Titration: An Experimental Technique

Titration

• Titration is an experimental technique used to determine the concentration or composition of a substance, often during acid-base or redox reactions.

Vocabulary

• Titrant: A substance with a known concentration and volume used as a standard to measure another component.
• Analyte: A substance with a known volume but unknown concentration, which is determined through titration.
• Burette: A long, narrow tube with precise gradations, typically used to hold the titrant.
• Erlenmeyer flask/Beaker: Containers used to hold the analyte during titration.
• Stirring bar: Used to stir the solution to ensure a consistent reaction.
• Indicator: A chemical substance that changes color at a specific pH or when all the analyte has been used up.

Indicators

• Indicators signal the endpoint of a titration through a color change.
• Examples:
  • Hydration paper: Paper containing multiple pH indicators to check the pH.
  • Phenolphthalein: A chemical indicator that is clear in acidic solutions and hot pink in basic solutions.
  • Litmus: An indicator that turns red in acidic solutions and blue in basic solutions.
  • Universal indicator: A combination of indicators showing a range of colors depending on the pH.
  • Red cabbage indicator: A homemade indicator that can be created using red cabbage.
• Redox Titration Indicators:
  • Transition metals often serve as indicators due to their inherent color changes during redox reactions; indicators are usually not required.
  • Example: Permanganate ion (purple) turns into manganese ion (pink).
• pH probe: An electronic device that measures pH by translating electrical conductivity in the solution.

Equivalence Point vs. Endpoint

• Equivalence Point: The point in the titration where the correct mole ratio is achieved (moles of acid = moles of base in acid-base titration).
• Endpoint: The point where the indicator changes color.
• For accurate titrations, the indicator should be chosen so that the endpoint matches the equivalence point.

Titration Calculations

• Titration calculations are essentially solution stoichiometry.
• Steps:
  • Calculate moles of titrant using the formula:
    $Moles = Volume \, \times \, Molarity$
  • Use the mole ratio from the balanced chemical equation to find moles of analyte.
  • Calculate the concentration of the analyte using the formula:
    $Molarity = \frac{Moles}{Liters}$
• This process can be done in three separate steps or through dimensional analysis.

Example Calculation

• Problem: 19.35 mL of $NaOH$ reacts with 40 mL of 0.15 M $H<em>2SO</em>4$.
• Balanced equation:
  $H<em>2SO</em>4 + 2NaOH \rightarrow Na<em>2SO</em>4 + 2H_2O$
• Convert volumes to liters:
  $40 \, mL = 0.04 \, L$
  $19.35 \, mL = 0.01935 \, L$
• Calculate moles of $H<em>2SO</em>4$.
  $0. 04 \, L \, H<em>2SO</em>4 \times 0.15 \frac{moles}{L} \, H<em>2SO</em>4 = 0.006 \, moles \, H<em>2SO</em>4$
• Use the mole ratio to find moles of $NaOH$.
  $0. 006 \, moles \, H<em>2SO</em>4 \times \frac{2 \, moles \, NaOH}{1 \, mole \, H<em>2SO</em>4} = 0.012 \, moles \, NaOH$
• Calculate the concentration of $NaOH$.
  $Molarity = \frac{0.012 \, moles \, NaOH}{0.01935 \, L} = 0.62 \, M$

Titration Experiment

• Titrant: Sodium hydroxide ($NaOH$).
• Analyte: Vinegar (acetic acid).
• Goal: Determine the molarity of acetic acid in vinegar.
• Reaction:
  $NaOH + CH<em>3COOH \rightarrow CH</em>3COONa + H_2O$
• Indicator: Phenolphthalein (clear in acid, pink in base).

Experiment Procedure

• The burette is loaded with 0.5 M $NaOH$.
• The volume of vinegar is 25 mL.
• A magnetic stirrer is used to mix the solution.
• Phenolphthalein indicator is added to the vinegar; it remains clear.
• $NaOH$ is slowly added from the burette until a faint pink color persists (the endpoint).

Data and Calculation

• Initial volume of $NaOH$ in the burette: 0 mL.
• Final volume of $NaOH$ in the burette: 22.32 mL.
• Volume of $NaOH$ used: 22.32 mL = 0.02232 L.
• Calculate moles of $NaOH$.
  $0. 02232 \, L \, NaOH \times 0.5 \frac{moles}{L} \, NaOH = 0.01116 \, moles \, NaOH$
• Use the mole ratio to find moles of acetic acid. The ratio is 1:1.
  $0. 01116 \, moles \, NaOH = 0.01116 \, moles \, CH_3COOH$
• Calculate the concentration of acetic acid in vinegar.
  $Molarity = \frac{0.01116 \, moles \, CH_3COOH}{0.025 \, L} = 0.4464 \, M$
• Considering significant figures, the concentration is approximately 0.4 M.