Nearly every vial of bacteriostatic water is Type I borosilicate glass. The choice is not tradition: it is the combination of chemical inertness toward water, resistance to the thermal shock of sterilization and depyrogenation, and the ability to be formed into thin-walled, dimensionally precise tubing vials that take a crimped closure.
What borosilicate glass is
Ordinary soda-lime glass is silica with sodium and calcium oxides as fluxes. Borosilicate glass replaces much of the sodium and calcium with boron oxide: a typical pharmaceutical borosilicate is about 80% silica, 10–13% boron trioxide, 4–7% sodium and potassium oxides and 2–3% alumina. Less alkali means less leaching into water, and the boron-silica network gives a coefficient of thermal expansion of roughly 3 to 5 × 10−6 per kelvin, compared with about 9 × 10−6 for soda-lime glass.
The USP glass types
| Type | Glass | Hydrolytic resistance | Typical use |
|---|---|---|---|
| Type I | Borosilicate | Highest; passes the USP <660> surface and powdered-glass tests at the strictest limits | Sterile aqueous products, including bacteriostatic water; products of any pH |
| Type II | Soda-lime, inner surface treated (dealkalized) | High surface resistance while the treated layer lasts | Some aqueous products below pH 7 |
| Type III | Soda-lime, untreated | Moderate | Dry powders and non-aqueous products |
The hydrolytic resistance tests measure how much alkali the glass releases into water under autoclave conditions. Type I releases the least, which is why water stored in it for years stays within its pH specification. See the pH of bacteriostatic water.
Why thermal expansion matters
Vials are steam-sterilized at 121 °C or more, depyrogenated empty by dry heat at around 250 °C, and later cooled, shipped and stored. Glass with a low expansion coefficient tolerates those temperature swings without cracking, and it lets the vial wall be thin enough to form precisely and light enough to ship. Soda-lime glass would crack under the same treatment.
The one failure mode: delamination
Under some conditions the inner surface of a borosilicate vial can shed microscopic glass flakes, a phenomenon called delamination. It is associated with alkaline products, certain buffers such as citrate and phosphate, high-temperature tubing conversion that leaves an altered surface near the base of the vial, and long storage. Slightly acidic, unbuffered water is among the lowest-risk contents, but delamination is one of the reasons manufacturers test particulate matter on every lot and why visible flakes in any vial are grounds for rejection. See particulate matter testing and what a vial should look like.
Clear vs. amber
Amber borosilicate contains iron and manganese oxides that absorb ultraviolet and short-wavelength visible light. Bacteriostatic water does not require light protection on its label and is supplied in clear glass, which also makes visual inspection easier. See does it need to be protected from light?
Recycling
Borosilicate melts at a higher temperature than container glass and contaminates ordinary glass recycling; empty vials go to laboratory glass waste. See recycling the packaging.
Related
Vials: glass vs. plastic · Vial stoppers explained · Flip-off caps and crimp seals
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.