General Chemistry Lab Experiment 11

General Chemistry Lab: Experiment 11 - Standardization of Sodium Hydroxide Solution

• Objective: To standardize a solution of NaOH
• Introduction:
  • A standard solution is a solution with a precisely known concentration.
  • It's used to determine the concentration of other solutions through titration.
  • Titration is a process where one solution (titrant) is slowly added to another (analyte) until a reaction endpoint is reached. This endpoint involves a color change using an indicator.
• Standard Solution Types:
  • Primary Standard: Highly pure, stable, not hygroscopic, high molar mass, crystalline compounds (e.g., KIO₃, Na₂CO₃, KHP). Primarily used for the preparation of accurate concentration solutions.
  • Secondary Standard: Does not have the properties of a primary standard and must be standardized using a primary standard (e.g., NaOH, HCl)
    • NaOH reacts with glass and CO₂, absorbs water.
    • HCl evaporates, not stable.
• Titration Requirements:
  • Stoichiometric reaction
  • Rapid reaction
  • No side reactions
  • Distinct endpoint (using an indicator)
• Titration Types:
  • Acid-base Titration (neutralization reaction)
  • Oxidation-Reduction Titration (redox reaction)
  • Gravimetric Titration (precipitation reaction)
• Acid-Base Titration:
  • Acid + Base → Salt + Water
  • Equivalent point (stoichiometric point): The point where reactants are present in stoichiometric ratios.
  • Endpoint is when an indicator shows a change in color. Equivalent and endpoint are usually close but not always the same.
• Indicator:
  • A dye that changes color in acidic and basic media.
  • The color change occurs at or near the endpoint.
  • It's a weak organic acid or base that changes color when protonated or deprotonated.
  • Chosen according to the pH at the endpoint (e.g., phenolphthalein changes color at pH 8-8.2).
• Volumetric Glassware:
  • Burette, pipette, volumetric flask
• Procedure:
  • Wash burette with distilled water.
  • Rinse burette with the 0.1 M NaOH solution, then fill with the solution.
  • Ensure the tip of the burette is filled with the solution.
  • Weigh a known amount of KHP.

Procedure (continued)

• Add the weighed KHP to a flask with 30 ml of water.
• Heat to dissolve the KHP.
• Add 1-2 drops of phenolphthalein indicator.
• Slowly add NaOH solution to the KHP solution from the burette until the color changes to a faint pink.
• Note the volume of NaOH used at the endpoint.
• Calculate the molar concentration of the NaOH solution.

Calculations

• The balanced chemical reaction equation is required to calculate molar concentration.
• The mass of KHP is known.
• The volume of NaOH needed is measured from the burette.
• Using the balanced chemical equation and the known measurements, the molar concentration of NaOH can be calculated.