Eduqas Chemistry for A Level Year 2: Student Bk

90 Eduqas A Level Chemistry Year 2: Component 1 Weak acid – strong base The pH titration curve of weak acid (CH 3 COOH) and strong base (NaOH) 14 12 10 8 6 2 4 0 5 15 25 35 45 55 pH Volume NaOH added (cm 3 ) Equivalence point Methyl Orange Phenolphthalein For a weak acid–strong base the pH increases sharply from 5 to 12 so phenolphthalein or bromothymol blue can be used as they change colour completely in the right pH range. Methyl orange would not change colour at all in this vertical range – it would have changed colour before reaching this point. Phenolphthalein is the most commonly used indicator for this type of titration. Strong acid – weak base Methyl Orange Phenolphthalein 12 10 8 6 2 4 0 5 15 25 35 45 55 pH Volume NH 3 added (cm 3 ) For a strong acid–weak base the pH increases sharply from 2 to 8. Methyl orange, litmus and bromothymol blue change fully in this range. Phenolphthalein starts to change at the end of this range, but it will not change fully, so we can’t use it for a titration using a weak alkali. Methyl orange is the most commonly used indicator for this type of titration. Weak acid–weak base There is no significant vertical area showing a significant and sudden change in pH. No indicator would therefore work and alternative methods must be used for titration. This is usually by using a pH probe. PRACTICAL CHECK The titration of a weak acid–weak base using a pH probe is a specified practical task. This section explains why a pH probe is used rather than an indicator as is the case for all other types of titration. A weak acid–weak base titration is only used when no other method is available as the technique is more challenging than titration using an indicator. Examtip The two most common indicators in use are phenolphthalein and methyl orange. If you have to suggest an indicator, then phenolphthalein is the indicator of choice for any titration using a strong base, as the colour change is sharp and clear. When the base is weak, phenolphthalein cannot be used and so methyl orange is chosen.