Essential Chemistry and Biochemistry Associated with Low Temperature and Freezing - Freeze-drying of Pharmaceuticals and Biopharmaceuticals - page 56

Figure 5 pH changes during the cooling (and freezing) and rewarming of an ice-seeded

citric acid-sodium phosphate buffer mixture, illustrating the effects of delayed
crystallisation of sodium phosphate; reproduced from Pikal (unpublished)

this temperature. During cooling, the solution became increasingly su-
persaturated until, at ca.-101C, Na 2 HPO 4 .12H 2 O began to precipitate,
but slowly. On completion, this produced a major pH shift to 3.5, since
only citric acid was left to act as buffer.

5.5 Effects of Freeze-Concentration on Reaction
Kinetics

Reaction rates in part-frozen solutions tend to exhibit complex temper-
ature dependencies. According to textbook chemistry, the Arrhenius
equation describes the effect of temperature on chemical rate constants.
It is therefore sometimes (mistakenly) assumed that reaction rates must
decrease during freezing. This slowing down of deleterious reactions is
often advanced as one of the benefits of freeze-drying. In fact, the
opposite is the case; reaction rates increase, sometimes dramatically,
during freezing. It is overlooked that the Arrhenius equation applies to
homogeneous systems of constant composition, whereas freezing pro-
duces a heterogeneous system and continuous changes in concentration,
or even composition. Probably because of experimental complexities,
few quantitative studies of reaction kinetics in part-frozen solutions are
on record, but rate enhancements of several orders of magnitude have
been reported, particularly for enzyme-catalysed reactions. One