What is pKa?
The pKa is a quantitative measure that determines how weak or strong an acid is. More precisely, pKa tells you how strongly a Brønsted acid holds on to a given proton (H+). It allows you to predict how each acid and base solution will react in a specific experimental setting.
Interpretation of pKa
- Lower pKa → Stronger acid. The H+ is held more loosely by the acid and can be donated more easily.
- Higher pKa → Weaker acid. The H+ is held more tightly and the acid does not easily donate a proton.
| Functional Group | Formula | pKa |
|---|---|---|
| Hydroiodic acid | HI | −10 |
| Hydrobromic acid | HBr | −9 |
| Hydrochloric acid | HCl | −6 |
| Sulfuric acid | H2SO4 | −3 |
| Hydronium ion | H3O+ | −1.7 |
| Sulfonic acids | R–SO3H | −1 |
| Hydrofluoric acid | HF | 3.2 |
| Carboxylic acid | R–COOH | 4 |
| Acetic acid | CH3COOH | 4.76 |
| Carbonic acid (1st) | H2CO3 | 6.35 |
| Carbonic acid (2nd) | HCO3− | 10.33 |
| Protonated amines | R–NH2 | 9–11 |
| Thiols | R–SH | 13 |
| Malonates | CH2(COOH)2 | 13 |
| Water | H2O | 14 |
| Alcohol | CH3CH2OH | 17 |
| Ketone / Aldehyde | R–CH2O / R–CHO | 20–24 |
| Nitrile | R–C≡N | 25 |
| Ester | R–COO–R' | 25 |
| Alkyne | R–C≡C–R' | 25 |
| Amines | R–NH2 | ~35 |
| Hydrogen | H | 36 |
| Alkene | R–C=C–R' | ~43 |
| Alkane | CnH2n+2 | ~50 |
pKa and pH — How to Calculate pKa from pH?
The relationship between pKa and pH is described by the Henderson–Hasselbalch equation:
Rearranging to find pKa:
Where:
- [A−] — molar concentration of the conjugate base (deprotonated form)
- [HA] — molar concentration of the undissociated acid
- pH — the measured pH of the solution
Note on units: Concentrations can be expressed in metric units (mol/L, mmol/L, μmol/L) or American units (mol/US gal, mol/fl oz). Since only the ratio [A−]/[HA] matters, any consistent unit gives the same pKa result.
pKa and Ka — How to Calculate pKa from Ka?
The acid dissociation constant Ka and pKa are related by a simple logarithmic formula:
And the reverse conversion:
The Ka value represents the equilibrium constant for the dissociation reaction:
Similarly, pKb and Kb relate to the conjugate base:
Examples of pKa Calculation
Example 1: pKa from Ka (acetic acid)
Acetic acid has Ka = 1.8 × 10−5.
pKa = −log10(1.8 × 10−5) = −(−4.745) = 4.74
pKb = 14 − 4.74 = 9.26
Example 2: pKa from pH using Henderson–Hasselbalch
A buffer has pH = 5.0, [CH3COO−] = 0.2 mol/L (metric), [CH3COOH] = 0.1 mol/L (metric).
pKa = 5.0 − log10(0.2 / 0.1) = 5.0 − log10(2.0) = 5.0 − 0.301 = 4.70
Example 3: American units — same result
The same buffer expressed in mol/US gal: [A−] = 0.757 mol/US gal, [HA] = 0.379 mol/US gal.
Ratio = 0.757 / 0.379 = 2.0 → pKa = 5.0 − log10(2.0) = 4.70 ✓ (identical result)
Frequently Asked Questions
What does a negative pKa mean?
A negative pKa means the acid is very strong (Ka > 1). Examples include HCl (pKa ≈ −6) and H2SO4 (pKa ≈ −3). These acids are essentially fully dissociated in aqueous solution.
What is the difference between pKa and pH?
pH describes the acidity of a solution and depends on concentration. pKa is an intrinsic property of an acid that does not depend on concentration — it only depends on the molecular structure and temperature.
At what pH is an acid half-dissociated?
From the Henderson–Hasselbalch equation: when [A−] = [HA], log10(1) = 0, so pH = pKa. This is the half-equivalence point in a titration.
What is pKb?
pKb measures the strength of the conjugate base. At 25°C, pKa + pKb = 14. A strong acid (low pKa) has a weak conjugate base (high pKb).
Can pKa be greater than 14?
Yes. Very weak acids like water (pKa ≈ 14), alcohols (~17), ketones (~20–24), and alkanes (~50) have pKa values well above 14. These compounds are essentially not acidic in aqueous solution.