# pH – Titration Curves ## Introduction to Titration Curves - Titration curves are graphs that track pH changes as a base is gradually added to an acid (or vice versa) - They provide a visual representation of neutralisation reactions - The curves reveal key information about: - The strength of acids and bases involved - The equivalence point of the reaction - Suitable indicators for the titration

A titration curve plots pH (y-axis) against volume of titrant added (x-axis) during a neutralisation reaction

## Required Materials - This experiment uses $1.0 \text{ mol dm}^{-3}$ solutions of: - **Acids**: hydrochloric acid (strong), ethanoic acid (weak) - **Bases**: sodium hydroxide (strong), ammonia solution (weak) - Four possible acid-base combinations can be investigated: - Strong acid + strong base - Strong acid + weak base - Weak acid + strong base - Weak acid + weak base ## Calibrating the pH Probe - Before any measurements, the pH probe must be calibrated - Over time, the glass electrode in a pH meter loses stability and accuracy - Calibration restores precision using standard buffer solutions

Always calibrate your pH probe before starting experiments to ensure reliable measurements

### Two-Point Calibration Method 1. Rinse the pH probe thoroughly with distilled water 2. Gently shake off excess water (don't wipe, as this can damage the electrode) 3. Immerse the probe tip in **pH 4 buffer solution** - Ensure the bulb is completely submerged - Wait for the reading to stabilise (this may take 30-60 seconds) - Adjust the meter to display exactly 4.0 4. Rinse again with distilled water 5. Immerse the probe tip in **pH 9 buffer solution** - Again, ensure full immersion - Wait for stabilisation - The meter should now read 9.0 if calibration was successful - This establishes two reference points across the pH range you'll be measuring ## Experimental Method ### Setup and Procedure 1. **Measure the acid precisely** - Use a volumetric pipette to transfer $25 \text{ cm}^3$ of acid into a clean beaker - Choose either hydrochloric acid or ethanoic acid 2. **Record the initial pH** - Place the calibrated pH probe in the acid - Allow the reading to stabilise - Record this starting pH value 3. **Prepare the base** - Fill a clean burette with either sodium hydroxide or ammonia solution - Record the initial burette reading - Ensure there are no air bubbles in the burette tip 4. **Add base in portions** - Add the base in $5 \text{ cm}^3$ increments - After each addition, swirl the beaker gently to ensure thorough mixing - Wait for the pH reading to stabilise before recording 5. **Continue the titration** - Keep adding base until you've added a total of $50 \text{ cm}^3$ - This ensures you pass through the equivalence point 6. **Repeat for all combinations** - Carry out the same procedure for all four acid-base combinations - This allows direct comparison of titration curves

Never stir the solution using the pH probe itself – always use a glass rod to avoid damaging the delicate electrode

## Typical Results ### Titration with Sodium Hydroxide - When adding NaOH to acids, the pH rises as neutralisation occurs - The rate of pH change varies depending on acid strength | Volume of NaOH / cm³ | HCl (pH) | CH₃COOH (pH) | |---------------------|----------|--------------| | 0 | 2.0 | 3.3 | | 5 | 2.1 | 4.3 | | 10 | 2.2 | 4.7 | | 15 | 2.3 | 5.0 | | 20 | 2.6 | 5.4 | | 25 | 11.2 | 6.0 | | 30 | 11.4 | 11.5 | | 35 | 11.8 | 11.5 | | 40 | 11.8 | 11.8 | | 45 | 11.9 | 11.9 | | 50 | 12.0 | 12.0 | The data above can be visualized to show the characteristic titration curve shapes:

**Key observations:** - HCl starts at lower pH (stronger acid) - Both acids end at similar pH (excess strong base) - The jump around 25 cm³ indicates the equivalence point - HCl shows a steeper pH jump (strong acid characteristics) ### Titration with Ammonia Solution - Ammonia is a weak base, so the final pH values are lower than with NaOH | Volume of NH₃(aq) / cm³ | HCl (pH) | CH₃COOH (pH) | |------------------------|----------|--------------| | 0 | 2.0 | 3.3 | | 5 | 2.2 | 4.4 | | 10 | 2.3 | 4.8 | | 15 | 2.4 | 5.0 | | 20 | 2.6 | 5.4 | | 25 | 7.0 | 5.8 | | 30 | 8.6 | 7.9 | | 35 | 9.0 | 8.9 | | 40 | 9.2 | 9.2 | | 45 | 9.3 | 9.2 | | 50 | 9.3 | 9.3 | The curves for titration with a weak base show different characteristics:

**Key observations:** - Final pH values plateau around 9-10 (weak base in excess) - The equivalence point is still around 25 cm³ - The pH jump is less pronounced than with NaOH - Weak acid + weak base shows the most gradual pH change ## Analysing the Results ### Constructing Titration Curves 1. **Set up your axes correctly** - x-axis: Volume of base added (cm³) - y-axis: pH (scale from 0 to 14) 2. **Plot your data points** - Mark each pH reading at its corresponding volume - Use crosses or small dots for precision 3. **Draw smooth curves** - Connect the points with a smooth curve (not straight lines) - The curve should show a gradual change, then a steep section, then leveling off 4. **Compare the four curves** - Strong acid + strong base: Shows the steepest, longest vertical section - Strong acid + weak base: Vertical section is shorter, levels off at lower pH - Weak acid + strong base: Starts at higher pH, still has clear vertical section - Weak acid + weak base: Most gradual throughout, shortest vertical section ### Identifying the Equivalence Point

The equivalence point is where stoichiometrically equal amounts of acid and base have reacted – it lies at the centre of the steep vertical section

- Located at the midpoint of the sharp pH rise - For 25 cm³ acid + base titrations, typically occurs around 25 cm³ addition - **Not the same as the end point** (where indicator changes colour) ### Improving Curve Resolution - To get more detail around the equivalence point: - Add smaller portions (e.g., 1 cm³ or even 0.5 cm³) near the steep section - This reveals the true shape of the vertical rise - Makes equivalence point determination more accurate - Think of it like zooming in on a photo – smaller volume increments give you a higher-resolution curve ## Characteristic Curve Shapes ### Strong Acid + Strong Base - Starts at very low pH (~1-2) - Changes very slowly at first - Shows dramatic vertical jump at equivalence point (pH change of ~8-10 units) - Levels off at high pH (~12-13) - Equivalence point is at pH 7 ### Weak Acid + Strong Base - Starts at moderate pH (~3-5) - Shows buffer region before equivalence point - Has clear vertical section, but slightly shorter than strong acid/strong base - Levels off at high pH (~12-13) - Equivalence point is above pH 7 ### Strong Acid + Weak Base - Starts at very low pH (~1-2) - Shows moderate pH rise - Has vertical section, but shorter than strong acid/strong base - Levels off at moderate pH (~9-10) - Equivalence point is below pH 7 ### Weak Acid + Weak Base - Starts at moderate pH (~3-5) - Shows gradual pH increase throughout - Has very short or no clear vertical section - Levels off at moderate pH (~9-10) - Equivalence point difficult to identify precisely ## Key Terminology ### Equivalence Point vs End Point | Aspect | Equivalence Point | End Point | |--------|------------------|-----------| | **Definition** | Stoichiometric completion of reaction | Visible indicator colour change | | **Location** | Centre of vertical section on curve | Where indicator changes | | **Dependence** | Fixed by chemistry of reaction | Depends on indicator chosen | | **Ideal scenario** | End point coincides with equivalence point | Choose indicator with appropriate pK_a | - In practice, we choose indicators whose colour change occurs within the vertical section of the pH curve - This ensures the end point closely matches the equivalence point - For weak acid/weak base titrations, no indicator works well because the vertical section is too short ## Practical Skills Developed ### Measurement Techniques - **Volumetric pipettes**: Transferring precise volumes of acid - **Burettes**: Delivering measured volumes of base - **pH probes**: Continuous pH monitoring with calibration ### Data Collection - Recording pH values at regular volume intervals - Recognising when to take more frequent readings (near equivalence point) - Maintaining clean technique to avoid cross-contamination ### Graph Analysis - Plotting smooth curves from experimental data - Identifying equivalence points from curve shape - Comparing strong/weak acid and base behaviour ### Experimental Design - Understanding how volume increment size affects curve resolution - Recognising the link between experimental decisions and data quality - Appreciating why the vertical section is crucial for indicator selection ## Common Exam Applications ### Recognising Curve Types - You should instantly identify which combination of acid/base strength produced a given curve - Look for: - Starting pH (low = strong acid, moderate = weak acid) - Ending pH (high = strong base, moderate = weak base) - Steepness of vertical section (steeper = both strong) ### Locating Equivalence Points - Find the centre of the steepest part of the curve - For equal volumes and concentrations, this should be at the halfway point - Remember: this is **not** always at pH 7 ### Understanding Indicator Choice - The indicator must change colour **within the vertical section** - This ensures a sharp colour change at the equivalence point - Different acid-base combinations require different indicators

Strong acid + strong base titrations have the longest vertical section, so almost any indicator works well

## Practical Considerations ### Achieving Accurate Results - **Calibration is crucial**: Always use fresh buffer solutions for calibration - **Mixing matters**: Swirl thoroughly after each addition but avoid splashing - **Stability is key**: Wait for pH readings to settle before recording - **Clean equipment**: Rinse all glassware with distilled water between measurements ### Common Sources of Error - **Uncalibrated pH probe**: Leads to systematic errors in all readings - **Insufficient mixing**: Creates pockets of high/low pH, giving unreliable readings - **Reading too quickly**: pH takes time to equilibrate after adding base - **Dirty glassware**: Contamination affects concentrations and pH values ### Extending the Investigation - Compare different concentrations of the same acid-base pair - Investigate how temperature affects titration curves - Use the curves to determine $K_a$ values for weak acids - Test the suitability of different indicators experimentally ## Real-World Connections - **Industrial quality control**: Monitoring acid/base concentrations in manufacturing - **Environmental testing**: Measuring acidity in water samples or soil - **Pharmaceutical development**: Ensuring correct pH in medicines - **Food science**: Controlling acidity in beverages and preserved foods Understanding titration curves gives you both practical lab skills and theoretical insight into acid-base chemistry – making it a cornerstone technique in analytical chemistry.