C3 - titrations

• finding concentration of alkali:

  1. add 10cm of 0.1 mol/dm sodium hydroxide to conical flask using measuring cylinder
  2. add two drops of phenolphtalein indicator to flask and swirl
  3. use measuring cylinder to add 20cm of hydrochloric acid to burette - do not pour over eye level
  4. open stopcock on burette to add acid to alkali - stop every 5cm and swirl flask
  5. when indicator changes colour, write doewn volume used from burette - this is rough titre
  6. refill burette with hydrochloric acid and add acid drop by drop until indicator changes colour
  7. repeat step 6 and write down each result until concordant results of volume of hydrochloric acid added
• • to increase accuracy and spot anomalous results, repeat experiment to get several consistent readings
  • calculate mean by ignoring any anomalous readings
• • universal indicators are made of a mixture of different indicators
  • So, there is a gradual colour change
  • titrations need a sudden colour change at end point
  • Single indicators are used to get sudden colour change
• indicators:

  • litmus - blue in alkali, red in acid
  • phenolphthalein - pink in alkali, colourless in acid
  • methyl orange - yellow in alkali, red in acid
• • a pipette is used to measure the volume of a reactant
  • the same volume can be measured each time, to help the results be repeatable
• • a burette is used to add small measured amounts of one reactant to other