Bacteriostatic water and sterile water look identical in the vial and are both sterile when sealed. The difference is a single ingredient: bacteriostatic water contains benzyl alcohol at 0.9% w/v as a preservative. That one addition decides how the container can be used and which laboratory applications it suits.
1. Side by side
| Sterile water (preservative-free) | Bacteriostatic water | |
|---|---|---|
| Composition | Water only | Water + benzyl alcohol 0.9% w/v |
| Preservative | None | Benzyl alcohol (inhibits growth of many bacteria) |
| Container | Single-use vials, ampoules or larger bottles | Multi-use septum vials, typically 10–30 mL |
| After opening | Use immediately; discard the remainder | Repeated aseptic entry for up to 28 days after first puncture |
| Suited to | Preservative-sensitive work: cell culture, microbiology, many enzymatic and UV assays | A preserved diluent or rinse drawn in repeated small aliquots for compatible reagents |
| Not suited to | Repeated withdrawals over days from one container | Cell culture, microbiological growth, assays sensitive to benzyl alcohol |
2. Why the preservative matters
Every time a septum vial is entered, there is a small chance of introducing microorganisms. In preservative-free sterile water, anything introduced can grow, which is why opened sterile water is treated as single use. Benzyl alcohol suppresses the growth of many bacteria introduced this way, which is what makes a multi-use vial practical over several weeks of careful use.
The same property is a drawback where living cells or microbial growth are the point of the experiment, and benzyl alcohol can interfere with some analytical methods. See TB-TL-003 for compatibility details.
3. How to choose
- Will living cells or microorganisms contact the water? If yes, use preservative-free sterile water or an appropriate buffer.
- Does your method detect in the low UV range, or involve preservative-sensitive proteins? If yes, validate with a diluent blank, or use preservative-free water.
- Do you need small, repeated aliquots from one sealed container over days or weeks? If yes, and the preservative is compatible, bacteriostatic water is the practical choice.
- Do you need a specific purity grade? Ultrapure (Type I) water for trace analysis and nuclease-free water for molecular biology are defined by different specifications. Neither sterile nor bacteriostatic water substitutes for them.
4. Handling either one
- Disinfect the stopper and use a new sterile transfer device for every entry (TB-TL-001).
- Store at controlled room temperature and do not freeze (TB-TL-002).
- Record the lot number of every container you use, so results can be traced back to its certificate of analysis.
5. Summary
Choose sterile water when the preservative could affect your system, and accept single use. Choose bacteriostatic water when you need repeated withdrawals from one sealed vial and benzyl alcohol is compatible with the work. Whichever you use, documented lot quality matters as much as the formulation. Every TrueBac lot is published with its independent laboratory report.