Bacteriostatic water vs sterile water

Three kinds of water sit in most laboratory supply cupboards:

• Purified or sterile water — water with impurities removed, and, if sterile, treated so that it contains no viable microorganisms at the time of sealing.
• Water for injection (WFI) — a pharmacopoeial grade of sterile, pyrogen-free water, defined by a standard that specifies its purity and how it is produced and tested.
• Bacteriostatic water — sterile water to which a preservative has been added, conventionally benzyl alcohol at 0.9%. The preservative inhibits the growth of bacteria in the water after the container has been opened.

Sterile water and WFI contain no preservative. Bacteriostatic water does. That is the whole distinction.

Bacteriostatic is not the same as sterile. Sterile describes the state of the water when it is sealed: no viable organisms present. Bacteriostatic describes a property the water keeps after it is opened: it inhibits bacterial growth. A sealed container of sterile water is sterile; once it is opened and the seal is gone, it is water, and whatever enters it can grow. A preservative is there to stop that growth. It does not sterilise; it holds the line.

Any laboratory that opens a container of water more than once has a choice: discard the remainder after each use, or use a grade that resists contamination between uses. Single-use sterile water is discarded after opening. Bacteriostatic water is supplied in multi-use containers precisely because the preservative allows the container to be opened more than once. Which grade a laboratory uses for a given task is a decision its own procedures make, on its own criteria.

A preservative is an ingredient, and any ingredient can be incompatible with something. Benzyl alcohol is a small organic molecule with its own chemistry, and laboratory procedures that specify water without additives specify it for a reason. Read what a procedure asks for and supply that. The presence of a preservative is the one fact about bacteriostatic water that every procedure should be checked against.

Like any laboratory supply, water is stored as its label states and its container is dated when opened. A multi-use container carries an open date and a discard-by date under the laboratory's own procedure. Record it in the inventory the way any consumable is recorded — see a lab notebook and chain of custody for reference materials. And a multi-use container is handled with the same aseptic care as anything else that is opened more than once.

The container label answers every question in this post. It states the grade — sterile water, water for injection, or bacteriostatic water — and, for bacteriostatic water, names the preservative and its concentration. It states the container type, single-use or multi-use, and the storage condition. It carries a lot number and an expiry date, as any laboratory supply should. Read it on arrival the way a vial of reference material is read: grade matches what was ordered, seal intact, lot number recorded, stored as stated. The receiving routine in inspecting a vial on arrival applies to supplies as much as to compounds, and a laboratory supply that arrives damaged is handled under the same returns policy.

This is a description of what bacteriostatic water is and how it differs from other grades of laboratory water. It is not a preparation, dilution or usage instruction for it or for any material sold on this site. We publish none, and we cannot advise on them. Everything sold here is for in-vitro laboratory research only — see research use only. Wholesale quantities of laboratory supplies are available through a wholesale account.