Bacteriostatic water (BAC water) is sterile water containing 0.9% benzyl alcohol. The benzyl alcohol inhibits the growth of bacteria, so you can take liquid from the same vial on several occasions without the contents becoming unusable once opened. In the lab, it is mainly used as a solvent for freeze-dried substances such as peptides. It is precisely this preservative that sets it apart from ordinary sterile water.
Composition: water and 0.9% benzyl alcohol
BAC water has two components: sterile, purified water and benzyl alcohol. Benzyl alcohol (phenylmethanol, C7H8O, molar mass 108.14 g/mol) is a simple aromatic alcohol [1]. A concentration of 0.9% (mass per volume) corresponds to 9 mg of benzyl alcohol per millilitre. In the pharmaceutical literature, 0.9% benzyl alcohol in water is referred to as a ‘bacteriostatic amount’ [2].
‘Bacteriostatic’ means growth-inhibiting. The benzyl alcohol stops bacteria from multiplying, but it does not make a contaminated vial sterile again. Good aseptic technique therefore remains essential.
According to the product specification, Clean Peptides BAC water consists of purified water with 0.9% benzyl alcohol and comes in vials of 3 ml and 10 ml.
How it differs from sterile water
| Property | Bacteriostatic water | Sterile water |
|---|---|---|
| Composition | Water with 0.9% benzyl alcohol | Water only |
| Preservative | Yes | No |
| After first opening | Inhibits bacterial growth if the vial is opened more than once | No protection; intended for single use |
| Note | Benzyl alcohol can react with some proteins and peptides | No additives that could affect the experiment |
Do not confuse BAC water with physiological saline either. Its label also says ‘0.9%’, but that refers to 0.9% sodium chloride: a salt solution without a preservative.
What it is used for in the lab
The main application is dissolving (reconstituting) freeze-dried substances, such as peptides in a vial. The preservative makes BAC water particularly suitable when you need to take liquid from the same vial on several occasions. For the same reason, antimicrobial preservatives are added to pharmaceutical products from which several portions are taken from a single container [3].
Two points to consider when designing an experiment:
- Use the same solvent in the control group as in the test group. Benzyl alcohol is itself biologically active and can have an effect of its own in sensitive cell or enzyme experiments.
- Record in your lab notebook which solvent you used, together with its batch number and the date you opened the vial.
For analyses such as HPLC, BAC water is usually not the first choice. Benzyl alcohol has an aromatic ring that absorbs UV light, so it can show up as an extra peak in the chromatogram. For samples you send for analysis, it is therefore best to use the solvent specified by the lab. How to read such an analysis is explained in How to read a certificate of analysis (COA).
When benzyl alcohol is less suitable
Benzyl alcohol is not compatible with every molecule. In a study of a protein (an interleukin-1 receptor antagonist), reconstitution in 0.9% benzyl alcohol led to more aggregation than reconstitution in water [2]. In another study, of a 31-amino-acid acylated peptide, benzyl alcohol did not cause aggregation, whereas the preservative m-cresol did [3]. The effect therefore depends on the molecule.
If you do not know how your peptide reacts to benzyl alcohol, check the literature first or run a small test: dissolve a small amount and check whether the solution stays clear. If it becomes cloudy or a precipitate forms, sterile water or another solvent may be a better choice.
Storage
According to the product specification, unopened Clean Peptides BAC water should be stored at room temperature (15–25 °C), out of direct sunlight. After first opening, keep the vial tightly closed and write the opening date on it. If the packaging states a period of use after opening, observe it. If the liquid is cloudy or contains particles, stop using the vial.
Note the difference between the solvent and the solution: BAC water itself can be kept at room temperature, but a peptide dissolved in it is usually kept in the fridge. For details, see Storing peptides.
Which size suits your experiment
The only difference between the 3 ml and the 10 ml vial is the volume. A small vial is useful if you need little solvent and will use it up quickly: the shorter the time an opened vial is in use, the lower the risk of contamination. A larger vial is practical for more frequent or larger preparations. The solvent is sold separately, even for retatrutide vials. Other questions about ordering and shipping are answered in the FAQ.
Further reading
- How do you store peptides so they stay stable?
- How to read a certificate of analysis (COA): HPLC and MS explained
- Retatrutide pen or vial: what are the differences?
- What are peptides? Explanation and key terms
- Knowledge base: peptides and research materials explained
Sources
- National Center for Biotechnology Information. PubChem Compound Summary for CID 244, Benzyl Alcohol. pubchem.ncbi.nlm.nih.gov (accessed 30 September 2026)
- Roy S, Jung R, Kerwin BA, et al. Effects of benzyl alcohol on aggregation of recombinant human interleukin-1-receptor antagonist in reconstituted lyophilized formulations. Journal of Pharmaceutical Sciences 2005;94(2):382–396. doi:10.1002/jps.20258
- Li M, Falk BT, Lu X, et al. Molecular Mechanism of Antimicrobial Excipient-Induced Aggregation in Parenteral Formulations of Peptide Therapeutics. Molecular Pharmaceutics 2022;19(9):3267–3278. doi:10.1021/acs.molpharmaceut.2c00449
For laboratory research use only. Clean Peptides products are not intended for use in humans or animals. This article provides general information and is not medical advice.
