Bacteriostatic water and bacteriostatic saline share a preservative, a vial format and most of a name, which is why they are so often confused. They are different products with different compositions, and they are not interchangeable. This guide explains the difference and how to tell which one a protocol needs.
Composition
| Bacteriostatic water | Bacteriostatic saline | |
|---|---|---|
| Base | Sterile water (WFI-grade) | Sterile 0.9% sodium chloride solution |
| Preservative | 0.9% benzyl alcohol | 0.9% benzyl alcohol |
| Sodium chloride | None | 9 mg/mL (0.9% w/v) |
| Tonicity | Hypotonic; essentially solute-free | Approximately isotonic (about 300 mOsm/kg) |
| Ionic strength / conductivity | Very low | High (about 154 mmol/L each of Na+ and Cl-) |
| pH | 4.5 to 7.0 | 4.5 to 7.0 |
| Container | multi-use vials up to 30 mL | multi-use vials up to 30 mL |
| Odor | Faint aromatic (benzyl alcohol) | Faint aromatic (benzyl alcohol) |
In short: both contain the same preservative at the same concentration; only the saline contains salt.
Why the salt matters
Nine milligrams of sodium chloride per millilitre sounds trivial, but it transforms the solution's physical chemistry:
- Osmolality. Bacteriostatic water is hypotonic, essentially pure water with a preservative. Bacteriostatic saline is approximately isotonic with physiological fluids. Any in vitro method that depends on osmotic conditions will behave differently in the two.
- Ionic strength. Salt affects the solubility and stability of many dissolved materials, shields electrostatic interactions between molecules, and changes the behavior of ionic compounds in solution. A material that dissolves cleanly in water may behave differently in saline, and vice versa.
- Conductivity. Saline is highly conductive; bacteriostatic water is not. This matters for any electrochemical or conductivity-based measurement and for methods sensitive to ionic background.
- Analytical interference. Chloride ions interfere with some assays and are incompatible with certain analytical hardware, while sodium can suppress signal in some spectroscopic methods. Bacteriostatic water introduces neither.
- Corrosion and residue. Saline leaves a salt residue on drying and is corrosive to some metals. Water does not.
What they have in common
Because both contain 0.9% benzyl alcohol, every caveat that applies to bacteriostatic water applies equally to bacteriostatic saline:
- Neither is suitable for cell culture; benzyl alcohol is cytotoxic to cultured cells.
- Both show a benzyl alcohol peak in UV-detected chromatography and absorb near 257 nm.
- Both are multi-use products with the same 28-day discard convention after first puncture and the same aseptic handling requirements. See storage and handling.
- Both are sterile only until the seal is broken; the preservative inhibits growth, it does not sterilize.
Which one does a protocol need?
The protocol decides, and the vocabulary is usually explicit:
- If it specifies water, sterile water, a solute-free or low-ionic-strength vehicle, or a diluent that must not add salt, use bacteriostatic water (or preservative-free sterile water if the preservative would interfere).
- If it specifies saline, normal saline, 0.9% sodium chloride, an isotonic vehicle, or a physiological ionic strength, use bacteriostatic saline (or preservative-free saline).
- If it specifies neither, the safer default for dissolving or diluting a material whose behavior in salt is unknown is water, because salt can be added later but cannot be removed.
A note on labels and names
The two products fall under separate pharmacopeial monographs and are labeled differently: one as bacteriostatic water with the preservative named, the other as bacteriostatic sodium chloride with both the salt and the preservative named. Read the label rather than relying on vial shape or color, because the vials are often visually identical. The preservative concentration and the sodium chloride content are the two numbers that distinguish them.
Related grades
- Sterile water (no preservative, single-use): see bacteriostatic water vs. sterile water.
- Sterile saline (0.9% sodium chloride, no preservative, single-use).
- Phosphate-buffered saline (PBS): isotonic and buffered to about pH 7.4, not preserved, and not a substitute for either product above.
Summary
Bacteriostatic water is preserved water; bacteriostatic saline is preserved 0.9% sodium chloride. They share the preservative and its limitations, and differ in osmolality, ionic strength, conductivity and analytical behavior. Use the one the protocol names.
BAC Water Supply stocks bacteriostatic water in sealed 3 mL, 10 mL and 30 mL vials. For a full description of the product, see what is bacteriostatic water.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.