Buffers: How They Work and Where They Stop Working
A buffer resists pH change only within a range, and knowing that range is what distinguishes a designed buffer from a hopeful one.
How a buffer resists change
A buffer contains a weak acid and its conjugate base in comparable amounts. Added strong acid is consumed by the conjugate base; added strong base is consumed by the weak acid. In both cases the ratio of the two components changes only slightly, so the pH barely moves.
The requirement is that both components be present in significant amount. A weak acid alone is not a buffer, and neither is its salt alone.
The Henderson–Hasselbalch equation
The equation is an approximation, valid where the concentrations are much larger than the amount of acid dissociating. At very low concentrations it fails, and the full equilibrium treatment is needed.
Buffer capacity and range
Capacity is the amount of strong acid or base a buffer can absorb before the pH shifts substantially. It depends on two things:
- Total concentration — more of both components means more capacity.
- The ratio — capacity is maximal when the two are equal, that is at pH equal to pK.
The useful range is conventionally taken as one pH unit either side of the pK, corresponding to ratios between one to ten and ten to one. Outside that range one component is largely depleted and the buffer fails quickly.
Choosing a buffer
- Identify the pH required.
- Choose a weak acid whose pK is close to that pH — ideally within one unit.
- Compute the ratio needed from the Henderson–Hasselbalch equation.
- Choose total concentration according to how much capacity is needed.
- Check the buffer does not interfere chemically with the system.
Step two is the essential one. Trying to buffer at a pH far from any available pK gives a solution with almost no capacity, and no adjustment of concentration fixes that.
Preparation
Two routes give the same result. Mix the weak acid with its salt in the calculated ratio, or take the weak acid and partially neutralise it with strong base — which generates the conjugate base in situ.
The second route is often more convenient and is the basis of a common calculation: given a volume of weak acid and a volume of strong base, find the resulting pH. The method is to compute how much acid was converted to conjugate base, then apply the equation to the remaining amounts.
Biological buffering
Blood pH is held within a narrow range chiefly by the carbonic acid and hydrogencarbonate system, which is unusual in being an open buffer — one component is a dissolved gas in equilibrium with the lungs.
That makes it far more effective than a closed buffer of the same composition, because carbon dioxide can be removed by breathing, shifting the equilibrium and restoring pH. Explaining why an open system buffers better is a good higher-order question, and the answer is that one component can be added or removed independently.
Frequently asked questions
Why does dilution not change buffer pH?
Because pH depends on the ratio of the two components, and dilution changes both equally. Capacity falls, but pH stays nearly the same.
When is buffer capacity greatest?
When the acid and conjugate base are present in equal amounts, which is when the pH equals the pK.
Can a strong acid be used to make a buffer?
No. A strong acid is fully dissociated, so there is no undissociated acid to absorb added base. Buffering requires a weak acid with a meaningful equilibrium.
Why is the blood buffer so effective?
Because it is open — carbon dioxide can be exhaled, allowing one component to be removed independently and the equilibrium to be reset, which a closed buffer cannot do.
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