Topic 12.2.1 Acid-base titrations, pH curves and indicators

• When an indicator changes colour, this is described as the 'end point' of a reaction.
• The equivalence point is when the acid and base have reacted together in the exact proportions dictated by the stoichiometric equation.
• When titrating a an aqueous solution of a MONOBASIC acid with an aqueous solution of a MONOACIDIC base, 25cm^3 of acid will react with exactly 25cm^3 of base.
• The pH at the equivalence point depends on the combination of acid and base used. E.g. dilute NaOH (aq) titrated with dilute HCl (aq) will give a pH of 7.00 at the equivalence point, because of the strong acid and strong base.
• If ethanoic acid (weak acid) and sodium hydroxide (strong base) were titrated together, the pH of the solution at the equivalence point would be greater than 7 (more alkaline solution).
• If hydrochloric acid (strong acid) is titrated against aqueous ammonia (weak base), the solution at the equivalence point will have a pH less than 7 (more acidic).
• pH titration curves are used to plot the pH of a solution against the volume of acid added (titrated) against the base. They show how the pH changes across an acid-base reaction.
• When an acid and base react, a neutralisation (equivalence) point is reached, whereby the graph has a large vertical section.
• To obtain a pH titration curve, alkali is added slowly to an acid (or vice versa) and the pH is regularly measured with a pH probe. The smaller the added volumes, the more accurate the curve produced.
• For a strong acid-base reaction, the neutralisation point occurs at around pH 7. Other combinations of strong and weak acids and bases result in different neutralisation points.
• The vertical region of a titration curve is the equivalence point. At the half-equivalence point, pH = pKa.
• The half-equivalence point is when half the volume of acid/base at the equivalence point is added.
• By reading the pH at the half-equivalence point, Ka can be easily calculated from pKa.