Principles of Chemistry: Back Titration Lab

Introduction

• Objective: Perform a back titration to analyze aspirin tablets.
• Titration Process: First, titrate with sodium hydroxide to determine its concentration, then use this data for further analysis.

Equipment and Initial Setup

• Equipment: Pipette, pipette filler, standard volumetric flask, burette, retort stand, funnel, litmus paper, scale, and burner.
• Pipetting: Use a glass pipette to measure 25 ml of 1 molar sodium hydroxide.
  • Rinse with deionized water.
  • Add to the standard flask and dilute with water to the white line.
  • Mix by inverting the flask.

Performing the Titration

Preparing the Sample

• Sample Preparation: Add 25 ml of standardized sodium hydroxide into a conical flask.
• Indicator: Add a few drops of phenolphthalein; it turns pink in a basic solution.

Preparing the Burette

• Burette Setup:
  • Rinse with deionized water and then with 0.1 molar hydrochloric acid.
  • Fill past zero mark, ensure no air bubbles, and adjust to zero.

Titration Procedure

• Titrate: Add hydrochloric acid to the sodium hydroxide solution until the pink color disappears (end point).
• Measurement: Record the volume of hydrochloric acid used.
  • Rough Titration: 26.6 cm³ of 0.1 molar HCl.
  • Accurate Titrations: Aim for readings within 0.2 cm³ of each other.
  • Example: 25.5 cm³ for both accurate titrations.

Analysis of Aspirin Tablets

Preparation

• Weigh Tablets: Use four 325 mg aspirin tablets (total 1.55 grams).
• Dissolve Tablets: Add 25 ml of 1 molar sodium hydroxide to dissolve the tablets.
• Check Solution: Use litmus paper to ensure the solution is basic.

Mixing and Heating

• Solution Preparation:
  • Add water and gently warm to ensure the aspirin dissolves.
  • Transfer to a standard flask and dilute to the mark.
  • Mix thoroughly by inverting.

Titration

• Titrate: Pipette 25 ml of the solution into a conical flask, add phenolphthalein.
• Burette Setup: Ensure it's filled to zero mark with 0.1 molar HCl.
• Titration Results:
  • Rough: 11.4 cm³.
  • Accurate: Two readings of 9.8 cm³.

Conclusion

• The lab process included preparing solutions, performing titrations, and recording accurate measurements.
• Ensure mixing and accurate readings for precision.
• Data will be shared by the instructor.

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Key Points to Avoid Errors

• Always rinse equipment appropriately.
• Ensure solutions are properly mixed.
• Remove the funnel during titrations.
• Record readings at eye level to avoid parallax errors.
• Perform accurate titrations within a narrow range (0.2 cm³).