Labconco FreeZone Triad Freeze Dry System 74000 Series - Manual (Page 8)

Chapter 1: Introduction
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In addition to these substances having an effect on the eutectic temperature,
they may increase the vapor pressure at the surface of the sample. Also,
compared to water, they will require the absorption of less heat for
sublimation to occur. Hence, samples that contain volatile substances will
have a greater tendency to melt, particularly when placed in flasks or exposed
to room temperature. If a sample containing a volatile substance tends to
melt when placed on a freeze dry system, dilution of the sample with more
water will help keep the sample frozen. For example, a 0.2M solution of
acetic acid is much easier to freeze dry than a 0.5M solution.