Performing a Titration & Volumetric Analysis (AQA AS Chemistry): Revision Note

Volumetric Analysis

REQUIRED PRACTICAL 1

Performing the Titration

• The main piece of apparatus used in a titration is the burette

• Burettes are typically graduated in 0.10 cm³ divisions

  • Because they are analogue instruments, the reading uncertainty is taken as ±0.05 cm³ (half the smallest scale division)

• The equivalence point is reached when the amount of titrant added is chemically equivalent to the substance in the flask

  • The endpoint is the point at which the indicator changes colour, signalling that the reaction is complete (or very close to complete)

    

• 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 as follows:

  • A known volume (usually 20.0 cm³ or 25.0 cm³) of one solution is measured using a volumetric pipette and transferred to a conical flask

  • The other solution is placed in a burette, which is typically filled close to 0.00 cm³ at the start

  • The initial burette reading is recorded

  • A few drops of a suitable indicator are added to the solution in the conical flask

  • The burette tap is opened carefully, and the solution is added to the conical flask while continuously swirling the flask

  • As the endpoint is approached, the solution is added dropwise

    • The tap should be closed immediately after a single drop causes the indicator to change colour

  • The final burette reading is recorded, and the titre is calculated

• The titration is repeated until concordant results are obtained

  • Concordant titres are usually defined as results that differ by no more than 0.10 cm³

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

• The titre (the volume delivered from the burette) is recorded with an uncertainty of ±0.10 cm³

  • This uncertainty is doubled because two burette readings are used to calculate the titre (final reading − initial reading)

  • Each reading has an uncertainty of ±0.05 cm³, so when subtracting the two values, the uncertainties are added according to the rules for propagation of uncertainties

• Concordant results are then averaged, and any results that are not concordant are excluded from the mean

• The average titre is then used in the relevant calculations, such as determining the number of moles or the concentration of a solution

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:

• Percentage uncertainty is used to show how significant an absolute uncertainty is compared to the size of the measurement

• It should not be confused with percentage error, which compares an experimental result with a known or literature value

• The percentage uncertainty is calculated using the formula:

Adding or subtracting measurements

• When adding or subtracting measurements, the absolute uncertainties are added together

• For example, this applies when:

  • Measuring the initial and final mass of a container using a balance

  • Measuring the temperature at the start and end of an experiment using a thermometer

  • Recording the initial and final readings from a burette to calculate a titre

• In each case, the instrument is read twice to obtain the final value

• If each reading has an uncertainty, the total uncertainty in the calculated quantity is the sum of the two individual uncertainties