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Buffer pH Calculator

Chemistry

Calculate buffer solution pH using the Henderson-Hasselbalch equation. Enter pKa and acid/conjugate base concentrations to get buffer pH and buffer capacity indicator.

4.74
0.1 mol/L
mol/L
0.1 mol/L
mol/L

Buffer pH

4.74
A⁻/HA Ratio
1
Effective Buffer Range
pH 3.74 – 5.74

This calculator computes your Buffer pH, A⁻/HA Ratio, Effective Buffer Range from the values you enter.

Inputs
pKa of Weak AcidConcentration of Acid (HA)Concentration of Conjugate Base (A⁻)
Outputs
Buffer pHA⁻/HA RatioEffective Buffer Range

What is a Buffer pH?

The Buffer pH Calculator computes the pH of a buffer solution using the Henderson-Hasselbalch equation: pH = pKa + log₁₀([A⁻]/[HA]). Enter the pKa of the weak acid and the molar concentrations of the acid (HA) and its conjugate base (A⁻), and the calculator returns buffer pH, the [A⁻]/[HA] ratio, and the effective buffer range (pKa ± 1).

A buffer solution resists pH change when small amounts of strong acid or base are added to it. This resistance arises from the equilibrium between a weak acid and its conjugate base. When acid is added, the conjugate base (A⁻) neutralises the H⁺; when base is added, the weak acid (HA) neutralises the OH⁻. The resulting pH shift is governed by the change in the [A⁻]/[HA] ratio, not by the absolute quantity of H⁺ added — which is why the pH barely shifts for reasonable additions of strong acid or base.

The Henderson-Hasselbalch equation quantifies this relationship. At equal concentrations of acid and conjugate base ([A⁻] = [HA]), the ratio is 1, log(1) = 0, and pH = pKa exactly. Increasing the proportion of conjugate base shifts pH above pKa; decreasing it shifts pH below pKa. The effective buffer range — where both components are present in useful amounts and the buffer can absorb either added acid or base — is pKa ± 1.

Common buffer systems in Indian chemistry and biology labs include the acetate buffer (acetic acid / sodium acetate, pKa 4.74, effective pH 3.7–5.7), the phosphate buffer (KH₂PO₄ / Na₂HPO₄, pKa 7.21, effective pH 6.2–8.2), and the carbonate buffer (H₂CO₃ / HCO₃⁻, pKa 6.35, effective pH 5.4–7.4). The physiological blood buffer at pH 7.4 uses the bicarbonate-carbonic acid system regulated by CO₂ exhalation.

For a deeper analysis of the Henderson-Hasselbalch equation — including different concentration combinations and the [H⁺] output — see the dedicated Henderson-Hasselbalch Calculator. To find the pKa of your acid from its Ka, use the pKa Calculator.

How to use this Buffer pH calculator

  1. Choose your weak acid and find its pKa — select an acid whose pKa is within ±1 of your target pH. Use the pKa Calculator to convert Ka to pKa if you have Ka from a reference. Common pKa values: acetic acid = 4.74, dihydrogen phosphate = 7.21, ammonium ion = 9.25.
  2. Enter pKa of Weak Acid — type the pKa value into the pKa of Weak Acid field. For acetic acid, enter 4.74.
  3. Enter Concentration of Acid (HA) — type the molar concentration of the weak acid (HA) into the Concentration of Acid (HA) field (unit: mol/L). For example, 0.1 mol/L for a 0.1 M solution.
  4. Enter Concentration of Conjugate Base (A⁻) — type the molar concentration of the conjugate base (typically the sodium or potassium salt) into the Concentration of Conjugate Base (A⁻) field (unit: mol/L).
  5. Read Buffer pH — the highlighted output shows the resulting pH. Compare this to your target. If it is too high, increase the acid concentration or decrease the base concentration to lower the [A⁻]/[HA] ratio.
  6. Check [A⁻]/[HA] Ratio and Effective Range — confirm the ratio is between 0.1 and 10 and that your target pH falls within the stated effective range. For finer control over the buffer composition, take these values to the Henderson-Hasselbalch Calculator.

Formula & Methodology

Henderson-Hasselbalch equation:

> pH = pKa + log₁₀([A⁻]/[HA])

Where:
- pKa = −log₁₀(Ka) of the weak acid
- [A⁻] = molar concentration of conjugate base (mol/L)
- [HA] = molar concentration of weak acid (mol/L)

Rearrangements for buffer design:

To find the required ratio for a target pH:
> [A⁻]/[HA] = 10^(pH_target − pKa)

Effective buffer range:

> pH_effective = pKa ± 1  (corresponding to [A⁻]/[HA] ratios of 0.1 to 10)

Worked example 1 — Acetate buffer preparation:

Prepare a buffer at pH 5.0 using acetic acid (pKa = 4.74):
- Required ratio: [A⁻]/[HA] = 10^(5.0 − 4.74) = 10^(0.26) = 1.82
- To prepare 0.1 M total buffer: use acid concentration = 0.1 ÷ (1 + 1.82) = 0.0355 M; conjugate base = 0.0645 M
- Verification: pH = 4.74 + log(0.0645/0.0355) = 4.74 + log(1.82) = 4.74 + 0.26 = 5.00 ✓

Worked example 2 — JEE-style problem:

A buffer contains 0.2 mol/L CH₃COOH and 0.3 mol/L CH₃COONa. Given Ka = 1.8 × 10⁻⁵ (pKa = 4.74):
- [A⁻]/[HA] = 0.3/0.2 = 1.5
- pH = 4.74 + log(1.5) = 4.74 + 0.176 = 4.92
- Effective range: pKa ± 1 = 3.74 to 5.74 — this pH falls within range ✓

Common buffers and their pKa values:

| Buffer system | Acid | pKa | Effective pH range |
|---|---|---|---|
| Acetate | CH₃COOH / CH₃COO⁻ | 4.74 | 3.7–5.7 |
| Phosphate | H₂PO₄⁻ / HPO₄²⁻ | 7.21 | 6.2–8.2 |
| HEPES | — | 7.55 | 6.8–8.2 |
| Tris-HCl | Tris-H⁺ / Tris | 8.07 | 7.0–9.0 |
| Borate | H₃BO₃ / B(OH)₄⁻ | 9.24 | 8.2–10.2 |

Frequently Asked Questions

A buffer solution is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes in pH when small amounts of strong acid or base are added. The buffer works because the weak acid can neutralise added base (HA + OH⁻ → A⁻ + H₂O) while the conjugate base can neutralise added acid (A⁻ + H⁺ → HA). This dual capacity keeps the pH relatively stable over a useful range.
The Henderson-Hasselbalch equation is pH = pKa + log₁₀([A⁻]/[HA]), where pKa is the acid dissociation constant of the weak acid, [A⁻] is the molar concentration of the conjugate base, and [HA] is the molar concentration of the weak acid. When [A⁻] = [HA], log(1) = 0 and pH = pKa. This is the midpoint of the buffer range — the pH where the buffer has maximum capacity.
A buffer is most effective when pH is within ±1 unit of the pKa of the weak acid: effective range = pKa ± 1. Within this range, the [A⁻]/[HA] ratio is between 0.1 and 10, which means both components are present in significant amounts and can neutralise either added acid or base. Outside pKa ± 1, the buffer capacity drops sharply because one component is nearly exhausted.
The [A⁻]/[HA] ratio (or base-to-acid ratio) determines where the buffer pH sits relative to the pKa. At ratio = 1 (equal concentrations), pH = pKa. At ratio = 10, pH = pKa + 1. At ratio = 0.1, pH = pKa − 1. The Buffer pH Calculator outputs this ratio alongside the pH, so you can verify whether the ratio is within the 0.1–10 range for effective buffering.
A strong acid or base solution has a pH determined solely by the concentration of the strong acid or base — adding more water changes pH predictably. A buffer solution maintains its pH when diluted or when small amounts of acid or base are added, because the equilibrium between HA and A⁻ adjusts to compensate. Buffers only work within their effective range (pKa ± 1) and have a limited capacity — adding too much acid or base exhausts the buffer.
pKa is a fixed property of the acid (it only changes with temperature). Buffer pH is the pH of the actual buffer solution, which depends on both pKa and the [A⁻]/[HA] concentration ratio. By adjusting the ratio of acid to conjugate base, you can prepare a buffer at any pH within pKa ± 1. For example, using acetic acid (pKa 4.74), you can prepare buffers ranging from pH 3.74 to 5.74 by varying the acetate-to-acid ratio.
Enter the pKa of your chosen acid (e.g., 4.74 for acetic acid), then enter the acid and conjugate base concentrations to see the resulting buffer pH. Adjust the concentrations until the output matches your target pH. The Henderson-Hasselbalch rearrangement gives the required ratio directly: [A⁻]/[HA] = 10^(pH_target − pKa). For example, to prepare pH 5.0 with acetic acid: [A⁻]/[HA] = 10^(5.0 − 4.74) = 10^0.26 = 1.82, meaning use 1.82 parts sodium acetate to 1 part acetic acid.
Common buffer systems used in Indian undergraduate labs include: acetate buffer (acetic acid / sodium acetate, pH 3.7–5.7), phosphate buffer (sodium dihydrogen phosphate / disodium hydrogen phosphate, pH 6.2–8.2), and borate buffer (boric acid / sodium borate, pH 7.8–10.0). The phosphate buffer at pH 7.4 is widely used in biochemistry practicals and cell culture, as it mimics physiological pH. Oxalic acid buffers are used in standardisation practicals.
Adding strong acid (H⁺) consumes conjugate base: A⁻ + H⁺ → HA, increasing [HA] and decreasing [A⁻], which lowers the [A⁻]/[HA] ratio and shifts pH downward slightly. Adding strong base (OH⁻) consumes weak acid: HA + OH⁻ → A⁻ + H₂O, increasing [A⁻] and decreasing [HA], raising pH. Because the pH change depends on the log of the ratio, small additions cause only small pH changes — until one component is exhausted and the buffer fails.
Buffer solutions are covered in NCERT Class 11 Chemistry Chapter 7 (Equilibrium), where the Henderson-Hasselbalch equation and the concept of pH stability are introduced. Class 12 Biology discusses physiological buffers (blood carbonate-bicarbonate system, haemoglobin buffer). In undergraduate analytical chemistry practicals across Indian universities, students prepare acetate and phosphate buffers and verify their pH stability — the Buffer pH Calculator is a useful pre-lab tool for calculating the required component ratios.
No — a specific buffer pair only works within pKa ± 1 of the weak acid's pKa. To buffer at pH 7, choose an acid with pKa near 7, such as dihydrogen phosphate (pKa 7.21) or HEPES (pKa 7.55, used in cell biology). To buffer at pH 9.5, choose boric acid (pKa 9.24) or carbonate (pKa 10.33). There is no single universal buffer for all pH values — different acid-base pairs are used for different pH ranges.
Buffer capacity (β) is the amount of strong acid or base that a litre of buffer can absorb before the pH changes by 1 unit. Buffer capacity is greatest at pH = pKa and increases with the total concentration of the acid-base pair. A 0.5 M acetate buffer has five times the buffer capacity of a 0.1 M acetate buffer at the same pH. The Buffer pH Calculator shows the effective range but not the numerical capacity — for capacity calculations, use the Henderson-Hasselbalch equation combined with the Koppel-Spiro formula.
Also known as
Henderson-Hasselbalch bufferbuffer solution pHweak acid buffer calculatorbuffer capacityacetate buffer calculator