Reading a bacteriostatic water certificate

A useful bacteriostatic water certificate answers three questions: is the water sterile, does it contain the stated amount of benzyl alcohol, and is the pH within range? These are the tests that matter for a preserved diluent. Peptide-style purity tests do not apply to it. Alphex bacteriostatic water lot 0011 is reported as negative for growth on a USP <71> sterility test, with 0.88% benzyl alcohol against a 0.72 to 1.08% limit, and pH 6.93 against a 4.0 to 7.0 limit. This note explains what each test means, how to read the limits, and what to look for on any supplier's certificate. All Alphex products are for laboratory and in-vitro research only.

The three tests at a glance

The earlier lot 0010, now marked Superseded in the library, was tested for all three on one certificate. It reported benzyl alcohol at 0.89%, pH 6.35 by AOAC 981.12 and sterility negative, no growth. Both certificates are in the COA library. Type the code on your vial label to open the certificate for your lot.

Sterility: USP <71>

USP General Chapter <71> Sterility Tests is the compendial test for articles that are required to be sterile. Portions of it are harmonised with the European and Japanese pharmacopoeias. Samples are either filtered through a membrane that is then placed in culture media, or added directly to the media. The media are then incubated and checked for microbial growth. "Negative for growth" means nothing grew in the samples tested.

Two points are worth understanding. First, a sterility test samples a small number of units, so it can detect a contaminated batch but cannot prove that every vial is sterile. Sterility of a batch rests mainly on the validated process used to fill and seal it, and the test is a check on that process. Second, sterility says nothing about endotoxin. A sterile solution can still carry endotoxin from organisms that were present before sterilisation. Alphex does not test endotoxin. See endotoxin testing and research peptides.

Benzyl alcohol assay

Benzyl alcohol is the preservative and the only thing that separates bacteriostatic water from plain sterile water. The nominal concentration is 0.9% w/v, which is 9 mg per mL. The limit on the certificate, 0.72 to 1.08%, is 80 to 120% of that nominal value. A result of 0.88% is about 98% of nominal.

Benzyl alcohol absorbs strongly in the UV range, so it is measured by HPLC with UV detection against a calibration standard. Why this matters at the bench:

• Too little preservative weakens protection across repeated punctures. See bacteriostatic water shelf life after first puncture.
• A product sold as bacteriostatic water with no benzyl alcohol is simply sterile water and should be treated as single-use once opened.
• The benzyl alcohol content is what you need to calculate its final concentration in a cell assay after dilution. See benzyl alcohol in bacteriostatic water.

pH

Bacteriostatic water is unbuffered, so its pH is sensitive to dissolved carbon dioxide and to the container. USP <791> is the compendial pH method. AOAC 981.12, used on the earlier lot, is another standard pH method. The certificate limit is 4.0 to 7.0. For reference, the published description of Hospira's Bacteriostatic Water, USP gives a pH of 5.7 with a range of 4.5 to 7.0. Both Alphex lots, at 6.93 and 6.35, sit inside that narrower range as well.

Because the water is unbuffered, it has almost no effect on the final pH of a peptide solution that contains buffer or counter-ions. If a peptide needs a defined pH to dissolve or stay stable, use a buffer. See how to make a buffer for reconstitution.

What else to check on any bacteriostatic water certificate

1. Lot number that matches the vial. Alphex lot 0011 vials read #0011-BAC. See why COA batch number matching matters.
2. A named laboratory and a laboratory ID. Alphex certificates are issued by Vanguard Laboratory, with IDs V260924-15 001 (sterility) and V260831-46 001 (benzyl alcohol and pH) for lot 0011.
3. Stated limits, not just results. A result with no limit cannot be judged.
4. Method references. A compendial method number, or a description of the method, tells you what was actually done.
5. The limitations statement. Results relate only to the portion of the sample tested. See what that means.
6. Accreditation scope. Check whether each test is on the laboratory's accredited scope or reported outside it. See ISO 17025 scope on a COA.

What a purity percentage would mean here

Some listings quote a "purity" figure for bacteriostatic water, as if it were a peptide. That number does not say much on its own. Water is not assayed by area-percent purity, and the meaningful attributes are the ones above: sterility, preservative content and pH. A certificate that shows only a purity figure, with no sterility result and no benzyl alcohol assay, is not telling you what you need to know.

Frequently asked questions

Why are sterility and benzyl alcohol on separate certificates for lot 0011?

The tests were submitted and reported at different times. The sterility result was reported in October 2026 and the benzyl alcohol and pH results in September 2026. Both are published for the same lot.

Does a pass mean the vial stays sterile after opening?

No. The certificate describes the water as supplied. After first puncture, aseptic technique and the 28-day convention take over.

Where do I order it?

Bacteriostatic water, 10 mL multi-use vial, ships from UK stock by Royal Mail Tracked 24.

Sources and further reading

• USP-NF General Chapter <71> Sterility Tests: the compendial sterility test, partly harmonised with the European and Japanese pharmacopoeias
• Hospira. Bacteriostatic Water, USP: product description: 0.9% (9 mg/mL) benzyl alcohol, pH 5.7 (4.5 to 7.0), multiple-withdrawal container
• CDC summary of USP General Chapter 797 on opened multi-use vials: the 28-day convention after first puncture

Alphex bacteriostatic water is supplied as a laboratory reagent for laboratory and in-vitro use only. It is not authorised for human or veterinary use.

Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.

Published 26 October 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.

Alphex Research Team

Alphex Limited, UK research peptide supplier

The Alphex research team writes and maintains these notes for UK laboratories buying research peptides. Every batch Alphex lists is tested by reverse-phase HPLC (RP-HPLC) at an independent laboratory, with a batch-matched Certificate of Analysis. Content covers laboratory and in-vitro research only.