pKa Calculator

Compute pKa ↔ Ka, visualize the mole fractions of HA (weak acid) and A⁻ (conjugate base) across any pH range. Instantly obtain the protonation state, buffer region, and exact ratios for biochemical, pharmaceutical, or educational applications.

Typical range: -2 to 16 for most weak acids. Note: pKa values are temperature‑dependent; standard reference is 25 °C. For precise work, use temperature‑corrected constants (ΔH° of dissociation).
Used to compute exact HA / A⁻ fractions at a specific pH.
? Acetic acid (pKa 4.76)
? Carbonic acid (pKa₁ 6.35)
⚗️ Ammonium ion (pKa 9.25)
? Citric acid (pKa₁ 3.14)
? Phosphoric acid (pKa₁ 2.15)
? HEPES buffer (pKa 7.2)
Privacy assured: All calculations are performed locally in your browser. No data is transmitted or stored.

Understanding pKa and the Henderson-Hasselbalch Equation

The acid dissociation constant (Ka) quantifies the strength of a weak acid (HA) in solution: HA ⇌ H⁺ + A⁻. The pKa is defined as pKa = –log₁₀(Ka). A lower pKa indicates a stronger acid. The relationship between pH, pKa, and the ratio of conjugate base [A⁻] to acid [HA] is given by the Henderson-Hasselbalch equation:

pH = pKa + log₁₀( [A⁻] / [HA] )

This equation is fundamental in biochemistry, pharmacology, and environmental chemistry. It allows prediction of the protonation state of ionizable groups in enzymes, drug molecules, and natural waters.

Species Distribution Curves

The diagram above shows the mole fractions αHA and αA⁻ as a function of pH:

  • αHA = 1 / (1 + 10(pH – pKa))
  • αA⁻ = 10(pH – pKa) / (1 + 10(pH – pKa)) = 1 – αHA

At pH = pKa, both species are equal (50% HA, 50% A⁻). This point defines the maximum buffer capacity region, typically effective within pKa ± 1 pH unit.

For polyprotic acids (e.g., H₃PO₄, citric acid): Each dissociation step has its own pKa (pKa₁, pKa₂, pKa₃). This calculator handles one pKa at a time (monoprotic or a single ionization step). For stepwise distributions, consider using a dedicated polyprotic simulator.
Real-world application: Drug absorption

Most drugs are weak acids or bases. Their ability to cross cell membranes depends on the proportion of the neutral (lipophilic) form. Using the Henderson-Hasselbalch equation, medicinal chemists predict intestinal absorption (pH ~6.5) or blood distribution (pH 7.4). For a weakly acidic drug with pKa = 4.5, at pH 7.4, the ratio [A⁻]/[HA] ≈ 10(7.4-4.5) ≈ 794, meaning the ionized form dominates, reducing passive diffusion. Our interactive graph helps visualize this shift instantly.

Application Example: Soil Chemistry & Plant Nutrition

The availability of many nutrients in soil (e.g., phosphate, organic acids) is controlled by soil pH. For instance, phosphate ions (H₂PO₄⁻/HPO₄²⁻, pKa₂≈7.2) exist predominantly as H₂PO₄⁻ in acidic soils (pH<6), which is readily taken up by plants. In alkaline soils, the equilibrium shifts to HPO₄²⁻, reducing availability. This tool allows agronomists to quickly estimate the phosphate speciation at a given soil pH.

Application Example: Analytical Chemistry & Chromatography

In reversed-phase liquid chromatography (RP-HPLC), the retention time of weak acids or bases depends strongly on the mobile phase pH. By adjusting the pH to near the compound's pKa, one can alter its ionization state and thus its hydrophobicity, achieving baseline separation of complex mixtures. This tool helps analytical chemists simulate the ionization fraction at different pH values to optimize chromatographic methods.

From pKa to Ka and vice versa

Ka = 10–pKa. This tool automatically converts between the two. For acetic acid (pKa = 4.76), Ka ≈ 1.74 × 10⁻⁵. Stronger acids (e.g., oxalic acid pKa₁ = 1.27) have Ka ~ 0.054.

Step-by-step usage guide

  1. Enter the pKa of your weak acid (or click any preset).
  2. Specify a pH value to obtain exact HA and A⁻ fractions at that pH.
  3. Click "Calculate & Draw Distribution" — the interactive curve updates automatically.
  4. Hover (or view) the marker on the plot: the green vertical line indicates your chosen pH, showing the exact fractions.
  5. Use the Copy button to export numerical results for reports or lab notebooks.

Common pKa reference table (25°C)

Acid / Base pKa (approx.) Ka (mol/L) Typical buffer range
Hydrochloric acid (strong) -6.0 10⁶ not a buffer
Oxalic acid (pKa₁) 1.27 5.4×10⁻² 0.3 – 2.3
Citric acid (pKa₁) 3.14 7.2×10⁻⁴ 2.1 – 4.1
Acetic acid 4.76 1.74×10⁻⁵ 3.8 – 5.8
Carbonic acid (pKa₁) 6.35 4.47×10⁻⁷ 5.4 – 7.4
HEPES (zwitterionic) 7.20 6.31×10⁻⁸ 6.2 – 8.2
Ammonium ion (NH₄⁺) 9.25 5.62×10⁻¹⁰ 8.3 – 10.3
Bicarbonate (pKa₂) 10.33 4.68×10⁻¹¹ 9.3 – 11.3
Ionic strength & activity effects: In concentrated solutions (>0.1 M), activity coefficients deviate from unity, altering the effective pKa. This calculator assumes ideal dilute conditions (activity = concentration). For high‑precision work (e.g., physiological buffers), use activity‑corrected constants or Davies equation.
Temperature dependence: pKa changes with temperature (ΔH° of dissociation). The values in the table are standard at 25 °C. For example, the pKa of Tris buffer decreases by ≈0.03 per °C increase. Always verify pKa under your experimental conditions.

Mathematical derivation of the distribution equations

From the equilibrium: Ka = [H⁺][A⁻]/[HA]. Defining αHA = [HA] / ([HA] + [A⁻]) and αA⁻ = [A⁻] / ([HA] + [A⁻]). Rearranging: [A⁻]/[HA] = Ka/[H⁺] = 10(pH – pKa). Then αHA = 1/(1 + [A⁻]/[HA]) = 1/(1 + 10(pH-pKa)). These equations generate the smooth transition curves you see on the canvas.

The inflection point at pH = pKa corresponds to the half‑neutralization point of a weak acid. The area of maximum buffer capacity (dpH/d(base) is minimal) lies exactly at pKa.

Frequently Asked Questions

pKa is the negative logarithm of the acid dissociation constant. Lower pKa = stronger acid (more willing to donate a proton). Higher pKa = weaker acid. It also defines the pH at which the acid is 50% ionized.

An effective buffer has pH within ±1 of its pKa. The Henderson-Hasselbalch equation lets you calculate the ratio of conjugate base to acid needed to achieve a target pH.

Yes, strong acids like HCl have pKa ≈ -6, indicating near-complete dissociation. Our calculator handles negative pKa values but the distribution curve may shift; for practical purposes most weak acids have positive pKa values.

The tool uses double-precision arithmetic (relative error < 1e-12). Fractions are exact given ideal solution assumptions (activity coefficients = 1). For dilute solutions this is highly accurate.

Yes, pKa varies with temperature due to enthalpy of dissociation. The values provided are standard at 25°C. For high-precision work, use temperature-corrected pKa constants (many buffers have known ΔpKa/ΔT).

Authoritative sources: IUPAC database, RSC pKa compilations, and literature like "Dissociation Constants of Organic Acids in Aqueous Solution" (Kortüm et al.). Many biochemistry textbooks also list side-chain pKa values of amino acids.

This tool is designed for monoprotic weak acids or a single dissociation step of a polyprotic acid (e.g., pKa₁ of carbonic acid). For simultaneous multiple equilibria, a polyprotic simulator is required. However, you can still use it stepwise: enter pKa₁ to see the first dissociation, then pKa₂ for the second, etc.

References & Further Reading

  • Atkins, P., & de Paula, J. Physical Chemistry (11th ed.). Oxford University Press. (Authoritative text on chemical equilibrium and pKa fundamentals.)
  • Haynes, W. M. (Ed.). CRC Handbook of Chemistry and Physics. CRC Press. (Comprehensive source of accurate pKa data.)
  • Harris, D. C. Quantitative Chemical Analysis. W.H. Freeman. (Standard analytical chemistry textbook with detailed buffer calculations.)
  • Segel, I. H. Biochemical Calculations. Wiley. (Classic reference for biological applications of acid-base equilibria.)
  • "IUPAC Analytical Chemistry Division – Compendium of Terminology". https://goldbook.iupac.org/ (International standard definitions for acid, base, pH, etc.)
  • National Institute of Standards and Technology (NIST) Chemistry WebBook. (Provides thermodynamic data including pKa.) https://webbook.nist.gov/chemistry/
  • Bordwell pKa Table. https://www.chem.wisc.edu/areas/reich/pkatable/ (Extensive compilation of pKa values.)

Content Review & Professional Background

This tool and its explanatory content were developed by getzenquery Tech team. The computational model and educational material have been reviewed against standard textbooks (Atkins, Harris, Segel) and authoritative databases (IUPAC, NIST, CRC Handbook). The implementation follows established physical chemistry principles and peer-reviewed methodologies.

For critical research or applications, we recommend cross-verifying the results with experimental data or dedicated commercial software. The tool is intended for educational use, preliminary research, and as a quick reference for acid-base speciation calculations.

Important Disclaimer

This tool is intended for educational and preliminary research purposes only. All calculations assume ideal dilute solution conditions and do not account for ionic strength, temperature variation (other than 25°C unless specified), solvent effects, or activity corrections. For polyprotic acids, only a single dissociation step is modeled. When used for drug development, environmental monitoring, clinical diagnosis, or any critical decision-making, always verify with experiments or consult a domain expert. We are not responsible for any direct or indirect consequences resulting from the use of this tool.

Page content last updated: March 2026

Recommended external resources: Bordwell pKa Table, PubChem, Illumina pKa guide.