Buffer pH Calculator
Henderson Hasselbalch pH from pKa and base acid ratio.
What it does
Buffer pH follows the base to acid ratio around the acid pKa: equal parts read exactly pKa, doubling the base lifts pH by 0.3, and tenfold lifts it a full unit. The pair absorbs added acid or base by shifting the ratio.
The tool above takes pKa with both concentrations. It returns the pH with the ratio and a buffer strength verdict beside it.
The default acetic pair at 1 M each with pKa 4.76 reads pH 4.76, a good buffer. Doubling the base to 2 M reads 5.06 while staying inside the effective zone.
The Henderson Hasselbalch form follows OpenStax Chemistry buffer chapters with Khan Academy acid base units matching them. Both flag pKa plus or minus 1 as the working range.
How to use
- Enter the acid pKa for your pair.
- Enter the conjugate base concentration.
- Enter the weak acid concentration.
- Read pH first for the solution.
- Check strength before trusting it against additions.
Formula
- pKa
- Acid strength anchor of the pair.
- base/acid
- Concentration ratio setting the offset.
- ±1
- Working zone holding 10:1 to 1:10 ratios.
Concentrations share units so they cancel; only the ratio matters. Strong acids and bases alone do not buffer and need the pH page instead.
Worked example
Example: acetic 1 M / 1 M, pKa 4.76
Defaults: pKa 4.76, base 1, acid 1.
- Ratio: 1 / 1 = 1.
- log10(1) = 0.
- pH: 4.76 + 0 = 4.76.
- Zone: exactly at pKa, good buffer.
- Tool shows 4.76, 1:1, Good buffer.
Reference table
| Pair | pKa | pH at 1:1 |
|---|---|---|
| Acetic / acetate | 4.76 | 4.76 |
| Carbonic / bicarbonate | 6.37 | 6.37 |
| Ammonia / ammonium | 9.25 | 9.25 |
| Phosphate second | 7.21 | 7.21 |
Limits
- Dilute ideal solutions assumed.
- Single pair only; polyprotic needs stepwise math.
- Outside ±1 the capacity fades fast.
- Temperature shifts pKa slightly.
Common mistakes
- Logging acid over base instead.
- Mixing pKa with pH as inputs.
- Trusting buffers past 10:1 ratios.
- Using volume instead of concentration.
- Expecting dilution to shift pH.
Try it yourself
Quick checks. Answers sit tucked under each question.
Base 2 M, acid 1 M, pKa 4.76?
4.76 + log10(2) = 4.76 + 0.30 = 5.06.
Base 1, acid 10, pKa 7.21?
7.21 - 1 = 6.21, edge of zone.
Why no dilution effect?
Both halve together, so the ratio stands still.
Questions
What is buffer pH?
pKa plus log of base over acid: 1:1 always reads pKa.
When is buffering best?
Within 1 of pKa, where ratios run 10:1 to 1:10.
Does adding water change pH?
No. Dilution keeps the ratio, so pH holds.
Blood buffer pKa?
Carbonic 6.37 pair tuned by lungs and kidneys near 7.4.
Base ten times acid?
pH sits exactly 1 above pKa.
How do I pick a pair?
Choose pKa nearest your target pH, then set the ratio.