Buffer Solution Calculator

Click a buffer to load its pKa value into the calculator.

The Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the weak acid and the ratio of conjugate base to acid concentrations:

pH = pKa + log₁₀([A⁻] / [HA])

What is a Buffer Solution?

A buffer solution resists changes in pH when small amounts of acid or base are added. It consists of:

• Weak acid (HA) and its conjugate base (A⁻), or
• Weak base (B) and its conjugate acid (BH⁺)

Buffer Capacity

A buffer works most effectively when:

• The pH is within ±1 unit of the pKa (effective range: pKa − 1 to pKa + 1)
• The ratio [A⁻]/[HA] is between 0.1 and 10
• Buffer capacity is maximum when pH = pKa (ratio = 1:1)
• Higher total buffer concentration provides greater buffering capacity

Key Relationships

• When [A⁻] = [HA]: pH = pKa (the log term is zero)
• When [A⁻] > [HA]: pH > pKa (more basic)
• When [A⁻] < [HA]: pH < pKa (more acidic)
• pKa + pKb = 14 (at 25°C)
• Ka = 10⁻ᵖᴷᵃ

Buffer Preparation Steps

1. Choose a buffer system with pKa close to your target pH
2. Calculate the required acid/base ratio using Henderson-Hasselbalch
3. Determine the masses of acid and conjugate base salt needed
4. Dissolve both components in less than the final volume of water
5. Adjust pH with dilute HCl or NaOH if needed
6. Bring to final volume with water and mix thoroughly

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