Sterile saline and bacteriostatic water differ on both axes that matter: one has salt and no preservative, the other has a preservative and no salt. They are not substitutes, and a protocol that names one means that one. This guide sets them side by side, and covers the third product, bacteriostatic saline, that combines features of both.
Composition
| Bacteriostatic water | Sterile saline | Bacteriostatic saline | |
|---|---|---|---|
| Sodium chloride | None | 0.9% (9 mg/mL) | 0.9% |
| Preservative | 0.9% benzyl alcohol | None | 0.9% benzyl alcohol |
| Tonicity | Hypotonic | Isotonic (~308 mOsm/kg) | Isotonic |
| Buffer | None | None | None |
| Container | Multi-use vial ≤ 30 mL | Single-use vials and bottles, any size | Multi-use vial ≤ 30 mL |
| After opening | 28-day convention | Use once, discard | 28-day convention |
| Sterile, endotoxin-tested | Yes | Yes | Yes |
What salt changes
Nine milligrams per millilitre of sodium chloride makes the solution approximately isotonic, highly conductive and about 154 mmol/L in each of sodium and chloride ions. Ionic strength affects the solubility and stability of many dissolved materials, screens electrostatic interactions, interferes with some assays, and leaves a residue on drying. Bacteriostatic water brings none of that, which is exactly why a protocol that calls for water rather than saline should not be given saline. See bacteriostatic water vs. bacteriostatic saline for the full treatment of ionic effects.
What the preservative changes
Benzyl alcohol lets a vial be entered repeatedly for up to 28 days, and rules the product out of cell culture and UV-based analysis. Sterile saline, having no preservative, is single-use but usable wherever an isotonic vehicle is needed and the preservative would interfere. See bacteriostatic water vs. sterile water for the preservative discussion on the water side.
Where sterile saline is used in a laboratory
Isotonic dilutions and washes for cell-based and tissue work, sample preparation where physiological ionic strength is required, and any single-use application where salt is wanted and a preservative is not. It is often bought in larger single-use bottles because those applications consume volume.
Where bacteriostatic water is used
Repeated small withdrawals of sterile, solute-free water over weeks, for dissolving or diluting research materials that will not touch living cells and will not go into UV-detected analysis. See laboratory uses.
Decision guide
- Protocol says saline or 0.9% sodium chloride, single use, preservative-sensitive: sterile saline.
- Protocol says saline, repeated withdrawals, preservative acceptable: bacteriostatic saline.
- Protocol says water, repeated withdrawals, preservative acceptable: bacteriostatic water.
- Protocol says water, single use or preservative-sensitive: preservative-free sterile water.
- Protocol says buffer or PBS: neither; see vs. PBS.
Labeling
The three products are visually identical in the vial. Read the label for sodium chloride content and preservative content; keep them on separate shelves with distinct labels. Mix-ups are among the most common laboratory errors with these products; see common mistakes.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.