7. What pH changes go on in a neutralisation reaction?

The graphs show how the pH changes when an alkali (soluble base) and an acid neutralise each other and what you see visually using universal indicator (univ. ind.).

This what is happening in the salt preparation method (a) above. Note: you can prepare a salt by doing the acid-alkali addition either way round but in either case the volume of acid or alkali needed for neutralisation = the volume reading X at pH 7 (univ. ind. green).

Red graph line: If you add acid to an alkali (univ. ind. = blue), the pH starts at about 13 and only falls little at first as the colour changes from purple ==> blue. Then the pH falls much more steeply as the indicator colour changes from 'bluey' green ==> dark green ==> pale green. The solution is then neutralised at pH 7. This is the point where the salt is 100% formed. With further addition of excess acid, the pH falls and then levels out to about pH 1 as the colour changes further from green ==> yellow ==> orange.

Blue graph line: If you add alkali to an acid (univ. ind. = red), the pH starts at about 1 and only rises a little at first with the colour still quite red. Then on further addition of alkali the pH rises more sharply as the colour changes from red ==> orange ==> yellow and eventually at the neutralisation point at pH 7 the univ. ind. is green. This is the point where the salt is 100% formed. With excess alkali the pH continues to rise and then levels out to about 13 as the indicator colour changes through dark green ==> blue ==> purple.

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.
• 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 accurately.
  • 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.
  • 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.
•  Section (2) lists common indicators.
• Acid-alkali titration calculations for GCSE students.
• Further theory, and examples of strong/weak acids/alkalis (soluble bases) are described on the section 10.
• Advanced level theory of indicators and titrations and advanced acid-alkali titration questions (GCE-AS-A2-IB students only!)