Cards (24)

  • 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
  • Titration method
    1. Fill burette with acid of known concentration
    2. Record volume of acid in burette
    3. Measure a known volume of alkali into a conical flask
    4. Add a suitable indicator
    5. Add acid from burette into conical flask slowly
    6. Stop adding acid when end point is reached
    7. Repeat until concordant results
  • Manganate (VII) ions are readily reduced to Mn 2+ ions under acidic conditions. The MnO4- ions are purple, Mn2+ are colourless.
    • Oxidation: Fe 2+ -> Fe 3+ + e-
    • Reduction: 8H+ + MnO4- + 5e– → Mn 2+ + 4H2O
    • Overall: 8H+ + MnO4- + 5Fe 2+ → Mn 2+ + 4H2O + 5Fe 3+
  • In iodine-thiosulphate titrations, iodine is reduced to form iodide ions and thiosulphate ions are oxidised. Iodine is often combined with starch, which makes it dark blue. As iodine is consumed the solution turns pale yellow at the end point
  • Oxidation: 2S2O3 2– (aq) → S4O6 2– (aq) + 2e–
    Reduction: I2 (aq) + 2e– → 2I– (aq)
    Overall: 2S2O3 2– (aq) + I2 (aq) → S4O6 2– (aq) + 2I– (aq)