Typical pH curves for acid–base titrations in all combinations of weak and…

The idea

Four shapes, and the key to all of them is which partner is weak. Strong acid with strong base starts near pH 1, has a long steep section running from about pH 3 to pH 11, and its equivalence point is at pH 7. Weak acid with strong base starts higher, near pH 3, shows a flat buffer region, and its steep section runs only from about pH 7 to pH 11, so equivalence lies above pH 7 because the salt formed is that of a weak acid. Strong acid with weak base starts near pH 1 and its steep section runs from about pH 3 to pH 7, with equivalence below pH 7. Weak acid with weak base has no steep section at all, just a gradual change through the equivalence region. Every curve levels off at the pH of whichever solution is in excess.

What earns the marks

• A starting pH of about 1 for a strong acid and about 3 for a weak acid of the same concentration.
• A steep section spanning roughly pH 3 to pH 11 for a strong acid with a strong base.
• An equivalence point above pH 7 for a weak acid with a strong base.
• An equivalence point below pH 7 for a strong acid with a weak base.
• Showing no steep section for a weak acid with a weak base.

Common mistakes and how to put them right

• Drawing every curve passing through pH 7 at the equivalence point.

• Starting a weak acid curve at pH 1.

• Drawing a steep section on the weak acid with weak base curve.

• Letting the curve keep rising instead of levelling off once the alkali is in excess.

• Drawing the buffer region as a flat horizontal line rather than a shallow rise.

Exam tips

• Decide the starting pH and the final pH first, then join them with the correct shape; the two ends fix most of the marks.
• Mark the equivalence volume on the axis before you draw, so the steep section ends up in the right place.