Last updated: June 2026
Use this buffer pH calculator to estimate the pH of an acidic or basic buffer solution. It is designed for A-Level and Singapore H2 Chemistry students working with weak acids, weak bases, conjugate pairs and the Henderson–Hasselbalch equation.
What is a buffer?
A buffer is a solution that resists a large change in pH when a small amount of acid or alkali is added. It does not keep the pH perfectly constant. Instead, it reduces the size of the change by using one component to remove added H+ and the other to remove added OH−.
Acidic buffers
An acidic buffer contains a weak acid, HA, and its conjugate base, A−. A common example is ethanoic acid mixed with sodium ethanoate.
The conjugate base removes added H+, while the weak acid removes added OH−. If [A−] equals [HA], the concentration ratio is 1 and pH equals pKa.
Basic buffers
A basic buffer contains a weak base, B, and its conjugate acid, BH+. A common example is ammonia mixed with ammonium chloride.
At 25°C, convert the calculated pOH into pH using pH = 14.00 − pOH. The calculator uses this 25°C relationship for basic buffers.
Using Ka, Kb, pKa and pKb
You may enter either the equilibrium constant or its logarithmic form. The calculator converts between them using:
A larger Ka corresponds to a smaller pKa and therefore a stronger weak acid. The same inverse logarithmic relationship applies to Kb and pKb.
Worked examples
Acidic buffer with equal concentrations
A buffer contains 0.100 mol dm−3 ethanoic acid and 0.100 mol dm−3 ethanoate ions. If pKa = 4.76, then:
Acidic buffer with more conjugate base
If [A−] = 0.200 mol dm−3 and [HA] = 0.100 mol dm−3, the ratio is 2. The pH becomes 4.76 + log102 = 5.06, so the pH is higher than pKa.
Basic buffer with equal concentrations
A buffer contains equal concentrations of NH3 and NH4+. If pKb = 4.75, then pOH = 4.75 and pH = 14.00 − 4.75 = 9.25 at 25°C.
Buffer capacity
Buffer capacity describes how much acid or alkali a buffer can absorb before its pH changes significantly. A more concentrated buffer generally has a larger buffer capacity, even when its component ratio and initial pH are unchanged.
Frequently asked questions
Why does pH equal pKa when the concentrations are equal?
When [A−] = [HA], their ratio is 1. Since log101 = 0, the logarithmic term disappears and pH = pKa.
Can I use moles instead of concentrations?
Yes, provided both buffer components are in the same final solution volume. The common volume cancels from the ratio, so the mole ratio is the same as the concentration ratio.
Can this calculator be used in a weak acid–strong base titration?
It can be used in the buffer region after the added alkali has partially neutralised the weak acid. You must first determine the remaining weak acid and the conjugate base formed by reaction.
Does pH + pOH always equal 14?
The value 14.00 applies at 25°C because pKw is approximately 14.00 at that temperature. At other temperatures, pKw changes.