Bacteriostatic water has exactly two ingredients: WFI-grade water and benzyl alcohol at 0.9% w/v (9 mg per mL). There is no sodium chloride, no buffer, no dextrose, no chelating agent and no second preservative. The simplicity is the point: the product is a preserved diluent, and everything about its behavior follows from those two components.
Ingredient 1: WFI-grade water
The water is produced by distillation or reverse osmosis with further purification to the pharmacopeial standard for water used in sterile products: conductivity at or below 1.3 µS/cm at 25 °C, total organic carbon at or below 0.5 mg/L and bacterial endotoxins at or below 0.25 EU/mL at the point of manufacture. It contributes nothing except water. See USP water grades explained and how bacteriostatic water is manufactured.
Ingredient 2: benzyl alcohol, 0.9%
Benzyl alcohol (phenylmethanol, CAS 100-51-6) is the preservative. At 0.9% it inhibits the growth of bacteria introduced when a vial is entered, which is what allows a multi-use vial to be used over a limited period. It also accounts for the faint sweet odor, the ultraviolet absorbance and the slightly acidic pH of the product. See what 0.9% benzyl alcohol means in mg/mL, ppm and molarity and benzyl alcohol properties.
What is deliberately absent
| Not present | Consequence |
|---|---|
| Sodium chloride or other salts | Near-zero conductivity and osmolality; not interchangeable with bacteriostatic saline. See does it contain salt? |
| Buffer | The pH (4.5 to 7.0) is set by dissolved carbon dioxide and moves when anything is dissolved in it. See the pH explained. |
| Antioxidant or chelator | Benzyl alcohol oxidizes slowly in an opened vial, which is one reason for the 28-day in-use convention. See benzyl alcohol oxidation. |
| Second preservative | Activity against yeasts and molds is weaker than against bacteria. See benzyl alcohol vs. other preservatives. |
How the composition is stated
The label lists the preservative and its proportion, as the USP monograph requires. The certificate of analysis reports the measured benzyl alcohol content for the lot alongside sterility, endotoxin, pH and particulate results. See how to read a COA.
Related
What is BAC water? · Physical properties of bacteriostatic water
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.