pKa Table: Common Acids and Conjugate Acids
Reference pKa values for 60 acids and conjugate acids in water at 25 °C, grouped by structural class. Lower pKa is a stronger acid. Values below 0 and above 14 are estimates from the leveling effect of water and indirect measurement; precision is typically ±0.3 in those tails and ±0.05 in the 2 to 11 window. Inorganic values from the CRC Handbook of Chemistry and Physics; organic and Good's-buffer values from the Bordwell tables at organicchemistrydata.org.
Inorganic
| Acid | Formula | pKa | Note |
|---|
| Hydroiodic acid | HI | −10 | strong; pKa is approximate |
| Hydrobromic acid | HBr | −9 | strong; pKa is approximate |
| Hydrochloric acid | HCl | −7 | strong; pKa is approximate |
| Sulfuric acid (pKa₁) | H₂SO₄ | −3 | strong first dissociation |
| Nitric acid | HNO₃ | −1.4 | strong |
| Perchloric acid | HClO₄ | −10 | strong |
| Sulfuric acid (pKa₂) | HSO₄⁻ | 1.99 | |
| Phosphoric acid (pKa₁) | H₃PO₄ | 2.15 | |
| Hydrofluoric acid | HF | 3.17 | |
| Nitrous acid | HNO₂ | 3.40 | |
| Carbonic acid (pKa₁) | H₂CO₃ | 6.35 | |
| Hydrogen sulfide (pKa₁) | H₂S | 7.05 | |
| Phosphoric acid (pKa₂) | H₂PO₄⁻ | 7.20 | |
| Hypochlorous acid | HOCl | 7.53 | |
| Hydrocyanic acid | HCN | 9.21 | |
| Boric acid | B(OH)₃ | 9.24 | |
| Carbonate (pKa₂) | HCO₃⁻ | 10.33 | |
| Phosphoric acid (pKa₃) | HPO₄²⁻ | 12.35 | |
| Water | H₂O | 15.7 | |
| Hydrogen sulfide (pKa₂) | HS⁻ | 19 | |
Carboxylic acid
| Acid | Formula | pKa | Note |
|---|
| Trifluoroacetic acid | CF₃COOH | 0.23 | |
| Trichloroacetic acid | CCl₃COOH | 0.66 | |
| Oxalic acid (pKa₁) | H₂C₂O₄ | 1.23 | |
| Dichloroacetic acid | CHCl₂COOH | 1.29 | |
| Pyruvic acid | CH₃COCOOH | 2.39 | |
| Malonic acid (pKa₁) | HOOC-CH₂-COOH | 2.83 | |
| Chloroacetic acid | CH₂ClCOOH | 2.87 | |
| Salicylic acid | C₆H₄(OH)COOH | 2.97 | |
| Tartaric acid (pKa₁) | C₄H₆O₆ | 3.04 | |
| Citric acid (pKa₁) | C₆H₈O₇ | 3.13 | |
| Mandelic acid | C₆H₅CH(OH)COOH | 3.41 | |
| Formic acid | HCOOH | 3.75 | |
| Glycolic acid | HOCH₂COOH | 3.83 | |
| Lactic acid | CH₃CH(OH)COOH | 3.86 | |
| Benzoic acid | C₆H₅COOH | 4.20 | |
| Acetic acid | CH₃COOH | 4.76 | |
| Citric acid (pKa₂) | C₆H₇O₇⁻ | 4.76 | |
| Propanoic acid | CH₃CH₂COOH | 4.87 | |
Phenol
| Acid | Formula | pKa | Note |
|---|
| 4-Nitrophenol | O₂N-C₆H₄-OH | 7.15 | |
| Phenol | C₆H₅OH | 10.00 | |
Alcohol
| Acid | Formula | pKa | Note |
|---|
| Methanol | CH₃OH | 15.5 | |
| Ethanol | CH₃CH₂OH | 15.9 | |
| tert-Butanol | (CH₃)₃COH | 18 | |
Amine (conjugate acid)
| Acid | Formula | pKa | Note |
|---|
| Anilinium | C₆H₅NH₃⁺ | 4.60 | |
| Pyridinium | C₅H₅NH⁺ | 5.25 | |
| Imidazolium | C₃H₅N₂⁺ | 6.95 | |
| Tris | (HOCH₂)₃CNH₃⁺ | 8.06 | |
| Ammonium | NH₄⁺ | 9.25 | |
| Trimethylammonium | (CH₃)₃NH⁺ | 9.81 | |
| Methylammonium | CH₃NH₃⁺ | 10.66 | |
| Guanidinium | (NH₂)₃C⁺ | 13.6 | |
Amino acid side chain
| Acid | Formula | pKa | Note |
|---|
| Aspartate (β-COOH) | Asp | 3.65 | side chain |
| Glutamate (γ-COOH) | Glu | 4.25 | side chain |
| Histidinium (imidazole) | His | 6.00 | imidazole side chain |
| Cysteine (thiol) | Cys | 8.33 | side chain SH |
| Tyrosine (phenol) | Tyr | 10.07 | phenol side chain |
| Lysine (ε-NH₃⁺) | Lys | 10.53 | side chain amine |
| Arginine (guanidinium) | Arg | 12.48 | guanidinium side chain |
Other
| Acid | Formula | pKa | Note |
|---|
| MOPS | C₇H₁₅NO₄S | 7.20 | Good's buffer |
| HEPES | C₈H₁₈N₂O₄S | 7.50 | Good's buffer |
Sources
- CRC Handbook of Chemistry and Physics, current edition (inorganic and physical acids).
- Bordwell pKa Tables at organicchemistrydata.org (Hans Reich, University of Wisconsin; organic acids).
- Amino acid side chain pKa values: standard biochemistry references (Lehninger, Voet & Voet).
- Good's buffer pKa values: Good, N. E. et al. Biochemistry (1966).