Bacteriostatic water is water containing 0.9% w/v benzyl alcohol, that is 9 mg per mL. Because the preservative is present at less than 1% and is a small non-ionic molecule, most physical properties of the solution are the properties of water to two or three decimal places. The exceptions are ultraviolet transparency and osmolality, where the preservative dominates. The values below are calculated or taken from the properties of the components; a certificate of analysis reports the measured values for a specific lot.
Concentration
9 g/L divided by the molar mass of benzyl alcohol (108.14 g/mol) gives 0.083 mol/L, or 83 mmol/L. Since the solution is almost entirely water, molality and molarity are the same to two significant figures: 0.083 mol per kg of water.
Density
Benzyl alcohol is only slightly denser than water (1.044 g/mL at 20 °C), and at 0.9% the solution density works out to about 0.999 g/mL at 20 °C compared with 0.998 g/mL for pure water. For any practical purpose, 1 mL of bacteriostatic water weighs 1.00 g, which is the basis of the weight-check on fill volume described in fill volume and overfill.
Freezing point and boiling point
A dissolved non-electrolyte lowers the freezing point of water by 1.86 °C per mole per kilogram. At 0.083 mol/kg the depression is 0.15 °C, so bacteriostatic water freezes at about −0.15 °C: the preservative gives no meaningful protection against freezing, which is why the label says to protect the product from freezing. The boiling point elevation is 0.512 °C per mole per kilogram, giving a boiling point of about 100.04 °C. See can bacteriostatic water be frozen?
Osmolality
Because benzyl alcohol does not dissociate, each dissolved molecule contributes one osmotically active particle, and the osmolality of bacteriostatic water is about 83 mOsm/kg. For comparison, a 0.9% sodium chloride solution is close to 290 mOsm/kg because each formula unit contributes two ions. Bacteriostatic water is therefore strongly hypotonic relative to saline, a difference that matters wherever osmolality is a controlled variable. See does bacteriostatic water contain salt?
Conductivity and pH
Benzyl alcohol contributes no ions, so the conductivity of bacteriostatic water is that of the WFI-grade water it is made from: on the order of 1 µS/cm at manufacture, rising to a few µS/cm as the water absorbs carbon dioxide from the headspace. The dissolved carbon dioxide is also the main reason the pH sits between 4.5 and 7.0 rather than at 7.0; the solution is unbuffered, so its pH moves easily when anything is dissolved in it. See the pH of bacteriostatic water.
Viscosity, refractive index and surface tension
At 0.9% the viscosity is that of water, about 1.0 mPa·s at 20 °C, and the refractive index differs from water only in the fourth decimal place. Benzyl alcohol is weakly surface active, so the surface tension is slightly below that of pure water. None of these differences is visible: a vial of bacteriostatic water looks identical to a vial of plain sterile water, and only the label distinguishes them.
Optical transparency
This is where the preservative dominates. Benzyl alcohol absorbs ultraviolet light between about 240 and 275 nm strongly enough that the undiluted solution is opaque in that region in a 1 cm cell, while it is fully transparent above 300 nm and across the visible spectrum. See bacteriostatic water as a UV-Vis blank.
Summary table
| Property | Bacteriostatic water (0.9% benzyl alcohol) | Pure water |
|---|---|---|
| Benzyl alcohol | 9 mg/mL, 83 mmol/L | none |
| Density, 20 °C | about 0.999 g/mL | 0.998 g/mL |
| Freezing point | about −0.15 °C | 0 °C |
| Boiling point | about 100.04 °C | 100 °C |
| Osmolality | about 83 mOsm/kg | 0 |
| Conductivity | a few µS/cm (from the water, not the preservative) | 0.055 µS/cm (theoretical) |
| pH | 4.5 to 7.0, unbuffered | about 5.5 to 7 when exposed to air |
| UV transparency | opaque 240–275 nm; transparent above 300 nm | transparent above 190 nm |
Related
Benzyl alcohol: chemical and physical properties · What is bacteriostatic water? · Bacteriostatic water vs. sterile water
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.