Written and maintained by CASRAI Editorial Board
Last updated
Short answer: A buffer solution is a solution that resists a change in pH when a small amount of strong acid or strong base is added. The usual recipe is a weak acid together with its conjugate base (for example acetic acid and acetate) in comparable amounts. This guide is written for researchers and lab staff who need to understand why a buffer holds pH, estimate how much it will move, and pick a sensible one for a bench protocol.
Definition and scope: what is a buffer solution in chemistry?
In analytical chemistry, buffer capacity is described as the ability of a buffer to resist a change in pH when a strong acid or a strong base is added to it (Harvey, Analytical Chemistry 2.1, section 6.8, LibreTexts). Acetic acid with sodium acetate is the standard example of an acid-base buffer.
The contrast with plain water shows what that means. In the same textbook section, adding about 0.1 mL of concentrated HCl to 1 L of water drops the pH from 7.0 to 3.0. Adding the same amount to 1 L of a solution that is 0.1 M in acetic acid and 0.1 M in sodium acetate changes the pH negligibly.
How does a buffer solution work?
A buffer holds two species in balance: a weak acid (HA) that can neutralise added base, and its conjugate base (A−) that can neutralise added acid. Added strong acid converts some A− into HA; added strong base converts some HA into A−. Because pH depends on the ratio of the two, not on their absolute amounts, a modest conversion shifts the ratio only slightly.
How does a buffer solution resist a change in pH?
The pH of a weak-acid buffer follows the Henderson-Hasselbalch relationship: pH = pKa + log([A−]/[HA]). When the two concentrations are equal the logarithm is zero and pH equals pKa, which is 4.76 for the acetic acid and acetate pair (Harvey, section 6.8).
How to calculate the pH of a buffer solution: a worked check
Take the acetate buffer above, with equal concentrations of acetic acid and acetate (ratio 1.00, pH 4.76). Suppose a strong acid converts 10% of the acetate to acetic acid. The ratio [A−]/[HA] becomes 0.90/1.10 = 0.818, log(0.818) = −0.087, and the pH falls to 4.67, a drop of only 0.09 units (the same figures Harvey gives). Compare that with the 4-unit drop in unbuffered water above. The same arithmetic is a quick bench check: estimate what fraction of one buffer component your sample or reagent will consume, convert that to a new ratio, and read off the pH shift.
What makes a good buffer solution? Capacity and range
Two properties matter most.
- pH near the pKa. Ratios between 1:10 and 10:1 give a useful range of about pKa ± 1 pH unit, and a buffer is most effective when its pH is close to its pKa. A buffer with equal acid and base needs more strong acid or base to move its pH by one unit (Harvey, section 6.8).
- Concentration. Higher buffer concentration gives better capacity but may introduce osmolarity or ionic-strength problems (Boston BioProducts, Good’s Buffer Overview). Temperature changes and high ionic strength are also listed there as causes of buffer precipitation.
What is buffer capacity, and what is buffer action?
Buffer action is the resistance to pH change described above. Buffer capacity is how much strong acid or base the buffer can absorb before its pH shifts appreciably. Capacity is not the same thing as pH: two buffers can sit at the same pH with very different capacities if their concentrations differ.
What is a buffer solution used for, and what are examples?
Buffers hold a working pH steady for enzymes, proteins and cells. The “Good’s buffers” commonly used in biochemistry cover the range from mildly acidic to alkaline. Typical pKa values and useful ranges from Boston BioProducts’ Good’s Buffer Overview (temperature not stated by the source) are:
| Buffer | Useful pH range | Typical pKa |
|---|---|---|
| MES | 5.5–6.7 | 6.15 |
| PIPES | 6.1–7.5 | 6.80 |
| MOPS | 6.5–7.9 | 7.20 |
| HEPES | 6.8–8.2 | 7.55 |
| HEPPS | 7.3–8.7 | 8.00 |
| Tricine | 7.4–8.8 | 8.15 |
| TAPS | 7.7–9.1 | 8.40 |
| Bicine | 7.8–8.8 | 8.35 |
| CAPS | 9.7–11.1 | 10.40 |
Acetic acid and acetate (pKa 4.76) cover the acidic end. For recipes and specifics see CASRAI’s guides on phosphate buffer (the answer to “what is phosphate buffer solution”), Tris-HCl buffer, HEPES and acetic acid pKa. For the selection criteria behind the Good’s series, read What Is a Biological Buffer?, and for making up stock solutions see buffer and solution preparation and molarity calculations.
Choosing a buffer: a checklist built from the sources above
- Start from the target pH of your experiment and pick a buffer whose pKa is within about 1 unit of it, ideally close (table above).
- Pick a concentration high enough for the capacity you need but low enough that osmolarity and ionic strength do not interfere (Boston BioProducts).
- Watch for the conditions the same source links to buffer precipitation: temperature shifts and high ionic strength.
- Use the Henderson-Hasselbalch check to estimate how far pH will move if your sample adds acid or base.
Common misunderstandings
- “Any acid in water is a buffer.” The comparison with plain water above shows why not: the resistance comes from having both a weak acid and its conjugate base present.
- “A buffer fixes the pH forever.” It resists change only until its capacity is used up; the Henderson-Hasselbalch ratio keeps moving as acid or base is added.
- “Capacity and pH are the same thing.” pH is set by the ratio and the pKa; capacity depends on how much buffer is present.
FAQ
What is a buffer solution?
A solution, typically a weak acid plus its conjugate base, that resists a change in pH when a small amount of strong acid or base is added.
What is a buffer solution in biochemistry?
The same idea applied to biological samples: buffers such as MES, MOPS, HEPES and Tricine hold the pH within the range where enzymes, proteins and cells are being studied.
How do I calculate the pH of a buffer?
Use pH = pKa + log([A−]/[HA]); see the worked acetate check above.
What pH range does a buffer work over?
About pKa ± 1 unit, and best close to the pKa.
Sources
- Harvey, D. Analytical Chemistry 2.1, section 6.8 “Buffer Solutions”, LibreTexts: chem.libretexts.org (read 2026-11-07).
- Boston BioProducts, “Good’s Buffer Overview”: bostonbioproducts.com (read 2026-11-07).
