titrations

• Litmus


  • Litmus solution: Alkaline = blue, Acidic = red
  • Litmus paper: Blue litmus paper goes red in acidic & stays blue in alkaline, Red litmus paper goes blue in alkaline & stays red in acidic
• Phenolphthalein


  Alkaline = pink, Acidic = colourless
• Methyl orange


  Alkaline = yellow, Acidic = red
• pH scale


  • Measures the acidity or alkalinity of a solution
  • pH 7 is neutral
  • pH < 7 is acidic (the closer to 0, the stronger the acid)
  • pH > 7 is alkaline (the closer to 14, the stronger the alkali)
• Using universal indicator to measure pH


  Add a couple of drops of solution to a piece of universal indicator paper and observe the colour (compare to pH scale)
• Acids in aqueous solution


  Source of hydrogen ions (H+)
• Alkalis in aqueous solution


  Source of hydroxide ions (OH-)
• Alkalis can neutralise acids
• Neutralisation reaction


  • Reaction between an acid and a base
  • Ionic equation: H+(aq) + OH-(aq) -> H2O(l)
• How to carry out an acid-alkali titration


  1. Wash burette using acid and then water

  2. Fill burette to 100cm3 with acid with the meniscus' base on the 100cm3 line

  3. Use 25cm3 pipette to add 25cm3 of alkali into a conical flask, drawing alkali into the pipette using a pipette filler

  4. Add a few drops of a suitable indicator to the conical flask (eg: phenolphthalein which is pink when alkaline and colourless when acidic)

  5. Add acid from burette to alkali until end-point is reached (as shown by indicator)

  6. The titre (volume of acid needed to exactly neutralise the acid) is the difference between the first (100cm3) and second readings on the burette

  7. Repeat the experiment until you get concordant results