pKa Calculator

Calculate pKa from Ka (acid dissociation constant) or vice versa. Use the Henderson-Hasselbalch equation to find pH and acid-base equilibrium ratios.

Core Formulas: pKa = -log₁₀(Ka) Ka = 10⁻ᵖᴷᵃ Henderson-Hasselbalch Equation: pH = pKa + log₁₀([A⁻]/[HA]) Where: • Ka = acid dissociation constant • pKa = negative logarithm of Ka • [A⁻] = concentration of conjugate base • [HA] = concentration of acid • pH = solution pH Relationships: pKa + pKb = 14 (conjugate acid-base pair) Ka × Kb = Kw = 10⁻¹⁴ Percent Dissociation: % deprotonated = (ratio / (1 + ratio)) × 100 % protonated = 100 - % deprotonated Buffer Range: Effective buffering: pKa ± 1 pH unit At pH = pKa: 50% dissociated
Example 1 (Acetic Acid - Calculate pKa): Ka = 1.74×10⁻⁵ pKa = -log₁₀(1.74×10⁻⁵) = 4.76 Acid strength: Weak acid pKb (conjugate base) = 14 - 4.76 = 9.24 Example 2 (Calculate Ka from pKa): pKa = 4.76 (acetic acid) Ka = 10⁻⁴·⁷⁶ = 1.74×10⁻⁵ This is a weak acid (pKa > 4) Example 3 (Henderson-Hasselbalch for pH): pKa = 4.76 (acetic acid buffer) [A⁻]/[HA] = 1.0 (equal concentrations) pH = 4.76 + log₁₀(1.0) = 4.76 Result: 50% dissociated at pH = pKa Example 4 (Find ratio from pH): pKa = 7.20 (phosphate buffer) pH = 7.40 (blood pH) [A⁻]/[HA] = 10^(7.40-7.20) = 10^0.20 = 1.58 Result: 61.3% deprotonated, 38.7% protonated Example 5 (Strong vs Weak Acid): Strong acid (HCl): pKa ≈ -7, Ka ≈ 10⁷ Weak acid (acetic): pKa = 4.76, Ka = 1.74×10⁻⁵ Difference: Ka differs by 10¹² times! Example 6 (Amino Acid): Glycine carboxyl group: pKa = 2.34 At pH 7.4 (physiological): [A⁻]/[HA] = 10^(7.4-2.34) = 1.15×10⁵ Result: >99.999% deprotonated

What is pKa?

pKa is the negative logarithm of the acid dissociation constant (Ka). pKa = -log₁₀(Ka). It measures the strength of an acid - lower pKa values indicate stronger acids. For example, HCl has a pKa of -7 (very strong), while acetic acid has a pKa of 4.76 (weak acid).

What is the relationship between pKa and Ka?

pKa and Ka are inversely related: pKa = -log₁₀(Ka) and Ka = 10⁻ᵖᴷᵃ. A change of 1 pKa unit represents a 10-fold change in Ka. For example, pKa 3 means Ka = 10⁻³ = 0.001, while pKa 4 means Ka = 10⁻⁴ = 0.0001.

How does pKa relate to pH?

The Henderson-Hasselbalch equation relates them: pH = pKa + log([A⁻]/[HA]). When pH = pKa, the acid is exactly 50% dissociated. If pH > pKa, the deprotonated form dominates; if pH < pKa, the protonated form dominates.

What is a good pKa value?

There's no single "good" value - it depends on application. For buffers, you want pKa near your target pH (±1 unit). For drug design, pKa affects absorption and distribution. Physiological buffers typically have pKa 6-8 (e.g., phosphate buffer pKa = 7.2).

How do I calculate Ka from pKa?

Use Ka = 10⁻ᵖᴷᵃ. For example, if pKa = 4.76 (acetic acid), then Ka = 10⁻⁴·⁷⁶ = 1.74×10⁻⁵. This is the acid dissociation constant at equilibrium.

What are typical pKa values?

Strong acids: HCl (pKa ≈ -7), H₂SO₄ (pKa₁ ≈ -3). Weak acids: Acetic acid (4.76), Carbonic acid (6.35), Phosphoric acid (2.15, 7.20, 12.35). Weak bases are characterized by their conjugate acid's pKa.

Why is pKa important in biochemistry?

pKa determines ionization state at physiological pH (7.4). This affects enzyme activity, protein structure, drug absorption, and metabolic reactions. Amino acids have multiple pKa values (carboxyl ~2, amino ~9, side chains variable).

How do I use the Henderson-Hasselbalch equation?

pH = pKa + log([A⁻]/[HA]). To find pH: enter pKa and concentration ratio. To find ratio at given pH: rearrange to [A⁻]/[HA] = 10^(pH-pKa). This is crucial for buffer calculations and acid-base equilibria.

What is the buffer range?

Buffers work best within pKa ± 1 unit. This range provides good buffering capacity because both acid and conjugate base are present in significant amounts (10-90% dissociation). Outside this range, buffering capacity drops sharply.

How does temperature affect pKa?

pKa changes with temperature due to changes in the dissociation equilibrium. Most pKa values increase slightly with temperature (0.01-0.03 units per °C). Standard pKa values are reported at 25°C unless specified otherwise.

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