Abstract
In this critical review, the significance of the term 'activity' is
examined in the context of the properties of aqueous solutions. The
dependence of the activity of water(l) at ambient pressure and 298.15
K on solute molality is examined for aqueous solutions containing
neutral solutes, mixtures of neutral solutes and salts. Addition
of a solute to water(l) always lowers its thermodynamic activity.
For some solutes the stabilisation of water(l) is less than and for
others more than in the case where the thermodynamic properties of
the aqueous solution are ideal. In one approach this pattern is accounted
for in terms of hydrate formation. Alternatively the pattern is analysed
in terms of the dependence of practical osmotic coefficients on the
composition of the aqueous solution and then in terms of solute-solute
interactions. For salt solutions the dependence of the activity of
water on salt molalities is compared with that predicted by the Debye-H�ckel
limiting law. The analysis is extended to consideration of the activities
of water in binary aqueous mixtures. The dependence on mole fraction
composition of the activity of water in binary aqueous mixtures is
examined. Different experimental methods for determining the activity
of water in aqueous solutions are critically reviewed. The role of
water activity is noted in a biochemical context, with reference
to the quality, stability and safety of food and finally with regard
to health science.
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