Development of certified reference materials of lithium chloride solution for water activity.

Water activity (a) is used to assess the microbiological stability, shelf life, and safety of food. Water activity meters using electric humidity sensors can measure water activity conveniently. To calibrate water activity meters, certified reference materials (CRMs) of lithium chloride (LiCl) solution for water acidity were prepared by using the gravimetric method. Both the mass balance approach and potentiometric titration were combined to determine the purity of LiCl, and the molality (m) was calculated. The osmotic coefficient (ϕ) was calculated using m and the standard data for ϕ expressed as the Debye-Hückel equation. The certified value of a was calculated using an exponential function of the product of ϕ, m, and molecular mass of water. The certified values and uncertainty (U, k = 2) of two CRMs were 0.250 ± 0.006 and 0.500 ± 0.007, respectively. These two CRMs can be used to calibrate water activity meters and validate analytical methods.