Phosphate-buffered saline (PBS) and bacteriostatic water sit next to each other on many benches and are occasionally confused because both are clear, sterile aqueous liquids. They are opposites in almost every property that matters. This guide sets them side by side.
Composition
| Bacteriostatic water | PBS (1×) | |
|---|---|---|
| Base | WFI-grade water | Purified water |
| Salts | None | ~137 mM NaCl, ~2.7 mM KCl |
| Buffer | None | ~10 mM phosphate (Na2HPO4 / KH2PO4) |
| pH | 4.5–7.0, unbuffered, typically ~5.7 | 7.4, buffered |
| Tonicity | Hypotonic | Isotonic (~290 mOsm/kg) |
| Preservative | 0.9% benzyl alcohol | None (some formulations add sodium azide for non-culture use) |
| Sterile | Yes, sealed vial | If purchased sterile or filtered/autoclaved |
| Container | Multi-use vials ≤ 30 mL | Bottles, 100 mL to 1 L; single-use once opened, or short refrigerated life |
| Endotoxin tested | Yes, per lot | Only for specific cell-culture grades |
What PBS is for
PBS is a physiological vehicle: it mimics the salt concentration and pH of biological fluids so that cells, proteins and tissue samples are not stressed by osmotic or pH shifts. It is the standard wash, dilution and suspension medium for cell-based work, immunoassays, and protein handling where ionic strength and pH must be held steady. The phosphate buffer resists pH change when small amounts of acid or base are introduced. It is also a nutrient-free medium, so it does not support cell growth on its own.
What bacteriostatic water is for
A general-purpose sterile solvent for repeated small withdrawals over weeks, with no ions and no buffer so that it imposes nothing on what is dissolved in it, and a preservative so the vial can be entered many times. Dissolving or diluting lyophilized research materials that will not be applied to living cells is the textbook use. See laboratory uses.
Why they cannot substitute for each other
- Cells. Bacteriostatic water is both hypotonic and cytotoxic (benzyl alcohol); either property alone would rule it out for anything touching living cells. PBS is designed for exactly that. See cell culture.
- pH-sensitive materials. Bacteriostatic water has no buffering capacity; the dissolved material sets the pH. PBS holds pH 7.4. See the pH of bacteriostatic water.
- Ionic strength. Salt changes solubility, aggregation and electrostatic interactions. A material specified for water may behave differently in PBS, and vice versa.
- Phosphate interference. Phosphate can precipitate with calcium and some metal ions and interferes with phosphate-based assays; water introduces none of that.
- Multi-use handling. Opened PBS has no preservative and is either used promptly or refrigerated briefly; bacteriostatic water is designed for 28 days of repeated entries.
Decision guide
- Protocol says PBS, buffer, isotonic, physiological, pH 7.4: use PBS.
- Protocol says water, sterile water, solvent with no buffer, and the material will not touch living cells: bacteriostatic water is appropriate if repeated withdrawals are needed; preservative-free sterile water if not.
- Protocol says saline without buffer: bacteriostatic or sterile saline, not PBS and not water. See bacteriostatic water vs. bacteriostatic saline.
- Unsure, and the material's behavior in salt is unknown: water first; salt and buffer can be added later but not removed.
Other comparisons: sterile water, distilled water, deionized water.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.