Bacteriostatic water and sterile water look identical in the vial, and both are sterile, endotoxin-controlled aqueous vehicles. The difference is a single ingredient, 0.9% benzyl alcohol, and that one ingredient changes how each product is packaged, stored, handled and used. This guide sets the two side by side, and adds the other water grades a lab is likely to have on the shelf.
The one-line difference
Sterile water is purified water that has been sterilized and sealed with no preservative. Bacteriostatic water is the same base water with 0.9% benzyl alcohol added as a bacteriostatic preservative. Everything else follows from that.
Side-by-side comparison
| Property | Sterile water | Bacteriostatic water |
|---|---|---|
| Preservative | None | Benzyl alcohol, 0.9% (9 mg/mL) |
| Sterile when sealed | Yes | Yes |
| Endotoxin limit | Yes (pharmacopeial limit) | Yes (< 0.5 EU/mL) |
| Container | single-use vials and larger bottles | multi-use vials, 30 mL maximum |
| After first opening | Use once, discard the remainder | Multiple withdrawals; common convention is discard after 28 days |
| Odor | None | Faint aromatic odor from benzyl alcohol |
| UV absorbance | None | Absorbs near 257 nm |
| Suitable for cell culture | Yes, if cell-culture grade | No (benzyl alcohol is cytotoxic to cultured cells) |
| Tonicity | Hypotonic (no solutes) | Hypotonic (no solutes other than the preservative) |
Why the preservative changes the packaging
A sealed vial of sterile water is sterile until the stopper is punctured. From that moment, every entry is an opportunity for organisms to be introduced, and nothing in plain water inhibits their growth. That is why sterile water is packaged for single use: open it, use it, discard what is left.
Bacteriostatic water is packaged in multi-use vials precisely because benzyl alcohol suppresses the growth of small numbers of organisms introduced during repeated entries. The pharmacopeial standard caps the container at 30 mL, which is why 3 mL, 10 mL and 30 mL vials are the only sizes you will find. The preservative extends the usable life of an opened vial; it does not make careless handling safe.
When a lab reaches for sterile water
- Preservative-sensitive work. Cell culture media and supplements, and any assay in which benzyl alcohol could interfere with the biology or the chemistry.
- UV and chromatographic methods. Benzyl alcohol produces a UV peak. Blanks, mobile phases and standards for HPLC-UV or spectrophotometric methods should be preservative-free.
- Large single preparations. If a protocol consumes 100 mL in one go, a single-use bottle of sterile water is cheaper and simpler than four 30 mL multi-use vials.
When a lab reaches for bacteriostatic water
- Small, repeated withdrawals. A bench that needs 0.5 to 2 mL of sterile water several times a week over a month is the textbook case. One sealed vial, entered aseptically each time, replaces a drawer full of opened-and-discarded single-use vials.
- Dissolving or diluting lyophilized research materials where the protocol calls for a preserved aqueous vehicle and does not involve cell-based or UV-based readouts.
- Reducing waste. Where sterile single-use vials are being opened for a fraction of their volume, switching to the right size of bacteriostatic vial usually cuts both cost and waste.
The other waters on the shelf
Several other grades get confused with the two above:
- Distilled water has been boiled and condensed to remove dissolved minerals. It is not sterile, not endotoxin-tested and not preserved. Fine for rinsing glassware; not a substitute for a sterile vehicle.
- Deionized (DI) water has had ions removed by exchange resins. Like distilled water it is purified, not sterile.
- Purified water is a pharmacopeial grade with limits on conductivity and total organic carbon, but without the endotoxin requirement of WFI-grade.
- WFI grade is the highly purified, endotoxin-controlled base from which both sterile water and bacteriostatic water are made.
- Bacteriostatic saline is 0.9% sodium chloride with 0.9% benzyl alcohol. It is isotonic, whereas bacteriostatic water contains no salt. See bacteriostatic water vs. bacteriostatic saline.
Can one substitute for the other?
Only in one direction, and only sometimes. Sterile water can replace bacteriostatic water in almost any protocol if the vial is treated as single-use, because it contains nothing the bacteriostatic product lacks apart from the preservative. Bacteriostatic water cannot replace sterile water where benzyl alcohol would interfere: cell culture, UV-based analysis, or any method validated with preservative-free water. When a protocol specifies one, use that one.
Storage and handling in brief
Both products are stored at controlled room temperature, 20 to 25 °C, protected from freezing and kept in the carton away from light. The difference is what happens after the seal is broken: sterile water is discarded, bacteriostatic water is dated and used for up to 28 days. Full details are in how to store bacteriostatic water.
BAC Water Supply stocks sealed 3 mL, 10 mL and 30 mL bacteriostatic water vials in packs of 4 to 36. Questions about which grade a workflow needs are covered in our FAQs.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.