What it is
Bacteriostatic water is sterile water for injection containing 0.9% benzyl alcohol as a bacteriostatic preservative. Its purpose in peptide work is straightforward: it is the diluent that turns a lyophilised cake into a solution, and the preservative allows a vial to be entered more than once without the contents becoming a culture medium.
Not an assayed peptide
This page states the obvious because the market frequently does not.
Bacteriostatic water has no purity specification, no HPLC result and no peptide Certificate of Analysis, because there is no peptide in it to assay. Listings that advertise "99% purity" bacteriostatic water are either copying a template from a peptide product page or inventing a number outright, and it is a useful tell about the rest of a vendor's documentation.
What it does have — and what a supplier should be able to speak to — are sterility and endotoxin specifications. Those are the meaningful quality attributes for a sterile solvent, and they are established by entirely different methods from the ones used on peptides. See endotoxin and sterility testing.
Bacteriostatic versus sterile water
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | None |
| Vial entries | Multiple | Single use |
| Suits | Solutions stored over days or weeks | Single-session preparation |
| Consideration | Benzyl alcohol is an additional variable in the system | No preservative to interact with the peptide |
The choice is not automatic. Where a reconstituted peptide will be drawn on repeatedly over a period, the preservative is doing necessary work. Where a working solution is prepared and consumed in one session — or where the peptide is aggregation-prone and you want the fewest possible variables — a preservative-free diluent is the cleaner option.
Handling and storage
Store at controlled room temperature, protected from light. Do not freeze. Unlike lyophilised peptides, there is no benefit to cold storage here and freezing risks the container.
Inspect before use: the solution should be clear and colourless, with no particulates and no discolouration. Discard any vial that fails that inspection regardless of its date. Wipe the stopper before each entry — the preservative inhibits growth, it does not sterilise what you introduce.
Once a vial is in use, note the date of first entry. Benzyl alcohol limits microbial growth for a period; it does not do so indefinitely.
Using it
The arithmetic of reconstitution is mass divided by volume, and the reconstitution calculator works it in both directions. The technique matters as much as the arithmetic: run the diluent down the inside wall of the vial rather than directly onto the lyophilised cake, and swirl rather than shake. See peptide aggregation and handling for why agitation is the specific thing to avoid.
What we supply
Bacteriostatic water is supplied as a sterile solution with sterility and endotoxin specifications, not as an assayed peptide. It carries no purity figure and no peptide Certificate of Analysis, and we will not print one.