Titration curves There are 4 main types of curve 1. Strong acid and strong base 2. Weak acid and strong base 3. Strong acid and weak base 4. Weak acid and weak base The Key points to sketching a curve: Initial and final pH Volume at neutralisation General Shape (pH at neutralisation) Strong acid – Strong base pH 7 1 13 25 cm3 of base Long steep part from around 3 to 9 pH at equivalence point = 7 You may also have to work out the neutralisation volume from titration data given in the question. These are done by standard titration calculations from module 1. The equivalence point lies at the mid point of the extrapolated vertical portion of the curve. e.g. HCl and NaOH Weak acid – Strong base e.g. CH3CO2H and NaOH pH 7 1 13 V cm3 of base At the start the pH rises quickly and then levels off. The flattened part is called the buffer region and is formed because a buffer solution is made Steep part of curve >7 (around 7 to 9) Equivalence point >7 pH starts near 3 Half neutralisation volume [H+ (aq)][A- (aq)] [HA (aq)] Ka= For weak acids At ½ the neutralisation volume the [HA] = [A-] So Ka= [H+ ] and pKa = pH If we know the Ka we can then work out the pH at ½ V or vice versa. If a pH curve is plotted then the pH of a weak acid at half neutralisation (½ V) will equal the pKa Constructing a PH curve Calibrate meter first by measuring known pH of a buffer solution. This is necessary because pH meters can lose accuracy on storage Can improve accuracy by maintaining constant temperature •Measure initial pH of the acid • Add alkali in small amounts noting the volume added • Stir mixture to equalise the pH • Measure and record the pH to 1 dp • When approaching endpoint add in smaller volumes of alkali •Add until alkali in excess. Strong acid – Weak base Equivalence point < 7 Steep part of curve <7 (around 4 to 7) Weak acid – Weak base e.g. CH3CO2H and NH3 e.g. HCl and NH3 pH 7 1 13 25 cm3 of base No Steep part of the curve Choosing an Indicator Indicators can be considered as weak acids. The acid must have a different colour to its conjugate base HIn (aq) In- (aq) + H+ (aq) colour A colour B We can apply Le Chatelier to give us the colour. In an acid solution the H+ ions present will push this equilibrium towards the reactants. Therefore colour A is the acidic colour. In an alkaline solution the OHions will react and remove H+ ions causing the equilibrium to shift to the products. Colour B is the alkaline colour. An indicator changes colour from HIn to In- over a narrow range. Different indicators change colours over a different ranges. The end-point of a titration is reached when [HIn] = [In-]. To choose a correct indicator for a titration one should pick an indicator whose end-point coincides with the equivalence point for the titration. An indicator will work if the pH range of the indicator lies on the steep part of the titration curve. In this case the indicator will change colour rapidly and the colour change will correspond to the neutralisation point. pH 7 1 13 25 cm3 of base strong base weak base strong acid weak acid pH range for phenolphthalein pH range for methyl orange Use methyl orange with titrations with strong acids but not weak acids Colour change: red acid yellow alkali (orange end point) Only use phenolphthalein in titrations with strong bases but not weak bases- Colour change: colourless acid pink alkali
22.214.171.124 pH curves, titrations and indicators (A-level only)
Typical pH curves for acid–base titrations in all combinations of weak and strong monoprotic acids and bases.
Students should be able to:
• sketch and explain the shapes of typical pH curves
• use pH curves to select an appropriate indicator