Two clear liquids in near-identical vials, and one of them keeps for 28 days after opening while the other is finished the moment its cap is pierced. The difference is a single ingredient. This article explains what it does, why it matters for laboratory work with lyophilised peptides, and how to choose between the two.
Sterile water
Sterile water is purified water that has been sterilised, usually by filtration and autoclaving, and sealed in a single-use vial. It contains nothing else: no salts, no buffers, no preservative. Because there is nothing in it to stop microbial growth, it is intended to be used once. After the seal is broken the vial is open to the air, and anything that enters it can multiply.
For a laboratory that reconstitutes a whole vial of peptide in one go and uses the solution the same day, sterile water is perfectly suitable.
Bacteriostatic water
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added. Benzyl alcohol is a preservative: it inhibits the growth of bacteria that enter the vial after opening. “Bacteriostatic” means exactly that, it stops bacteria growing; it does not sterilise a contaminated solution, and it is not effective against everything.
The practical consequence is that a vial of bacteriostatic water can be entered with a needle repeatedly over a period of up to 28 days, which is the standard period suppliers and pharmacopoeias assign to the preservative’s effectiveness. That is why it is the default solvent for reconstituting research peptides: a 10 mg vial dissolved in 2 ml will usually be drawn from many times over several weeks, and the solution needs a preservative to stay usable.
The 0.9% figure
The 0.9% refers to benzyl alcohol by volume, and it is a coincidence that it matches the 0.9% of sodium chloride in normal saline. Bacteriostatic water contains no salt. Bacteriostatic saline, a separate product, contains both. For dissolving lyophilised peptides, water is preferred because some sequences are less soluble in the presence of salt.
Compatibility with peptides
Benzyl alcohol is compatible with the great majority of peptide sequences at the concentrations used in laboratory work. Two cautions from the literature: it can promote aggregation of some proteins at high concentration, and it is a solvent for certain plastics, so solutions should be held in glass or polypropylene rather than polystyrene. Neither is a concern for the short peptides in a research catalogue at typical concentrations.
Choosing between them
| Sterile water | Bacteriostatic water | |
|---|---|---|
| Preservative | None | 0.9% benzyl alcohol |
| After opening | Single use | Up to 28 days, refrigerated |
| Typical use | One-off reconstitution used same day | Multi-draw reconstitution over weeks |
| Storage unopened | Room temperature, dark | Room temperature, dark |
| Storage opened | Discard | 2 to 8°C, discard at 28 days |
If the whole reconstituted volume is used at once, either works. If the solution will be drawn from more than once, bacteriostatic water is the correct choice.
Vial sizes
We stock bacteriostatic water in 3 ml and 10 ml vials. The 3 ml vial is enough for a single reconstitution at the usual 1 to 3 ml volumes. The 10 ml vial suits laboratories running several reconstitutions over a month; because the preservative clock starts at first opening, buy the size you will finish within 28 days rather than the largest.
Orders of five or more research peptide vials include a free 10 ml vial automatically, and both sizes are covered by buy one, get one free.
Storage
Unopened: room temperature, away from light, in the box. Opened: refrigerate at 2 to 8°C, mark the date of first opening on the label, and discard after 28 days. Do not freeze; freezing can crack the vial and does nothing useful for a preserved solution.
Reading the label
A bacteriostatic water label should state “0.9% benzyl alcohol” and “multi-use”. A sterile water label will say “single-use” or “single use” and “no preservative”. If a vial sold as bacteriostatic water does not list benzyl alcohol, it is not bacteriostatic water.
