Unit 2: Quantitative Analysis
Course Code: CHM-DSC-352
Paper Name: Analytical Chemistry
Table of Contents
1. Introduction to Quantitative Methods
Quantitative analysis determines the numerical amount or concentration of a substance in a sample. The two primary classical methods are Gravimetry and Volumetry.
2. Gravimetric Analysis: Principles
Gravimetric analysis is a technique where the amount of an analyte is determined through the measurement of mass.
- Process: The analyte is converted to a sparingly soluble precipitate, which is then filtered, washed, dried/ignited, and weighed.
- Requirements: The precipitate must be highly insoluble, easily filtered, and have a known, constant chemical composition.
3. Mechanism of Precipitation
Precipitation occurs in two distinct stages: Nucleation and Particle Growth.
- Nucleation: A minimum number of atoms or molecules come together to form a stable solid phase.
- Particle Growth: Existing nuclei grow by the addition of more atoms or molecules.
Von Weimarn Equation: Relative Supersaturation = (Q - S) / S
Where Q is the concentration of mixed reagents and S is the equilibrium solubility. To get large, filterable crystals, we want low relative supersaturation.
4. Co-precipitation and Post-precipitation
These processes introduce impurities into the final precipitate, affecting the accuracy of the analysis.
| Feature | Co-precipitation | Post-precipitation |
|---|---|---|
| Definition | Soluble impurities are pulled down during precipitate formation. | Impurity deposits on the surface after the precipitate has formed. | Adsorption, Occlusion, or Mixed-crystal formation. | Surface primary adsorption of ions. |
| Time Effect | Decreases with Digestion (aging). | Increases with time left in contact with mother liquor. |
5. Volumetric Analysis (Titrimetry)
Volumetric analysis involves measuring the volume of a solution of known concentration (titrant) required to react completely with the analyte.
- Standard Solutions: Primary standards must be 100% pure, stable, and have high molecular weight.
- End Point vs. Equivalence Point: The equivalence point is the theoretical completion; the end point is the actual observed change (color change).
6. Exam Focus: Tips and FAQs
- Digestion: Explain how "aging" a precipitate in hot mother liquor (digestion) improves purity and filterability by dissolving small particles and recrystallizing onto larger ones.
- Washing: Use a wash liquid containing a common ion or a volatile electrolyte to prevent peptization (re-dispersion of the precipitate).
- Relative Supersaturation: To minimize Q and maximize S (for better crystals), perform precipitation from hot, dilute solutions with slow addition and stirring.
Frequently Asked Questions
Q: What is the difference between occlusion and inclusion?
A: Occlusion involves trapping impurities within a growing crystal. Inclusion (mixed-crystal) occurs when an impurity ion replaces an analyte ion in the crystal lattice.
Q: Why is ignition necessary in gravimetry?
A: To convert the precipitate into a stable, anhydrous form of known stoichiometric composition (e.g., converting Fe(OH)3 to Fe2O3).
Q: Define Peptization.
A: It is the process where a coagulated precipitate reverts back to a colloidal state, usually during washing with pure water. It is prevented by using an electrolyte in the wash liquid.