entrapped in the glass formed at further freeze-concentration. The
consequence of specific substances crystallizing is that the reaction mixture
is altered, which can affect reaction rates and pathways.
An example is the effect of crystallization of buffer components on the
pH. Consider a buffer of NaH 2 PO 4 and Na 2 HPO 4 , giving a pH of 6.8. Upon
cooling to0.5 8 C, the eutectic point of NaH 2 PO 4 is reached; the pH then is
6.6. At 108 C, where the other phosphate starts precipitating, the pH then
drops to 3.6. The changes depend, of course, on the type of buffer. For
instance, the pH of potassium phosphate buffers is less sensitive to freeze-
concentration. The pH change will also depend on the ionic strength, the
presence of proteins, and other compositional variables. Altogether, it is
very difficult to predict pH values after freeze-concentration.
Finally, ongoing freeze-concentration leads to astrong increase of
viscosityand possibly the formation of a glass. Besides a direct effect on
the consistency of the product, it goes along with slowing down, and
possibly stopping, of reactions, as discussed. The decrease of reaction
rates at low temperatures shown in Figure 16.11 will at least be partly
due to this effect. Moreover, the association of ionized components may
play a part.
FIGURE16.12 (a) The equilibrium vapor pressure of water, ice, and two aqueous
solutions differing in molar concentration, as a function of temperature. Highly
schematic and not to scale. (b) The water activity of aqueous solutions in the
presence of ice as a function of temperature, assuming equilibrium.