Acid-Base Titrations with Indicators (Edexcel International AS Chemistry): Revision Note

Acid-Base Titrations with Indicators

• Acid-base titrations are used to find the unknown concentrations of solutions of acids and bases

• Acid-base indicators give information about the change in chemical environment

• They change colour reversibly depending on the concentration of H⁺ ions in the solution

• Indicators are weak acids and bases where the conjugate bases and acids have a different colour

• Many acid-base indicators are derived from plants, such as litmus

Common Indicators Table

• A good indicator gives a very sharp colour change at the equivalence point

• In titrations is it not always possible to use two colour indicators because of this limitation, so for example litmus cannot be used successfully in a titration

• When phenolphthalein is used, it is usually better to have the base in the burette because it is easier to see the sudden and permanent appearance of a colour (pink in this case) than the change from a coloured solution to a colourless one

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

Concentration (mol dm³) = 

• 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

• When carrying out calculations involve concentrations in mol dm^-3 the following points need to be considered:

  • Change mass in grams to moles

  • Change cm³ to dm³ 

• To calculate the mass of a substance present in solution of known concentration and volume:

  • Rearrange the concentration equation

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

• Multiply the moles of solute by its molar mass

mass of solute (g) = number of moles (mol) x molar mass (g mol^-1)

Uncertainty - Calculations

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 of these examples, you have to read the instrument twice to obtain the quantity

• If each time you read the instrument the measurement is ‘out’ by the stated uncertainty, then your final quantity is potentially ‘out’ by twice the uncertainty

Total experimental uncertainty

• Some experiments use multiple pieces of equipment, which each have a measure of uncertainty in their use

• For example, a titration against a standard solution will have the following uncertainties:

  • Using a balance to measure the mass of the solid used to make the standard solution

  • Using a volumetric flask to make the standard solution

  • Using a volumetric pipette to measure the sample that is being titrated against

  • Using a burette to find the initial reading and final reading that determine the titre volume

• If each piece of equipment used in the experiment has its own associated uncertainty, you can determine the total experimental uncertainty by adding the total individual uncertainties

• For example, a titration against a standard solution may have:

  • An uncertainty of ±0.5% for the use of the balance

  • An uncertainty of ±0.1% for the use of the volumetric flask

  • An uncertainty of ±0.2% for the use of the volumetric pipette

  • An uncertainty of ±0.4% for the use of the burette

  • Therefore, the total experimental uncertainty would be 0.5 + 0.1 + 0.2 + 0.4 = ±1.2%