Bacteriostatic water for peptides (often called "bac water") is laboratory water containing 0.9% benzyl alcohol (9 mg/mL) as a preservative, used as the solvent to reconstitute lyophilized research peptides into stock solutions that will be sampled more than once in vitro. This guide covers what is in it, why the benzyl alcohol is there, vial sizes, the concentration math for common stocks, storage after the first puncture, and the cases where a different solvent is the better choice.
What is bacteriostatic water for peptides?
Bacteriostatic water is a water-based solvent with a preservative added to limit the growth of contaminating microorganisms in a vial that is opened and sampled repeatedly. "Bacteriostatic" means it inhibits growth rather than killing organisms outright. In a research setting it is the usual first-choice solvent for lyophilized peptides: the freeze-dried cake dissolves into a solution of known concentration (mg/mL or molarity), and the same vial can be sampled across a series of assay set-ups. Searches such as "bac water for peptides," "bacteriostatic water peptides" and "bacteriostatic water for peptide reconstitution" all describe this same laboratory use.
It is a solvent, not an active reagent: it adds no biological activity of its own and is only one half of reconstitution. For the procedure itself, see how to reconstitute lyophilized peptides.
What is in bacteriostatic water, and why is benzyl alcohol there?
The composition is two things: water and 0.9% benzyl alcohol (CAS 100-51-6). Since 0.9% w/v is 9 mg per mL, that is roughly 83 mM benzyl alcohol (MW 108.14 g/mol). Benzyl alcohol is a mild aromatic alcohol that partitions into microbial membranes and slows growth. It is the reason a multi-withdrawal vial resists contamination between uses: every pierce of the rubber closure is a chance to introduce microbes, and the preservative is a backstop for clean technique, never a substitute for it.
What it does not do is equally important. It does not sterilize a contaminated solution, remove endotoxin, or protect the peptide from chemical degradation. The chemistry and its limits are covered in benzyl alcohol in bacteriostatic water: what 0.9% does and doesn't do.
How does bac water compare with sterile water and acetic acid?
| Solvent | Preservative | Best fit | Watch for |
|---|---|---|---|
| Bacteriostatic water | 0.9% benzyl alcohol | Most peptides; stock sampled repeatedly | Benzyl alcohol carry-over into sensitive assays |
| Sterile water (no preservative) | None | Single-use preparations; preservative-sensitive assays | No protection once opened |
| 0.6% acetic acid | None (low pH) | Basic peptides that resist neutral water | Lower pH; check assay compatibility |
The full comparison lives in sterile water vs bacteriostatic water for peptides; the acid route is explained in choosing a reconstitution solvent.
What vial sizes does bacteriostatic water come in?
Research-supply bac water is typically listed in 3 mL, 5 mL, 10 mL and 30 mL vials, and Happy Peptides lists its Reconstitution Solution in all four. The right size depends on how many milliliters your lab goes through before an opened vial reaches its discard window: small vials minimize leftover volume, while a large vial lowers the cost per mL only if it is used up in time. The waste and puncture-count arithmetic is in 10 mL vs 30 mL bacteriostatic water.
How much bacteriostatic water do you add for common stock concentrations?
This is concentration math only. Stock concentration (mg/mL) equals the peptide mass in the vial divided by the solvent volume added, so the volume to add is the peptide mass divided by the concentration you want. The small volume of the dissolved cake is ignored at these scales.
| Peptide in vial | Bac water added | Stock concentration |
|---|---|---|
| 5 mg | 1 mL | 5 mg/mL |
| 5 mg | 2 mL | 2.5 mg/mL |
| 10 mg | 1 mL | 10 mg/mL |
| 10 mg | 2 mL | 5 mg/mL |
| 10 mg | 4 mL | 2.5 mg/mL |
| 15 mg | 3 mL | 5 mg/mL |
To express a stock as molarity, divide mg/mL by the molecular weight: for a 4,000 g/mol peptide, 2.5 mg/mL is 0.625 mM. The peptide concentration and reconstitution math guide works through more examples.
How should bacteriostatic water be stored after the first puncture?
- Write the date of first puncture on the vial and apply the discard window set by your SOP or the supplier's label; 28 days is a widely used convention for multi-withdrawal vials.
- Keep the closure clean and dry, wipe it with 70% alcohol and let it dry before each withdrawal, and keep the number of punctures low.
- Keep the solvent vial separate from peptide stock vials and label both clearly.
- Discard any vial that turns cloudy, shows particles or has a damaged closure.
Unopened and opened-vial conditions are laid out in storing bacteriostatic water before and after opening.
When should you not use bacteriostatic water?
- The peptide will not dissolve. Basic or hydrophobic peptides can stay cloudy in neutral water; a dilute acetic acid solution is the usual next step. See why some peptides won't dissolve.
- The assay is sensitive to benzyl alcohol. A 1:1,000 dilution of a bac-water stock still carries about 83 µM benzyl alcohol, which can matter in cell-based and enzymatic work. Preservative-free water or buffer plus a solvent-matched vehicle control is the safer design.
- The readout is optical or chromatographic. Benzyl alcohol absorbs in the UV and shows up in reverse-phase HPLC traces at low wavelengths.
- The preparation is single-use. If a vial will be opened once and discarded, a preservative adds nothing.
Where do you source bacteriostatic water as a research material?
Happy Peptides lists Reconstitution Solution (bacteriostatic water) in 3, 5, 10 and 30 mL sizes as a research material for in vitro use. Review the product page for the specification stated for the material, and use the solvent sourcing guide as a checklist when comparing suppliers.
Related research guides
- Reconstitution-solution buyer's guide
- 0.6% acetic acid: spec and when labs use it
- Bacteriostatic water: a short explainer
- Choosing a solvent volume: 3 mL vs larger
Research-use note
All materials discussed are supplied for in vitro laboratory research use only, and are not for human or animal use.
