ANALYTICAL & LABORATORY EQUIPMENT
Novasina off ers a range of standards for water activity
As with any laboratory test method, good laboratory practices require routine validation and verifi cation assessments for water activity. T is can be accomplished by running tests on standards of known value and comparing the test results to the actual values. Although this can be diffi cult for some common quality assurance tests if they do not have independent standards, water activity standards exist in the form of electrolytic (salt) solutions. T ese salt standards are used to verify water activity instruments by placing them in the instrument and performing a water activity measurement. T e 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 verifi cation. If not, the instrument is inspected for cleanliness, or the calibration is adjusted until successful verifi cation is achieved. Salt standards that cover
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A REVIEW S
ince water activity testing is critical to ensure the quality and safety of manufactured products, it is a common quality laboratory test.
WATER ACTIVITY STANDARDS:
Dr Brady Carter discusses the two standards options for testing water activity
the full water activity range are available and typically two or more standards will be used for verifi cation. 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. T ese salts have well defi ned thermodynamic properties including that at saturation, they will maintain a constant water activity if temperature remains constant. T e water activity value depends on the type of salt and the temperature. T e 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. T e water activity of an unsaturated salt solution is calculated by the following equation:
Where v is the number of dissociation ions for the salt, Φ is the osmotic coeffi cient for the salt, c is the molal concentration, and Mw is the molecular mass of water.[2] As would be expected both standard options have advantages and disadvantages. T e advantage of saturated slurry standards is that they can be re-used indefi nitely if maintained properly and are easily prepared without a precision balance and complicated recipes. T eir disadvantage is that they take longer to equilibrate and are temperature sensitive. T e advantage of unsaturated salt standards is that they can be made to any water activity value, are faster to equilibrate and less temperature sensitive. T e disadvantage of unsaturated standards is that they are single-use only and must be purchased as a consumable. T e availability of both standards ultimately allows the user to choose based on individual preference.
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References (1) Greenspan, L. (1977). Humidity fi xed 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 Diff usion in
Solutions of Simple Electrolytes. London: Butterworth.
Adapted from Greenspan, L. (1977). Humidity fi xed points of binary saturated solutions. Research of the National Bureau of Standards -
A.Physics and Chemistry. 81A:89-96. Numbers rounded to nearest thousandth.
Water activity of selected saturated electrolyte slurries
Dr Brady Carter is an application scientist with Novasina.
www.novasina.com
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