Pharmaceutical glass containers are classified by a common standard set by the United States Pharmacopeia (USP). This standard regulates the chemical composition and chemical durability of the glass, or the glass’ ability to resist attack by the contents of the container. The two types of glass react differently when exposed to solutions and vapors depending on their manufactured formulation. With these criteria, the USP separates glass into two primary types, Type I and Type III. Type II glass is simply a modification of Type III. The primary ion removed from the glass’ surface is sodium, but other types of ions may also leach from the glass. As ions leach from the surface of the glass, the pH of the solution in the container changes. Hydrolytic resistance is a glass’ resistance to this leaching.
When choosing between USP Type I and USP Type III glass, it is important to consider the USP’s standards. These standards regulate how well a container will resist chemical attack by its contents. USP Type I borosilicate glass offers the best chemical resistance of any type of glass. This means that USP Type I glass has a higher hydrolytic resistance than Type III glass. USP Type I glass is the least reactive glass container available and can be used for any application, but is commonly used for packaging parenteral and non-parenteral drug products, as well as products with extreme pH values. Type I glass’ coefficient of expansion means it can withstand greater shifts in temperature, with a smaller chance of breaking when exposed to rapid fluctuations in temperature. This doesn’t mean you can take it out of a hot autoclave and set it on a cold tabletop, but it does mean it will withstand both extremely high and extremely low temperatures better than other types of glass. In terms of autoclaving, USP Type I containers can be autoclaved.
USP Type III glass has modest chemical resistance and is typically used when packaging dry powders or buffers with low pH sensitivity. While type III glass has a lesser hydrolytic resistance than type I, it is still safe to use in appropriate laboratory applications. It is common to find type III glass where exposure to harsh conditions is brief or nonexistent and in metal to glass applications. Because of its higher coefficient of expansion, type III glass is more susceptible to breakage when exposed to rapid changes in temperature. Type II containers can be autoclaved; however they do not withstand the high heat and humidity as well as type I glass containers. In some rare cases, there is a special internal surface treatment applied to type III containers. This treatment utilizes sulfur compounds that de-alkalize the interior of the container, improving its chemical durability. This treatment creates what we call USP Type II glass.
In summary, type I glass provides superior chemical and thermal resistance than type III. These factors guide the decision on which type of glass to choose for every application. Which type of glass do you use most, type I or type III?
Chapter <660> Containers-Glass/Physical Tests. (2014). In The United States Pharmacopeia: The National Formulary. (Vol. 1, pp. 318-323). Rockville, Md.: The United States Pharmacopeial Convention.