Acid-base Titrations (OCR A Level Chemistry A): Revision Note

Acid-base Titrations

Volumetric analysis

• Volumetric analysis determines the concentration of an unknown solution by reacting it with a solution of known concentration (a standard solution)

• The most common method is a titration

  • This uses precise volume measurements

  • The two main pieces of equipment used are a volumetric pipette (or graduated pipette) and a burette

• A standard solution must first be prepared carefully before the titration can be performed

• Accurate apparatus is essential throughout the process to minimise uncertainty

Key apparatus

• Some key pieces of apparatus used in preparing a standard solution and performing a titration include:

  1. Beaker

  2. Burette

  3. Volumetric pipette

  4. Conical flask

  5. Volumetric flask

Making a standard solution

• Chemists often need to prepare solutions with accurately known concentrations

  • These are called standard solutions or volumetric solutions

• To ensure precision, a 3 d.p. balance and a volumetric flask are used

  • This helps reduce uncertainty in measurement

Method

1. Weigh out the solute

   • Use a 3 d.p. balance to accurately weigh the required mass of solid

2. Dissolve the solute

   • Transfer the solid into a beaker containing a small volume of distilled water

   • Stir using a glass rod until completely dissolved

3. Transfer to a volumetric flask

   • Pour the solution into a clean volumetric flask using a funnel

4. Rinse the beaker

   • Rinse the beaker and glass rod with distilled water

   • Add the rinsings to the volumetric flask to ensure no solute is lost

5. Make up to the mark

   • Add distilled water until the bottom of the meniscus sits exactly on the scratch mark

   • Stopper the flask and invert to mix thoroughly

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, i.e. ±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

Method

1. Measure a known volume (typically 25 cm³) of one solution using a volumetric pipette

2. Transfer it into a conical flask

3. Add a few drops of indicator to the solution in the conical flask

4. Place the conical flask on a white tile

   • The white tile makes it easier to observe the colour change

5. Fill the burette with the second solution and record the initial volume

6. Slowly add the solution from the burette to the conical flask while swirling

7. As the end point approaches, add the solution drop by drop until the colour changes

7. Record the final burette volume

8. Repeat the titration until you obtain concordant titres

   • Concordant results are within 0.10 cm³ of each other

Recording and processing titration results

• Record initial and final burette readings to 2 decimal places, ending in .00 or .05

  • This is because the precision/uncertainty of the burette is ±0.05 cm³

• Calculate the titre by subtracting the initial from the final volume

  • The uncertainty of the titre is ±0.10 cm³

  • This is because two readings with 0.05 cm³ uncertainty are used to determine the titre

• Discard any rough or inconsistent results

  • The rough titre is usually far over the end point and should be discarded

• Calculate an average titre using only concordant values

Example titre table

• ✓ = Concordant results used to calculate the average titre

• Run 3 is discarded as an outlier since it is outside the concordant range

• The average titre is calculated from Run 1 and Run 2 only:

= 22.775 → 22.78 cm³

Percentage uncertainties

• Percentage uncertainty helps assess how significant an error is compared to the measured value

• The percentage uncertainty formula is:

percentage uncertainty = x 100

Adding or subtracting measurements

• When adding or subtracting quantities, add the absolute uncertainties

• This applies to:

  • Mass measurements (initial and final)

  • Temperature changes

  • Burette readings (initial and final)

• Each instrument is read twice, so uncertainties are doubled

Acid-base titration calculations

Volumes & concentrations of solutions

• The concentration of a solution is the amount of solute in 1 dm³ of solution

  • The solute is the substance being dissolved

  • The solvent is the liquid doing the dissolving (usually water)

• A concentrated solution contains a high amount of solute

  • A dilute solution contains a small amount of solute

• Concentration can be expressed as:

  • Moles per dm³

  • Mass per dm³

  • Parts per million (ppm), often used for trace substances in environmental chemistry

Using concentration in calculations

• To calculate values involving concentration, use:

concentration (mol dm^-3) =

• This rearranges to:

number of moles (mol) = concentration (mol dm^-3) x volume (dm³)

• To convert between mass and moles:

mass (g) = amount (mol) × molar mass (g mol^-1)