AquaLab 17 APPENDIX C
where M
w
is the molecular weight of water (0.018 mol/kg
−1
). When
equations 1 and 2 are combined a simplified equation for water ac-
tivity is obtained;
a
w
= exp(−φγcM
w
) (3)
For example, equation 3 gives the a
w
in a 6 mol/kg NaCl solution,
(M
w
= 0.018 kg mol
−1
, φ 2, and = 1.271; from tables in Robinson
and Stokes, 1965) as
a
w
= exp(−2 × 1.271 × 6 × 0.018) = 0.760 (4)
It is important to note that equation 3 has no explicit temperature
dependence. Available data on temperature dependence of γ indi-
cates variation is less than ±2% over the range 0 to 50
◦
C for NaCl
(Lang, 1967) and KCl (Campbell and Gardner, 1971) and no other
terms have any temperature dependence.
A further advantage of unsaturated salts is that there is no solid
phase present to affect the water activity of the solution. Salt in sat-
urated solutions can exist in different states and result in uncertainty
in the water activity values.
Instructions for Using Decagon’s Verification Standards
Simply empty one vial of standard solution into a sample dish and
place the dish immediately into the AquaLab for measurement. Each
vial will fill a sample dish to just less than half full. Table 6 shows
the expected values.
Note: If you need to obtain a Material Safety Data Sheet (MSDS)
for any of these standards, a printable version is available on our
website at www.decagon.com/msds.
105
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