Module 3

Gravimetric Analysis Overview

  • Gravimetric analysis is based on the measurement of mass using an analytical balance.

  • The quantification of the analyte is accomplished through a chemical reaction represented by a balanced equation.

Learning Outcomes

  1. Theoretical Basis: Understand gravimetric analysis theory.

  2. Types of Precipitates: Differentiate between colloidal and crystalline precipitates.

  3. Particle Size Prediction: Use the von Weimarn equation to predict particle size.

  4. Colloid Stability: Explain stability factors of colloids.

  5. Colloidal Handling: Describe proper handling methods for colloidal precipitates.

  6. Coprecipitation Errors: Identify different coprecipitation errors encountered in precipitation.

  7. Minimizing Errors: Determine techniques to minimize coprecipitation errors.

  8. Gravimetric Factors (GF): Calculate the correct gravimetric factors.

  9. Gravimetric Calculations: Utilize GFs in various gravimetric calculations.

  10. Problem Solving: Solve gravimetric analysis problems.

Types of Gravimetric Analysis

  • Precipitation Gravimetry: Analyte is separated into a precipitate which is converted to a compound with a known composition.

  • Volatilization Gravimetry: Analyte is separated by converting it to a gas of known composition.

  • Electrogravimetry: Analyte is separated through deposition on an electrode via electrical current.

Steps in Precipitation Gravimetry

  1. Solution Preparation: Convert the analyte into an aqueous form; prepare the precipitating reagent.

  2. Precipitation: Allow precipitation to occur.

  3. Digestion: Let the precipitate settle to enhance purity.

  4. Filtration: Filter the precipitate from the solution.

  5. Washing: Wash the precipitate to remove impurities.

  6. Drying/Igniting: Dry or ignite the precipitate to remove moisture.

  7. Weighing: Weigh the dried precipitate.

  8. Calculation: Determine the correct gravimetric factor for final analysis.

Gravimetric Factor (GF)

  • Definition: A factor that quantifies the relationship between the amount of analyte and the formed precipitate.

  • Calculation: GF is the ratio of the molar mass of the precipitate to the molar mass of the original analyte.

    Equation:mass of analyte = (mass of precipitate weighed) x (GF)

Desirable Properties of Precipitates

  • Precipitating Agent Requirements: Should react specifically/ selectively with the analyte.

  • Precipitate Characteristics:

    • Easily filtered

    • Low solubility

    • Unreactive with surrounding constituents

    • Known chemical composition

Factors Affecting Particle Size of Precipitates

  • Relative Supersaturation: Determined by the von Weimarn equation.

    • High supersaturation favors colloidal precipitates.

    • Low supersaturation favors crystalline solids.

  • Particle Size Calculation:

    Relative supersaturation (𝑄 - 𝑆)/𝑆; where Q= concentration at any instant and S= equilibrium solubility.

Mechanism of Precipitation

  • Competing Mechanisms: Nucleation vs. Particle Growth

    • Nucleation: Formation of stable nuclei from ions or molecules.

    • Particle Growth: Growth occurs upon existing nuclei.

Controlling Particle Size Experimentally

  • Temperature Control: Higher temperatures increase solubility.

  • Dilute Solutions: Reduction in concentration to decrease supersaturation.

  • Reagent Addition: Slowly add the precipitating agent while stirring to minimize oversaturation.

  • pH Control: Adjust pH to influence solubility.

Coprecipitation Errors

  • Definition: Occurs when a precipitate unintentionally contains impurities from soluble substances.

    • Positive/Negative errors based on atomic masses of contaminants.

Types of Coprecipitation Errors

  1. Surface Adsorption (SA): Larger surface areas, especially in colloids, are prone to contamination.

    • Solution: Wash colloids with solutions containing volatile electrolytes, digest, and reprecipitate.

  2. Mixed Crystal Formation: Replacing ions in the crystal lattice.

  • Solution: Selective precipitating agents and reprecipitation.

  1. Occlusion: Foreign ions trapped within a crystal.

  • Solution: Digest.

  1. Mechanical Entrapment: Solutions trapped within crystals.

  • Solution: Digest.

Precipitation from Homogenous Solutions

  • Precipitating agent generated by slow chemical reactions in the analyte's solution.

  • No excess reagent; maintains low supersaturation (e.g., using urea to generate OH-).

Drying and Ignition of Precipitates

  • Heating until mass becomes constant eliminates solvent and volatiles.

  • Ignition often decomposes the precipitate into a stable compound with known composition.

Summary

  • Gravimetric analysis converts the analyte into a precipitate of known composition.

  • The gravimetric factor relates the masses of reactants and products in the analysis.

  • Understanding elementary principles such as the von Weimarn equation, nucleation, and growth mechanisms are crucial for effective application and problem-solving in gravimetric techniques.