Buffer pH Calculator
The pH of a buffer from its pKa and the concentrations of weak acid and conjugate base, or the base-to-acid ratio for a target pH, by the Henderson–Hasselbalch equation.
Result
pH: 5.2371
An approximation that ignores activity. For precise work, measure the pH of the made-up buffer with a calibrated meter.
- Base-to-acid ratio, [A⁻] ÷ [HA]
- 3
- log₁₀ of the ratio
- 0.477121
- Conjugate base, [A⁻]
- 0.15
- Weak acid, [HA]
- 0.05
https://calcgallery.com/chemistry/buffer-ph-calculator/
Results come from the published equations applied to the values you enter. They are not electrical design, conductor sizing, laboratory safety or medical guidance. Weather indices describe a standard person in standard conditions and are not a forecast. Where codes apply, local rules vary and must be checked. Read the full disclaimer
How to use the Buffer pH Calculator
- Choose whether to find the pH of a buffer or the ratio of base to acid that gives a target pH.
- Enter the weak acid's pKa, then either the two concentrations or the target pH, with an optional total concentration.
- Read the pH or the ratio, and for a total, how much of each component to use.
The formulas
pH = pKa + log₁₀([A⁻] ÷ [HA])[A⁻] ÷ [HA] = 10^(pH − pKa)for a total concentration C and ratio r: [A⁻] = C × r ÷ (1 + r) [HA] = C ÷ (1 + r)The useful range
A buffer resists pH change because it has both components to react with added acid or base. OpenStax notes that once one component is less than about a tenth of the other, the buffer has generally lost its usefulness: that is a pH more than 1 away from the pKa. The page still gives the pH there and adds a note.
What the diagram shows
The pH scale around the pKa, with the range from pKa − 1 to pKa + 1 shaded and the buffer’s pH marked. Left of the pKa the acid form is the larger share; right of it, the base form.
With your numbers
- pH = pKa + log₁₀([A⁻] ÷ [HA]) = 4.76 + log₁₀(0.15 ÷ 0.05) = 5.237121
Quick reference
An approximation that ignores activity. For precise work, measure the pH of the made-up buffer with a calibrated meter.
| Example | Result |
|---|---|
| pKa 4.76, 0.15 base and 0.05 acid | pH: 5.2371 |
| pKa 4.76, equal amounts | pH: 4.76 |
| pKa 6.4, 0.024 base and 0.0012 acid | pH: 7.701 |
| Ratio for pH 5.0 at pKa 4.76, 0.1 total | Base-to-acid ratio, [A⁻] ÷ [HA]: 1.737801 |
| Ratio for pH 7.4 at pKa 7.21 | Base-to-acid ratio, [A⁻] ÷ [HA]: 1.548817 |
| pKa 9.25, 0.2 base and 0.3 acid | pH: 9.0739 |
Common questions about the Buffer pH Calculator
- What is the Henderson–Hasselbalch equation?
- pH = pKa + log₁₀([A⁻] ÷ [HA]): the pH of a buffer made of a weak acid, HA, and its conjugate base, A⁻, depends on the acid's pKa and the ratio of base to acid. With equal amounts of each, the log term is zero and the pH equals the pKa.
- How do I make a buffer at a particular pH?
- Choose a weak acid whose pKa is within about 1 of the pH you want, then use the ratio 10^(pH − pKa). For pH 5.0 with a pKa of 4.76 the ratio of base to acid is about 1.74, so a 0.1 M total splits into about 0.0635 M base and 0.0365 M acid.
- Do I need concentrations or moles?
- Either, as long as both components use the same one. Only the ratio enters the equation, and when acid and base are in the same solution the volume cancels. The same applies to millimoles or to grams converted to moles.
- How accurate is it?
- It takes the amounts of acid and base you put in to be the amounts present once the solution settles, ignoring the acid's own dissociation and that of water. That holds when both components are far more concentrated than the hydrogen and hydroxide ions those reactions make, so it drifts for very dilute buffers and for acids with a very low or very high pKa, where the true pH sits closer to neutral than the equation says. It also treats concentrations as if they were activities, which is a better approximation the more dilute the solution. pKa itself changes with temperature and with the other salts present. For precise work, make up the buffer and adjust it against a calibrated pH meter.