Acid-base titration curve
Add sodium hydroxide to hydrochloric acid, drop by drop, and watch the pH curve trace itself out. Drag the slider and watch. The pH barely moves for most of the titration — then near the equivalence point it rockets through neutral in a whisker of a millilitre.
The ideas you're seeing
The equivalence point
The point where moles of acid exactly equal moles of base added. For a strong acid + strong base, this point is always at pH 7 — perfectly neutral, no exceptions.
Why the curve is so steep
Right around the equivalence point, there's almost no excess acid or base left to buffer the change — so adding even a tiny bit more base causes a huge pH swing.
Before vs after equivalence
Before equivalence, pH is set by the leftover acid. After equivalence, it's set by the excess base. The calculation flips from a [H⁺] problem to an [OH⁻] problem right at the crossover.
Choosing an indicator
Because the pH jumps sharply through 7, almost any indicator that changes colour in that steep region — like phenolphthalein, colourless → pink around pH 8.3 — shows a sharp endpoint very close to the true equivalence point.
Part of the Equilibrium chapter — read the notes, grab the formula sheet and take the quiz. One of Priodemy for School, free with every EduSuite school.
Reading a titration curve
Why the curve is flat, then vertical, then flat again
The striking feature of a titration curve is its shape: long gentle stretches at each end and an almost vertical jump in the middle. Both come from the logarithmic definition of pH.
Early on, the flask holds a lot of unreacted acid. Each drop of base neutralises a small fraction of it, so the hydrogen ion concentration falls only slightly and the pH barely moves. Near the equivalence point almost all the acid has been consumed, so a single drop can remove most of what remains — cutting [H⁺] by a factor of ten or a hundred, which is a jump of one or two whole pH units. Past equivalence, excess base accumulates and again dilutes the effect of each further drop. The steepness in the middle is not a chemical event so much as arithmetic on a logarithmic scale.
Equivalence is not always pH 7
The equivalence point is where the moles of acid and base are exactly equal. That is a stoichiometric statement, not a pH of 7, and the two coincide only for a strong acid with a strong base, where the salt formed is neutral.
Titrate a weak acid with a strong base and the salt is that of a weak acid, whose anion hydrolyses to give a basic solution — equivalence lands above 7, typically around 8 to 9. Titrate a weak base with a strong acid and equivalence falls below 7. Switch the combination in the simulator and watch the midpoint of the vertical section move up or down.
Weak-acid curves also show a buffer region before equivalence, where pH changes very slowly. At the half-equivalence point exactly half the acid has been converted to its conjugate base, and the Henderson–Hasselbalch relationship reduces to pH = pKa. This is the standard experimental route to measuring pKa, and reading it off the graph is a common exam task.
Choosing the indicator
An indicator must change colour within the vertical portion of the curve, so that one drop produces the colour change. Phenolphthalein turns between about pH 8.3 and 10, which suits weak acid against strong base. Methyl orange changes between about 3.1 and 4.4, suiting strong acid against weak base. Either works for a strong–strong titration, because its vertical section spans both ranges.
Mistakes that cost marks
Assuming equivalence means pH 7. True only for strong–strong. Stating it generally is one of the most commonly penalised claims in this chapter.
Confusing the equivalence point with the end point. Equivalence is the true stoichiometric point; the end point is where the indicator actually changes. A well-chosen indicator makes them nearly coincide, and the difference between them is the titration error.
Expecting a sharp jump for weak against weak. Such a titration has no steep section at all, so no indicator can mark it reliably — which is why it is not done in practice.