Bacteriostatic water: use, shelf life and volumes
Bacteriostatic water is sterile water with 0.9 percent benzyl alcohol added, so a vial can be opened more than once. The preservative slows bacterial growth in the water. It does nothing for the peptide, so an opened vial of water and a mixed peptide have two different shelf lives.
What is in the vial
Bacteriostatic water is sterile water for injection to which benzyl alcohol has been added at 0.9 percent, which is 9 mg per mL. It is supplied in multi-use vials, most often 30 mL, with a rubber stopper that closes again after a needle is withdrawn. The benzyl alcohol is a bacteriostatic agent: it inhibits the growth of bacteria that reach the water through repeated punctures. It does not sterilize water that was contaminated, and it is not an antifungal.
Sterile water for injection is the same water without the preservative. It is intended to be used once, because nothing in it stops growth after the first puncture. Bacteriostatic sodium chloride is normal saline with the same 0.9 percent benzyl alcohol, and it is a different liquid with a different salt balance, chosen for different reasons. Dilute acetic acid appears in some laboratory protocols for peptides that will not dissolve at neutral pH, and it is covered in the solubility guide.
Four kinds of water, and how they differ
Bacteriostatic water
Water for injection with 0.9 percent benzyl alcohol. Multi-use. The usual choice for reconstituting a lyophilized peptide that will be drawn from more than once.
Sterile water for injection
The same water without preservative. Single use. Chosen when a protocol excludes benzyl alcohol or when the whole vial is used at once.
Bacteriostatic saline
0.9 percent sodium chloride with 0.9 percent benzyl alcohol. Isotonic, which matters for some assay conditions and not for dissolving a powder.
Dilute acetic acid
Used at 0.1 percent or thereabouts for peptides that are poorly soluble at neutral pH. Not a preservative, and not interchangeable with the others.
The numbers on the label
- Preservative
- Benzyl alcohol, 0.9 percent w/v (9 mg/mL)
- After first puncture
- Discard 28 days after the vial is first entered. This is the convention for multi-use vials with a preservative and it is about the preservative and the stopper, not about the water spoiling.
- Unopened
- To the expiry date printed on the vial, stored at controlled room temperature, 20 to 25 C.
- Storage
- Room temperature. Refrigeration is not required and freezing is to be avoided, since a frozen vial can crack and a thawed one has been through a temperature cycle for no benefit.
- Vial size
- 30 mL is the common size. 10 mL vials exist. Either is entered with a fresh sterile needle each time.
- Appearance
- Clear and colorless. A vial that is cloudy, has particles, or smells of anything other than a faint sweet note from the benzyl alcohol is discarded.
These describe the water, not the peptide dissolved in it. The stability of the peptide in solution is a separate question, covered in the stability guide.
Why 28 days, and what the number is not
The 28-day figure comes from the practice for multi-use vials that carry a preservative: the preservative has a demonstrated window in which it holds bacterial counts down under repeated entry, and the stopper has a limited number of punctures before it stops resealing cleanly. After that window the vial is discarded whether or not it looks fine, because the failure it guards against is invisible.
It is not the shelf life of a reconstituted peptide. A peptide in solution degrades by hydrolysis, deamidation and oxidation on its own schedule, and that schedule depends on the sequence, the concentration, the temperature and the light it sees. Most reconstituted material is kept cold and dark and used within days to a few weeks, which is shorter than 28 days for some sequences and longer for others. The stability guide covers how to think about it. The water's clock and the peptide's clock run separately. The shorter one wins.
Using it with a vial of freeze-dried peptide
- 01
Bring both vials to room temperature
A cold peptide vial opened straight from the refrigerator pulls condensation onto the powder. Twenty minutes on the bench.
- 02
Swab both stoppers
Alcohol swab, allowed to dry. The bacteriostatic vial is entered many times; the peptide vial only once or twice.
- 03
Draw the volume you decided on
From the calculator or the arithmetic below. The volume sets the concentration; there is no correct amount, only a convenient one.
- 04
Run it down the inside wall
Slowly, against the glass, never onto the cake. Squirting water into the powder is where clumping starts.
- 05
Let it dissolve on its own
Tilt and roll if needed. Do not shake. A few minutes is normal; longer, and the solubility guide is the next stop.
- 06
Label the vial
Date reconstituted, concentration, and the compound name, which is what ties the solution back to its certificate.
How much to draw
- 5 mg vial, 1 mL
- 5 mg/mL
- 5 mg vial, 2 mL
- 2.5 mg/mL
- 10 mg vial, 1 mL
- 10 mg/mL
- 10 mg vial, 2 mL
- 5 mg/mL
- 10 mg vial, 3 mL
- 3.33 mg/mL
- Vial capacity
- A 3 mL glass vial takes 2 mL comfortably and 3 mL only with care. More liquid than the vial holds is not an option; a lower concentration is reached by diluting a drawn aliquot instead.
Worked from peptide mass. Where a certificate reports net peptide content, correct the label mass first. The reconstitution calculator does this for you.
What bacteriostatic water should come with
The same thing as any other item: a certificate from a named lab. For water, the results that matter are sterility and endotoxin, since contaminated water contaminates every vial it goes into, plus the benzyl alcohol content, which is what the label promises. A certificate that shows only an appearance check is not enough for water you will mix everything else with.
Water with no certificate behind it cannot be traced to anything and should not be used to reconstitute material that can.
Questions
Common questions
Can sterile water be used instead of bacteriostatic water?
Yes, if the vial will be used once. Sterile water for injection has no preservative, so a reconstituted vial that will be entered again over days should be made up with bacteriostatic water. Where a protocol excludes benzyl alcohol, sterile water and single use is the answer.
How long does bacteriostatic water last after opening?
Twenty-eight days after the first puncture, by the convention for preserved multi-use vials. Write the date on the vial when it is first entered. Unopened, it lasts to the printed expiry at room temperature.
Does bacteriostatic water need to be refrigerated?
No. It is stored at controlled room temperature, 20 to 25 C. Refrigeration does not extend the 28 days and freezing risks cracking the vial.
Why is the benzyl alcohol concentration 0.9 percent?
It is the concentration at which benzyl alcohol has a demonstrated bacteriostatic effect in water under repeated entry while remaining compatible with most materials dissolved in it. It is a preservative amount, not a solvent amount.
How much bacteriostatic water goes into a 10 mg vial?
Whatever volume gives a convenient concentration and fits the vial. One mL gives 10 mg/mL, two mL gives 5 mg/mL. The peptide mass may be lower than the label mass where the certificate reports net peptide content, and the reconstitution calculator corrects for that.
Is the reconstituted peptide good for 28 days too?
Not necessarily. The 28 days is the liquid's window. The peptide has its own stability in solution, which depends on the sequence, the concentration and the temperature, and is often shorter. Keep reconstituted material cold and dark and use the stability guide to judge it.
In the catalog
Compounds this applies to
Cite as: SBz Industries research library, "Bacteriostatic water: use, shelf life and volumes", https://www.sbzindustries.com/research/bacteriostatic-water, updated 2026-09-18.
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