Sterile water and bacteriostatic water are both sterile, both nonpyrogenic and both made from WFI-grade water. The only difference is 0.9% benzyl alcohol, and the choice between them comes down to whether that preservative helps or hurts in a particular use. Five questions settle it.
1. Will the container be entered more than once?
If the whole container will be used at once, a preservative adds nothing, and preservative-free sterile water in a single-use container is the simpler choice. If withdrawals will be spread over days or weeks, the preservative is what makes that safe to do, and bacteriostatic water is the right product, discarded 28 days after first opening. See multi-use vs. single-use vials.
2. Is benzyl alcohol compatible with the application?
Benzyl alcohol is not inert in every setting. It is not used in cell-culture work, it interferes with some enzymatic and microbiological assays, and it partitions into organic phases. If the method or the material’s data sheet specifies preservative-free water, that settles it. See bacteriostatic water and cell culture and when the preservative interferes.
3. Will anything be measured below 290 nm?
Benzyl alcohol is opaque between about 240 and 275 nm and still absorbs at 280 nm. For UV spectrophotometry in that region, use preservative-free water for both sample and blank. See bacteriostatic water as a UV-Vis blank.
4. How much is needed per use?
Bacteriostatic water is only supplied in vials of 30 mL or less. A laboratory that needs hundreds of millilitres at a time is better served by sterile water in larger containers; a laboratory that needs a few millilitres at a time, repeatedly, is better served by bacteriostatic water in the smallest vial it will finish within 28 days. See why bacteriostatic water is only sold in 30 mL or smaller vials and how labs choose a vial size.
5. What does the documentation require?
Both products come with a certificate of analysis. The difference is that bacteriostatic water carries one more test (the benzyl alcohol assay) and one more record for the laboratory to keep (the date each vial was opened). A quality system that already tracks opened-container dates absorbs this easily; one that does not may prefer single-use sterile water for that reason alone. See how to read a COA.
Summary
| Situation | Choice |
|---|---|
| Small volumes withdrawn repeatedly over days or weeks; preservative tolerated | Bacteriostatic water |
| Entire container used at once | Sterile water (single-use) |
| Cell culture, preservative-sensitive assays, UV work below 290 nm | Sterile water or buffer |
| Large volumes per use | Sterile water in larger containers |
| Preserved solution wanted but benzyl alcohol specifically excluded | Neither; a diluent with the specified preservative system |
Related
Bacteriostatic water vs. sterile water · Laboratory uses of bacteriostatic water · Is bacteriostatic water the same as sterile water?
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.