Acid-Base Titration (OCR AS Chemistry A): Revision Note

PAG 2: Acid-base titration

• There are a series of suggested practicals for PAG 2 - Acid-base titration

  • PAG 2.1: Determination of the concentration of hydrochloric acid

    • Making a standard solution of sodium hydrogencarbonate

    • Titrating this against hydrochloric acid

    • Determining the concentration of the acid

  • PAG 2.2: Determination of the molar mass of an acid

    • Making a solution of an unknown hydrated acid (e.g. citric acid monohydrate)

    • Titrating this against a sodium hydroxide solution of known concentration.

    • Determining the molar mass of the acid and, with additional information, the formula of the acid

  • PAG 2.3: Identification of an unknown carbonate

    • Making a solution of an unknown Group 1 carbonate

    • Titrating this solution against hydrochloric acid of a known concentration

    • Determine the molar mass of the carbonate and the identity of the metal

• All three suggested practicals use volumetric analysis

  • This requires making standard solutions and completing titration experiments

  • The main difference between the practicals is how the titration results are processed

Volumetric analysis

• Volumetric analysis uses the volume and concentration of one chemical reactant (a volumetric solution) to determine the concentration of another unknown solution

• The technique most commonly used is a titration

• The volumes are measured using two precise pieces of equipment:

  • A volumetric or graduated pipette 

  • A burette

• Before the titration can be done, the standard solution must be prepared

• Specific apparatus must be used both when preparing the standard solution and when completing the titration, to ensure that volumes are measured precisely

1. Beaker

2. Burette

3. Volumetric Pipette

4. Conical Flask

5. Volumetric Flask

Making a volumetric solution

• Chemists routinely prepare solutions needed for analysis, whose concentrations are known precisely

• These solutions are termed volumetric solutions or standard solutions

• They are made as accurately and precisely as possible using three decimal place balances and volumetric flasks

  • This reduces the impact of measurement uncertainties

• The steps are:

Volumes & concentrations of solutions

• The concentration of a solution is the amount of solute dissolved in a solvent to make 1 dm³ of  solution

  • The solute is the substance that dissolves in a solvent to form a solution

  • The solvent is often water

• A concentrated solution is a solution that has a high concentration of solute

• A dilute solution is a solution with a low concentration of solute

• Concentration is usually expressed in one of three ways:

  • moles per unit volume (mol dm^-3)

  • mass per unit volume (g dm^-3)

  • parts per million (ppm)

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

Titrating 

• 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 opened and the solution is slowly added 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

  • This is because two burette readings are made to obtain the titre (V final – V initial)

  • This follows the rules for propagation of uncertainties

• Concordant results are then averaged, and non-concordant results are discarded

• The appropriate calculations are then done

Practical skills reminder

• These titration-based practicals help develop essential experimental techniques, including:

  • Accurately measuring volumes using volumetric pipettes, burettes, and volumetric flasks

  • Preparing standard solutions with appropriate equipment and minimising measurement uncertainties

  • Recording burette readings to the correct precision and accounting for instrumental uncertainty

  • Performing titrations safely and effectively, using indicators and controlling flow rates near the endpoint

  • Calculating concentrations, molar masses, and formulae using balanced equations and volume data

  • Interpreting titration data to identify unknown compounds, following correct stoichiometric reasoning