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3. How do you choose a correct indicator for an acid - alkali titration

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Titration index  *  Full chemistry calculations index

Quiz 12 The basics of acid-alkali titration calculations Good exam practice questions for GCSE level

3. How do you choose the right indicator for an acid - alkali titration?

  • Important question! What is the best indicator to use for a particular titration?
  • Universal indicator, and most other acid-base indicators, work for strong acid and alkali titrations, but universal indicator is a somewhat crude indicator for other acid-alkali titrations because it gives such a range of colours for different pH's. Examples of more accurate and 'specialised' indicators are:
    • titrating a strong alkali with a strong acid (or vice versa):
      • e.g. for sodium hydroxide (NaOH) - hydrochloric/sulphuric acid (HCl/H2SO4) titrations, use ...
      • phenolphthalein indicator (pink in alkali, colourless in acid-neutral solutions), the end-point is the pink <==> colourless change.
      • Litmus works too, the end point is the red <==> purple/blue colour change AND with universal indicator you see green at the endpoint, but these two indicators are NOT as accurate as phenolphthalein.
        • They are ok as simple demonstration to illustrate the principles of a titration, BUT, they should not be used for quantitative work.
    • titrating a weak alkali with a strong acid:
      • e.g. for titrating ammonia (NH3) with hydrochloric/sulfuric acid (HCl/H2SO4), use ...
      • methyl orange indicator (red in acid, yellowish-orange in neutral-acid), the end-point is an 'orange' colour, not easy to see without experience.
      • screened methyl orange indicator is a slightly different dye-indicator mixture that is reckoned to be easier to see than methyl orange, the end-point is a sort of 'greyish orange', but still not easy to do accurately.
    • titrating a weak acid with a strong alkali:
      • e.g. for titrating ethanoic acid (CH3COOH) with sodium hydroxide (NaOH), use ...
      • phenolphthalein indicator (pink in alkali, colourless in acid-neutral solutions, pink in alkali), the end-point is the first permanent pink. when the solution just becomes alkaline.
      • methyl red indicator (red in acid, yellow in neutral-alkaline), the end-point is 'orange'.
    • titrating a weak acid with a weak alkali (or vice versa):
      • These are NOT practical titrations because the pH changes at the end-point are not great enough to give a sharp colour change with any indicator.

Indicator

 colour in acid pH<7 colour in neutral pH=7 colour in alkali pH >7

litmus

 red 'purple' blue
phenolphthalein colourless colourless >9 pink
methyl orange <3.5 red, orange about pH 5, > 6 yellow yellow yellow
methyl red <5 red, orange, >6 yellow yellow yellow
bromothymol blue <6 yellow green >8 blue

* Used in titrations - see section 10, litmus is NOT a good indicator for use in accurate titrations.

titration indicator colours

Summary: Three common indicators for titrations and their colours:

litmus: red in acid, blue in alkali, doesn't give that sharp a colour change except with moderately concentrated solutions, the neutralisation end-point is a fairly sharp change red<=>blue

phenolphthalein: colourless in acid, pink in alkali, the neutralisation end-point is the sharp change colourless<=>pink

methyl orange: red in acid, yellow in alkali, the neutralisation end-point is an orange

Titration index  *  Full chemistry calculations index

Quiz 12 The basics of acid-alkali titration calculations Good exam practice questions for GCSE level

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