The United States Pharmacopeia (USP) defines a family of water grades, each with its own monograph and specification. They build on one another: each grade adds requirements to the one before. Knowing the ladder makes it obvious why bacteriostatic water is packaged, tested and priced the way it is.
Purified Water
The base grade for most pharmaceutical and laboratory use. Produced from drinking water by distillation, deionization, reverse osmosis or a combination, it must meet limits on conductivity (a three-stage test; the first-stage limit is 1.3 µS/cm at 25 °C) and total organic carbon (0.50 mg/L). It has no sterility requirement and no endotoxin requirement. It is stored and distributed in bulk systems.
WFI grade
Purified Water with one more requirement: bacterial endotoxins not more than 0.25 EU/mL. Because endotoxins are the heat-stable remnants of Gram-negative bacteria, meeting this limit means controlling microbial growth throughout the production and distribution system, which is why WFI-grade water is traditionally made by distillation or by membrane processes validated to remove endotoxin, and is held hot or circulated to prevent biofilm. It is the base from which the sealed sterile grades below are made.
Sterile Water
WFI-grade water that has been sterilized and sealed in single-use containers. No preservative. Same endotoxin limit as WFI grade for standard container sizes. Once the container is opened it is used immediately and the remainder discarded, because nothing in it inhibits microbial growth.
Bacteriostatic Water
WFI-grade water, sterilized, with one or more suitable antimicrobial agents added, in practice 0.9% benzyl alcohol, and sealed in multi-use containers of not more than 30 mL. Its own monograph requirements include sterility, an endotoxin limit of 0.5 EU/mL, pH 4.5 to 7.0, verification of the labeled preservative content, and particulate matter limits. The label must state the antimicrobial agent and its proportion. The preservative is what allows repeated withdrawals over a limited period, conventionally 28 days after first puncture.
The ladder at a glance
| Grade | Conductivity / TOC | Endotoxin | Sterile | Preservative | Container |
|---|---|---|---|---|---|
| Purified Water | Yes | No limit | No | No | Bulk |
| WFI grade | Yes | ≤ 0.25 EU/mL | No (bulk) | No | Bulk, hot or circulated |
| Sterile Water | Yes | ≤ 0.25 EU/mL | Yes | No | Single-use |
| Bacteriostatic Water | Yes | ≤ 0.5 EU/mL | Yes | 0.9% benzyl alcohol | Multi-use, ≤ 30 mL |
Grades that are not on the ladder
Distilled, deionized and reverse-osmosis water are processes, not pharmacopeial grades; they may or may not meet Purified Water limits and carry no sterility or endotoxin claim. Cell-culture-grade and molecular-biology-grade waters are vendor specifications, typically endotoxin-tested and nuclease-free but preservative-free and intended for single use. Bacteriostatic saline is a separate monograph (0.9% sodium chloride with 0.9% benzyl alcohol). See vs. distilled, vs. deionized and vs. bacteriostatic saline.
Why this matters when buying
A supplier who describes bacteriostatic water as "distilled water with benzyl alcohol" is describing a process, not the grade. The grade is defined by the tests on the certificate of analysis: sterility, endotoxin below 0.5 EU/mL, pH, preservative assay and particulates, lot by lot. Ask for the COA and read it against the table above; how to read a COA walks through each line.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.