Library / Foundations: Chemistry / FCH-25
Foundations: Chemistry
pH of a buffer
pH of a mixture of a weak acid and its conjugate base.
- Ref
- FCH-25
- Rev
- v1
- Field
- Foundations: Chemistry
- Result
- pH
pKa = −log₁₀Ka. [A⁻] is the concentration of the conjugate base and [HA] that of the weak acid. Valid for moderate concentrations when both are present in significant amounts.
A buffer resists changes in pH; its pH sits near the acid’s pKa and shifts by how lopsided the mix is.
Calculator
- Base-to-acid ratio2
- Ka1.738 10⁻⁵ mol/dm³
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- When [A⁻] = [HA] the pH equals the pKa, which is where the buffer is most effective.
- A buffer works well within about one pH unit of its pKa.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing () | Local step | Change in result |
|---|---|---|---|---|---|---|
| pKa Acid pKa | 0 to 14 | 0.301 | 14.3 | 14 | +10% | +0.476 |
| [A⁻] Conjugate base | 0.001 to 5 mol/dm³ | 2.76 | 6.459 | 3.699 | +10% | +0.0414 |
| [HA] Weak acid | 0.001 to 5 mol/dm³ | 3.362 | 7.061 | 3.699 | +10% | −0.0414 |
Sweep of Conjugate base
| Conjugate base (mol/dm³) | pH |
|---|---|
| 0.001 | 2.76 |
| 0.0029 | 3.222 |
| 0.008409 | 3.685 |
| 0.02438 | 4.147 |
| 0.07071 | 4.609 |
| 0.205 | 5.072 |
| 0.5946 | 5.534 |
| 1.724 | 5.997 |
| 5 | 6.459 |
Sweep one variable
Variables and how each one contributes
pH
What the equation gives you. Everything else feeds it.
Acid pKa
pKa of the weak acid (ethanoic 4.76). Raises pH one-for-one.
Conjugate base
Concentration of the conjugate base. More base raises the pH.
Weak acid
Concentration of the weak acid. More acid lowers the pH.
Worked example with the values above
- Ratio [A⁻]/[HA] = 0.2 / 0.1 = 2.
- log₁₀(2) = 0.301.
- pH = 4.76 + 0.301 = 5.061.
Assumptions and limits
- Both forms present at moderate concentration.
- Activity effects ignored.
Common mistakes
- Inverting the ratio.
- Using Kₐ instead of pKₐ.
Statistics and uncertainty
A pH meter is typically ±0.02 pH, so the Henderson equation is validated to about that level.
Where it is used
- Maintaining stable pH in solutions.
- Biological and analytical chemistry.
Maths you need
- pH: −log[H⁺]
- Logarithms: log of a ratio