Last updated: July 3, 2026
Henderson-Hasselbalch Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The Henderson-Hasselbalch calculator estimates buffer pH with pH = pKa + log₁₀([A⁻]/[HA]). It takes pKa, conjugate base concentration, and weak acid concentration, then returns pH plus the ratio and pH shift. Equal acid and base concentrations give pH = pKa.
The Henderson-Hasselbalch equation says buffer pH equals pKa plus the base-ten logarithm of conjugate base concentration divided by weak acid concentration.
Key Takeaways
- Use pH = pKa + log₁₀([A⁻]/[HA]) for weak-acid/conjugate-base buffers.
- Equal conjugate base and weak acid concentrations make pH equal to pKa.
- A ratio of 2 raises pH by 0.301 units; a ratio of 1/3 lowers it by 0.477 units.
- The best practical buffer range is usually pKa ± 1 pH unit.
- Use final mixed concentrations or consistent mole amounts, not mismatched stock units.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
pH = pKa + log10([A⁻]/[HA])
Where:
- pH=Acidity of the buffer solution(dimensionless)
- pKa=Negative log₁₀ of the acid dissociation constant(dimensionless)
- [A⁻]=Conjugate base concentration(mol/L)
- [HA]=Weak acid concentration(mol/L)
Worked Examples
Equal acetate buffer components
Equal conjugate base and weak acid concentrations make the logarithm term zero.
- 1Use pH = pKa + log₁₀([A⁻]/[HA]).
- 2Compute the ratio: [A⁻]/[HA] = 0.10 / 0.10 = 1.00.
- 3log₁₀(1.00) = 0, so pH = 4.76 + 0 = 4.76.
Phosphate buffer with twice as much conjugate base
A base-to-acid ratio of two raises pH by log₁₀(2).
- 1Compute the ratio: [A⁻]/[HA] = 0.20 / 0.10 = 2.00.
- 2Calculate the logarithm term: log₁₀(2.00) = 0.30103.
- 3Add to pKa: pH = 7.21 + 0.30103 = 7.51103, reported as 7.511.
Ammonium buffer with acid excess
A base-to-acid ratio of one third lowers pH below pKa.
- 1Compute the ratio: [A⁻]/[HA] = 0.05 / 0.15 = 0.33333.
- 2Calculate the logarithm term: log₁₀(1/3) = −0.47712.
- 3Add to pKa: pH = 9.25 − 0.47712 = 8.77288, reported as 8.773.
Introduction
The Henderson-Hasselbalch calculator estimates the pH of a weak-acid buffer from pKa and the concentration ratio of conjugate base to weak acid. It is the focused form of the broader buffer pH calculator for acidic buffer pairs such as acetate/acetic acid, phosphate, and ammonium/ammonia. The equation is derived from the acid dissociation expression and the IUPAC pH definition.
Henderson-Hasselbalch equation
For a weak acid HA and its conjugate base A⁻, the working equation is pH = pKa + log₁₀([A⁻]/[HA]). The ratio term is dimensionless, so the two concentrations must use the same units and refer to the same final solution. When [A⁻] equals [HA], log₁₀(1) is zero and pH equals pKa.
[A⁻]/[HA] = 1 gives pH = pKa.
A tenfold excess of A⁻ raises pH by 1 unit.
A tenfold excess of HA lowers pH by 1 unit.
Use the pKa calculator when you need to infer pKa from measurements.
How to calculate buffer pH
Enter the pKa for the acid pair, then enter the conjugate base concentration and the weak acid concentration after mixing. The calculator divides A⁻] by [HA], takes the base-ten logarithm, and adds that shift to pKa. If you are preparing the buffer from stocks, first compute final concentrations after dilution; the [concentration calculator can help check those values.
If both species are in the same final volume, you may use moles instead of mol/L because the common volume cancels in the ratio.
Best buffer range
The equation is most useful for practical buffers when the ratio is between about 0.1 and 10. That corresponds to pH within approximately pKa ± 1, where both HA and A⁻ remain present in meaningful amounts. Outside that range the pH estimate may still be mathematically valid, but buffer capacity becomes weak and small additions of acid or base can cause large pH changes.
For maximum buffer capacity, choose a conjugate pair with pKa close to your target pH and keep [A⁻]/[HA] near 1.
Assumptions and activity effects
The classroom Henderson-Hasselbalch form uses molar concentrations as approximations for chemical activities. In concentrated salt solutions, high ionic strength media, mixed solvents, or very precise analytical work, activity coefficients and temperature-dependent pKa values can shift the measured pH. The LibreTexts Henderson-Hasselbalch discussion explains when the approximation is appropriate for routine aqueous buffers.
Common buffer pairs and pKa values
Several acid/base pairs are widely used because their pKa values land near important laboratory pH regions. Acetate buffers cover mildly acidic work, phosphate buffers cover near-neutral work, and ammonium/ammonia covers mildly basic work. Check authoritative tables such as the NIST Chemistry WebBook when temperature and ionic strength matter.
| Buffer pair | Typical pKa | Useful region |
|---|---|---|
| Acetic acid / acetate | 4.76 | pH 3.8–5.8 |
| H₂PO₄⁻ / HPO₄²⁻ | 7.21 | pH 6.2–8.2 |
| NH₄⁺ / NH₃ | 9.25 | pH 8.3–10.3 |
| Carbonic acid / bicarbonate | 6.35 | pH 5.4–7.4 |
| Tris / Tris-H⁺ | 8.1 | pH 7.1–9.1 |
Laboratory workflow
Select a buffer pair with pKa close to the target pH, calculate the required ratio, convert the ratio to reagent amounts, prepare the solution in a volumetric flask, and verify the pH with a calibrated meter. For titration experiments, combine this calculation with the titration calculator to understand equivalence regions and buffer zones before the endpoint.
Quick Reference Card
Henderson-Hasselbalch — Quick Reference
Quick reference • Henderson-Hasselbalch Calculator
pH = pKa + log₁₀([A⁻]/[HA])Valid range: Best practical buffer range: 0.1 ≤ [A⁻]/[HA] ≤ 10, approximately pKa ± 1
Common Values
⚠ Watch Out
- •Do not divide concentrations with different units or different final volumes.
- •Do not use the equation for strong acid/strong base mixtures.
- •Very dilute buffers may have poor capacity even when the ratio looks ideal.
- •High ionic strength makes activity corrections more important.
- •Use temperature-corrected pKa values for precision work.
Pro Tips
- →Choose a pKa within one pH unit of your target before adjusting the ratio.
- →If both species are diluted together, pH changes little because the ratio is unchanged.
- →For recipes, calculate moles from the target total buffer concentration and volume.
- →Calibrate pH meters with standards bracketing the target pH.
- →Report pH with realistic significant figures based on pKa and concentration precision.
FAQs
What does the Henderson-Hasselbalch equation calculate?
It calculates the approximate pH of a buffer from the acid pKa and the conjugate base to weak acid concentration ratio: pH = pKa + log₁₀([A⁻]/[HA]).
Why is pH equal to pKa when [A⁻] equals [HA]?
Because [A⁻]/[HA] equals 1 and log₁₀(1) equals 0, leaving pH = pKa.
Can I use moles instead of molar concentrations?
Yes, if the weak acid and conjugate base are in the same final volume. The common volume cancels, so the mole ratio equals the concentration ratio.
What happens if the conjugate base concentration is larger?
The ratio [A⁻]/[HA] becomes greater than 1, the logarithm term is positive, and pH rises above pKa.
What is the useful range of a Henderson-Hasselbalch buffer?
A common practical range is pKa ± 1 pH unit, equivalent to a conjugate base to acid ratio from about 0.1 to 10.
Why might measured pH differ from the calculator result?
Temperature, ionic strength, activity coefficients, calibration error, and using stock concentrations instead of final mixed concentrations can all shift measured pH.