of CRMs were suitable for the validation of KFT methods for the water contents in solids.