Our pH Buffer Calculator uses the Henderson-Hasselbalch equation to calculate the pH of a buffer solution or find the acid/base ratio needed to prepare a buffer at a target pH — instantly, no log tables needed.
pH Buffer Calculator
Henderson-Hasselbalch Equation
pH = pKa + log₁₀([A⁻] / [HA])
Where [A⁻] is the concentration of the conjugate base and [HA] is the concentration of the weak acid.
Common Buffer Systems
| Buffer System | pKa | Useful pH Range |
|---|---|---|
| Acetic acid / Sodium acetate | 4.75 | 3.75 – 5.75 |
| Phosphate (H₂PO₄⁻/HPO₄²⁻) | 7.20 | 6.20 – 8.20 |
| Citric acid / Sodium citrate | 4.77 / 5.19 | 3.0 – 6.2 |
| Tris (THAM) | 8.07 | 7.0 – 9.0 |
| Carbonate / Bicarbonate | 10.33 | 9.2 – 11.0 |
Buffer Capacity and Limitations
A buffer works best when pH = pKa ± 1. Beyond this range the buffer capacity drops sharply. For the buffer to resist pH changes effectively:
- Keep [A⁻]/[HA] between 0.1 and 10
- Use a buffer with pKa within 1 unit of your target pH
- Use a high enough total concentration (0.05–0.5 M) for adequate capacity
Worked Example: 1 Litre of 0.1 M Acetate Buffer at pH 5.0
Acetic acid has a pKa of 4.76. Rearranging Henderson-Hasselbalch for the ratio:
- log([A-]/[HA]) = pH – pKa = 5.00 – 4.76 = 0.24
- [A-]/[HA] = 10 raised to 0.24 = 1.74
- Total concentration must be 0.1 M, so [HA] + [A-] = 0.1
- Solving: [HA] = 0.1 / (1 + 1.74) = 0.0365 M acetic acid
- [A-] = 0.1 – 0.0365 = 0.0635 M sodium acetate
Converting to quantities for 1 litre, using molar masses of 60.05 g/mol for acetic acid and 82.03 g/mol for sodium acetate:
- Acetic acid: 0.0365 x 60.05 = 2.19 g, or 2.09 mL of glacial acetic acid
- Sodium acetate: 0.0635 x 82.03 = 5.21 g
Dissolve both in about 900 mL of water, check the pH with a calibrated meter, adjust with dilute acid or base as needed, then make up to 1 litre.
Choosing the Right Buffer
Work through these in order:
- Match the pKa to your target pH. The pKa should be within 1 unit of your working pH, and ideally within 0.5.
- Check chemical compatibility. Phosphate precipitates with calcium and magnesium and inhibits several enzymes. Tris interferes with some protein assays and reacts with aldehydes.
- Pick a concentration. 0.05 to 0.5 M covers most work. Higher gives more capacity but adds ionic strength that can affect the system you are studying.
- Consider the working temperature. See below, because this is the trap.
Temperature Changes pH More Than People Expect
pKa values are temperature dependent, and the size of the effect varies enormously between buffers.
| Buffer | Change in pKa per degree C | Practical consequence |
|---|---|---|
| Tris | About -0.028 | A buffer set to pH 8.0 at 25 C reads about 8.6 at 4 C |
| Phosphate | About -0.0028 | Very stable, shifts only slightly |
| Acetate | About 0.0002 | Essentially temperature independent |
| HEPES | About -0.014 | Moderate shift, worth accounting for |
Practical Preparation Notes
- Calibrate the meter first, with fresh standards bracketing your target pH.
- Dissolve fully before adjusting. Reading the pH of a partly dissolved solution gives a misleading value.
- Adjust before making up to volume, then top up to the final mark, since adding acid or base changes the volume.
- Prepare fresh or store cold. Phosphate and Tris buffers support microbial growth readily. Filter-sterilise for anything sensitive.
Common Buffer Mistakes
- Using a buffer more than 1 pH unit from its pKa. Capacity collapses outside that window, and the solution no longer meaningfully buffers.
- Adjusting Tris at the wrong temperature. The single most common source of unexplained pH error in the lab.
- Ignoring dilution. Diluting a buffer reduces its capacity proportionally and can shift pH slightly through ionic strength effects.
- Using phosphate with divalent cations. Calcium and magnesium phosphates precipitate, quietly removing buffer from solution.
Frequently Asked Questions
What is a buffer solution?
A buffer resists changes in pH when small amounts of acid or base are added. It contains a weak acid and its conjugate base, or a weak base and its conjugate acid, in comparable concentrations. Adding acid is absorbed by the conjugate base and adding base is absorbed by the weak acid, so the pH moves far less than it would in unbuffered water.
What is the Henderson-Hasselbalch equation used for?
It relates the pH of a buffer to the pKa of the weak acid and the ratio of conjugate base to acid: pH = pKa + log10([A-]/[HA]). You can use it in either direction, to predict the pH of a known mixture or, more commonly, to work out what ratio of the two components you need to hit a target pH.
How do I prepare an acetate buffer at pH 5.0?
Acetic acid has a pKa of 4.76, so the required ratio is 10 raised to (5.00 – 4.76) = 1.74 parts acetate to 1 part acetic acid. For 1 litre of 0.1 M buffer that is 0.0365 M acetic acid (2.19 g) and 0.0635 M sodium acetate (5.21 g). Dissolve in about 900 mL, check the pH with a calibrated meter, adjust, then make up to 1 litre.
What pH range can a buffer effectively control?
Within about 1 pH unit either side of its pKa, which is where the ratio of base to acid stays between 0.1 and 10. Outside that window one component is nearly exhausted and buffer capacity falls sharply. For the best performance, choose a buffer whose pKa is within 0.5 units of your target pH.
Why does my Tris buffer change pH when I cool it?
Because the pKa of Tris is strongly temperature dependent, shifting by about -0.028 pH units per degree Celsius. A Tris buffer adjusted to pH 8.0 at 25 C will read roughly 8.6 at 4 C. Always adjust the pH at the temperature the buffer will actually be used at. Phosphate and acetate are far less affected.
What concentration should my buffer be?
Between 0.05 and 0.5 M covers most laboratory work. Higher concentrations provide more buffering capacity but also raise ionic strength, which can affect enzyme activity, protein solubility and binding interactions. Choose the lowest concentration that holds your pH stable through the experiment.
When should I avoid phosphate buffer?
Avoid it when calcium or magnesium ions are present, since their phosphates precipitate and quietly remove buffer from solution. Phosphate also inhibits a number of enzymes, including some kinases and polymerases, and it can interfere with protein assays. HEPES or Tris are common alternatives in that pH range.