Decagon Devices AquaLab 4TE Operator's Manual page 122

AquaLab
Appendix C
better standards than saturated salts. Robinson and Stokes (1965)
give activity coeffi cient for various salt solutions. Th ese can be used
to compute the water potential, or partial specifi c Gibbs free energy,
of the water in the solution using;
  - 
cRT
where  is the water potential,  is the number of active particles
per molecule of solute (i.e., 2 for NaCl), is the activity coeffi cient,
1
c is the concentration of the solute (mol kg ), R is the gas constant
-1 -1
(8.314 J mol K ), T is the Kelvin temperature. Water potential is
related to water activity by the equation;

M
w

a exp( )
w
RT
-1
is the molecular weight of water (0.018 kg mol ). When
where M
w
equations 1 and 2 are combined a simplifi ed equation for water ac-
tivity is obtained;
 
a exp(- cM )
w w
For example, equation 3 gives the a in a 6M NaCl solution, (M =
w
w
,  = 2, and = 1.271; from tables in Robinson and
-1
0.018 kg mol
Stokes, 1965) as
    
a exp(-2 1.271 6 0.018) 0.760
w
It is important to note that equation 3 has no explicit temperature
dependence. Available data on temperature dependence of  indi-
cates that its variation is less than ± 2% over the range 0 to 50°C for
NaCl (Lang, 1967) and KCl (Campbell and Gardner, 1971) and no
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