Titration Curves & Equivalence Points (College Board AP® Chemistry)

Titration Curves & Equivalence Points

• During a titration, a pH meter can be used and a pH curve plotted

• A pH curve is a graph showing how the pH of a solution changes as the acid (or base) is added

Graph to show the pH curve for the addition of a alkali to an acid

The features of a pH curve

• All pH curves show an s-shape curve

• There are four main points in a titration curve:

  • The start where the solution only contains either acid or base

  • The region where the titrant is added up to the equivalence point, and the solution now contains a mixture of reactant and products

  • The equivalence point occurs when the number of moles of titrant (the solution that has been added) added is equal to the number of moles of analyte originally present

  • The region after the equivalence point where the solution contains product and excess titrant

• pH curves yield useful information about how the acid and alkali react together with stoichiometric information

• The midpoint of the inflection is called the equivalence point

• From the curves, you can:

  • Determine the pH of the acid by looking at where the curve starts on the y-axis

  • Find the pH at the equivalence point

  • Find the volume of base at the equivalence point

  • Obtain the range of pH at the vertical section of the curve

pH curve of a strong acid - strong base

• For example odium hydroxide, NaOH (aq), is being added to hydrochloric acid, HCl (aq)

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

Strong acid - strong base pH curve 

The pH increases as the alkali is added to the base