Bacteriostatic water: what it is, what it is for in a laboratory, and how it is kept

Written 24 September 2026 by the trade desk. About 4 minutes to read.

The diluent most lyophilised lines are made up with, and the one line on the list that is stocked by the hundred. What it is, why it is preserved, and how it is kept before and after the first puncture.

What it is

Bacteriostatic water is low-endotoxin water of pharmacopoeial grade that has been filtered, sterilised and filled into a glass vial with 0.9 percent weight in volume of benzyl alcohol dissolved in it. Benzyl alcohol is a small aromatic alcohol, soluble in water to about 4 percent, so at 0.9 percent it is fully dissolved, and its job in the vial is to hold back the growth of microorganisms. The water is otherwise plain: no salt, no buffer, no other additive. We list it as BAC3 (3 mL) and BAC10 (10 mL), ten vials to a box.

Why a preserved water for a vial entered more than once

Every time a stopper is pierced there is a chance of carrying organisms into the vial, and water with no preservative has nothing to stop them growing between one entry and the next. Benzyl alcohol at 0.9 percent does. That is the whole reason for choosing it: not the first entry but the second and later ones. A vial that will be opened once, emptied and discarded gains nothing from the preservative, which is why sterile water without one exists alongside it.

Unopened storage

Sealed vials are kept at room temperature in their carton. The storage line on the certificate and the product page reads 15 to 25 degrees Celsius; the USP term for a US-facing article is controlled room temperature, 20 to 25 degrees Celsius with excursions between 15 and 30 permitted, and the stated range sits within it. They do not need a refrigerator, and they must never be frozen: ice expanding inside a filled glass vial can crack it, and the label says do not freeze. Each vial carries a batch number and a retest date, and the certificate for the batch is found on the certificate page like any other line.

After the first puncture

The convention is 28 days. Write the date of first entry on the label, and discard whatever is left 28 days after that date regardless of the volume remaining. The preservative is still present on day 29, but the convention bounds the number of entries and the time the stopper has been in use, and it is the figure most written laboratory procedures adopt unless the label states something shorter. Between entries the opened vial is kept as the label directs: upright, in its carton, at room temperature. A cloudy vial, or one with anything visible in it, is discarded whatever the date.

What the certificate carries

The certificate for a batch of bacteriostatic water reads differently from a peptide certificate: there is no purity line, and identity is conformance to the article specification rather than a measured mass. The lines are the six on the product page's release table, and the identity line is where the tests on the water itself sit.

Batch number, date of manufacture, date of analysis and retest date sit at the top as on any certificate, and the batch number on the vial is what is looked up.

When preservative-free sterile water is used instead

Benzyl alcohol is not inert to every method. It absorbs in the ultraviolet, so a sample made up in bacteriostatic water and run on HPLC with detection at 220 nm shows a solvent peak that has to be kept apart from the peptide peaks. It has a mass of 108 and can appear in a mass spectrum. Some written protocols simply specify a preservative-free diluent. In those cases sterile water without preservative is used, the vial is opened once, and the remainder is discarded at the end of the session rather than kept. Preservative-free sterile water is not on our list at present, so it is sourced separately.

Handling in short

• Room temperature unopened, never frozen.
• Date the label at first puncture; 28 days from then.
• Upright, in its carton, between entries.
• One batch number per vial, checked on the certificate page.
• Preservative-free water when the method says so, and then a single entry.

Applies to

This guide is about Bacteriostatic Water and no other line; the full list is on the compounds page.

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