Research-use information only. The materials discussed are laboratory reference reagents, not for human or animal consumption.
Bacteriostatic water and sterile water look identical in the vial and are frequently confused when ordering. They are not interchangeable. Choosing the wrong one is one of the most common reasons a reconstituted peptide degrades days after it was prepared. Here is the difference and when each applies.
The single difference: benzyl alcohol
Sterile water is purified water that has been sterilised. It contains nothing else. Once the seal is broken and air enters, there is nothing preventing microbial growth, so it is a single-use reagent.
Bacteriostatic water is the same sterile water with 0.9% benzyl alcohol added as a preservative. Benzyl alcohol is bacteriostatic — it suppresses bacterial reproduction rather than killing outright. That preservative is what makes the vial multi-use, typically for around 28 days after first puncture when stored correctly.
Why it matters for peptide work
A lyophilised peptide vial usually holds far more material than a single experiment requires. Reconstituting with sterile water means the remainder is unpreserved from the moment it is mixed, and any organism introduced by the needle has an ideal growth medium. Reconstituting with bacteriostatic water gives the working solution a usable window across repeated withdrawals.
This is why bacteriostatic water is the default diluent in most peptide reference protocols, and why it is worth ordering alongside the peptide rather than after.
When sterile water is the correct choice
- The entire vial is consumed at once. No preservative is needed if nothing is stored.
- The assay is incompatible with benzyl alcohol. Some analytical and cell-based work is sensitive to it; check the protocol.
- The peptide is documented as unstable in benzyl alcohol. Uncommon, but supplier documentation governs.
Handling notes
- Add the diluent slowly, letting it run down the vial wall rather than jetting onto the powder. See how to reconstitute research peptides.
- Never shake. Swirl gently. Agitation denatures peptides and produces foam.
- Refrigerate after reconstitution and record the date of first puncture on the vial.
- Do not top up a partially used bacteriostatic vial from another vial.
Quick reference
- Sterile water: no preservative, single use, discard remainder.
- Bacteriostatic water: 0.9% benzyl alcohol, multi-use, ~28 days refrigerated after first puncture.
- Default for reconstituting lyophilised peptides: bacteriostatic water.
- Both: store below 25°C, protect from light, inspect for cloudiness before use.
What we stock
We supply bacteriostatic water in 3 ml and 10 ml vials, 0.9% benzyl alcohol, sterile-filled. Most researchers order one alongside each peptide vial. See the catalogue, or contact us if you need a diluent not listed. Related reading: peptide storage and shelf life.