Bacteriostatic water is sterile water containing 0.9% benzyl alcohol as a bacteriostatic preservative, which allows a vial to be entered more than once. Sterile water and Water for Injection (WFI) contain no preservative and are single-use. For laboratory reconstitution of lyophilized peptides where a vial will be accessed repeatedly, bacteriostatic water is the standard choice.
The three waters, side by side
| Bacteriostatic water | Sterile water | Water for Injection (WFI) | |
|---|---|---|---|
| Preservative | 0.9% benzyl alcohol | None | None |
| Multi-entry | Yes | No | No |
| Typical window after opening | Commonly stated as up to 28 days | Single use | Single use |
| Tonicity | Hypotonic | Hypotonic | Hypotonic |
| Typical laboratory use | Reconstituting material accessed repeatedly | Single reconstitution, discard remainder | Preparation of solutions, single use |
What benzyl alcohol does
Benzyl alcohol is bacteriostatic, not bactericidal — the distinction matters. It inhibits bacterial reproduction rather than killing an established population. That is enough to keep a repeatedly entered vial from developing growth between accesses, but it is not a sterilising agent and it does not rescue a vial that has already been contaminated.
Because it is a preservative rather than a steriliser, technique still matters: wipe the stopper before each entry, use a fresh needle, and keep entry count low.
Compatibility considerations
Benzyl alcohol is a small organic molecule and is compatible with the great majority of research peptides. There are two situations where a preservative-free water is preferred:
- Single-use preparations where the entire vial will be consumed in one experiment — no preservative is needed, so none should be introduced.
- Assays sensitive to organic solvents, where even 0.9% benzyl alcohol may interfere with the readout. Cell-based assays are the usual case.
Some peptides with limited aqueous solubility require a small volume of dilute acetic acid or another co-solvent before dilution. This is documented per compound rather than being a general rule.
Volume, concentration and why it is arithmetic, not guesswork
Reconstitution concentration is determined entirely by the mass in the vial and the volume of solvent added:
concentration (mg/mL) = mass in vial (mg) ÷ solvent volume (mL)
A 100 mg vial reconstituted with 2 mL of bacteriostatic water gives 50 mg/mL. The same vial with 5 mL gives 20 mg/mL. Nothing about the peptide changes — only the concentration of the working solution. Choosing a larger volume produces a more dilute, easier-to-measure solution; a smaller volume produces a more concentrated one. Full procedure in our reconstitution guide.
Storage after reconstitution
Once a lyophilized peptide is in solution, its stability clock starts. Dry powder stored cold and dark is stable for long periods; the same material in aqueous solution is not. Reconstituted material should be refrigerated, protected from light, and not subjected to repeated freeze-thaw cycles. See peptide storage and stability.
What a laboratory reconstitution setup contains
- The lyophilized peptide vial, at room temperature before opening
- Bacteriostatic water as the solvent
- Alcohol wipes for the stoppers
- A graduated syringe appropriate to the volume being transferred
- Refrigerated storage for the reconstituted vial
Bacteriostatic Water 20 mL is stocked alongside the peptide catalog, so a single order covers both the compound and the solvent.
Summary
Bacteriostatic water is the default for reconstituting lyophilized research peptides that will be accessed more than once, because the 0.9% benzyl alcohol suppresses bacterial growth between entries. Sterile water and WFI are preservative-free and appropriate only for single-use preparations or assays where the preservative would interfere.
All products are intended for laboratory and research use only. They are not medicines, are not for human or veterinary use, and are not for diagnostic or therapeutic application.