Acid-Base Titrations
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge AS Level Physics 9702
What is a Titration? Titration A quantitative analytical technique used to determine the exact concentration of an unknown solution by reacting it with a solution of known concentration. End Point The point in a titration where the indicator changes color, signaling that the reaction is complete.
In an acid-base titration, an acid and base react together in a neutralization reaction to form a salt and water. General equation: Acid + Base → Salt + Water Example: HCl + NaOH → NaCl + H₂O Apparatus Required Apparatus Purpose Burette (50 cm³) Delivers variable volumes of solution accurately Pipette (25 cm³) + pipette filler Measures exact fixed volume of solution Conical Flask Container for the solution being titrated White Tile Placed under flask to see color change clearly Indicator Shows when neutralization is complete Funnel For filling burette (remove before titration) Beakers Hold stock solutions Interactive Diagram Labeled diagram showing complete titration setup: Burette clamped to retort stand, filled with solution (labeled "Solution in burette - known concentration"), tap visible.
Below: conical flask on white tile containing solution (labeled "25 cm³ from pipette + indicator drops"). Show hand turning tap to add drops. Include labels pointing to: burette readings (0.00 cm³ at top, graduations visible), meniscus reading point, conical flask (why conical - easy to swirl), white tile beneath (makes color change visible), clamp and stand.
Add annotation: "Add solution dropwise near end point while swirling flask". Indicators for Acid-Base Titrations Indicator Color in Acid Color in Alkali pH Range Best Used For Methyl Orange Red Yellow 3.1 - 4.4 Strong acid + weak base Phenolphthalein Colorless Pink 8.3 - 10.0 Weak acid + strong base Litmus Red Blue 4.5 - 8.3 General use (less precise) Most Common at IGCSE Methyl orange or phenolphthalein are most commonly used.
For strong acid + strong base titrations (e.g., HCl + NaOH), either indicator works well. Titration Procedure Step-by-Step Method: Rinse the pipette with the solution to be pipetted (do NOT use water) Use the pipette to transfer exactly 25.0 cm³ of solution into a clean conical flask Add 2-3 drops of suitable indicator to the flask Rinse the burette with the solution to be used in it Fill the burette with solution using a funnel, then remove funnel Open the tap briefly to fill the jet (removing air bubbles) Record the initial burette reading to 2 decimal places (e.g., 0.05 cm³) Place the flask on a white tile under the burette Add solution from burette while swirling the flask continuously Near the end point , add solution drop by drop until indicator just changes color Record the final burette reading to 2 decimal places Calculate the titre (volume used) = Final reading - Initial reading Repeat the titration until you get concordant results (within 0.10 cm³) Key Technique Swirl continuously to ensure thorough mixing Add dropwise near the end point - even half a drop can make a difference!
Watch for permanent color change - swirling should not reverse the color Recording Results A typical results table: Titration Rough 1 2 3 Initial reading (cm³) 0.00 0.05 0.10 0.00 Final reading (cm³) 26.80 25.60 25.55 25.50 Titre (cm³) 26.80 25.55 25.45 25.50 Analyzing Results: Rough titration: Quick first attempt to find approximate end point (26.80 cm³) Accurate titrations: Titrations 1, 2, 3 done carefully near the end point Concordant results: Titrations 2 and 3 are within 0.10 cm³ (25.45 and 25.50) Ignore outliers: Titration 1 (25.55) is slightly different - may be ignored Calculate mean: Average of concordant results = (25.45 + 25.50) ÷ 2 = 25.48 cm³ Important Always record burette readings to 2 decimal places (e.g., 25.50 cm³, not 25.5 cm³) Only average concordant results (within 0.10 cm³ of each other) Calculations from Titrations Once you have the mean titre, you can calculate unknown concentrations using the equation: Formula: n(acid) / n(base) = Volume(acid) × Concentration(acid) / Volume(base) × Concentration(base) Or use: Moles = Concentration × Volume (in dm³) Example Calculation: 25.0 cm³ of sodium hydroxide solution was titrated with 0.10 mol/dm³ hydrochloric acid.
The mean titre was 24.50 cm³. Calculate the concentration of the sodium hydroxide. Solution: Equation: HCl + NaOH → NaCl + H₂O Mole ratio: 1 HCl : 1 NaOH Step 1: Calculate moles of HCl used Volume of HCl = 24.50 cm³ = 24.50 ÷ 1000 = 0.02450 dm³ Moles = Concentration × Volume = 0.10 × 0.02450 = 0.00245 mol Step 2: Use mole ratio to find moles of NaOH From equation: 1 mol HCl reacts with 1 mol NaOH So moles of NaOH = 0.00245 mol Step 3: Calculate concentration of NaOH Volume of NaOH = 25.0 cm³ = 0.0250 dm³ Concentration = Moles ÷ Volume = 0.00245 ÷ 0.0250 Concentration of NaOH = 0.098 mol/dm³ Common Titration Reactions Reaction Equation Mole Ratio HCl + NaOH HCl + NaOH → NaCl + H₂O 1 : 1 H₂SO₄ + NaOH H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O 1 : 2 HNO₃ + KOH HNO₃ + KOH → KNO₃ + H₂O 1 : 1 HCl + Ca…
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