Types II and III glass compounds are composed of relatively
high proportions of sodium oxide (~14%) and calcium oxide
(~8%). This makes the glass chemically less resistant. Both
types melt at a lower temperature, are easier to mold into various shapes, and have a higher thermal coefficient of expansion.
Although there is no one standard formulation for glass among
manufacturers of these USP type categories, Type II glass has a
lower concentration of the migratory oxides than Type III. In
addition, Type II has been treated under controlled temperature
and humidity conditions, with sulfur dioxide or other dealkalizers to neutralize the interior surface of the container. Although
it remains intact, this surface increases substantially the chemical resistance of the glass. However, repeated exposures to sterilization and alkaline detergents break down this dealkalized
surface and expose the underlying soda-lime compound.
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