Titrations in Analytical Chemistry

Titration Methods

• Titration methods are quantitative methods that measure the quantity of a reagent of known concentration required to react completely with the analyte.
• The reagent may be a standard solution of a chemical or an electric current of known magnitude.

Volumetric Titrations

• Volumetric titrations involve measuring the volume of a solution of known concentration needed to react completely with the analyte.
• The analyte is a solution of unknown concentration but known volume.
• The standard solution (titrant) is a reagent of known concentration and volume.
• Back-titration is a process in which the excess of a standard solution used to consume an analyte is determined by titration with a second standard solution.

Key Concepts

• Equivalence point: the point in a titration when the amount of added standard reagent is equivalent to the amount of analyte.
• End point: the point in a titration when a physical change occurs that is associated with the condition of chemical equivalence.
• Titration error: the difference in volume between the equivalence point and the end point.

Indicators

• Indicators are added to the analyte solution to produce an observable physical change (detect end point) at or near the equivalence point.
• Indicator changes include the appearance or disappearance of a color.

Types of Standard Solutions

• Primary standard solutions (ex. Na2CO3 and H2C2O4): high purity, stability, rapid reaction with analyte, complete reaction, and selective reaction with analyte.
• Secondary standard solutions (ex. HCl and NaOH): less pure and less stable than primary standards.

Types of Volumetric Titrations

• Neutralization titration (acid-base titration)
• Complexometric titration
• Precipitation titration
• Oxidation-reduction titration

Neutralization Titration

• Standard reagents are always strong acids or strong bases.
• Acid-base indicators are weak organic acids or bases whose color changes according to pH value.
• Types of acid-base titrations:
  • Strong acid + strong base (equivalence point pH = 7)
  • Strong acid + weak base (equivalence point pH < 7)
  • Weak acid + strong base (equivalence point pH > 7)

Complexometric Titration

• Method based on complex formation due to reaction between metal ion (M2+) and ligand (L).
• Ligands: H2O, NH3, Cl-, Br-, I-, and F-.
• Coordination number: the number of covalent bonds a metal forms with electron donors.
• Chelate: a metal ion coordinates with two or more donor groups of a single ligand to form a five- or six-membered heterocyclic ring.
• Types of ligands:
  • Unidentate: a ligand with a single donor group (ex. NH3)
  • Bidentate: a ligand with two donor groups (ex. glycine)
  • Tridentate, tetradentate, pentadentate, and hexadentate
• EDTA (Ethylene Diamine Tetraacetic Acid) is a widely used complexometric titrant that analyzes metal ions as a hexadentate ligand.

Precipitation Titration

• Method based on the formation of a slightly soluble precipitate.
• Standard reagent is a precipitating agent (ex. AgNO3).
• Methods of detection of end point:
  • Mohr Method: uses chromate ion (CrO4 -2) as an indicator.
  • Fajans Method: uses adsorption indicators (ex. Fluorescein-).
  • Volhard Method: uses iron III (Fe+3) as an indicator.

Oxidation-Reduction Titration

• Oxidation: an increase in oxidation number resulting from loss of electrons or increase in oxygen content or decrease in hydrogen content.
• Reduction: a decrease in oxidation number resulting from gain of electrons or decrease in oxygen content or increase in hydrogen content.
• Oxidizing agent (oxidant): a substance that accepts electrons and gets oxidized.
• Reducing agent (reductant): a substance that donates electrons and gets reduced.
• Reaction where a transfer of electrons is involved is called an oxidation-reduction reaction.
• Types of redox indicators:
  • Self-indicators: standard reagents with oxidized and reduced forms that differ in color.
  • Specific indicators: form a colored compound with a specific oxidized or reduced form of the analyte or standard reagent.
  • True redox indicators: have oxidized and reduced forms that differ in color.