titrations

• End point

  The point in a titration where the indicator changes colour, the end point indicates when the reaction is just complete
• what is meant by concordant titres?

  when they have a difference of 0.1 cm3
• A titration is a technique used to accurately measure the volume of one solution that reacts exactly with another solution. Titrations can be used for:

  • finding the concentration of a solution
  • identification of unknown chemicals
  • finding the purity of a substance
• A standard solution is a solution of known concentration
• A volumetric flask is used to make up a standard solution very accurately
• The volumetric flasks that you will use are manufactured to the typical tolerances below:

  • a 100 cm3 volumetric flask: + / - 0.20 cm3
  • a 250 cm3 volumetric flask: + / - 0.30 cm3
• Preparing standard solutions

  1. Solid is first weighed accurately
  2. Solid is dissolved in a beaker using less distilled water than will be needed to fill the volumetric flask to the mark
  3. This solution is transferred to a volumetric flask. The last traces of the solution are rinsed into the flask with distilled water
  4. The flask is carefully filled to the graduation line by adding distilled water a drop at a time until the bottom of the meniscus lines up exactly with the mark
  5. Finally, the volumetric flask is slowly inverted several times to mix the solution thoroughly
• When preparing standard solution if too much water is added, the solution will be too dilute and must be prepared again
• When preparing standard solution you should view the graduation mark and meniscus at eye level for accuracy
• Acid-base titrations apparatus

  • pipette
  • burette
• A burette reading is recorded to the nearest half division, with the bottom of the meniscus on a mark or between two marks
• • Each burette reading is measured to the nearest 0.05 cm3
  • So the reading always has two decimal places
  • The last place being either a 0 or 5, for example 25.40 cm3 or 26.25 cm3
• Titre = final reading - initial reading
• A quick, trial titration is carried out first to find the approximate titre
• Further titrations are carried out until two accurate titres are concordant - agreeing to within 0.10 cm3
• When filling a burette, run excess solution out through the tap to remove any air bubbles. If a bubble is left in the neck of the burette, the air could be released during the titration, leading to an error in the titre
• When working out the mean titre, it is important to use only your closest accurate titres
• From the results of a titration, you will know the following:

  • both the concentration and the reacting volume of one of the solutions
  • only the reacting volume of the other solution
• The method for analysing the results follows a set pattern:

  1. Work out the amount, in mol, of the solute in the solution for which you know both the concentration and volume (n = c x V)
  2. Use stoichiometry to work out the amount, in mol, of the solute in the other solution
  3. Work out the unknown information about the solute in the other solution e.g. c = mol/volume