Buffer pH Calculator
Our buffer pH calculator provides you with an effortless way to compute the pH of any kind of buffer solution. To decide whether your buffer is based on an acid & its conjugate base or the other way around, enter the data, and let the calculator do the job for you.
How to use the buffer pH calculator?
Our buffer calculator requires 5 easy steps:
- The big choice: is your buffer based on an acid or a base?
- Do you know the K, pK, or neither?
- If you don't know the pK, check out our list of popular buffer pKa values below.
- Enter the molar concentrations of your reagents — available in mol/L (M), mmol/L (mM), and μmol/L (μM).
- Your pH is shown at the right of the tool!
Optionally, enter the volume of your solution (metric: mL, L; or US: fl oz, cups, pints, quarts) to calculate exactly how many moles of each reagent you need to prepare the buffer. You can also enter the temperature (°C or °F) for reference — note that pKw = 14 is standard at 25 °C and changes at other temperatures.
Examples of pKa for different buffers — find your own
To make things even easier for you, here is a list of popular buffers and their pKa values at 25 °C, arranged from most acidic to most alkaline:
| Buffer | pKa (25 °C) | Ka |
|---|---|---|
| TFA (trifluoroacetic acid) | 0.5 | 3.16 × 10⁰ |
| Sulfonate | 1.8 | 1.58 × 10⁻² |
| Aspartic acid | 2.1 / 3.9 / 9.8 | — |
| Phosphate (H₃PO₄ / H₂PO₄⁻) | 2.1 / 7.2 / 12.3 | — |
| Chloroacetate | 2.9 | 1.26 × 10⁻³ |
| Citrate | 3.1 / 4.8 / 6.4 | — |
| Gluconic acid | 3.6 | 2.51 × 10⁻⁴ |
| Formate | 3.8 | 1.58 × 10⁻⁴ |
| Lactate | 3.9 | 1.26 × 10⁻⁴ |
| Acetate (acetic acid) | 4.76 | 1.74 × 10⁻⁵ |
| Carbonate (H₂CO₃ / HCO₃⁻) | 6.4 | 3.98 × 10⁻⁷ |
| Phosphate (H₂PO₄⁻ / HPO₄²⁻) | 7.2 | 6.31 × 10⁻⁸ |
| HEPES | 7.5 | 3.16 × 10⁻⁸ |
| Tris | 8.1 | 7.94 × 10⁻⁹ |
| Carbonate (HCO₃⁻ / CO₃²⁻) | 10.3 | 5.01 × 10⁻¹¹ |
| Ammonia / Ammonium | 9.2 (pKb = 4.8) | 6.31 × 10⁻¹⁰ |
| Phosphate (HPO₄²⁻ / PO₄³⁻) | 12.3 | 5.01 × 10⁻¹³ |
Example: To compute the pH of a sodium phosphate buffer, enter the pKa = 7.2 (for the H₂PO₄⁻ / HPO₄²⁻ pair). Remember: buffers work best if the solution's pH is close to the buffer's pKa (±1).
How to calculate the pH of a buffer solution?
🅰 Acid buffer — Henderson-Hasselbalch formula
The standard buffer pH equation is based on the Henderson-Hasselbalch formula:
pH = pKa + log₁₀([A⁻] / [HA])
where:
- pH = −log₁₀[H⁺]
- Ka — acid dissociation constant
- [HA] — molar concentration of the weak acid
- [A⁻] — molar concentration of the conjugate base
- pKa = −log₁₀(Ka)
This equation works on solutions made of an acid & its conjugate base. When [A⁻] = [HA], the ratio equals 1 and log(1) = 0, so pH = pKa. This is why buffers are most effective at pH ≈ pKa.
🅱 Base buffer — converting pKb to pKa
If your buffer is based on a weak base B and its conjugate acid BH⁺, enter the pKb of the base. The calculator automatically converts:
pKa(BH⁺) = pKw − pKb = 14 − pKb (at 25 °C) pH = pKa(BH⁺) + log₁₀([B] / [BH⁺])
For example, ammonia (NH₃) has pKb = 4.8, so pKa(NH₄⁺) = 14 − 4.8 = 9.2. An ammonia/ammonium buffer works best around pH 9.2.
What is a buffer?
A buffer solution is an aqueous solution consisting of a weak acid (or base) and its conjugate that resists significant pH changes when a small amount of strong acid or base is added. Buffers are critical in:
- Biological systems (blood pH ≈ 7.4 maintained by carbonate buffer)
- Laboratory experiments (enzyme assays, electrophoresis, PCR)
- Industrial processes (fermentation, pharmaceutical manufacturing)
- Food preservation and swimming pool chemistry
A buffer is most effective when the pH is within ±1 of the pKa of the weak acid. Outside this range, the buffering capacity drops sharply.
Measurement units supported
📐 Metric system
- Concentration: mol/L (M), mmol/L (mM), μmol/L (μM)
- Volume: mL, L
- Temperature: °C
🇺🇸 American (US) system
- Concentration: mol/L (same, internationally standard)
- Volume: fl oz, cups, pints, quarts
- Temperature: °F
FAQs
- What is the pH of a 0.1 M acetic acid / 0.1 M sodium acetate buffer?
- pKa of acetic acid = 4.76. Ratio = 0.1/0.1 = 1, log(1) = 0. So pH = 4.76 + 0 = 4.76.
- How do I prepare a phosphate buffer at pH 7.4?
- Use the H₂PO₄⁻/HPO₄²⁻ pair (pKa = 7.2). Enter pKa = 7.2 and adjust concentrations until pH = 7.4. From the formula: log([HPO₄²⁻]/[H₂PO₄⁻]) = 7.4 − 7.2 = 0.2, so [HPO₄²⁻]/[H₂PO₄⁻] ≈ 1.58.
- Does temperature affect buffer pH?
- Yes. The pKa of most buffers changes with temperature (typically 0.01–0.03 pH units per °C). The standard pKw = 14 applies at 25 °C. For precise work at other temperatures, use temperature-corrected pKa values.
- What is the buffering capacity?
- Buffering capacity (β) is maximized when [A⁻] = [HA] (i.e., pH = pKa) and decreases as the ratio deviates from 1:1. The effective buffering range is pH = pKa ± 1.
- Can I use this for HEPES or Tris buffers?
- Yes. Enter pKa = 7.5 for HEPES or pKa = 8.1 for Tris (at 25 °C). These are zwitterionic buffers commonly used in biochemistry. Note that Tris pKa decreases significantly with temperature (about −0.03/°C).