Buffers and titration curves
Walk a weak acid across its whole range and watch where the pH actually goes.
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Description of this simulation
25 mL of 0.1 mol/L CH3COOH starts at pH 2.88 and reaches equivalence after 25 mL of titrant. Halfway there, at 12.5 mL, the pH is 4.76 — which is the pKa of CH3COOH. That is what the half-equivalence point is for. Equivalence is at pH 8.73, not 7. All the acid has been converted to its conjugate base, and that base takes a proton back from the water, so the solution is alkaline at the very moment the acid is used up.
- pKa
- 4.76
- Equivalence volume
- 25 mL
- pH at equivalence
- 8.73alkaline
- pH at half-equivalence
- 4.76 = pKa
- Useful buffer range
- 3.76 – 5.76
Equivalence is not pH 7. All the acid has been converted to its conjugate base, and that base takes a proton back from the water, so the solution is alkaline at the very moment the acid is used up. A curve that crosses 7 at equivalence is the classic wrong plot.
What a buffer actually does
The same 0.01 mol of strong acid, added to a litre of buffer and to a litre of pure water.
Into the buffer
4.76 → 4.67
±0.09
Into pure water
7 → 2
±5
A buffer resists change; it does not prevent it. Both pH values moved — one by about a tenth of a unit, the other by five.
Which indicator would work
- Methyl orange3.1–4.4wrong for this one
- Litmus5–8wrong for this one
- Bromothymol blue6–7.6wrong for this one
- Phenolphthalein8.3–10suits this titration
An indicator has to change colour where the curve is vertical. Methyl orange on a weak acid changes far too early, which shows up as a titre that is consistently short.
Where this came from
Acids, bases and pHWhat this model shows — and what it simplifies
Educational ModelThe curve is solved point by point from the charge and mass balances together with Ka, not sketched from a formula that only holds in the buffer region.
Where do we see this in real life?
Blood is held between pH 7.35 and 7.45 by a carbonate buffer, and a tenth of a unit either way is a medical emergency. The same arithmetic decides how a swimming pool is dosed and which crops a soil will grow.