AQA A Level Chemistry复习笔记4.1.2 Performing a Titration & Volumetric Analysis

REQUIRED PRACTICAL 1

Performing the Titration

• The key piece of equipment used in the titration is the burette
• Burettes are usually marked to a precision of 0.10 cm3
  • Since they are analogue instruments, the uncertainty is recorded to half the smallest marking, in other words to ±0.05 cm3

   

• The end point or equivalence point occurs when the two solutions have reacted completely and is shown with the use of an indicator

The steps in a titration

• A white tile is placed under the conical flask while the titration is performed, to make it easier to see the colour change

• The steps in a titration are:
  • Measuring a known volume (usually 20 or 25 cm3) of one of the solutions with a volumetric pipette and placing it into a conical flask
  • The other solution is placed in the burette
    • To start with, the burette will usually be filled to 0.00 cm3

     

  • A few drops of the indicator are added to the solution in the conical flask
  • The tap on the burette is carefully opened and the solution added, portion by portion, to the conical flask until the indicator starts to change colour
  • As you start getting near to the end point, the flow of the burette should be slowed right down so that the solution is added dropwise
    • You should be able to close the tap on the burette after one drop has caused the colour change

     

  • Multiple runs are carried out until concordant results are obtained
    • Concordant results are within 0.1 cm3 of each other

     

   

Recording and processing titration results

• Both the initial and final burette readings should be recorded and shown to a precision of  ±0.05 cm3, the same as the uncertainty

A typical layout and set of titration results

• The volume delivered (titre) is calculated and recorded to an uncertainty of ±0.10 cm3
  • The uncertainty is doubled, because two burette readings are made to obtain the titre (V final – V initial), following the rules for propagation of uncertainties

   

• Concordant results are then averaged, and non-concordant results are discarded
• The appropriate calculations are then done

Percentage Uncertainties

• Percentage uncertainties are a way to compare the significance of an absolute uncertainty on a measurement
• This is not to be confused with percentage error, which is a comparison of a result to a literature value
• The formula for calculating percentage uncertainty is as follows:

Adding or subtracting measurements

• When you are adding or subtracting two measurements then you add together the absolute measurement uncertainties
• For example,
  • Using a balance to measure the initial and final mass of a container
  • Using a thermometer for the measurement of the temperature at the start and the end
  • Using a burette to find the initial reading and final reading

   

• In all these example you have to read the instrument twice to obtain the quantity
• If each you time you read the instrument the measurement is ‘out’ by the stated uncertainty, then your final quantity is potentially ‘out’ by twice the uncertainty