Last updated: July 3, 2026
Buffer pH Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The buffer pH calculator uses the Henderson–Hasselbalch equation. For acidic buffers, pH = pKa + log₁₀([A⁻]/[HA]); equal acid and conjugate base amounts give pH = pKa. It can also solve the required ratio for a target pH and handle basic buffers through pOH = pKb + log₁₀([BH⁺]/[B]).
For an acidic buffer, pH equals pKa plus the base-ten logarithm of conjugate base divided by weak acid. Equal amounts give pH equal to pKa, and a target pH uses the ratio ten raised to pH minus pKa.
Key Takeaways
- Acidic buffers use pH = pKa + log₁₀([A⁻]/[HA]).
- Equal weak acid and conjugate base amounts give pH = pKa.
- The target ratio is [A⁻]/[HA] = 10^(pH − pKa).
- Buffers are most useful when the ratio is between 0.1 and 10, about pKa ± 1.
- Basic buffers are calculated in pOH form and converted with pH = 14 − pOH at 25 °C.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
Acidic buffer: pH = pKa + log₁₀([A⁻]/[HA]); required ratio = 10^(pH−pKa); basic buffer: pH = 14 − (pKb + log₁₀([BH⁺]/[B]))
Where:
- pH=Acidity of the buffer solution(dimensionless)
- pKa=Negative log₁₀ of the acid dissociation constant(dimensionless)
- [A⁻]=Concentration or moles of conjugate base(mol/L or mol)
- [HA]=Concentration or moles of weak acid(mol/L or mol)
- pKb=Negative log₁₀ of the base dissociation constant(dimensionless)
- [BH⁺]=Conjugate acid amount in a basic buffer(mol/L or mol)
- [B]=Weak base amount in a basic buffer(mol/L or mol)
Worked Examples
Acetate buffer with equal acid and base
Sodium acetate and acetic acid at equal analytical amounts place the buffer pH at pKa.
- 1Use the acidic Henderson–Hasselbalch equation: pH = pKa + log₁₀([A⁻]/[HA]).
- 2Compute the ratio: [A⁻]/[HA] = 0.10 / 0.10 = 1.00.
- 3log₁₀(1.00) = 0, so pH = 4.74 + 0 = 4.74.
Acetate buffer with twice as much conjugate base
Increasing acetate relative to acetic acid raises pH by log₁₀(2).
- 1Ratio = 0.20 / 0.10 = 2.00.
- 2pH = 4.74 + log₁₀(2.00).
- 3log₁₀(2.00) = 0.301, so pH = 5.041 ≈ 5.04.
Acetate buffer with twice as much weak acid
A base-to-acid ratio below one lowers pH below pKa.
- 1Ratio = 0.10 / 0.20 = 0.50.
- 2pH = 4.74 + log₁₀(0.50).
- 3log₁₀(0.50) = −0.301, so pH = 4.439 ≈ 4.44.
Phosphate buffer at equal amounts
For the H₂PO₄⁻/HPO₄²⁻ pair, equal conjugate acid and base gives pH equal to pKa₂.
- 1Ratio = 0.05 / 0.05 = 1.00.
- 2pH = pKa + log₁₀(1.00).
- 3The logarithm term is zero, so pH = 7.21.
Required acetate ratio for target pH 5.00
Solve the inverse Henderson–Hasselbalch relation for [A⁻]/[HA].
- 1Rearrange: [A⁻]/[HA] = 10^(pH − pKa).
- 2Substitute pH = 5.00 and pKa = 4.74.
- 3Ratio = 10^(0.26) = 1.82, so use about 1.82 parts acetate per 1 part acetic acid.
Introduction
A buffer resists pH change because it contains both a weak acid and its conjugate base. This buffer pH calculator applies the Henderson–Hasselbalch equation, pH = pKa + log₁₀(A⁻]/[HA]), so you can calculate an acidic buffer pH, design the [base-to-acid ratio needed for a target pH, or back-calculate pKa from a measured buffer. It also supports basic buffers through pOH = pKb + log₁₀([BH⁺]/[B]).
Henderson–Hasselbalch equation for acidic buffers
For an acidic buffer containing weak acid HA and conjugate base A⁻, the working equation is pH = pKa + log₁₀([A⁻]/[HA]). If [A⁻] = [HA], the logarithm term is zero and pH equals pKa. If the conjugate base amount is ten times the acid amount, pH is one unit above pKa; if it is one tenth as large, pH is one unit below pKa.
Use concentrations after mixing, not stock bottle labels, when volumes differ.
You may use moles instead of concentrations when both species are in the same final volume.
The ratio term is dimensionless, so units cancel only if both amounts use the same unit.
For stoichiometric preparation, pair this tool with the molarity calculator.
Solving the ratio needed for a target pH
Rearranging the equation gives [A⁻]/[HA] = 10^(pH − pKa). A target pH above pKa requires more conjugate base than acid; a target pH below pKa requires more acid. For acetate at pKa 4.74 and target pH 5.00, the required ratio is 10^0.26 = 1.82, meaning 1.82 moles acetate for every mole acetic acid.
Choose a buffer acid whose pKa is within about 1 pH unit of your desired pH before adjusting the ratio.
Basic buffers and pOH
For a weak base B and its conjugate acid BH⁺, the Henderson–Hasselbalch equation is written in pOH form: pOH = pKb + log₁₀([BH⁺]/[B]). The calculator then converts to pH using pH = 14 − pOH at 25 °C. If you know the conjugate acid pKa instead, you can often use pKa + pKb = 14 for a conjugate pair in water at 25 °C.
At temperatures far from 25 °C, pKw is not exactly 14; use temperature-corrected data for high-precision basic buffers.
Best buffer range and capacity
Buffers work best near their pKa because both acid and conjugate base are present in substantial amounts. A practical rule is 0.1 ≤ A⁻]/[HA] ≤ 10, equivalent to pH within pKa ± 1. Outside this range the solution still follows the equation approximately, but it has much less capacity to neutralize added acid or base. The [bleach dilution calculator is a reminder that dilution changes concentrations but not the acid/base ratio if both components are diluted together.
- Ratio 1:
maximum buffer capacity and pH = pKa.
- Ratio 0.1:
pH = pKa − 1, acid-rich edge.
- Ratio 10:
pH = pKa + 1, base-rich edge.
Very dilute buffers can fail because added acid/base is no longer small relative to buffer amount.
Assumptions, activities, and ionic strength
The classic equation uses concentrations as a convenient approximation for activities. At high ionic strength, in non-aqueous solvent, or with very concentrated salts, activity coefficients can shift the apparent pH. Authoritative definitions of pH and activities are given by the IUPAC Gold Book. For teaching labs and routine aqueous buffers, the concentration form is usually accurate enough if the acid is weak and the solution is not extremely dilute.
Use measured pH after calibration when preparing biological or analytical buffers that need tight tolerances.
Common buffer systems
Different chemical systems cover different pH regions. Acetate works well near pH 3.7–5.7, phosphate near neutral pH, ammonium/ammonia near pH 9.25, and carbonate/bicarbonate near physiological CO₂ equilibria. Reference tables from LibreTexts and NIST Chemistry WebBook are useful for checking pKa values.
| Buffer pair | Useful pH range | Typical pKa |
|---|---|---|
| Acetic acid / acetate | 3.74–5.74 | 4.74 |
| H₂PO₄⁻ / HPO₄²⁻ | 6.21–8.21 | 7.21 |
| NH₄⁺ / NH₃ | 8.25–10.25 | 9.25 |
| HCO₃⁻ / CO₃²⁻ | 9.33–11.33 | 10.33 |
| Tris / Tris-H⁺ | 7.1–9.1 | 8.1 |
Laboratory workflow for preparing a buffer
Start by selecting a conjugate acid-base pair with pKa close to the target pH. Calculate the required ratio, convert the ratio to moles using your desired total buffer concentration and volume, then weigh or pipette the components. After dissolving and bringing to final volume, verify pH with a calibrated meter and make small adjustments only if needed. Concentrations for related electrochemical or spectroscopic work may also feed into tools such as the Nernst equation calculator and Beer–Lambert law calculator.
Quick Reference Card
Buffer pH — Quick Reference
Quick reference • Buffer pH Calculator
pH = pKa + log₁₀([A⁻]/[HA]); ratio = 10^(pH − pKa)Valid range: Best practical range: 0.1 ≤ [A⁻]/[HA] ≤ 10, approximately pKa ± 1 pH unit
Common Values
⚠ Watch Out
- •Do not mix units in the ratio; both amounts must use the same unit basis.
- •The equation is approximate when activities differ strongly from concentrations.
- •Very dilute buffers may not resist added acid or base even if the ratio is ideal.
- •pKa and pKw depend on temperature; use corrected values for precise work.
- •Strong acid/strong base pairs are not Henderson–Hasselbalch buffers.
Pro Tips
- →Choose pKa within one unit of the target pH before adjusting component amounts.
- →If both components are diluted together, pH stays nearly constant because the ratio is unchanged.
- →Use moles for recipe design, then divide by final volume to check concentration.
- →Calibrate the pH meter with standards bracketing your target pH.
- →For biological buffers, verify compatibility with metal ions, enzymes, and assay reagents.
FAQs
What is a buffer pH calculator?
It uses the Henderson–Hasselbalch equation to estimate the pH of a solution containing a weak acid and its conjugate base, or a weak base and its conjugate acid.
Can I enter moles instead of concentrations?
Yes. If acid and conjugate base are in the same final volume, the concentration ratio equals the mole ratio, so either moles or concentrations can be used consistently.
Why does pH equal pKa when the acid and base amounts are equal?
When [A⁻]/[HA] = 1, log₁₀(1) = 0, so the Henderson–Hasselbalch equation simplifies to pH = pKa.
What ratio gives the best buffer?
Maximum buffer capacity occurs near a 1:1 conjugate base to weak acid ratio. In practice, buffers are useful from about 0.1 to 10, or pKa ± 1 pH unit.
How do I calculate the ratio for a target pH?
Use [A⁻]/[HA] = 10^(target pH − pKa). For target pH 5.00 with pKa 4.74, the ratio is 10^0.26 = 1.82.
How are basic buffers handled?
For a weak base B and conjugate acid BH⁺, calculate pOH = pKb + log₁₀([BH⁺]/[B]), then convert to pH with pH = 14 − pOH at 25 °C.