Acid-Base Titration

Acid-Base Titrations (Neutralization)

• Acid-base titration is based on the neutralization reaction between an acid (usually analyte) and a base (usually standard).
• The reaction is: H+ (acid) + OH- (base)
• The type of indicator used in the titration depends on the pH of the solution.

Indicators

• An indicator has three colors: color before the pH range, color within the pH range, and color after the pH range.
• Examples of indicators:
  • Methyl orange: strong acid with weak base, pH at end point ≈ 2-5
  • Phenolphthalein: strong acid with strong base, pH at end point ≈ 6-9
  • Phenolphthalein or Alizarin: weak acid with strong base, pH at end point ≈ 9-13

Equivalence Point and End Point

• Equivalence point: the point at which the acid reacts completely with the base.
• End point: the point at which the indicator changes its color, and the color of the solution changes.
• The difference between the two points is negligible, and the volume difference is ignored in calculations.

Neutralization Reactions Definitions

• Equivalence point: the point in the reaction at which the number of moles of acid is equal to the number of moles of base.
• End point: the point at which the indicator changes its color, and the color of the solution changes.
• Titrant (standard solution): the solution with a known concentration, usually filled in the burette.
• Analyte: the solution with an unknown concentration, usually filled in the flask.

Calculations

• Molarity (mol/L) = number of moles (mol) / volume (L)
• At the equivalence point: facid × Moles acid = fbase × Moles base
• facid × (M × V)acid = fbase × (M × V)base
• M = Molarity, V = Volume of solution, facid = number of reacting hydrogen (H+), fbase = number of reacting hydroxide (OH-)

Titration Basics

• Titration is a common laboratory method of quantitative chemical analysis used to determine the concentration of a solution with unknown concentration (analyte) using another solution with known concentration (standard).
• The end point is detected by the indicator.
• Volume measurements play a key role in titration, and it is also known as volumetric analysis.

Quantitative Analysis

• Objectives:
  • Determine the concentration of a solution with unknown concentration (analyte) using another solution with known concentration (standard).
  • Practice the titration (volumetric method).
• Acid-base titration is a type of titration used to determine the concentration of an acid or base.

Evolution of the Burette

• In an acid-base titration experiment, a solution of accurately known molar concentration (standard base) is added gradually from a burette to another solution of unknown concentration (analyte acid) in a flask, until the chemical reaction (neutralization) between the acid and the base is complete.
• The end point of the reaction is detected using an indicator.

Experiment Procedure

• Titration of unknown concentration H2SO4 using standard NaOH:
  1. Clean a burette and rinse it with distilled water and fill the burette with the standard NaOH solution.
  2. Make sure that the air bubbles are removed from the tip of the burette, and that the initial reading is within the scale of the burette.
  3. Rinse a 10.00 ml pipette with distilled water and then with about 1 ml of the unknown sulfuric acid solution.
  4. Pipette a 10.00 ml of the unknown sulfuric acid into a clean 125 ml Erlenmeyer flask.
  5. ...