The page titrates 25 mL of a weak monoprotic acid with a strong base at 25 °C. Move “base added” and follow the pH. The default pKa of 4.76 matches ethanoic acid. The curve is calculated for an ideal dilute solution with pKw = 14. Equivalence volume, in mL, is (acid concentration × 25) / base concentration.
A weak acid is only partly dissociated. The starting pH is estimated from the acid concentration and Ka, which is why a larger Ka (a smaller pKa) gives a lower starting pH. This is not the pH = −log of the full acid concentration used for a strong acid.
Adding OH⁻ converts HA into A⁻. While both are present, pH = pKa + log([A⁻]/[HA]), the Henderson–Hasselbalch equation. The mixture is a buffer: added OH⁻ is consumed by HA, and added H⁺ is consumed by A⁻, so the pH changes slowly.
Half of the acid has been converted, so [A⁻] = [HA] and the log term is zero. Therefore pH = pKa. A measured titration curve can be used to estimate pKa by reading the pH halfway to equivalence.
At equivalence the solution contains the conjugate base, which reacts slightly with water, so the pH is above 7. Past equivalence, the excess strong base sets the pH and the curve flattens again at a high value.
GCSE titrations are usually strong-acid work and an indicator colour change. The shape of a weak-acid curve, buffers and pKa are A level and IB. The level control on the page is there because the early sliders are enough for a first look and the pKa comparison is the advanced point.
The acid is monoprotic, the base is strong, and the acid sample is fixed at 25 mL. Activity coefficients, dissolved carbon dioxide and a real indicator are not included. The pH is the ideal calculated value.