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pKa Calculator

Ready to calculate
Henderson-Hasselbalch.
Two Methods (pH or Ka).
14 Reference Acids.
100% Free.
No Data Stored.

How it Works

01Pick a Method

From pH (Henderson–Hasselbalch) or directly from Ka

02Enter Inputs

pH + [A⁻] + [HA] for buffer, or just Ka if known

03Calculate pKa

pKa = pH − log10([A⁻]/[HA]) or pKa = −log10(Ka)

04Identify the Acid

Match against 14 reference acids + strength class

What is the pKa Calculator?

The pKa Calculator finds the acid dissociation constant in its log form, pKa, using either of the two standard methods. Method 1 — From pH: apply the Henderson-Hasselbalch equation, pKa = pH − log10([A⁻]/), when you have a buffer with known pH and known concentrations of conjugate base and weak acid. Method 2 — From Ka: use the simple relation pKa = −log10(Ka) when you already know the acid dissociation constant.

pKa is one of the most important constants in chemistry, biochemistry, pharmacology, and biology. It tells you the pH at which a weak acid is half-dissociated (50% HA, 50% A⁻), and indirectly tells you the strength of an acid: lower pKa = stronger acid, higher pKa = weaker. Strong acids (HCl, H₂SO₄) have pKa < 0; carboxylic acids 3–5; phenols 9–10; alcohols 16+. The calculator classifies your result against 6 strength bands and finds the closest matching common acid.

Built for chemistry students, biochemistry researchers, pharmaceutical scientists studying drug ionization, and instructors. Free, fast, mobile-friendly, fully client-side.

Pro Tip: A buffer is most effective at pH = pKa ± 1 — choose your weak acid with a pKa near your target buffer pH for maximum buffering capacity.

How to Use the pKa Calculator?

Pick the Method: Both sections are visible. Use the pKa from pH block when you have a buffer; use the pKa from Ka block when you have the dissociation constant.
For pH method: Enter the measured pH, the conjugate base concentration [A⁻], and the weak acid concentration . Both concentrations can be in any molarity unit (M down to yM).
For Ka method: Enter the acid dissociation constant. Use scientific notation for very small values (e.g., 1.8e-5 for acetic acid).
Press Calculate: If both sections are filled, the pH/Henderson-Hasselbalch method takes priority. Otherwise the Ka method is used.
Read the Result: pKa value, calculated Ka, strength classification, and closest matching reference acid from a 14-acid library.

How is pKa calculated?

Two formulas, both core to acid-base chemistry: Henderson-Hasselbalch: pKa = pH − log₁₀([A⁻]/) and the trivial pKa = −log₁₀(Ka). The first relates pKa to a buffer's pH; the second is just the definition.

Both equations are exact within the assumptions of dilute aqueous solution. The Henderson-Hasselbalch equation works for any monoprotic weak acid HA ⇌ H⁺ + A⁻. For polyprotic acids, apply it stepwise per dissociation.

pKa Math — Step by Step:

1. Henderson-Hasselbalch

Buffer-based formula:

  • pH = pKa + log10([A⁻]/)
  • Rearranged: pKa = pH − log10([A⁻]/)
  • When [A⁻] = : pKa = pH

A buffer at half-titration point has pH = pKa exactly.

2. From Ka

Direct logarithmic relation:

  • pKa = −log10(Ka)
  • Ka = 10^(−pKa)
  • Ka in mol/L (M)

Example: Ka = 1.8 × 10⁻⁵ → pKa = 4.74 (acetic acid).

3. Strength Interpretation

pKa scale interpretation:

  • pKa < 0 → strong acid
  • 0 ≤ pKa < 5 → moderate
  • 5 ≤ pKa < 10 → weak
  • pKa ≥ 10 → very/extremely weak

Lower pKa = stronger acid. Each 1-unit drop = 10× more dissociated.

4. Buffer Design Rule

Effective buffer range:

  • pKa − 1 ≤ pH ≤ pKa + 1
  • Outside this range, capacity drops sharply
  • Pick your acid with pKa near target pH

For pH 7.4 (blood): use phosphate (pKa2 = 7.2) or HEPES (pKa = 7.55).

Common Acid pKa Reference:

Strong Acids
  • HCl: −7
  • H₂SO₄ (1st): −3
  • HSO₄⁻: 1.99
  • H₃PO₄: 2.15
Carboxylic / Mid-Range
  • HCOOH: 3.75
  • CH₃COOH: 4.76
  • H₂CO₃: 6.35
  • H₂PO₄⁻: 7.20
Weak / Very Weak
  • HCN: 9.21
  • NH₄⁺: 9.25
  • Phenol: 9.95
  • HCO₃⁻: 10.33
  • H₂O: 15.7

Why pKa Matters in Different Fields:

Pharmaceutical Sciences

Drug ionization at physiological pH (~7.4) determines absorption, distribution, and bioavailability. Drugs need certain pKa ranges for optimal pharmacokinetics.

Biochemistry

Amino acid pKa values determine protein structure, enzyme catalysis, and substrate binding. The pKa of histidine (~6) makes it a versatile catalytic residue.

Analytical Chemistry

Titration end-point selection, indicator choice, and buffer preparation all depend on pKa. The most accurate titrations occur near the pKa of the analyte.

Environmental Chemistry

Ocean acidification depends on the carbonate system pKa values (6.35 and 10.33). Soil acidity, river chemistry, and groundwater treatment all use pKa.

Real-World Example

Real Lab Scenarios

Sample pKa calculations using both methods:

Scenario Method Inputs pKa Identification
Acetic acid from KaKaKa = 1.8e−54.74Acetic acid (4.76) ✓
Half-titration pointpHpH=4.76, [A⁻]=4.76Acetic acid ✓
Phosphate bufferpHpH=7.2, ratio=1:17.20H₂PO₄⁻ ✓
Imbalanced bufferpHpH=4.5, [A⁻]/=0.54.80Acetic ≈
Carbonic acidKaKa = 4.45e−76.35H₂CO₃ ✓
Phenol from bufferpHpH=10, [A⁻]=10mM, =100mM11.00≈ HCO₃⁻ (10.33)

Notice the half-titration row: when [A⁻] = , log10(1) = 0, so pKa = pH directly. This is why titration curves identify pKa at the inflection point — the pH at half-equivalence equals pKa.

Who Should Use the pKa Calculator?

1
🧪 Chemistry Students: Solve textbook acid-base problems with confidence. Verify your manual calculations.
2
🔬 Biochem Researchers: Design buffers, predict ionization at experimental pH, study enzyme catalysis.
3
💊 Pharm Scientists: Predict drug ionization at physiological pH for absorption/distribution modeling.
4
🎓 Educators: Generate or verify problem sets, demonstrate Henderson-Hasselbalch and acid-strength classification.
5
🌊 Environmental Chemists: Carbonate system math, water-quality assessment, soil chemistry.
6
🧠 USMLE / Med Students: Acid-base physiology depends on the carbonate buffer pKa (6.35 and 10.33).

Technical Reference

Key Takeaways

pKa is the single most useful number in acid-base chemistry. Use the ToolsACE pKa Calculator to compute it via Henderson-Hasselbalch (from a buffer's pH and component concentrations) or directly from Ka. The closest-acid match against 14 references helps identify unknowns. Combined with the 6-band strength classification, you get instant context: is this a strong acid, a buffer-grade weak acid, or barely acidic at all?

Frequently Asked Questions

What is pKa?
pKa = −log₁₀(Ka), where Ka is the acid dissociation constant. It's the pH at which a weak acid is exactly 50% dissociated. Lower pKa means stronger acid (more dissociated at any given pH); higher pKa means weaker acid. The scale typically runs from −10 (very strong acids like HClO₄) to +50 (alkanes, hardly acidic at all).
What is the Henderson-Hasselbalch equation?
pH = pKa + log₁₀([A⁻]/), where [A⁻] is the conjugate base concentration and is the weak acid concentration. It relates the pH of a buffer to the pKa of the weak acid component plus the log of the buffer's component ratio. Rearranged: pKa = pH − log₁₀([A⁻]/).
When is a buffer most effective?
When the buffer's pH equals the pKa of the weak acid (within ±1 unit). At pH = pKa, [A⁻] = , and the buffer can absorb both added acid and added base equally well. Outside the pKa ± 1 range, buffer capacity drops dramatically. So choose your weak acid with a pKa near your target buffer pH.
What units should I use for concentrations?
Both [A⁻] and should be in the same units, but the actual unit doesn't matter — only the ratio appears in the formula. Common choices: M (molar), mM (millimolar), or μM (micromolar). The calculator supports 9 molarity units (M to yM).
Why is the half-titration point important?
At the half-titration point of a weak acid (where exactly half the acid has been neutralized), [A⁻] = . The Henderson-Hasselbalch equation simplifies to pH = pKa. This is why titration curves are used to measure pKa: find the inflection point on the buffer plateau and read off the pH.
What's the difference between Ka and pKa?
Ka is the acid dissociation equilibrium constant — a small number for weak acids (10⁻⁵ for acetic acid). pKa = −log₁₀(Ka) is the same information on a logarithmic scale, with intuitive integer values (4.76 for acetic acid). pKa is much easier to think about and compare; that's why it's the standard.
Can I use this for polyprotic acids?
For polyprotic acids (H₂SO₄, H₂CO₃, H₃PO₄), apply Henderson-Hasselbalch separately for each dissociation step, with each step's own pKa. For example, phosphoric acid has pKa1 = 2.15 (H₃PO₄/H₂PO₄⁻), pKa2 = 7.20 (H₂PO₄⁻/HPO₄²⁻), pKa3 = 12.35 (HPO₄²⁻/PO₄³⁻). The calculator gives you one pKa per calculation.
What's the difference between strong and weak acids?
Strong acids (pKa < 0) fully dissociate in water — HCl, H₂SO₄, HNO₃, HClO₄. They're "100% ionized". Weak acids (pKa > 0) dissociate only partially. Acetic acid (pKa 4.76) is only ~1% ionized in 1M solution. Most biological and pharmaceutical acids are weak.
Why do amino acids have different pKa values?
Each ionizable group on an amino acid has its own pKa. Glycine has α-COOH (pKa ~2.3) and α-NH₃⁺ (pKa ~9.6). At physiological pH (~7.4), it exists as the zwitterion (⁺H₃N-CH₂-COO⁻). Side-chain pKa values determine charge state: histidine ~6, lysine ~10.5, arginine ~12.5. These are critical for protein structure and enzyme function.
Does temperature affect pKa?
Yes, but usually only slightly. Most pKa values are reported at 25°C. Temperature dependence varies — for water (Kw = [H⁺][OH⁻]), pKw drops from 14.94 at 0°C to 13.26 at 50°C (a 1.7-unit change). For most weak acids, the temperature dependence is smaller (typically ≤ 0.1 unit per 10°C). For precise work, look up T-specific pKa values.
What does "closest acid match" tell me?
The calculator compares your computed pKa to 14 common reference acids (HCl, H₂SO₄, HCOOH, CH₃COOH, H₂CO₃, H₂PO₄⁻, NH₄⁺, phenol, HCO₃⁻, H₂O, ethanol, etc.) and identifies the nearest match. Strong match (Δ < 5%): probably this acid. Reasonable (Δ < 25%): same family. Different gas (Δ > 25%): likely an unfamiliar compound or a mixture.
What does buffer capacity depend on?
Buffer capacity (β) measures how much added strong acid or base a buffer can absorb. It depends on (1) the pKa of the weak acid relative to the target pH, (2) the total concentration + [A⁻], and (3) the ratio [A⁻]/. Maximum capacity occurs at pH = pKa with [A⁻] = . Higher total concentration = larger capacity but possible solubility/ionic strength issues.
Is my data private?
All calculations happen locally in your browser. Nothing is sent to a server, saved, or logged. The tool is free and requires no sign-up.

Author Spotlight

The ToolsACE Team - ToolsACE.io Team

The ToolsACE Team

Our chemistry tools team implements the two standard pKa calculations: Henderson-Hasselbalch for buffer-derived pKa (pKa = pH − log10([A⁻]/[HA])) and the direct relationship pKa = −log10(Ka). Strength classification follows the standard 6-band scheme used in undergraduate analytical chemistry curricula.

Henderson-Hasselbalch EquationAcid-Base EquilibriaSoftware Engineering Team

Disclaimer

Henderson-Hasselbalch assumes ideal behavior — accurate within ~0.1 pKa for dilute aqueous buffers near 25°C. For high ionic strength, non-aqueous solvents, or extreme pH values, use activity coefficients (Davies / Debye-Hückel) for greater precision.