Required Practical 2

Required Practical 2: Acid-Alkali Titration

Revision guide containing method, variables, safety, sample calculations, and model exam answers.

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Scientific Principles & Theory

Scientific Background: Titration is a quantitative chemical analysis technique used to find the concentration of an unknown acid or alkali solution. It involves neutralising a known volume of one reactant with a measured volume of another reactant of known concentration.

A single-stage indicator (such as methyl orange or phenolphthalein) is used to detect the end point, where the reaction is completed. Acid-base indicators change colour sharply at a specific pH. Hydrochloric acid neutralises sodium hydroxide as follows:

HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)

Experimental Variables

Independent Variable

The volume of acid added from the burette.

Dependent Variable

The volume of acid required to neutralise the alkali (concordant titres).

Control Variables

The volume of alkali pipetted (25.0 cm3), the concentration of alkali standard solution, and the number of drops of indicator.

⚠️ Lab Risk Assessment

Hazard Associated Risk Control Measure
Acid and alkali solutions Corrosive and irritating to skin and eyes Wear safety goggles; wash spills off skin immediately; use a pipette filler.
Burette filling at height Liquid splashing into eyes or face Always fill the burette below eye level by placing the burette stand on the floor.

Apparatus & Procedure

Required Apparatus

  • Burette (50 cm3)
  • Volumetric pipette (25 cm3)
  • Pipette filler
  • Conical flask (250 cm3)
  • White tile
  • Funnel
  • 0.100 mol/dm3 sodium hydroxide solution
  • Hydrochloric acid of unknown concentration
  • Methyl orange indicator
  • Wash bottle with distilled water

Step-by-Step Procedure

  1. Rinse the burette with hydrochloric acid solution, and rinse the volumetric pipette with sodium hydroxide solution.
  2. Use the pipette and a pipette filler to measure exactly 25.0 cm3 of the sodium hydroxide solution and transfer it into a clean conical flask.
  3. Add 3 to 4 drops of methyl orange indicator to the conical flask. Place the flask on a white tile under the burette (the solution turns yellow).
  4. Fill the burette with hydrochloric acid. Adjust the level so the bottom of the meniscus is on the 0.00 cm3 mark. Remove the funnel from the burette.
  5. Perform a rough titration: add the acid from the burette in small portions while swirling the flask. Stop when the indicator changes colour permanently from yellow to peach-orange. Record this rough volume.
  6. Discard the flask contents and rinse the conical flask thoroughly with distilled water.
  7. Pipette another 25.0 cm3 of sodium hydroxide into the conical flask and add indicator.
  8. Perform an accurate titration: add the acid quickly until you are about 2 cm3 before the rough titre volume, then add dropwise, swirling constantly, until the end point is reached (permanent yellow to peach-orange colour change). Record the final volume.
  9. Repeat the accurate titrations until you obtain at least two concordant results (titres within 0.10 cm3 of each other).
  10. Calculate the mean titre using only the concordant results.
Acid-Alkali Titration Setup Burette (Acid) Conical Flask (Alkali + Indicator) White tile

Fig 1. Laboratory experimental setup for Required Practical 2.

Sample Data & Calculations

This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:

Titration run Initial reading (cm3) Final reading (cm3) Titre volume (cm3) Concordant?
Rough Run 0.00 24.30 24.30 No (rough run)
Run 1 24.30 46.65 22.35 Yes
Run 2 0.50 22.95 22.45 No
Run 3 22.95 45.25 22.30 Yes

Data Processing & Analysis

  1. Identify concordant titres (Run 1 and Run 3 are within 0.10 cm3 of each other). Run 2 is excluded.
  2. Calculate mean titre: Mean Titre = (22.35 cm3 + 22.30 cm3) / 2 = 22.325 cm3 = 0.022325 dm3.
  3. Calculate moles of NaOH pipetted: Moles = Concentration * Volume (dm3) = 0.100 * 0.0250 = 0.00250 mol.
  4. Reaction stoichiometry is 1:1, so moles of HCl in mean titre = 0.00250 mol.
  5. Calculate concentration of HCl: Concentration = Moles / Mean Volume (dm3) = 0.00250 / 0.022325 = 0.112 mol/dm3.

Conclusion & Evaluation

Chemical Explanation: Evaluating the experimental outcomes against known values ensures validity. Understanding the source of systematic and random deviations allows for better experimental designs in future trials.

Experimental Error Analysis

Error Type & Source Effect on Final Result Mitigation Strategy
Systematic Error
Air bubble left in the burette tip before titration		 The air bubble fills with acid during the titration, making the recorded titre volume artificially larger than the actual reacting volume. Open the burette tap fully for a moment to flush out any air bubbles in the tip before starting the initial reading.
Random Error
Parallax error when reading the burette scale		 Causes small random variations in initial and final volume measurements. Ensure your eyes are at the same level as the liquid level when reading, and record the value from the bottom of the meniscus.

Exam Practice

Exam-Style Design Question (6 Marks)

Plan an experiment to determine the concentration of a solution of hydrochloric acid by titrating it against 25.0 cm3 of a 0.100 mol/dm3 standard solution of sodium hydroxide. Describe the procedure and explain how you would calculate your results.

View Model Answer & Mark Scheme

Model Answer (6/6 Marks):

  1. Preparation: Rinse the volumetric pipette with sodium hydroxide and the burette with hydrochloric acid. This prevents dilution of the reactants by water left in the glassware.
  2. Alkali measurement: Pipette exactly 25.0 cm3 of the sodium hydroxide solution into a clean conical flask using a pipette filler. Add 3 to 4 drops of methyl orange indicator (which turns yellow).
  3. Burette setup: Fill the burette with the hydrochloric acid solution using a funnel. Ensure there are no air bubbles in the tip, then remove the funnel and record the initial reading from the bottom of the meniscus at eye level.
  4. Titration runs: Place the conical flask on a white tile under the burette. Perform a rough titration to find the approximate neutralisation volume. Repeat the process accurately, adding acid dropwise near the end point until the solution changes from yellow to peach-orange.
  5. Concordancy: Repeat titrations until two concordancy values (within 0.10 cm3 of each other) are obtained. Average these concordant values to find the mean titre.
  6. Calculations: Find moles of NaOH using Moles = Concentration * Volume. Since HCl and NaOH react in a 1:1 ratio, moles of HCl equal moles of NaOH. Calculate HCl concentration using Concentration = Moles / Mean Volume (in dm3).
Examiner Tip:

Rinsing the burette with acid and the pipette with alkali, removing the funnel, and placing the flask on a white tile are crucial practical steps that are heavily awarded marks.