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Buffer pH Calculator

Ready to calculate
pH = pKa + log([salt]/[acid]).
Acid + Base Modes.
16 Reference Buffers.
100% Free.
No Data Stored.

How it Works

01Pick Acid or Base Buffer

Acid buffer = weak acid + conjugate base salt; Base buffer = weak base + conjugate acid salt

02Enter Ka or Kb

Dissociation constant of the weak acid (Ka) or base (Kb) — typically scientific notation like 1.8e-5

03Enter Both Concentrations

Weak acid/base + conjugate salt — supports M, mM, μM, nM

04Henderson-Hasselbalch

pH = pK + log([salt]/[weak]) — get pH + buffering capacity + closest reference buffer

What is a Buffer pH Calculator?

A buffer is a solution that resists pH changes when small amounts of acid or base are added — an essential property for biochemistry (cells maintain pH 7.35–7.45 to nanomolar precision), pharmaceutical formulation, analytical chemistry, and any process where pH stability matters. The Henderson-Hasselbalch equation, derived in 1908 (Henderson) and rearranged into its modern form in 1916 (Hasselbalch), tells you exactly what pH a buffer will produce from the concentrations of the weak acid (or base) and its conjugate salt. Our Buffer pH Calculator handles both acid buffers (e.g., acetic acid + sodium acetate) using pH = pKa + log([salt]/[acid]) and base buffers (e.g., ammonia + ammonium chloride) using pOH = pKb + log([salt]/[base]); pH = 14 − pOH.

The 5-band buffering-capacity classification translates abstract Henderson-Hasselbalch numbers into practical chemistry: balanced ([salt] = [weak], pH = pKa exactly) gives maximum buffer strength; well-buffered (ratio between 0.1 and 10) keeps pH within ±1 unit of pKa where buffering is reliable; weak-buffering regimes (ratio <0.1 or >10) are asymmetric — they resist additions only on one side; out-of-range (ratio <0.01 or >100) means the buffer is essentially exhausted and you should pick a different system.

The calculator includes a 16-buffer reference library covering the most-used buffers in biology and chemistry — acetate, MES, MOPS, HEPES, phosphate (PBS), Tris, borate, ammonia, glycine, and more — each with its standard pKa and effective pH range. The closest reference buffer to your computed pH is auto-highlighted, so you can see at a glance whether your buffer matches a familiar system.

Pro Tip: Pair this with our pKa Calculator for the inverse problem (find pKa from pH and concentrations), or our Molarity Calculator to convert your concentration data.

How to Use the Buffer pH Calculator?

Pick Buffer Type: "Acid" for a weak acid + conjugate base salt (acetic acid + sodium acetate, formic acid + sodium formate, etc.). "Base" for a weak base + conjugate acid salt (ammonia + ammonium chloride, methylamine + methylammonium chloride).
Enter Ka or Kb: The dissociation constant of the weak acid/base. Use scientific notation: acetic acid Ka = 1.8e-5; ammonia Kb = 1.8e-5; HEPES Ka = 2.8e-8 (pKa 7.55). The calculator computes pKa or pKb internally.
Enter Concentrations: The weak acid/base concentration and the conjugate salt concentration. Both in M, mM, μM, or nM (defaults to M). Use the formal (analytical) concentrations — what you actually weighed out.
Press Calculate: The tool applies the Henderson-Hasselbalch equation. For acid buffers: pH = pKa + log([salt]/[acid]). For base buffers: pOH = pKb + log([salt]/[base]), then pH = 14 − pOH.
Read Results: pH (with visual pH-scale gauge), pOH, pKa/pKb, [salt]/[weak] ratio, distance from pK, 5-band buffering-capacity classification, and the closest matching reference buffer from the 16-buffer library.

How do I calculate buffer pH using Henderson-Hasselbalch?

The Henderson-Hasselbalch equation captures buffer behavior in a single elegant log-linear formula. Here's the complete derivation and interpretation:

Think of a buffer like a financial cushion: the weak acid (or base) and its conjugate salt are two reserves that get depleted when you add base (or acid). The buffer holds pH steady as long as both reserves remain substantial — when one runs out, the buffering collapses.

The Henderson-Hasselbalch Equation (Acid Buffer)

pH = pKa + log₁₀([A⁻] / )

where pKa = −log₁₀(Ka), is the weak-acid concentration, and [A⁻] is the conjugate-base salt concentration. Originally derived from the equilibrium expression Ka = [H⁺][A⁻]/ by taking −log₁₀ of both sides and rearranging. The "salt" form [A⁻] is what you add to an acetic acid solution as sodium acetate, for example.

For Base Buffers

pOH = pKb + log₁₀([BH⁺] / ), then pH = 14 − pOH

where is the weak-base concentration and [BH⁺] is the conjugate-acid salt concentration. Same Henderson-Hasselbalch logic, but in pOH space because the equilibrium B + H₂O ⇌ BH⁺ + OH⁻ involves OH⁻ rather than H⁺. The pH = 14 − pOH conversion uses the water self-ionization Kw = 10⁻¹⁴.

Three Critical Cases

  • [salt] = [weak]: pH = pKa exactly. Maximum buffering capacity in both directions.
  • [salt] = 10 × [weak]: pH = pKa + 1. Above pKa by 1 unit.
  • [salt] = 0.1 × [weak]: pH = pKa − 1. Below pKa by 1 unit.

The useful pH range of a buffer is typically pKa ± 1 — corresponding to [salt]/[weak] ratios from 0.1 to 10. Outside this range, the buffer becomes asymmetric (works only against acid OR base, not both).

Buffering Capacity (β)

Quantitatively, β = dC_acid / dpH (moles of strong acid added per pH unit change). Maximum β occurs when [salt] = [weak] (i.e., pH = pKa). For a buffer of total concentration C_total = [salt] + [weak]: β_max ≈ 0.576 · C_total at pH = pKa. Buffer "strength" scales linearly with total concentration — a 1 M acetate buffer at pH 4.76 has 10× the buffering capacity of a 0.1 M acetate buffer.

Choosing a Buffer for a Target pH

Pick a buffer with pKa within ±1 unit of your target pH. Use the 16-buffer reference table:

  • pH 4–5: acetate (pKa 4.76)
  • pH 6–7: MES (pKa 6.10), MOPS (pKa 7.20)
  • pH 7–8: phosphate (pKa 7.20), HEPES (pKa 7.55), Tris (pKa 8.07)
  • pH 9–10: borate (pKa 9.24), ammonia (pKa 9.25)

Then adjust the [salt]/[acid] ratio to fine-tune pH around the pKa.

Real-World Example

Buffer pH Calculator – Acid & Base Buffers In Practice

Consider an acetate buffer made from acetic acid (Ka = 1.8 × 10⁻⁵) at 0.1 M and sodium acetate at 0.1 M. This is a 50:50 mix at the pKa.
  • Step 1: Pick "Acid" buffer type. Enter Ka = 1.8e-5, [acid] = 0.1 M, [salt] = 0.1 M.
  • Step 2: Compute pKa. pKa = −log₁₀(1.8 × 10⁻⁵) = 4.74.
  • Step 3: Compute the ratio. [salt]/[acid] = 0.1/0.1 = 1. log₁₀(1) = 0.
  • Step 4: Apply Henderson-Hasselbalch. pH = 4.74 + log(1) = 4.74 + 0 = 4.74.
  • Step 5: Classify. Ratio = 1 → "Balanced" band. Maximum buffering capacity in both directions. This is the optimal acetate buffer composition.
  • Step 6: Read the closest reference: Acetic acid (pKa 4.76) — exact match. Useful pH range 3.8–5.8 (i.e., pKa ± 1).

Now consider an ammonia buffer for protein purification at pH 9: Kb = 1.8 × 10⁻⁵, [NH₃] = 0.5 M, [NH₄Cl] = 0.5 M. pKb = 4.74. Ratio = 1, log = 0. pOH = 4.74 + 0 = 4.74; pH = 14 − 4.74 = 9.26. Falls in "Balanced" band.

For a phosphate buffer at pH 8.0: pKa2 = 7.20, target pH = 8.0. Solving Henderson-Hasselbalch: 8.0 = 7.20 + log(ratio) → log(ratio) = 0.8 → ratio = 6.31. So you need 6.31× more dibasic phosphate (HPO₄²⁻) than monobasic (H₂PO₄⁻). For 0.1 M total phosphate: ~0.087 M HPO₄²⁻ + 0.014 M H₂PO₄⁻. The calculator quickly verifies any buffer composition you propose.

Who Should Use the Buffer pH Calculator?

1
Biochemistry Researchers: Design buffers for enzyme assays, protein purification, cell culture media, gel electrophoresis (Tris-glycine, MOPS, HEPES).
2
Pharmaceutical Scientists: Formulate IV solutions, oral drug formulations — buffer pH controls drug solubility, stability, and bioavailability.
3
Cell Biologists: PBS for tissue culture, HEPES-buffered media, MOPS for tissue fixation. Get exact pH for delicate cellular work.
4
Analytical Chemists: Buffer prep for HPLC mobile phases, capillary electrophoresis, ion-selective electrode calibration.
5
Chemistry Students: Solve Henderson-Hasselbalch problems on coursework, understand buffer capacity, work with weak acid-base equilibria.
6
Brewing & Wine Chemistry: Beer mash pH (carbonate-bicarbonate), wine acid balance (citric/tartaric/malic acid buffers) — pH controls flavor and yeast performance.

Technical Reference

Origin (Henderson 1908, Hasselbalch 1916). Lawrence Joseph Henderson published the equilibrium relation in 1908; Karl Albert Hasselbalch rearranged it into the now-standard pH-explicit form in 1916, motivated by his work on blood acid-base balance. The equation is the foundation of acid-base physiology, pharmacology, and biochemistry.

Reference pKa Values for Common Buffers (at 25°C):

  • Glycine (pKa1, COOH): 2.34
  • Citric acid (pKa1): 3.13
  • Formic acid: 3.75
  • Acetic acid: 4.76
  • Citric acid (pKa2): 4.76
  • MES: 6.10
  • Citric acid (pKa3): 6.40
  • Carbonic acid (pKa1): 6.35 (blood bicarbonate buffer)
  • PIPES: 6.76
  • MOPS: 7.20
  • Phosphate (pKa2): 7.20 (PBS, intracellular)
  • HEPES: 7.55 (cell culture)
  • Tris: 8.07 (molecular biology)
  • Glycylglycine: 8.40
  • Boric acid: 9.24
  • Ammonia (NH₃/NH₄⁺): 9.25
  • Glycine (pKa2, NH₃⁺): 9.60
  • Carbonate/bicarbonate (pKa2): 10.33
  • Phosphate (pKa3): 12.38

"Good's Buffers" (Norman Good, 1966) — a family of zwitterionic buffers designed to be: (1) pKa between 6 and 8 (physiological range), (2) high water solubility, (3) low membrane permeability, (4) minimal interference with biochemical reactions, (5) photo- and enzymatically stable. Examples: MES (6.10), PIPES (6.76), MOPS (7.20), HEPES (7.55), TES (7.40), TRIS (8.07 — added by extension). These have largely replaced phosphate buffers in cell biology.

Buffering Capacity (β): Quantitatively defined as β = dC_strong-base / dpH (moles of strong base added per pH unit change). For a single weak-acid-conjugate-base buffer at total concentration C_total: β = 2.303 · C_total · ([H⁺]·Ka / ([H⁺] + Ka)²). Maximum β at pH = pKa: β_max ≈ 0.576 · C_total. Higher buffer concentration = stronger buffer. Higher buffer concentration also = higher ionic strength, which can affect downstream applications.

Limitations of Henderson-Hasselbalch. The equation assumes ≈ formal analytical concentration of acid, [A⁻] ≈ formal salt concentration. This breaks down when: (1) very dilute buffers (where dissociation contributes meaningfully to equilibrium concentrations), (2) very strong or very weak acids (Ka outside ~10⁻³ to 10⁻¹¹), (3) extreme pH values far from pKa (ratio outside 0.01–100), (4) high ionic strength (where activities differ from concentrations). For these cases, solve the exact equilibrium expression directly using the quadratic formula.

Key Takeaways

Buffers are the silent workhorses of chemistry and biology — they keep pH stable when you can't afford for it to drift. The Henderson-Hasselbalch equation makes buffer pH calculations effortless: pH = pKa + log([salt]/[acid]) for acid buffers, pOH = pKb + log([salt]/[base]); pH = 14 − pOH for base buffers. Use the ToolsACE Buffer pH Calculator to compute pH instantly from Ka/Kb and concentrations, get buffering-capacity classification across 5 bands, and match your buffer against a 16-buffer reference library covering acetate, phosphate, Tris, HEPES, MES, MOPS, borate, ammonia, glycine, and more. Bookmark it for biochemistry, pharmaceutical formulation, analytical chemistry, and any context where pH stability matters.

Frequently Asked Questions

What is the Buffer pH Calculator?
A buffer is a solution that resists pH changes when small amounts of acid or base are added — essential for biochemistry, pharma, analytical chemistry, and any process where pH matters. Our calculator uses the Henderson-Hasselbalch equation to compute the pH of any buffer from the dissociation constant (Ka or Kb) and the concentrations of the weak acid/base and its conjugate salt. Supports both acid buffers (pH = pKa + log([salt]/[acid])) and base buffers (pOH = pKb + log([salt]/[base])).

Designed for biochemistry researchers, pharmaceutical scientists, cell biologists, analytical chemists, students, and brewing/wine chemists, the tool runs entirely in your browser — no data is stored or transmitted.

Pro Tip: For more chemistry tools, try our pKa Calculator.

What is the Henderson-Hasselbalch equation?
pH = pKa + log₁₀([A⁻] / ) for an acid buffer (HA = weak acid, A⁻ = its conjugate base salt). For base buffers: pOH = pKb + log([BH⁺]/); pH = 14 − pOH. Derived from the dissociation equilibrium Ka = [H⁺][A⁻]/ by taking −log₁₀ and rearranging. Henderson 1908; Hasselbalch 1916. The single most-used equation in acid-base biochemistry.
When does pH = pKa?
When [salt] = [weak acid] (or [conjugate acid salt] = [weak base]). At this composition, log(ratio) = log(1) = 0, so the Henderson-Hasselbalch equation reduces to pH = pKa. This is the operational definition of pKa: the pH at which a weak acid is exactly half-dissociated. Also the point of maximum buffer capacity — the buffer resists pH changes equally well in both directions.
What's the useful pH range of a buffer?
pKa ± 1 pH unit — corresponding to [salt]/[weak] ratios between 0.1 and 10. Within this range, the buffer has good capacity in both directions. Outside ±1 (ratio < 0.1 or > 10), the buffer is asymmetric — it works against added base but poorly against added acid (or vice versa). For target pH outside the ±1 range of any single buffer's pKa, switch to a different buffer system.
How do I choose the right buffer for my pH?
Pick a buffer with pKa within ±1 unit of your target pH. For pH 4.5: acetate (pKa 4.76). For pH 7.4 (physiological): HEPES (pKa 7.55), phosphate (pKa 7.20), or Tris (pKa 8.07 — slightly above range, watch the asymmetry). For pH 9.5: ammonia (pKa 9.25) or borate (pKa 9.24). The 16-buffer reference table in the calculator highlights the closest match to your computed pH.
What's a 'Good's buffer'?
A family of zwitterionic biological buffers designed by Norman Good (1966) for cell biology and biochemistry. Selection criteria: pKa 6–8 (near physiological), high water solubility, low membrane permeability, minimal interference with biochemistry, stability. Examples: MES (pKa 6.10), MOPS (7.20), HEPES (7.55), PIPES (6.76), TES (7.40). These largely replaced phosphate buffers in cell culture because phosphate complexes with calcium and other metals, interfering with cellular processes.
How do I make a buffer of higher concentration?
Just multiply both [weak] and [salt] by the same factor. The pH stays the same (the ratio [salt]/[weak] is what matters), but the buffering capacity increases linearly with total concentration. A 1 M acetate buffer at pH 4.76 has 10× the resistance to pH change as a 0.1 M acetate buffer at the same pH. The trade-off: higher concentration means higher ionic strength, which can affect downstream measurements.
What does 'buffering capacity' mean?
Buffering capacity (β) is the moles of strong acid or base needed to change the buffer pH by 1 unit. Mathematically: β = 2.303 · C_total · ([H⁺]·Ka / ([H⁺] + Ka)²). Maximum β at pH = pKa, where β_max ≈ 0.576 · C_total. So a 0.5 M buffer at pH = pKa can absorb ~0.29 mol of acid/base per liter before pH shifts by 1 unit. The calculator's 5-band classification reports buffer capacity qualitatively (balanced, well-buffered, weak, out-of-range).
Does temperature affect buffer pH?
Yes — Ka (and therefore pKa) depends on temperature. Tris is the most temperature-sensitive common buffer: pKa drops ~0.03 per °C, so a Tris buffer prepared at 25°C will have pH ~0.6 units lower at 4°C (cold-room conditions). HEPES is much more temperature-stable. Always prepare buffers at the temperature you intend to use them, or correct using van't Hoff equation for that buffer's ΔH of dissociation.
What if my Ka is for a polyprotic acid?
Each ionization step has its own Ka (Ka1, Ka2, Ka3). Use the Ka closest to your target pH. For example, phosphoric acid has Ka1 = 7.5 × 10⁻³ (pKa 2.12), Ka2 = 6.2 × 10⁻⁸ (pKa 7.21), Ka3 = 4.8 × 10⁻¹³ (pKa 12.32). Use Ka2 for a phosphate buffer near pH 7. Treat each step independently with the Henderson-Hasselbalch equation.
When does the Henderson-Hasselbalch equation fail?
Several cases: (1) Very dilute buffers (concentrations near or below Ka) — dissociation contributes meaningfully to and [A⁻], breaking the formal-concentration assumption. (2) Very strong or weak acids (Ka outside 10⁻³ to 10⁻¹¹). (3) Extreme pH values far from pKa (ratio outside 0.01–100). (4) High ionic strength — activities differ from concentrations, requiring activity coefficients γᵢ via Debye-Hückel. For these cases, solve the exact equilibrium expression directly.

Author Spotlight

The ToolsACE Team - ToolsACE.io Team

The ToolsACE Team

Our chemistry tools team implements the Henderson-Hasselbalch equation for both acid buffers (weak acid + conjugate base salt: pH = pKa + log([salt]/[acid])) and base buffers (weak base + conjugate acid salt: pOH = pKb + log([salt]/[base]); pH = 14 − pOH). The calculator returns pH in 3 decimal precision, the [salt]/[weak] ratio, distance from pK, buffering-capacity classification across 5 bands (balanced → out-of-range), and matches against a 16-buffer reference library covering acetate, MES, MOPS, HEPES, phosphate, Tris, borate, and other standard biology/chemistry buffers.

Acid-Base EquilibriaHenderson-Hasselbalch (1908/1916)Software Engineering Team

Disclaimer

Henderson-Hasselbalch assumes ≈ formal analytical concentration and [A⁻] ≈ formal salt concentration — accurate when both concentrations are much greater than the dissociation contribution (typically when concentrations exceed ~10× Ka). For very dilute buffers, extreme pH values, or high ionic strengths, solve the exact equilibrium expression. Activity coefficients are assumed to be 1 — true only at low ionic strength.