The vial is half the product. Sterility, the endotoxin limit and the preservative assay are all tested on the liquid, but it is the container and its closure that keep those properties intact from the filling line to the last withdrawal. This guide covers what a bacteriostatic water vial is made of and why.
The container: glass or plastic
Type I borosilicate glass is the traditional material. Borosilicate has low thermal expansion and the highest hydrolytic resistance of the pharmacopeial glass types, meaning it releases almost no alkali or other extractables into the water over shelf life. It is clear, allowing inspection for particles and color, and it is inert toward benzyl alcohol. Its disadvantages are weight and breakage.
Plastic vials, typically a semi-rigid polyolefin (a copolymer of ethylene and propylene) or polypropylene, are lighter and do not shatter. They are qualified for compatibility with benzyl alcohol, which can be absorbed by some polymers; a vial material that sorbed the preservative would lower its concentration over time, so the manufacturer's stability data must cover the specific plastic used. Plastic vials are slightly less clear than glass, which matters for particulate inspection.
Both materials are used for bacteriostatic water. Neither is inherently superior; what matters is that the manufacturer has stability and extractables data for the container actually used, and that the container is a sealed, single-fill vial rather than a re-used or screw-capped bottle.
The stopper
The stopper is a molded elastomer, usually bromobutyl or chlorobutyl rubber, chosen for low gas permeability, low extractables and the ability to reseal after a needle is withdrawn. It has a thin central target area designed to be punctured. Two properties matter to the user:
- Resealing. A good stopper closes behind the needle. Repeated punctures in the same spot, or large-gauge needles, degrade this and can eventually leave a channel.
- Coring. Each puncture can shave a tiny fragment of rubber into the vial. Entering at a slight angle with the bevel up, and limiting the number of punctures per vial, reduces coring. Fewer punctures per vial is one argument for smaller vials on low-volume benches; see choosing a vial size.
The crimp seal
An aluminum ferrule is crimped over the stopper flange and the vial neck, compressing the stopper against the glass or plastic to form the actual seal. The crimp cannot be removed and replaced; a lifted, torn or loose ferrule is permanent evidence that the seal has been compromised. Manufacturers test closure integrity, by dye ingress, vacuum decay or helium leak, to show that the assembled vial excludes microorganisms through shelf life.
The flip-off cap
The colored plastic cap over the crimp keeps the stopper's target surface clean and provides tamper evidence: once flipped off it cannot be reattached. It is not the seal. Removing the cap exposes a stopper that should still be wiped with 70% isopropyl alcohol and allowed to dry before the first entry, because the surface under the cap is clean but not sterile.
Why 30 mL is the largest size
The pharmacopeial standard limits bacteriostatic water containers to 30 mL. The preservative is validated to protect a limited number of entries over a limited period, and a larger container would invite more of both. Hence 3, 10 and 30 mL, and nothing bigger; see USP water grades.
Receiving inspection
- Cap present and seated on every vial.
- Ferrule crimped flat and tight, with no lift or tears.
- Vial free of cracks and chips, especially at the neck.
- Contents clear, colorless, no particles, no fill-level variation between vials in a tray.
- Lot number and expiration date legible on each label and matching the certificate of analysis.
A tray that passes goes to room-temperature storage in its carton; see how to store bacteriostatic water. A vial that fails is set aside and reported against its lot number.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.