Bacteriostatic water, sterile water and acetic water look identical in the vial: clear, colourless liquids in small sealed containers. Chemically they are different products with different compositions and storage characteristics. This guide explains what each one is, for anyone sourcing bacteriostatic water in the UK or comparing laboratory solvents for a research inventory. It covers composition and properties only.
For laboratory research use only. Not for human or veterinary use. See our research use only policy.
At a glance
- Name: Bacteriostatic Water
- Synonyms: BAC water, preserved sterile water
- Composition: sterile water containing 0.9% w/v benzyl alcohol
- Grade: USP grade, 10ml sterile vial
- CAS number: N/A, consumable (benzyl alcohol component: 100-51-6)
- Storage: Store at room temperature, away from light. Discard 28 days after first puncture.
- Name: Acetic Water
- Composition: sterile water containing a low concentration of acetic acid
- Grade: USP grade, 10ml sterile vial
- CAS number: N/A, consumable (acetic acid component: 64-19-7)
- Storage: Store at room temperature, away from light. Discard 28 days after first puncture.
Why the choice of solvent matters in the lab
Water is the most common solvent in biochemistry, but "water" covers a wide range of grades. Purified water, deionised water, sterile water and preserved water differ in microbial quality, the presence of additives and pH. For work with sensitive biomolecules such as peptides, those differences can matter to experimental consistency, so laboratories tend to specify the grade they hold in stock and record it alongside the materials it is used with.
The three products compared here are all sterile, aqueous and supplied in sealed vials. What separates them is a single added component: a preservative in one case, an acid in another, and nothing at all in the third.
Sterile water
Sterile water is purified water that has been sterilised and packaged so that it is free from viable microorganisms. It contains no preservative, no buffer and no added salts. Its pH is close to neutral, although unbuffered water readily absorbs carbon dioxide from the air and so tends to drift slightly acidic once opened.
Because there is no preservative, sterile water offers no protection against contamination after the container has been opened. Pharmacopoeial sterile water products are generally intended for single use, with any remainder discarded. This is the main practical difference between sterile water and bacteriostatic water.
Bacteriostatic water
Bacteriostatic water is sterile water to which 0.9% w/v benzyl alcohol has been added. Benzyl alcohol (C7H8O, CAS 100-51-6) is a simple aromatic alcohol that has a long history as a pharmaceutical preservative. At this concentration it is bacteriostatic: it inhibits the growth of many bacteria rather than killing them outright. That is where the product gets its name.
The presence of the preservative is what allows a vial of bacteriostatic water to be entered more than once over a limited period while limiting microbial growth. It does not make the contents permanently sterile, and it is not a substitute for aseptic technique. Our bacteriostatic water is USP grade, supplied as a 10ml sterile vial, stored at room temperature away from light and discarded 28 days after first puncture.
Things to be aware of with benzyl alcohol
Benzyl alcohol is a reactive organic molecule. Over time and with exposure to air and light it can slowly oxidise, for example to benzaldehyde. That is one reason storage away from light and a defined discard period are specified. Researchers working with materials known to be sensitive to organic solvents or preservatives will usually take the presence of benzyl alcohol into account when choosing which solvent to keep on hand, and should consult the relevant literature for their specific material.
Acetic water
Acetic water is sterile water containing a small amount of acetic acid (CH3COOH, CAS 64-19-7), the same weak organic acid found in vinegar. The added acid lowers the pH of the solution, so acetic water is mildly acidic rather than neutral. Exact concentrations vary between manufacturers, so the label of the specific product should always be checked.
Dilute acetic acid is a familiar solvent in peptide chemistry. Many synthetic peptides are isolated as acetate salts after purification, and the solubility of a peptide in water depends strongly on its sequence and net charge at a given pH. Peptides rich in basic residues such as arginine and lysine carry a positive charge at acidic pH, and their behaviour in acidic aqueous media is described in the analytical and preparative literature. Our acetic water is USP grade, supplied as a 10ml sterile vial and stored at room temperature away from light, with any remainder discarded 28 days after first puncture.
Side by side comparison
- Additive: sterile water has none, bacteriostatic water contains 0.9% benzyl alcohol, acetic water contains dilute acetic acid
- pH: sterile water is approximately neutral, bacteriostatic water is approximately neutral, acetic water is mildly acidic
- Preservative effect: present in bacteriostatic water only
- Typical vial use: sterile water single use, bacteriostatic and acetic water as specified by the label
- Storage of our products: room temperature, away from light, discard 28 days after first puncture
Storage and handling of laboratory solvents
Unopened vials should be kept in their outer packaging at room temperature and out of direct light. Once a vial has been punctured, label it with the date so that the 28 day discard period can be tracked. Visually inspect the liquid before use: any cloudiness, particles or discolouration is a reason to discard it. Keep vials closed, upright and away from heat sources, and dispose of them in line with your laboratory's waste procedures.
Solvents sit alongside the peptides they are stocked with, so the same discipline applies to both. Our guide on how to store lyophilised peptides covers temperature, light and moisture control for the powders themselves, and the research peptides UK buyer's guide explains how we describe quality across the range.
View bacteriostatic water or acetic water in the catalogue.
Frequently asked questions
What is bacteriostatic water?
Bacteriostatic water is sterile water containing 0.9% w/v benzyl alcohol as a preservative. The benzyl alcohol inhibits the growth of many bacteria, which is why the product is described as bacteriostatic.
What is the difference between bacteriostatic water and sterile water?
Both are sterile, but sterile water contains no preservative and is generally intended for single use, while bacteriostatic water contains benzyl alcohol, which limits microbial growth after the vial is first opened.
What is acetic water?
Acetic water is sterile water containing a low concentration of acetic acid. This makes it mildly acidic. Concentrations differ between manufacturers, so the product label should be checked.
How should bacteriostatic water be stored?
Our bacteriostatic water should be stored at room temperature, away from light, and discarded 28 days after first puncture.
Can I buy bacteriostatic water in the UK?
Yes. We supply USP grade bacteriostatic water and acetic water in 10ml sterile vials in the UK, for laboratory research use only.
References
- 1. United States Pharmacopeia. Bacteriostatic water monograph, USP-NF.
- 2. PubChem. Benzyl alcohol, CID 244. National Center for Biotechnology Information.
- 3. PubChem. Acetic acid, CID 176. National Center for Biotechnology Information.
- 4. Rowe RC, Sheskey PJ, Quinn ME (eds). Handbook of Pharmaceutical Excipients, 6th edition. Pharmaceutical Press, 2009. Monograph: Benzyl alcohol.
For laboratory research use only. Not for human or veterinary use. See our research use only policy.