Decagon Devices AquaLab 4TE Operator's Manual page 121

AquaLab
Appendix C
verifi ed by an independent third party and are shelf stable for one year.
Order your calibrtion salt standard of similar water activity today.
Uncertainties Using Saturated Salt Solutions
Th e water activity values listed in our operator's manual for satu-
rated salts were reprinted from Greenspan (1977). His method for
determining water activity was to combine all of the available data
from tests by other researchers. He did not set up any experiments
of his own. Th e uncertainty he published is due to variation among
the results from the diff erent methods. Th ere are, therefore, limita-
tions to the accuracy of these values. Th e instrumentation available
for making water activity measurements is much better now than it
was in 1977, so improved standards are needed.
Saturated salt solutions can be prepared by several methods. Th e
AOAC method involves starting with salt and adding water in small
increments, stirring well with a spatula after each addition, until salt
can absorb no more water as evidenced by free liquid (where it will
take on the shape of the container but will not easily pour). Th is
method gives the most accurate readings, but only for a short time
unless great care is taken to prevent water gain or loss. When a salt
standard is prepared so that it consists mostly of liquid with a few
crystals in the bottom, it can result in a layer of less than saturated
solution at the surface which will produce a higher reading than
anticipated. Conversely, solid crystals protruding above the surface
of the liquid can lower the readings. To comply with Good Labora-
tory Practices (GLP), a saturated salt solution must read within rea-
sonable analytical error of the accepted published value for a given
temperature.
Why AquaLab Verifi cation Standards are Superior
Our research indicates that unsaturated salt solutions make much
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