Applied Chemistry Gravimetry Study Notes

1. Pre-reading Overview

• Introduction to Matter and Measurement: Concepts of matter and methods for separating mixtures.

• Chemical Reactions and Reaction Stoichiometry:

  • Understanding Formula Weights.

  • Identifying Limiting Reactants.

• Reactions in Aqueous Solution:

  • Precipitation Reactions.

  • Concentration of Solutions and Solution Stoichiometry.

• Properties of Solutions:

  • Characteristics and behaviors of solutions.

2. Understanding Gravimetry

• Application Scenario: An analyst's task in determining the total suspended solids in water from a sewage treatment facility.

• Suspended Solids: Definition of solid materials that remain in suspension and do not settle out of the solution matrix.

• Analytical Techniques: Discussed techniques include filtration and methods for analyzing substances in solutions (like salt in seawater and moisture in food).

3. Principle of Gravimetry

• Definition: Gravimetric analysis estimates the mass percent of an ion or substance in an impure compound by determining its mass in a pure compound.

• Key Process:

  • The analyte (ion or substance) is converted to a product of known composition.

  • Mass of the product is measured after complete isolation through precipitation or vaporization.

• Typical Steps:

  1. Prepare a solution with the known amount of sample analyte.

  2. Separate the desired ion or element.

  3. Weigh the pure insoluble compound formed.

  4. Calculate the individual component of interest using the weight.

4. Types of Gravimetric Analysis Techniques

• Precipitation Gravimetry:

  - Analyte conversion to a precipitate that can be filtered and measured.

  Example Reaction:
  $2I^-(aq) + Pb^{2+}(aq) <br>ightarrow PbI_2(s)$

• Volatilisation Gravimetry:

  - Separation of the analyte by converting it to a gas of known composition, measuring the mass of the gas.

  Example Reaction:
  $CaCO_3(s) <br>ightarrow CaO(s) + CO_2(g)$

• Other Methods:

  • Electrogravimetry: Quantifies ions via electrodeposition, measuring mass difference.

  • Thermogravimetry: Measures changes in physical and chemical properties as temperature varies.

5. Precipitation Gravimetry Techniques

• Overview: Relies on the mass of solid precipitate to calculate the mass of analyte.

• Key Components:

  • Reagents & Precipitating Agents: Specific reagents such as AgNO3 can precipitate certain ions like Cl-, Br-, I-, and SCN-.

  • Dimethylglyoxime selectively precipitates Ni2+ from alkaline solutions.

6. Steps in Precipitation Gravimetry

• Weighing the sample for analysis.

• Dissolving the sample in a suitable solvent (e.g., water).

• Adding excess precipitating reagent to precipitate the analyte.

  • The reaction is typically a simple metathesis (exchange or double replacement).

• Filtering, washing, and drying the precipitate.

• Weighing and calculating the mass of the original analyte.

7. Filtration Process in Gravimetric Determination

• Filtration Setup:

  • After precipitate drying, weighing may not be accurate if the precipitate is on filter paper.

  • Heating the precipitate in a crucible will allow for accurate mass measurement.

• Cooling in Desiccator: Prevents moisture absorption from the atmosphere.

8. Product Form Impact

• Importance that the form of product may differ post-precipitation due to heating or drying.

• Example Analysis of PO4^{-3}: Precipitated as Mg(NH4)PO4•6 H2O but weighed as Mg2P2O7.

9. Ideal Properties of Precipitate

• Characteristics:

  • Low solubility.

  • Easily filterable and washable (large crystals).

  • Stability against atmospheric constituents.

  • Consistent and known composition.

10. Solubility Effects on Precipitation

• Key Relationships:

  • Influence of conditions such as solubility product and pH on the formation and stability of precipitate.

  • Example: Barium sulfate (BaSO4) Ksp = 1.5 x 10^{-9}.

11. pH Dependence of Solubility

• Low pH raises solubility for precipitates like Fe(OH)3.

• Reactions with acids (e.g., HCl) shift solubility dynamics.

12. Equilibrium and Intrinsic Solubility Considerations

• Example with AgCl dissolution:

  • $AgCl(s) <br>ightleftharpoons Ag^+(aq) + Cl^-(aq)$

  • Cooperative balance by introducing excess Cl- minimizes solubility losses.

13. Factors Influencing Particle Size

• Nucleation and Growth Mechanism:

  • Nucleation—initial small particles formation.

  • Growth—advancement of stable particles.

• Larger particles are easier to filter whereas small particles risk clogging filters.

14. Mechanism Control Factors

• Adjustments in temperature, reactant concentrations, and mixing rates can influence precipitate size and yield.

15. Coprecipitation Implications

• Occurs when soluble substances bind with precipitate, leading to contamination.

• Types Include:

  • Surface adsorption.

  • Mixed crystal formation, occlusion, and mechanical entrapment.

16. Digestion Process

• Definition: Heating precipitate in mother liquor to enhance purity.

17. Volatilisation Gravimetry Overview

• Sample decomposition under thermal or chemical means to measure mass change.

• Example Process: Removal of water from a hydrated salt to derive molar quantities.

18. Gravimetric Calculations

• Example 1: For a sample containing chloride yielding AgCl.

  • Steps to find percent mass of Cl in the original sample include:

  1. Finding the % of Cl in AgCl (based on molar mass).

  2. Using mass ratios for conversions.

19. Additional Gravimetric Calculation Examples

• Additional scenarios exemplifying analytical calculations, including calculating impurities and determining compositions in various complex systems.

20. Applications of Gravimetry

• Gravimetric analysis applications across quality control, inorganic and organic analyses, assessing environmental pollutants, and determining material compositions.

21. Total, Suspended, and Dissolved Solids in Environmental Analysis

• Definitions:

  • Total Dissolved Solids (TDS): Solids passing through a 2-micron filter.

  • Total Suspended Solids (TSS): Solids that do not pass through.

  • Total Solids (TS): Sum of TSS and TDS.

22. Conclusion and Further Practice

• Practice Problems: Additional scenarios for calculation skills and understanding concepts of gravimetric methods.