15 APPENDIX A AquaLab
15 Appendix A
15.1 Preparing Salt Solutions
If you choose to mix a saturated salt solution for use as a verification
standard, we recommend that you use the approved AOAC method.
Steps 1 through 4 detail the AOAC method.
1. Select a reagent-grade salt and place it in a test container to
a depth of about 4 cm for more soluble salts (lower a
w
), to a
depth of about 1.5 cm for less soluble salts (high a
w
), and to
an intermediate depth for intermediate salts.
2. Add steam distilled water in increments of about 2 mL, stirring
constantly.
3. Add water until the salt can absorb no more water, evidenced
by the presence of free liquid. Keep the amount of free liquid to
the minimum needed to keep the solution saturated with water.
If you plan on using this solution over a long term period, seal
the solution well to prevent losses from evaporation. Table 4
shows saturated salt solutions and their respective water ac-
tivities at various temperatures. Please note that these values
are based on averaged published data, and the standard errors
shown reflect Greenspan’s standard error for each salt solution,
not the AquaLab accuracy in measuring the salt. The AquaLab
measures all samples with an accuracy of ±0.003 a
w
using the
dew point sensor and ± 0.015 a
w
using the capacitance sensor.
106
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