PBS stands for phosphate buffered saline, a water-based salt solution used in nearly every biology and biomedical lab. It contains a precise mix of salts that keeps the solution at a pH of 7.4 and an overall salt concentration that matches the inside of mammalian cells. This makes it safe to use around living cells, tissues, and proteins without damaging them.
What PBS Contains
Standard PBS is made from four salts dissolved in water: sodium chloride (NaCl), potassium chloride (KCl), disodium phosphate (Na₂HPO₄), and potassium dihydrogen phosphate (KH₂PO₄). Sodium chloride is by far the dominant ingredient at 137 mM, which is close to the salt concentration in human blood. The two phosphate salts (10 mM Na₂HPO₄ and 1.8 mM KH₂PO₄) act as the buffering system, meaning they absorb small additions of acid or base and keep the pH locked near 7.4. Potassium chloride rounds out the recipe at 2.7 mM.
Labs typically keep a concentrated 10x stock on the shelf and dilute it with water right before use: 100 mL of the 10x stock plus 900 mL of water gives one liter of working-strength (1x) PBS.
Why pH 7.4 Matters
Human blood and most body fluids sit right around pH 7.4. Proteins change shape, enzymes stop working, and cells start dying when pH drifts even a fraction of a point away from what they’re built for. The phosphate pair in PBS acts like a chemical sponge: when something acidic enters the solution, one phosphate species absorbs the extra hydrogen ions, and when something basic enters, the other phosphate species releases hydrogen ions. The net effect is a solution that resists pH swings during experiments, which is exactly what researchers need when handling biological material outside the body.
How PBS Protects Cells
Beyond pH, PBS also matches the osmotic pressure of mammalian cells, sitting at roughly 313 mOsm/L. Osmotic pressure is the force that drives water in or out of a cell depending on the salt concentration around it. Place cells in pure water and they swell until they burst, because water floods inward trying to equalize the salt difference. Place them in a very salty solution and they shrivel as water is pulled out. PBS avoids both extremes by presenting cells with a salt environment nearly identical to what they experience inside the body. This makes PBS the default choice any time researchers need to rinse, transport, or briefly store cells without killing them.
Common Lab Uses
PBS shows up at almost every stage of a biology experiment. Its most frequent role is as a wash solution: after staining cells with fluorescent markers or antibodies, researchers rinse away the excess with PBS so only the specifically bound signal remains. It also serves as a dilution medium for proteins, antibodies, and drugs when those reagents need to be at a precise concentration but still in a cell-friendly environment.
Tissue samples headed for analysis are often kept in cold PBS during transport to prevent drying and degradation. In forensic biology, PBS has been validated for extracting cells and sperm from swabs without interfering with DNA profiling. Cell culture labs use it to rinse cell layers before detaching them from their growth surface, clearing away old culture medium and dead cells.
PBS With and Without Calcium and Magnesium
You’ll sometimes see PBS labeled as “+/+” or “−/−,” referring to whether calcium and magnesium ions have been added. The standard recipe described above is the calcium- and magnesium-free version. The supplemented version adds roughly 0.9 mM calcium chloride and 0.5 mM magnesium chloride.
The distinction matters because calcium is a powerful signaling molecule. It helps cells stick to surfaces and to each other, triggers muscle contraction, and influences immune responses. Research has shown that even the modest calcium concentration in +/+ PBS can prompt white blood cells to ramp up secretion of inflammatory signaling molecules. If the goal is to detach cells from a dish, you want −/− PBS so calcium-dependent adhesion proteins lose their grip. If the goal is to keep tissue architecture intact during a wash, +/+ PBS helps cells maintain their connections.
When PBS Is Not the Right Choice
PBS works for the vast majority of routine lab tasks, but a few situations call for alternatives. The phosphate in PBS directly interferes with alkaline phosphatase, an enzyme commonly used as a detection tag in experiments like Western blots. If your detection system relies on alkaline phosphatase, the free phosphate in PBS competes with the reaction and weakens the signal. In that case, Tris-buffered saline (TBS) is the standard substitute.
The same problem arises when studying phosphorylated proteins, which are proteins that have had a phosphate group attached as a biological “on/off switch.” Antibodies designed to detect that phosphate tag can mistakenly bind the phosphate floating in PBS, drowning out the real signal. TBS-based buffers eliminate this background noise. For most other applications, PBS and TBS are interchangeable.
Storage and Shelf Life
Prepared PBS is stable at room temperature (15 °C to 30 °C) and does not need refrigeration unless a specific protocol calls for cold washes. Commercial bottles carry shelf lives ranging from 12 to 36 months depending on the formulation and packaging. Contamination is the main risk over time, so labs that make their own PBS from powder typically autoclave it or pass it through a sterile filter, then label it with a preparation date. If the solution turns cloudy or develops visible particles, it should be discarded.