What Is Bacteriostatic Water? Composition and Chemistry

Bacteriostatic water is sterile water that contains 0.9% benzyl alcohol as a preservative. Chemically it is a simple aqueous solution of two components: water, H2O, and a small amount of dissolved benzyl alcohol, C6H5CH2OH. The benzyl alcohol is what sets it apart from plain sterile water, because it inhibits the growth of bacteria, which is what "bacteriostatic" means.

This page covers bacteriostatic water as a material: what it contains, the chemistry of benzyl alcohol, how it differs from sterile water and from laboratory reagent water, and how a sealed vial is stored and labelled.

What bacteriostatic water contains

A percentage for a solution can be expressed on more than one basis, and a specification should say which one it uses. For benzyl alcohol at 0.9% the choice makes little practical difference:

• Weight in volume (w/v): 0.9 g in every 100 mL of solution, which is exactly 9 mg per mL.
• Volume in volume (v/v): 0.9 mL in every 100 mL. Benzyl alcohol is slightly denser than water, 1.0419 g/cm³ at 24 °C, so this comes to about 9.4 mg per mL.

Either way the solution holds about 9 mg of benzyl alcohol per millilitre, or roughly 0.08 mol/L. On the weight-in-volume basis, a 3 mL vial holds about 27 mg of benzyl alcohol and a 10 mL vial about 90 mg.

That concentration sits well below benzyl alcohol's solubility in water, reported as 42.9 g/L at 25 °C and 35 g/L at 20 °C. The benzyl alcohol is therefore fully dissolved, and the result is a single clear, colourless phase. With less than one per cent of an organic solute, the solution looks like water, and its bulk physical properties are close to those of water.

What "bacteriostatic" means

The term comes from microbiology. A bacteriostatic agent inhibits the growth and multiplication of bacteria without necessarily killing them, whereas a bactericidal agent kills them.

It helps to keep three terms apart:

• Sterile describes a state: the material contains no viable microorganisms. Sterility comes from the manufacturing process, for example sterilisation by heat or by filtration, and it applies to the contents of a sealed container.
• Preservative describes an ingredient: a substance added so that the solution resists microbial growth. Adding a preservative does not make a solution sterile.
• Bacteriostatic describes what that preservative does: it holds bacterial growth in check.

Bacteriostatic water is therefore both things at once. It is sterile while sealed, and it is preserved by its benzyl alcohol content. Plain sterile water has only the first property.

The chemistry of benzyl alcohol

Benzyl alcohol, systematic name phenylmethanol, is the simplest aromatic alcohol. It consists of a benzene ring attached to a CH2OH group. The hydroxyl group sits on a carbon next to the ring (a benzylic carbon), not on the ring itself, which makes benzyl alcohol a primary alcohol rather than a phenol. That distinction shows in its acidity. Phenols are weak acids, while benzyl alcohol, with a pKa of 15.4, is no more acidic than water in any practical sense and stays un-ionised in aqueous solution.

Values from the PubChem record for benzyl alcohol (CID 244).

Several of these numbers explain how the compound behaves in water. A log Kow of 1.10 means benzyl alcohol partitions about 13 times more into octanol than into water, so it is only moderately polar. That is consistent with its limited solubility in water, a few per cent, compared with its free miscibility with ethanol and ether. A boiling point of 205 °C means it is far less volatile than water.

How benzyl alcohol changes over time

On exposure to air, benzyl alcohol is slowly oxidised. As a primary alcohol it oxidises in two steps: first to the aldehyde, benzaldehyde (C7H6O, 106.12 g/mol), and then to the carboxylic acid, benzoic acid (C7H6O2, 122.12 g/mol). The reaction is slow, and PubChem's record notes that benzyl alcohol remains stable for a long time for that reason. Even so, oxidation is the main chemical change to expect in a preserved solution that is left open to air, and it is one reason the vial should stay sealed until needed.

How benzyl alcohol is measured

Benzyl alcohol in water is routinely measured by gas chromatography. Standard environmental methods pair it with mass spectrometry, and one such method (EPA 8270D) gives a detection limit of 20 µg/L in water samples. Because the benzene ring absorbs ultraviolet light, liquid chromatography with UV detection is another option. Benzaldehyde and benzoic acid, the oxidation products, can be looked for alongside it.

Sterile water, bacteriostatic water and reagent water

All three materials are "water", but they are not interchangeable, and the differences are chemical.

Reagent water is defined by specifications such as ASTM D1193. That standard sets four types of reagent water, with three additional grades that address contaminants of microbiological origin. Its parameters cover electrical conductivity and resistivity, pH, silica, sodium, chloride, total organic carbon, endotoxin and microbiological contamination.

Two quick calculations show how far bacteriostatic water sits from that kind of material:

• Organic carbon. Benzyl alcohol is 77.7% carbon by mass (seven carbon atoms at 12.011 g/mol each, out of 108.14 g/mol). At 9 mg per mL, the solution carries about 7 mg of organic carbon per mL, or about 7 g per litre.
• UV absorbance. The tabulated maximum at 258.5 nm has log ε = 2.26, a molar absorption coefficient of about 180 L mol⁻¹ cm⁻¹. At 0.083 mol/L, the calculated absorbance in a 1 cm cell is about 15, far beyond what a spectrophotometer can read directly.

Storage and labelling

Store the sealed vial at 2–8 °C, protected from light. Keeping it sealed does two jobs: it keeps the contents sterile, and it limits contact with air, which is what slowly oxidises benzyl alcohol.

A label or specification for bacteriostatic water should make the material unambiguous. The points to look for are:

• Name and composition, including the benzyl alcohol content and, ideally, the basis of the percentage (w/v or v/v).
• Nominal volume of the container.
• Batch or lot number, so that the vial can be traced.
• Expiry or retest date.
• Storage temperature and any protection from light.
• Supply terms, such as a research-use statement.

Our Bacteriostatic Water is supplied in sealed glass vials of 3 mL and 10 mL, and its product page gives the composition, the storage temperature and the research-use terms. It is a laboratory consumable rather than a research compound, so it carries no HPLC purity specification.

Chemical terms used on this page are collected in the glossary, and questions about ordering and dispatch are answered in the FAQ. Other articles on the chemistry of research materials are listed in the Learn section.