Eduqas Chemistry for A Level Year 2: Student Bk

89 PI5.2 Acid-base equilibria Indicators Indicators are substances that change colour as the pH changes. To do this they must be weak acids, with the original molecule and dissociated ions having different colours. S N N N O O OH CH 3 H 3 C S N N N O O O - CH 3 H 3 C Methyl orange (red form) Methyl orange (orange form) In acid In alkali The solutions of these ions change colour as the pH of the solution changes, due to the relative amounts of the two forms changing. In acid, most of the indicator exists as the neutral form (HInd). In alkali, most of the indicator exists as the anion (Ind – ). The indicator does not suddenly change from one form to another at a specific pH – it changes over a range of pH values. This range is different for each indicator. The reason we see a sharp colour change in a titration is that the pH of the solution changes sharply at the end point. To make sure that the indicator is suitable, we have to check that the range that the indicator changes colour lies within this sharp increase in pH value. Indicator Approximate colour change range Phenolphthalein 8.3–10.0 Bromothymol blue 6.0–7.5 Litmus 4.0–6.5 Methyl orange 3.2–4.4 Strong acid – strong base 14 12 10 8 6 2 4 0 0 10 20 30 40 50 Methyl Orange Phenolphthalein pH Volume of NaOH (cm 3 ) Strong acid – strong base For a strong acid–strong base the pH increases sharply from 2 to 12. All the indicators change fully in this range, so any of them could be used, although the colour change of phenolphthalein means it is frequently the one chosen. Key Terms An indicator is a substance that has different colours in low and high pH solutions. This can be used to differentiate between an acid and an alkali but cannot distinguish between acids of different pH or alkalis of different pH. Universal indicator is a solution that shows a range of colours as the pH changes, so it can be used to find the pH of a particular solution. Acid-base titrations in Topic C2.1 in the year 1 book. Link & S C Stretch & Challenge Dibasic acids and bases show two different equivalence points, both having small vertical regions. Indicators can be selected that match each separate vertical region, which allows a titration to be performed with one indicator to find the first equivalence point, and then a second indicator can be added to find the second equivalence point. A similar approach can be used to analyse a mixture of a strong and weak acid (or a strong and weak alkali). YOU SHOULD KNOW › › › ››› how an indicator works ››› how to select an appropriate indicator for any titration