Dr Brady Carter discusses the two standard options for testing water activity
Since water activity testing is critical to ensure the quality and safety of manufactured products, it is a common quality laboratory test. As with any laboratory test method, good laboratory practices require routine validation and verification assessments for water activity. This can be accomplished by running tests on standards of known value and comparing the test results to the actual values. Although this can be difficult for some common quality assurance tests if they do not have independent standards, water activity standards exist in the form of electrolytic (salt) solutions.
These salt standards are used to verify water activity instruments by placing them in the instrument and performing a water activity measurement. The result is then compared to the actual value at the appropriate temperature. If the test result is within the instrument tolerance of the actual value, the instrument passes verification. If not, the instrument is inspected for cleanliness, or the calibration is adjusted until successful verification is achieved. Salt standards that cover the full water activity range are available, and typically two or more standards will be used for verification. Water activity salt standards can also be used to assess the accuracy of an instrument since their value is known independent of instrument measurement.
Independent water activity standards are made possible by the unique characteristics of crystalline electrolytes. These salts have well defined thermodynamic properties, including that at saturation, they will maintain a constant water activity if temperature remains constant. The water activity value depends on the type of salt and the temperature. The table below provides a short list of the water activity values of various saturated slurries and a more complete list is available from Greenspan.(1)
In addition to saturated slurry standards, unsaturated salt solutions can also be used as water activity standards. For these standards, the water activity value is determined by both the salt and the solution concentration. The water activity of an unsaturated salt solution is calculated by the equation in Figure 1 above:
Where v is the number of dissociation ions for the salt, Φ is the osmotic coefficient for the salt, c is the molal concentration, and Mw is the molecular mass of water.
As would be expected, both standard options have advantages and disadvantages. The advantage of saturated slurry standards is that they can be re-used indefinitely if maintained properly and are easily prepared without a precision balance and complicated recipes. Their disadvantage is that they take longer to equilibrate and are temperature sensitive. The advantage of unsaturated salt standards is that they can be made to any water activity value, are faster to equilibrate, and less temperature sensitive. The disadvantage of unsaturated standards is that they are single-use only and must be purchased as a consumable. The availability of both standards ultimately allows the user to choose based on individual preference.
(1) Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards. A. Physics and Chemistry. 81A:89-96.
(2) Robinson, R.A. and Stokes, R.H. (1965). Electrolyte Solutions: the Measurement and Interpretation of Conductance, Chemical Potential, and Diffusion in Solutions of Simple Electrolytes. London: Butterworth.
Dr Brady Carter is an application scientist with Novasina