Two bottles of "pure water" on a molecular biology bench can have entirely different meanings of pure. Nuclease-free water is pure in the sense that matters to nucleic acids; bacteriostatic water is pure in the sense that matters to a sealed, multi-use sterile vehicle. Neither claim covers the other.
What nuclease-free water claims
Nuclease-free (also molecular-biology-grade) water is tested and certified to be free of detectable RNase and DNase activity, the enzymes that degrade RNA and DNA. It is usually also filtered to 0.1 or 0.2 µm, autoclaved or otherwise sterilized, and often stated to be free of DNA, protease and, for some grades, endotoxin. It contains no preservative, because any additive would be a contaminant in nucleic acid work, and it is packaged in single-use or short-life bottles and tubes. Some suppliers treat with DEPC (diethyl pyrocarbonate) to inactivate RNases and then autoclave to remove the DEPC; others rely on ultrapure filtration and certified nuclease-free handling.
What bacteriostatic water claims
Sterility, bacterial endotoxins below 0.5 EU/mL, pH 4.5 to 7.0, particulate limits, and verified 0.9% benzyl alcohol content, in a sealed multi-use vial. Nothing about nucleases. A lot of bacteriostatic water may well be low in nuclease activity, since it is made from WFI-grade water under controlled conditions, but it is not tested for it and does not claim it. See what is bacteriostatic water.
Why bacteriostatic water is wrong for molecular work
- No nuclease certification. The one claim molecular work needs is absent.
- Preservative present. Benzyl alcohol is an organic additive that can affect enzymes, interfere with UV quantification of nucleic acids at 260/280 nm (it absorbs strongly near 257 nm), and is not validated in any molecular protocol.
- Multi-use handling. Repeated entries into one vial over weeks are contrary to the single-use discipline that keeps nuclease contamination out.
See analytical interference for the UV issue.
Why nuclease-free water is wrong for the bacteriostatic water job
- No preservative. An opened bottle is not protected against microbial growth; it is used and discarded or refrigerated briefly.
- Often no endotoxin claim. Many molecular grades are not endotoxin-tested, because nucleic acid work does not require it.
- Not a multi-use container. Screw-cap bottles and tubes are opened to the air each time.
Side by side
| Claim | Nuclease-free water | Bacteriostatic water |
|---|---|---|
| RNase / DNase-free | Yes, certified | Not tested |
| Sterile | Usually | Yes, tested per lot |
| Endotoxin limit | Some grades | < 0.5 EU/mL, tested |
| Preservative | None | 0.9% benzyl alcohol |
| Multi-use | No | Yes, 28 days |
| Container | Bottles, tubes | Sealed vials ≤ 30 mL |
Practical rule
Nucleic acids: nuclease-free water, single use, never bacteriostatic water. Sterile, preserved, multi-use solvent for materials that are not nucleic acids and will not touch cells or UV analysis: bacteriostatic water. Keep them apart and labeled. The broader map of water grades is in USP water grades explained, and the other comparisons are linked from the knowledge base.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.