pHcurve

A pH curve, also called a titration curve, is a graphical representation of how the pH of a solution changes as a titrant is added during an acid–base titration. It is used in analytical chemistry to study acid–base systems, determine endpoint conditions, calculate concentrations, and estimate dissociation constants.

Construction and measurement: The curve is generated by recording pH at incremental additions of titrant, typically with a calibrated pH meter or electrode linked to a burette. The horizontal axis shows the volume of titrant added, while the vertical axis shows pH. The curve reflects the chemical changes occurring as species are neutralized or converted, including buffering steps and acid–base equilibria.

Typical features: The curve starts at the initial pH of the analyte, includes a buffering region where pH changes slowly, and shows a steep change near the equivalence point. The half-equivalence point, at which the added amount equals half the stoichiometric amount for neutralization, often corresponds to pH = pKa for weak acids (by Henderson–Hasselbalch). The equivalence point is where the moles of titrant equal the moles of analyte.

Effect of titrant types: For a strong acid–strong base titration, the equivalence point pH is near 7 at 25°C. Titrating a weak acid with a strong base yields an equivalence pH above 7, while titrating a strong acid with a weak base yields an equivalence pH below 7. Curve shape and position depend on acid and base strengths and temperature.

Applications: pH curves are used to determine pKa values, calculate concentrations, design buffer systems, select appropriate indicators for endpoint detection, and verify stoichiometry in reactions and analyses.