Add alkali to acid one drop at a time and the pH hardly moves, then leaps by several units in a fraction of a cubic centimetre. The shape of that jump depends on whether the acid and base are strong or weak, and it decides which indicator will work.
pH = −log10[H+]. For a strong monobasic acid, [H+] equals the acid concentration, so 0.0100 mol dm−3 HCl has pH 2.00; each tenfold dilution raises the pH by one. A titration curve plots pH against volume added. Near the equivalence point the pH changes sharply, and the vertical section sits around pH 7 for strong acid/strong base, below 7 for strong acid/weak base, and above 7 for weak acid/strong base. A suitable indicator must change colour entirely within that vertical section: methyl orange (3.1 to 4.4) for strong acid/weak base, phenolphthalein (8.3 to 10.0) for weak acid/strong base, either for strong/strong, and none for weak/weak.
pH is a logarithmic scale: one unit is a factor of ten in hydrogen ion concentration.
| [H+] / mol dm−3 | pH | Example |
|---|---|---|
| 0.100 | 1.00 | 0.100 mol dm−3 HCl |
| 0.0500 | 1.30 | 0.0500 mol dm−3 HNO3 |
| 0.0100 | 2.00 | 0.0100 mol dm−3 HCl |
| 1.00 × 10−7 | 7.00 | pure water at 25 °C |
A weak acid at the same concentration has a much smaller [H+], so a higher pH. Calculating it needs Ka, which comes in the A Level year.
Run all four combinations in the simulator, adding 0.100 mol dm−3 base to 25.0 cm3 of 0.100 mol dm−3 acid. Note the starting pH, the pH at equivalence and the height of the vertical section.
| Acid / base | Start pH | Vertical section | Equivalence pH | Indicator |
|---|---|---|---|---|
| strong / strong | about 1 | about 3 to 11 | 7 | methyl orange or phenolphthalein |
| strong / weak | about 1 | about 3 to 7 | below 7 | methyl orange |
| weak / strong | about 3 | about 7 to 11 | above 7 | phenolphthalein |
| weak / weak | about 3 | no sharp change | about 7 | none suitable |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the pH of 0.0100 mol dm−3 hydrochloric acid?
A solution of a strong acid is diluted by a factor of 10. Its pH:
The pH at the equivalence point of a weak acid titrated with a strong base is:
Which indicator should be used to titrate ethanoic acid with sodium hydroxide?
An indicator changes colour over a range of about two pH units. It gives a sharp end point only if that whole range falls inside the vertical section of the curve.
The equivalence point is where the acid and base have reacted in exactly the ratio of the equation; it is not always pH 7. And the end point of an indicator is where it changes colour, which is only a good estimate of equivalence if the indicator's range lies inside the steep part of the curve.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
What is the pH of 0.0500 mol dm−3 nitric acid?
Why is no indicator suitable for titrating ethanoic acid with aqueous ammonia?
Which indicator is suitable for titrating hydrochloric acid with aqueous ammonia?
Two acids of equal concentration are titrated with the same NaOH solution. Acid X starts at pH 1 and acid Y at pH 3. Both need 25.0 cm3 to reach equivalence. What can be deduced?
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