To use buffer solutions effectively, you need a concrete understanding of different buffering systems. Below are several common buffer solutions and their uses to help deepen your understanding of buffers.
Phosphate Buffer Solutions
Because phosphoric acid is triprotic, phosphate buffering pairs can be H₃PO₄/H₂PO₄⁻, H₂PO₄⁻/HPO₄²⁻, or HPO₄²⁻/PO₄³⁻. In biochemical systems, commonly used phosphate buffers are typically prepared from sodium or potassium salts of the H₂PO₄⁻/HPO₄²⁻/ pair. pH stability in solution is mainly maintained by the equilibrium:
H₂PO₄⁻ + H₂O ⇌ HPO₄²⁻ + H₃O⁺, with an effective pH range of approximately 6.2–8.2.
In biochemical applications, NaCl or KCl is often added to prepare phosphate-buffered saline (PBS) (commonly available in two versions: without/with Ca²⁺ and Mg²⁺).
PBS is compatible with many biological samples and is commonly used for solution preparation, sample washing, and short-term suspension. Note:
a) In systems involving Ca²⁺/Mg²⁺, phosphate may precipitate; choose PBS without divalent cations according to your experimental purpose.
b) Cryopreservation of cells generally requires osmoprotectants (e.g., DMSO) and nutrients; PBS alone is not suitable as a standard freezing medium.
c) PBS does not rely on the CO₂–HCO₃⁻ buffer; it is suitable for washing/short holds, not for maintaining long-term cell culture.
Borate Buffer Solutions
Borate buffer systems commonly use boric acid (H₃BO₃)/tetrahydroxyborate (B(OH)₄⁻) as the principal acid–base pair, with the fundamental equilibrium:
H₃BO₃ + H₂O ⇌ B(OH)₄⁻ + H₃O⁺.
Typical formulations include borax buffer (based on sodium tetraborate, Na₂B₄O₇·10H₂O) and boric acid–borax buffers (boric acid and borax mixed in defined ratios). The effective pH range is usually about 8.0–10.0 (commonly 7.5–9.5); the exact range depends on component ratios and ionic strength. For biochemical applications, an appropriate amount of NaCl may be added to maintain isotonicity.
Applications and notes: The pH range of the boric acid/borax system is adjacent to that of tear fluid; consequently, it is used in some ophthalmic drops as a pH/isotonicity adjuster (typically together with other buffering/osmotic agents). In addition, borax buffer can serve as a release or solubilizing medium for certain drugs (particularly those containing vicinal diol structures). Use cautiously or avoid in cell culture and in systems involving polyols/sugar assays.
Tris(hydroxymethyl)aminomethane–Hydrochloric Acid Buffer (Tris-HCl)
 |
Buffering principle and range: The primary amine (–NH₂) in Tris is protonated to form the ammonium salt (R–NH₃⁺), which, together with the unprotonated form, constitutes the buffer pair: Tris·H⁺ + H₂O ⇌ Tris + H₃O⁺, with an effective pH range of approximately 7.0–9.0. The pH of Tris-HCl is strongly affected by solution concentration and temperature, and alkaline Tris solutions readily absorb CO₂ from air, causing pH to decrease; fresh preparation is recommended.
Additional note: Tris contains a primary amine and may interfere with coupling chemistries involving amines (e.g., NHS esters). Avoid Tris buffers in such experiments.
Applications: Tris exerts minimal interference in many biochemical processes and does not precipitate with Ca²⁺, Mg²⁺, or most heavy metal ions. It is commonly used for dissolving and extracting biological macromolecules such as DNA and proteins and also appears in certain biopharmaceutical formulations (refer to product labeling).
HEPES (4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid)
HEPES (C₈H₁₈N₂O₄S) is a Good’s buffer and a zwitterionic molecule. Its buffering capacity arises mainly from reversible protonation/deprotonation at one amine site on the piperazine ring (pKₐ ≈ 7.5 at 25 °C); the other amine contributes little near pH 7 ± 0.5. The sulfonic acid group is essentially fully dissociated to –SO₃⁻ at pH 7.2–7.6. Buffering capacity is maximal when the solution pH is close to the pKₐ. To prepare, dissolve HEPES and fine-tune to the target pH with NaOH (or HCl). The effective pH range is about 6.8–8.2. Compared with many buffers, HEPES exhibits relatively small pH dependence on concentration and temperature and shows good pH stability.
In aqueous solution, the sulfonic acid group (–SO₃H) is essentially fully dissociated to –SO₃⁻ under physiological pH, while buffering is actually provided by reversible protonation/deprotonation at the tertiary amine site (R₃N ⇌ R₃NH⁺, pKₐ ≈ 7.5 at 25 °C). Upon addition of acid, R₃N is protonated to R₃NH⁺, consuming H⁺ and inhibiting a drop in pH; upon addition of base, R₃NH⁺ loses a proton to form R₃N, consuming OH⁻ and inhibiting a rise in pH. The resulting conjugate acid/base pair coexists in comparable amounts at pH ≈ pKₐ (Henderson–Hasselbalch relationship), yielding maximal buffering capacity. The distribution of chemical species shifts with pH: in the neutral region the zwitterion (–SO₃⁻ with R₃NH⁺, overall neutral) predominates; under acidic conditions one or both amines become further protonated to give cations; under basic conditions both amines are unprotonated, giving an anion (–1). Thus, HEPES resists acid–base perturbations and stabilizes solution pH via a dynamic equilibrium between amine protonation states and bulk pH.
Applications and notes: HEPES has low cytotoxicity, crosses biological membranes poorly, and minimally perturbs many intracellular processes. It is suitable for open-air cell handling and microscopy (short operations outside a CO₂ incubator) and for a broad range of nucleic-acid/protein experiments. In studies involving metals or material corrosion/degradation, evaluate potential interactions between HEPES and metal ions that may affect the system.
Aladdin-Related Product List
Product Name | CAS No. | Notes |
Orthophosphoric acid (Phosphoric acid) | — | |
Sodium dihydrogen phosphate | Common hydrate: monohydrate 10049-21-5 | |
Disodium hydrogen phosphate | Common hydrate: dodecahydrate 10039-32-4 | |
Potassium dihydrogen phosphate (KH₂PO₄) | — | |
Dipotassium hydrogen phosphate (K₂HPO₄) | — | |
Boric acid | — | |
Sodium tetraborate decahydrate (borax) | Related: anhydrous 1330-43-4 | |
Tris (tris(hydroxymethyl)aminomethane) | — | |
Tris·HCl (tromethamine hydrochloride) | — | |
4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid | — | |
Sodium chloride | PBS component | |
Potassium chloride | Optional PBS component | |
Hydrochloric acid | 7647-01-0 | For pH adjustment |
Sodium hydroxide | For pH adjustment |
Aladdin：https://www.aladdinsci.com/