Bacteriostatic water has a narrow, well-defined job: to supply small, repeated volumes of sterile, endotoxin-controlled water from one sealed container over a period of weeks. Every appropriate use follows from that description, and so does every inappropriate one. This guide lists both.
The properties that define its uses
- Sterile in the sealed vial, and endotoxin-controlled below 0.5 EU/mL.
- Preserved with 0.9% benzyl alcohol, so the vial can be entered repeatedly for up to 28 days.
- Solute-free apart from the preservative: no salts, no buffer, hypotonic.
- Small volumes: 3, 10 or 30 mL per vial.
Appropriate laboratory uses
Dissolving lyophilized research materials
Freeze-dried research reagents and reference materials that are supplied as powders in vials are commonly brought into solution with a small volume of sterile water. When the same material is prepared repeatedly over weeks, a multi-use vial of bacteriostatic water replaces a stream of single-use sterile water vials, each opened for a fraction of its volume. The material's own documentation determines whether a preserved solvent is acceptable; many specify it, some exclude it.
Preparing dilution series
Serial dilutions for non-cellular assays, calibration curves and working solutions need sterile water in small aliquots on demand. A dated, aseptically handled vial on the bench serves that need for a month.
Working stock of sterile water
Any workflow that draws a few hundred microlitres to a few millilitres of sterile water several times a week, and where the preservative is harmless to the downstream use, is a candidate. The vial size should match the 28-day consumption; see choosing a vial size.
Diluting preserved formulations
Where a formulation already contains benzyl alcohol or a similar preservative, diluting it with bacteriostatic water keeps the preservative system intact rather than diluting it away. One caution: formulations containing nonionic surfactants reduce the effectiveness of benzyl alcohol, so the preserved status of the diluted product should not be assumed.
Where it does not belong
Cell culture
Benzyl alcohol at 0.9% is cytotoxic to cultured cells. Media, supplements, washes and any reagent that will contact living cells must be prepared with preservative-free, cell-culture-grade water. This is the most important exclusion; see can bacteriostatic water be used for cell culture.
UV-detected analytical methods
Benzyl alcohol absorbs in the ultraviolet with a maximum near 257 nm and appears as a peak in HPLC-UV chromatograms and as background in UV spectrophotometry. Blanks, mobile phases, diluents and standards for these methods use preservative-free water. Methods with mass-spectrometric, fluorescence or electrochemical detection may tolerate it, but the method validation must say so.
Anything that must be preservative-free or isotonic
Protocols that specify preservative-free water, saline, PBS or a buffered vehicle mean what they say. Bacteriostatic water is neither isotonic nor buffered nor preservative-free, and substituting it changes the system. The comparisons are in vs. sterile water and vs. bacteriostatic saline.
Large-volume preparations
Not an exclusion, but a mismatch. A protocol that consumes 100 mL at once is better served by a single-use bottle of sterile water; bacteriostatic water's advantage is repeated small withdrawals, not volume.
Handling rules that apply to every use
- Date the vial at first puncture; discard at 28 days.
- New sterile needle and syringe for every withdrawal; wipe the stopper first.
- Store at room temperature in the carton; never freeze.
- Record the lot number in the preparation record.
Details: storage and handling, lot traceability, what is bacteriostatic water.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.