Teoría Ejercicios

Acid-Base Volumetry

Introduction to Acid-Base Volumetry

Volumetry or volumetric analysis is a quantitative analytical technique that allows determining the concentration of a substance through precise measurement of volumes of reacting solutions. In this practice, we will determine the acetic acid content in commercial vinegar through titration with sodium hydroxide (NaOH).

Vinegar is an aqueous solution of acetic acid (CH₃COOH) obtained by acetic fermentation of wine or other alcoholic solutions. Commercial vinegar typically contains between 4% and 6% acetic acid by mass, and this concentration must be verified for quality control.

Theoretical Foundations

Chemical Reaction

The determination is based on the neutralization reaction between acetic acid (weak acid) and sodium hydroxide (strong base):

CH₃COOH + NaOH → CH₃COONa + H₂O

This reaction has the following characteristics:

  • 1:1 stoichiometry: One mole of acid reacts with one mole of base
  • Complete reaction: Practically irreversible under experimental conditions
  • Fast reaction: Allows easy detection of endpoint
  • Basic equivalence point: pH ≈ 8.7 due to acetate hydrolysis

Fundamental Concepts

Equivalence point: Moment when stoichiometrically equivalent amounts of reactivos have been added.

Endpoint: Moment when indicator changes color, ideally coincides with equivalence point.

Acid-base indicator: Substance that changes color in a specific pH range. For this titration we use phenolphthalein (colorless → pink, pH ≈ 8.2-10).

Materiales y reactivos

Volumetric Materials

  • 25 or 50 mL burette: To precisely measure NaOH volume used
  • 25 mL volumetric pipette: To measure exactly the sample volume
  • 250 mL Erlenmeyer flask: As container for titration
  • 100 mL volumetric flask: To prepare dilutions if necessary
  • Funnel: To fill burette without spilling

Other Materials

  • Universal stand with burette clamp
  • Pipette bulb or automatic pipettor
  • 100 mL beaker
  • Stirring rod or magnetic stirrer
  • Filter paper or paper towels

Reagents

  • Commercial vinegar: Sample to analyze (4-6% acetic acid)
  • 0.1 M NaOH: Standardized solution (titrant)
  • Phenolphthalein: Acid-base indicator (1-2 drops per titration)
  • Distilled water: For dilutions and rinses

Experimental Procedure

Equipment Preparation

  1. Material cleaning: Wash all glassware with soap and water, rinse with distilled water
  2. Burette preparation:
    • Rinse with small portions of 0.1 M NaOH (conditioning)
    • Fill completely with NaOH, eliminating air bubbles
    • Fill exactly to 0.00 mL
  3. Pipette preparation: Condition with small portions of vinegar

Titration Procedure

  1. Sample taking:
    • Pipette exactly 25.00 mL of vinegar
    • Transfer to 250 mL Erlenmeyer flask
    • Add approximately 50 mL of distilled water
  2. Indicator addition: Add 2-3 drops of phenolphthalein
  3. Preliminary titration:
    • Add NaOH rapidly at first
    • Observe transitory color changes
    • Approach the endpoint (permanent pink change)
  4. Precision titration:
    • Near endpoint, add drop by drop
    • Stir after each drop
    • Stop when pink color persists 30 seconds
  5. Data recording: Note exact volumen de NaOH used
  6. Repetitions: Perform at least 3 titrations to obtain reproducible data

Calculations and Data Treatment

Molar Concentration Calculation

At the equivalence point, moles of acetic acid equal moles of NaOH:

n(CH₃COOH) = n(NaOH)

Therefore: M₁ × V₁ = M₂ × V₂

Where:

  • M₁ = Molarity of acetic acid (unknown)
  • V₁ = Volume of vinegar = 25.00 mL
  • M₂ = Molarity of NaOH = 0.1000 M
  • V₂ = Volume of NaOH used (mL)

M₁ = (M₂ × V₂) / V₁

Aplicaciones y relevancia

Food Quality Control

Determination of acidity in vinegar is fundamental in the food industry to:

  • Verify compliance with commercial specifications
  • Control quality during fermentation process
  • Guarantee producto consistency
  • Comply with sanitary regulations

Other Volumetry Applications

  • Pharmaceutical analysis: Determination of active principles
  • Environmental analysis: Acidity of water, soil, and precipitation
  • Chemical industry: Process control and producto purity
  • Research: Characterization of new compounds