Core Practical 2.34

Core Practical 2.34: Acid-Alkali Titration

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

Scientific Principles & Theory

Scientific Background: Titration is a quantitative analytical technique used to determine the concentration of an unknown acid or alkali solution by reacting it completely with a solution of known concentration.

The equivalence point is the point at which stoichiometric amounts of acid and alkali have reacted completely. The end point is the experimental observation when the indicator changes colour sharply. A suitable single-stage indicator (such as phenolphthalein or methyl orange) is chosen so that its end point coincides as closely as possible with the equivalence point, minimising titration error. Universal indicator is unsuitable because its gradual colour change over a wide pH range makes identifying a precise end point impossible.

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 reach the end point (titre).

Control Variables

Volume of alkali pipetted (25.0 cm³), indicator type, concentration of acid.

⚠️ Lab Risk Assessment

Hazard Associated Risk Control Measure
Acid/Alkali solutions Corrosive / irritant to eyes and skin Wear safety goggles; use a pipette filler (never pipette by mouth); wash spills immediately.
Glassware (burette/pipette) Cuts from broken glass Clamp glassware securely; handle with care; do not force pipettes into fillers.

Apparatus & Procedure

Required Apparatus

  • Burette (50 cm³)
  • Volumetric pipette (25 cm³)
  • Pipette filler
  • Conical flask (250 cm³)
  • White tile
  • Funnel
  • Acid solution (0.100 mol/dm³ HCl)
  • Alkali solution (unknown concentration NaOH)
  • Phenolphthalein indicator
  • Wash bottle with distilled water

Step-by-Step Procedure

  1. Rinse the burette with distilled water and then with a small amount of the acid solution.
  2. Fill the burette with the acid solution using a funnel, making sure the area below the tap is filled with liquid and contains no air bubbles. Remove the funnel.
  3. Record the initial burette reading to the nearest 0.05 cm³ at the bottom of the meniscus.
  4. Rinse the volumetric pipette with distilled water and then with the sodium hydroxide solution.
  5. Use the pipette and a pipette filler to measure exactly 25.0 cm³ of the sodium hydroxide solution into a clean conical flask.
  6. Add 5 drops of phenolphthalein indicator to the conical flask. The solution will turn pink.
  7. Place the conical flask on a white tile directly underneath the burette tip.
  8. Perform a rough titration: add the acid in 1-2 cm³ portions while swirling the flask continuously until the solution changes from pink to colourless. Record the rough titre.
  9. Refill the burette. Repeat the titration: add the acid rapidly until you are within 2 cm³ of the rough end point. Then add the acid drop by drop, swirling constantly, until a single drop causes a permanent colour change from pink to colourless.
  10. Record the final burette reading. The difference between initial and final readings is the titre.
  11. Repeat the titration until you obtain at least two concordant results (titres within 0.10 cm³ of each other). Calculate the mean of these concordant titres.
Acid-Alkali Titration Apparatus Setup Enlarged apparatus layout: vertical clamped burette filled with acid, stopcock controlling dropwise delivery into a conical flask containing alkali and indicator, placed on a white tile for clear end-point observation. Acid-Alkali Titration Apparatus Burette (Acid) Filled with acid of known concentration Read at bottom of meniscus to 0.05 cm³ Retort Stand & Clamp Holds burette securely Stopcock / Tap Controls flow (deliver dropwise near end point) Conical Flask 25.0 cm³ alkali (measured by pipette) + indicator (phenolphthalein / methyl orange) White Tile Provides white background for clear end-point observation

Fig 1. Laboratory experimental setup for Core Practical 2.34.

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 (cm³) Concordant?
Rough Titre 0.00 24.30 24.30 No (rough run)
Run 1 24.30 48.15 23.85 Yes
Run 2 0.50 24.40 23.90 Yes
Run 3 24.40 48.20 23.80 Yes

Data Processing & Analysis

  1. Average titre = (23.85 + 23.90 + 23.80) / 3 = 23.85 cm³ [Average all concordant titres; all three runs are within 0.10 cm³ of each other]
  2. Moles of HCl = Concentration × Volume (dm³) = 0.100 × (23.85 / 1000) = 0.002385 mol
  3. Reacting Ratio: 1 mol HCl reacts with 1 mol NaOH, so moles of NaOH = 0.002385 mol
  4. Concentration of NaOH = Moles / Volume (dm³) = 0.002385 mol / (25.0 / 1000) dm³ = 0.0954 mol/dm³

Conclusion & Evaluation

Chemical Explanation: Titration is a volumetric analysis technique based on a neutralisation reaction between an acid and a base. By measuring the exact volume of acid (titre) required to neutralise a known volume of alkali using an indicator, the moles of acid reacted can be calculated, and the unknown concentration of the alkali determined using the reaction stoichiometry.

Experimental Error Analysis

Error Type & Source Effect on Final Result Mitigation Strategy
Systematic Error
Burette tip not filled before starting titration		 The initial titre value reads as higher than actual because some acid is used to fill the space in the tip before the level drops. Ensure a small volume of liquid is run through the tap to completely fill the jet space and remove air bubbles before recording the starting reading.
Random Error
Overshooting the end point (adding too much acid)		 The titre is too large, resulting in an overestimation of the concentration of the alkali. Near the end point, add the acid extremely slowly (drop-by-drop) while continuously swirling the conical flask.

Exam Practice

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Exam-style design question

6 marks
[6 marks]

A student needs to find the concentration of a sodium hydroxide solution using a 0.100 mol/dm³ standard solution of hydrochloric acid. Describe how the student should carry out the titration to obtain reliable results.

View Model Answer & Mark Scheme

Marking guidance: award 1 mark for each correct, clearly explained step in the model answer below, up to 6 marks.

Model Answer (6/6 Marks):

  1. Preparation: Rinse the burette with hydrochloric acid and fill it, ensuring no air bubbles are trapped in the tip. Record the initial reading.
  2. Alkali Pipetting: Rinse a 25.0 cm³ volumetric pipette with the sodium hydroxide solution. Use a pipette filler to transfer exactly 25.0 cm³ of the sodium hydroxide into a clean conical flask.
  3. Indicator: Add 3 to 5 drops of phenolphthalein indicator to the flask, which will turn pink. Place the flask on a white tile.
  4. Rough Run: Run the acid into the flask while swirling until the color changes sharply to colorless. Record this rough titre.
  5. Accurate Runs: Refill the burette. Add acid quickly until within 2 cm³ of the rough end point, then add dropwise with continuous swirling. Stop at the exact point the solution turns colorless.
  6. Reliability: Repeat the titration until two concordant results (within 0.10 cm³ of each other) are obtained. Calculate the mean of these concordant titres.
Examiner Tip:

Always mention rinsing the apparatus with the respective solutions they will contain, adding the acid dropwise near the end point, and averaging only concordant titres.

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