Bacteriostatic water: what it is and what to check

Bacteriostatic water is water containing 0.9% benzyl alcohol as a preservative. The alcohol inhibits bacterial growth, which is what allows a sealed vial to be entered more than once instead of being discarded after a single withdrawal. It is a solvent, not a peptide: no HPLC purity figure and no chromatogram apply to it, and a supplier offering one has misunderstood the product.

What is in it, and what the preservative does

The composition fits in one sentence: water, plus benzyl alcohol at 0.9% as a bacteriostatic agent. Benzyl alcohol is a well-characterised small molecule, formula C₇H₈O, mass 108.14 g/mol, PubChem CID 244, CAS 100-51-6. Bacteriostatic means it inhibits the multiplication of bacteria; it does not mean bactericidal, and it does not sterilise anything that was not sterile to begin with. That distinction is the whole reason the product exists as a separate item: the preservative is what makes a multiple-entry vial sensible, and nothing more.

Two things follow that are worth saying plainly. Water with no preservative is a different product with a different in-use life. And bacteriostatic water is not a pharmacopoeial grade of water, is not offered here under any pharmacopoeial monograph name, and is not presented as suitable for administration to people or animals.

Why a certificate for a solvent looks different

Everything else on this site is a peptide, and for a peptide the certificate questions are purity by HPLC, measured mass against theoretical mass, peptide content and water by Karl Fischer titration. None of those transfer here. A solvent has no chromatogram, no peptide content and no theoretical mass to check a measurement against, and a seller who prints a purity percentage on a bottle of water is either confused or counting on the buyer being so.

What does apply to a solvent is different analysis entirely: sterility, bacterial endotoxins, identification and content of the preservative, pH, particulate matter and container closure integrity. If a supplier offers documentation for bacteriostatic water, those are the headings to look for. Purity by HPLC is not one of them.

Handling, pH and the in-use period

The specification beside this text gives a fill of 10 ml in a vial closed with a butyl rubber stopper and an aluminium crimp cap, a pH range of 4.5 to 7.0, and storage at 15 to 25 degrees Celsius away from light. The in-use period conventionally quoted for a preserved multiple-entry vial is 28 days from first entry. That figure is convention for this class of product rather than the result of a study on any particular lot, and it is stated here as convention - which is a distinction most listings blur.

The volume of solvent used is what fixes the concentration of a reconstituted solution, and nothing else does: a 5 mg vial of lyophilisate made up to 2.5 ml gives 2 mg/ml, and the same vial made up to 5 ml gives 1 mg/ml. The peptide calculator converts between vial content, solvent volume and concentration. It recommends no quantity for any purpose, and neither does this page.

Regulatory position

This item is supplied as a laboratory solvent. It is not a medicinal product, not a food supplement and not a medical device, and it is not offered under a pharmacopoeial monograph. In the United Kingdom, regulation 2 of the Human Medicines Regulations 2012 defines a medicinal product in two limbs, one of which turns on how a product is presented - so a solvent described as being for administration to a person is a different legal object from the same liquid described as a laboratory reagent. Nothing here describes it as the former, and regulation 46 is the provision that makes that distinction matter.

"Research use only" is a purpose, not a permission

As with everything on this site, the phrase records the seller's intended purpose. It is not a legal category in the UK or the EU, not a licence, and not a review by any authority. It is not a sterility claim.

What Peptorium publishes about this item

This page publishes composition: what the solvent is, which preservative it carries, and what the container closure has to do for repeated withdrawal to be possible. Sterility, endotoxin content, preservative assay and pH describe one filled lot rather than a formulation. Those results belong on documentation issued by the laboratory that tested that lot, and they travel with the material itself.

What to ask before buying a solvent

• What is the stated concentration of benzyl alcohol, and is it identified and assayed in any documentation offered?
• Is there a sterility result and a bacterial endotoxin result, with a batch number and a date?
• Is the container closure described - stopper material, crimp seal - and is the vial genuinely designed for multiple entry?
• Is a pH range stated?
• Is anyone quoting an HPLC purity figure for it? That is a sign the documents were copied from a peptide listing.

What is not known

• Whether a particular vial is sterile cannot be known from a page. It is knowable only from a sterility test on that batch.
• The 28-day in-use period is convention for preserved multiple-entry vials, not a measurement on this or any other lot.

Frequently asked questions

This item is a laboratory solvent: it is not a medicinal product, not a food supplement and not a medical device, and it is not offered under any pharmacopoeial monograph name. It is listed together with a peptide as well as on its own, and the guide to reconstitution and storage covers handling in more detail. See also the research declaration.