What each one is
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Contents | Water plus 0.9% benzyl alcohol | Water only |
| Standard | USP monograph, "Bacteriostatic Water for Injection" | USP monograph, "Sterile Water for Injection" |
| Container | Multi-dose vial with a rubber stopper | Single-use vial or ampoule |
| After first puncture | 28 days by convention | Use once and discard |
| Sterility | Sterile when sealed; the preservative slows growth of organisms introduced afterwards | Sterile when sealed; nothing slows growth once opened |
| Tonicity | Hypotonic | Hypotonic |
What benzyl alcohol does and does not do
Benzyl alcohol at 0.9% is a bacteriostatic agent: it stops bacteria that get into the vial from multiplying. It does not sterilise a solution that is already contaminated, and it does nothing to slow the chemical degradation of a peptide. A reconstituted vial in bacteriostatic water still loses purity on the schedule its own chemistry sets; what the preservative buys is the ability to draw from the vial more than once without each puncture becoming a culture. The storage guide covers the chemistry side.
The pharmacopoeial label on bacteriostatic water carries a warning against use in newborns because of the benzyl alcohol. That is a property of the product worth knowing even in a laboratory context.
Where the 28 days comes from
USP General Chapter 797 assigns a beyond-use date of 28 days to an opened multi-dose container unless the manufacturer states otherwise. It is a limit on how long the preservative can be relied on after the seal is broken, not a statement about what is dissolved in the water. A peptide that is unstable in solution is unstable in bacteriostatic water too.
When sterile water is the better choice
- When the whole vial will be used at once, so preservation buys nothing.
- When a downstream assay is sensitive to benzyl alcohol, which absorbs in the ultraviolet and can interfere with some spectrophotometric measurements.
- When working with a protein known to be sensitive to benzyl alcohol. For a few therapeutic proteins the preservative has been reported to promote aggregation; for short peptides at research concentrations this is rarely reported, but it is a reason to prefer plain water when in doubt.
When bacteriostatic water is the better choice
- When a reconstituted vial will be sampled repeatedly over days or weeks.
- When the volume needed per sample is small relative to the vial, so a single-use diluent would waste most of the material.
Practical notes
- Store both at room temperature, boxed, out of light. Do not freeze bacteriostatic water; freezing can crack the vial and does nothing useful.
- Write the date of first puncture on the bacteriostatic vial. The 28 days runs from then, not from opening the box.
- Neither water is a buffer. A peptide solution in plain water sits at whatever pH the peptide and its counter-ion give it, usually mildly acidic for a TFA salt. Where pH matters to an assay, use a buffer, not water.
- Concentration arithmetic is on the reconstitution calculator. Why the vial is a dry cake in the first place is on why peptides are freeze-dried.
In our catalog
Bacteriostatic water in 3 ml and 10 ml vials. We do not currently stock sterile water for injection.