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Short answer
A biological buffer is a solution of a weak acid (or base) and its conjugate partner that holds pH steady when small amounts of acid or base are added. In the lab the term usually means a buffer chosen for use with enzymes, proteins, cells or nucleic acids, and most often a “Good’s buffer” such as MES, MOPS or HEPES. The practical rule: pick one whose pKa sits within about 1 unit of your target pH.
Definition and scope
Boston BioProducts, a laboratory reagent supplier, defines a biological buffer as “a solution composed of a weak acid (or base) and its conjugate partner” that is designed to maintain a constant pH. Biomol, another supplier, describes the same job in terms of protons: biological buffers let the researcher control acidity “by capturing or releasing protons.”
The word “biological” refers to the use, not to a special chemistry. The same weak-acid/conjugate-base logic applies to any buffer; what changes is the extra constraints that biological work adds, such as keeping the buffer from interfering with enzymes, membranes or optical readings. For how to make, adjust, sterilize and store a buffer, see the buffer and solution preparation lab guide; this page covers what the buffer is and how to choose one.
What is a biological buffer? How Good’s buffers fit in
Good’s buffers are “a set of zwitterionic buffering agents introduced by Norman E. Good and colleagues in the 1960s” (Boston BioProducts). The original paper is Good NE, Winget GD, Winter W, Connolly TN, Izawa S, Singh RM, “Hydrogen ion buffers for biological research,” Biochemistry 1966;5(2):467-77 (PubMed record 5942950).
Boston BioProducts summarizes the properties that define the family:
- A pKa between roughly 6.0 and 8.0, “a range where many biological processes take place.”
- High water solubility and low solubility in lipids and organic solvents, which limits unwanted partitioning.
- Low permeability through biological membranes, so the buffer stays out of cells.
- Minimal metal-ion binding, so metalloproteins and enzyme cofactors are not disturbed.
- Minimal absorbance in the UV and visible range, which matters for spectrophotometric assays.
- Stability across changes in temperature, ionic strength and concentration.
Buffers by pKa and useful range
The table lists nine Good’s-type buffers in order of pKa, with the useful pH range and typical pKa from Boston BioProducts. The last column gives a metal-binding note only where the supplier states one; a dash means no statement.
| Buffer | Useful pH range | Typical pKa | Supplier’s metal-binding note |
|---|---|---|---|
| MES | 5.5 – 6.7 | 6.15 | Minimal metal-ion binding |
| PIPES | 6.1 – 7.5 | 6.80 | Low metal chelation |
| MOPS | 6.5 – 7.9 | 7.20 | – |
| HEPES | 6.8 – 8.2 | 7.55 | – |
| HEPPS (EPPS) | 7.3 – 8.7 | 8.00 | – |
| Tricine | 7.4 – 8.8 | 8.15 | Low metal binding |
| Bicine | 7.8 – 8.8 | 8.35 | – |
| TAPS | 7.7 – 9.1 | 8.40 | – |
| CAPS | 9.7 – 11.1 | 10.40 | – |
Biomol’s listings give the same pKa for MOPS (7.20) and for Bicine (8.35 at 20 °C).
Choosing by target pH: a four-step order
Three decision points drive the choice: match pKa to the target pH, watch for metal binding, and watch concentration. In the order you would apply them:
- Set the target pH of the working solution, at the temperature it will be used.
- Keep buffers whose pKa is within about 1 unit of that pH. Boston BioProducts: “Always choose a buffer whose pKa is within ~1 unit of your target pH.”
- Screen for metal binding if the system has metal-dependent enzymes or metalloproteins. Some buffers bind metal ions and can reduce the activity of metal-dependent enzymes; Boston BioProducts lists MES, PIPES and Tricine as low-binding.
- Set the concentration. Capacity depends on both the concentration and how close the pH is to the pKa; a higher concentration gives better capacity but “may introduce osmolarity or ionic strength issues.”
Worked example, target pH 7.4. A pKa within 1 unit means 6.4 to 8.4. From the table that keeps PIPES (6.80), MOPS (7.20), HEPES (7.55), HEPPS (8.00), Tricine (8.15), Bicine (8.35) and TAPS (8.40), and drops MES (6.15) and CAPS (10.40). If the assay uses a metal-dependent enzyme, the metal-binding notes in the table point to PIPES or Tricine from that shortlist.
Worked example, target pH 6.5. The window is 5.5 to 7.5: MES (6.15), PIPES (6.80) and MOPS (7.20) qualify; the other six do not.
Where it matters for labs and supply decisions
Because the screening criteria above are properties of the specific reagent, the buffer’s identity belongs in the written record. For each buffer solution used in a method, note the buffer name, the pKa and target pH you chose it for, the concentration, and the temperature at which pH was adjusted. A methods section or SOP that says only “pH 7.4 buffer” cannot be reproduced exactly, and a substitution from the table (say, MOPS for HEPES) is then a change to the method that should be recorded as one. The instrument used to set the pH is explained in what is a pH meter, and the pH arithmetic in how to calculate pH with molarity. A lysis-specific formulation is worked through in the red cell lysis buffer recipe.
Common misunderstandings
“All Good’s buffers work between pH 6 and 8.”
The family is described as having a pKa “between roughly 6.0 and 8.0,” but the same supplier’s table runs from MES at 6.15 to CAPS at 10.40, and five of the nine have pKa values of 8.00 or above. Treat 6 to 8 as the centre of the family, not a rule for each member.
“A higher concentration is always safer.”
More buffer gives more capacity, but the same supplier warns of osmolarity and ionic-strength effects. Pick the concentration the system tolerates, not the highest one that dissolves.
“Any buffer with the right pH is interchangeable.”
Two buffers can reach the same pH and still differ in metal binding, membrane permeability and optical absorbance, which are the reasons the Good’s criteria exist.
FAQ
What is a biological buffer?
It is a weak acid (or base) with its conjugate partner, dissolved so that the pH changes little when acid or base is added, and chosen so that it does not interfere with the biological system under study. Good’s buffers such as MES, MOPS and HEPES are the best-known examples.
What is a biological buffer made of?
A weak acid and its conjugate base (or a weak base and its conjugate acid) in water. Good’s buffers are zwitterionic compounds, listed in the table above.
How close should the pKa be to my target pH?
Within about 1 unit, according to Boston BioProducts. Buffer capacity depends on both concentration and how close the pH is to the pKa.
Where can I read about how to make one?
The buffer and solution preparation guide covers pH adjustment, sterilization and storage, including how pH readings for some buffers change with temperature.
Sources
- Boston BioProducts, “Good’s Buffer Overview” (supplier technical page): https://www.bostonbioproducts.com/goods-buffer-overview/ (checked 2026-10-29).
- Biomol, “Biological Buffers” (supplier product page): https://www.biomol.com/products/chemicals/biological-buffers/ (checked 2026-10-29).
- Good NE, Winget GD, Winter W, Connolly TN, Izawa S, Singh RM. Hydrogen ion buffers for biological research. Biochemistry. 1966;5(2):467-77. PubMed: https://pubmed.ncbi.nlm.nih.gov/5942950/ (record checked 2026-10-29).
