GRAVIMETRIC ANALYSIS

Introduction

• Gravimetric analysis is an analytical method based on the measurement of the weight of a substance of known composition.

• It is recognized for its accuracy and precision within macro quantitative techniques.

• The substance of known composition must chemically relate to the analyte.

Major Types of Gravimetric Analysis

1. Precipitation Gravimetric Analysis

2. Volatilization Gravimetric Analysis

1. Precipitation Gravimetric Analysis

• This is the most commonly used type for gravimetric analysis.

• Process: Involves making the analyte chemically react with a reagent to yield a product of limited solubility. After filtration and treatment, the solid residue of known composition is weighed.

Important Concept: Precipitation Equilibria

• Definition: When substances have limited solubility and their solubility is exceeded, ions of the dissolved portion exist in equilibrium with the solid (undissolved portion).

• Clarification: It doesn't mean they are completely insoluble; they exhibit slight solubility where some dissolved portion exists in equilibrium with the solid.

• Crucial Rule for Precipitation: The product of the concentrations of ions involved should exceed the solubility product constant (Ksp).

  • Ksp is defined as: $K_{sp} = [Ag^+][Cl^-]$

  • If the product equals Ksp, all ions remain in the solution.

• Example: The Ksp for Ag2CrO4 is: $K<em>{sp} = [Ag^+]^2[CrO</em>4^{2-}]$

2. Volatilization Gravimetric Analysis

• Involves separating a substance to be determined in the gas form from the remaining sample.

• The weight of the volatile component is compared with the weight of the non-volatilized portion. This process is also referred to as gravimetric combustion analysis.

• Involves passing a partially combusted product through catalysts at elevated temperatures to generate CO2 and H2O.

Steps in Gravimetric Analysis

• Requires two major measurements: weight of the sample and weight of the product derived from the sample.

Steps Involved:

1. Preparation of Solution

   • Adjust conditions to encourage precipitation (temperature, pH, volume, concentration).

   • Remove interfering substances by introducing reagents that selectively mask them.

2. Precipitation

   • The precipitate should be insoluble with large crystals for easy filtration.

   • Desired properties for precipitating agents:

     • Low solubility to minimize loss

     • Readily filterable and wash clean

     • Remain inert and of known composition.

   • Process of Precipitation:

     • Occurs through supersaturation, nucleation, and crystal growth.

     • Relative supersaturation can be calculated as: $Relative ext{ } Supersaturation = Q - S$

     • Where Q = concentration and S = solubility.

     • To achieve preferable crystal size:

     • Keep Q low and S high.

     • Slowly add precipitating agent to a dilute solution with effective stirring.

     • Conduct precipitation at low pH.

3. Digestion of Precipitate

   • Allows larger crystals to grow at the expense of smaller ones.

   • Colloidal precipitates may form, affecting purity through coprecipitation (e.g., occlusion, surface adsorption, and post precipitation).

4. Washing and Filtration of Precipitates

   • Conducted to remove co-precipitated impurities and wetting liquids.

   • Washing may cause peptization; hence, electrolytes like HNO3 or NH4NO3 are added to prevent this.

5. Drying and Igniting of Precipitate

   • The product is treated at 110-120°C for one or two hours until a constant weight is achieved (variation within ±0.3 or 0.4 mg).

6. Calculation

   • Calculated on a percentage basis:

     • $$

Application of Gravimetry as a Separation Method of Metals

• Gravimetric analysis can separate metals from a material with high precision if conditions are optimal.

• Factors Influencing Precipitate Solubility:

  1. Temperature

  2. Solvent Used

  3. Rate of precipitation formation

Types of Precipitating Agents

• Inorganic Precipitating Agents:

  • Examples:

  • NH3 (aq) → Precipitates Be, Al, Fe, Sc

  • H2S → Precipitates Cu, Zn, As

  • (NH4)2MoO4 → Precipitates Cd, Pb

  • HCl → Precipitates Ag, Na, Si

• Reducing Agents:

  • Transform analytes to elemental form for direct weighing, e.g., SO2 for Se, SnCl2 for Ag.

• Organic Precipitating Agents:

  • Generally have low water solubility and favorable gravimetric factors. Includes coordination compounds and bonding agents.

Self-Assessment Exercise

1. A compound weighing 4.0 mg produces 1.25 mg of CO2 and 2.41 mg of H2O. Calculate the weight percent of C and H in the sample.

2. Highlight the necessary steps for successful gravimetric analysis.

3. Calculate the concentration of Ag+ and Cl- in a saturated solution of AgCl given Ksp at 25°C is 1.0 x 10^-10 and the molar solubility of AgCl.

4. Assess the importance of gravimetric analysis in modern analytical exercises.