LAUSR

Certified reference materials (CRMs) for water content are used to calibrate or validate Karl Fischer titrations for accuracy and consistency. Ampoules of CRMs with low water content are typically prepared by a continuous packaging method, and the water content may decrease or increase over time as the CRM is dispensed. In the present study, we measured the water activities of solutions of water in xylene with various water contents. We found the equilibrated humidity of the solution increased as the water content increased. An improved continuous packaging method using a pre-equilibrated bottle was employed to prepare the CRM of xylene for water content. The average water contents of the solutions dispensed into the ampoules are 73 mg/kg (relative standard deviation 1.6 %) and 221 mg/kg (relative standard deviation 0.4 %). There was no obvious shift in the water contents in ampoules of these CRMs over time. Analysis of variance indicated that the water contents of these two CRM candidates in ampoules were homogeneous, and the relative uncertainties in the water contents because of the inhomogeneity were 1.2 and 0.3 % for the low and high water contents, respectively. This simple method dispenses xylene under its equilibrated humidity condition, which reduces the exchange of water between the solution and headspace. This method will be useful for the preparation of CRMs for low water content.