Deionized (DI) water is the workhorse water of most laboratories, produced on tap by ion-exchange cartridges or a mixed-bed polisher. It is very pure in one specific sense, ionic content, and that is exactly the sense in which it is easy to mistake it for a sterile product. Bacteriostatic water is purified in a different way, to a different standard, and packaged for a different purpose.
What deionization removes
Ion exchange resins swap dissolved cations and anions for hydrogen and hydroxide ions, which combine to form water. The result has very low conductivity, often below 1 µS/cm from a good system, and is free of the calcium, magnesium, sodium, chloride and sulfate that make tap water "hard". That is all deionization does. Uncharged organic molecules pass through, and so do bacteria, which in fact colonize resin beds and tubing readily. DI water dispensed from a wall unit routinely carries measurable bacterial counts and, with them, bacterial endotoxins.
What bacteriostatic water adds
Bacteriostatic water is prepared from WFI-grade water, which is purified to tight conductivity and total organic carbon limits and, critically, to an endotoxin limit. It is then sterilized, preserved with 0.9% benzyl alcohol and sealed in vials no larger than 30 mL. Every lot is tested for sterility, endotoxins (below 0.5 EU/mL), pH, preservative content and particulates. The vial is a controlled, documented, sealed environment; a DI tap is not.
Comparison
| Property | Deionized water | Bacteriostatic water |
|---|---|---|
| Ionic purity | Very high | Very high (WFI-grade base) |
| Organic carbon control | Not by DI alone | Yes (WFI-grade limit) |
| Sterile | No | Yes, sealed |
| Endotoxin control | No; often elevated | < 0.5 EU/mL per lot |
| Preservative | None | 0.9% benzyl alcohol |
| Packaging | Dispensed on demand | Sealed 3, 10, 30 mL vials |
| Documentation | System logs, if any | Lot-specific certificate of analysis |
When DI water is the right choice
Rinsing, buffer and media preparation that will be sterilized afterward, instrument feed water, dilutions for non-sterile analytical work, and any application where a subsequent step (autoclaving, sterile filtration) provides the sterility. DI water is cheap, plentiful and correct for the majority of bench chemistry.
When bacteriostatic water is the right choice
When the water must already be sterile and endotoxin-controlled at the point of use, in small volumes, drawn repeatedly from one container over days or weeks, and when the 0.9% benzyl alcohol preservative will not interfere with the work. Dissolving or diluting lyophilized research materials is the typical case. For preservative-sensitive work, use preservative-free sterile water instead; see bacteriostatic water vs. sterile water.
A common mistake
Sterile-filtering DI water through a 0.22 µm filter produces water that is free of bacteria but not of endotoxins. Endotoxins are far smaller than bacteria and pass straight through sterilizing filters. Only distillation, reverse osmosis or ultrafiltration reduce endotoxin, and only lot testing verifies it. That is the gap between "I filtered it" and a WFI-grade specification, and it is why endotoxin-sensitive work uses tested, sealed product. See endotoxin testing explained.
Related: vs. distilled water, USP water grades explained.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.