Benzyl alcohol is the only ingredient in bacteriostatic water other than water itself, so its properties explain most of what makes the product behave the way it does: the faint odor, the ultraviolet absorbance, the slow oxidation in an opened vial and the reason it stays fully dissolved at 0.9%. This page collects the reference data in one place.
Identity
| IUPAC name | Phenylmethanol |
| Common name | Benzyl alcohol |
| CAS number | 100-51-6 |
| Formula | C7H8O (C6H5CH2OH) |
| Molar mass | 108.14 g/mol |
| Structure | A benzene ring bearing a hydroxymethyl group; the simplest aromatic alcohol |
| Compendial grade | Benzyl Alcohol NF (USP–NF monograph) |
Physical constants
| Appearance | Clear, colorless liquid with a faint sweet, aromatic odor |
| Melting point | about −15 °C |
| Boiling point | about 205 °C at atmospheric pressure |
| Density | 1.044 g/mL at 20 °C (slightly denser than water) |
| Refractive index | about 1.540 at 20 °C |
| Vapor pressure | about 0.1 mmHg (roughly 12 Pa) at 25 °C |
| Flash point | above 90 °C (closed cup), well above any laboratory storage temperature |
| log P (octanol/water) | about 1.1: moderately lipophilic, which is part of why it interacts with bacterial membranes |
| pKa | about 15 (the hydroxyl group); effectively non-ionizing in water, so it does not act as a buffer |
Solubility
Benzyl alcohol dissolves in water to about 4 g per 100 mL at 20 °C. The 0.9 g per 100 mL used in bacteriostatic water is therefore well inside the solubility limit, and the preservative stays in solution at every temperature the label permits, including the low end of controlled room temperature. It is miscible with ethanol, diethyl ether, chloroform and most organic solvents, which matters when bacteriostatic water is used alongside organic cosolvents: the preservative partitions into the organic phase.
Spectroscopy
The benzene ring gives benzyl alcohol a characteristic ultraviolet absorption band centered near 257 nm, with the fine structure typical of a monosubstituted benzene between about 250 and 270 nm. The molar absorptivity at the maximum is roughly 200 L·mol−1·cm−1. That is a weak band by the standards of conjugated chromophores, but at the 83 mmol/L concentration present in bacteriostatic water it makes the undiluted solution effectively opaque between about 240 and 275 nm. Above 300 nm and throughout the visible range benzyl alcohol is transparent. See bacteriostatic water as a UV-Vis blank.
Stability and reactivity
In the presence of air, light or trace metal ions, benzyl alcohol oxidizes slowly to benzaldehyde and then to benzoic acid. Benzaldehyde is responsible for the sharper bitter-almond note sometimes noticed in a vial that has been open for a long time, and because benzaldehyde absorbs ultraviolet light far more strongly than benzyl alcohol (its maximum near 249 nm has a molar absorptivity around 12,000), even a small fraction of oxidation changes the UV baseline of the solution noticeably. Benzyl alcohol is incompatible with strong oxidizing agents. It is otherwise unreactive under laboratory storage conditions. See benzyl alcohol oxidation.
Why these numbers matter for bacteriostatic water
- The low vapor pressure means the preservative is not lost from a sealed vial, but an open container held at elevated temperature does lose it slowly with the water vapor. See can bacteriostatic water be autoclaved?
- The density and refractive index of a 0.9% solution differ from water only in the third or fourth decimal place, which is why bacteriostatic water cannot be told from plain sterile water by eye. See physical properties of bacteriostatic water.
- The ultraviolet absorbance is the main analytical interference to plan around. See when the preservative interferes.
Related
Benzyl alcohol as a preservative · What is benzyl alcohol used for in bacteriostatic water? · Reading the safety data sheet
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