gravimetric analysis

• Gravimetric Analysis


  A class of laboratory techniques under pharmaceutical analysis used to determine the mass or concentration of a substance by measuring a change in mass
• Different Types of Gravimetric Analysis

  • Precipitation Gravimetric Analysis or Precipitation Gravimetry
  • Volatilization
• Precipitation Gravimetric Analysis or Precipitation Gravimetry


  1. Analyte must be first converted to a solid ppt with an appropriate reagent

  2. The ppt from the solution is filtered, washed, purified, and weighed
• Volatilization


  It simply involves separating components of our mixture thought the use of heating or chemical decomposition of the sample
• Criteria for a Successful Determination Gravimetric Analysis

  • The desired substance must be completely precipitated
  • The weight of the product should be of known composition
  • Ease in handling
• Steps in a Gravimetric Analysis

  1. Preparation of Analyte Solution

  2. Precipitation Process

  3. Digestion of Precipitate

  4. Filtration

  5. Washing

  6. Drying on Ignition

  7. Weighing
• Different Types of Crucibles

  • Silica - transparent
  • Porcelain - The most common thing seen in the laboratory
  • Sintered - glass
• Preparation of Analyte Solution

  1. Sampling

  2. Dissolution
• Sampling


  You will take a sample from the bulk, meaning you will take a representative sample from a larger quantity
• Dissolution


  The actual step of preparing the analyte solution
• Preliminary Separation


  The goal is to separate potential interferences before precipitating the analyte
• Adjustment of solution condition


  Matters in our environment (e.g., pH, temperature, and the concentration of the test substance as well as the volume) are adjusted to maintain a low solubility of the precipitate to maximize precipitation
• Precipitation Process


  1. The precipitation reagent is added at a concentration that favors the formation of a good precipitate

  2. Large excess of precipitant should be avoided
• Digestion of Precipitate


  Also known as Ostwald Ripening, it is a phenomenon in which smaller particles in a solution dissolve and then deposit to form larger particles to reach a more thermodynamically stable state
• Types of impurities

  • Adsorption
  • Occlusion
  • Inclusion
• Filtration

  Filtration medium/filtration media is dependent on the nature of the precipitate, the cost of media, and the heating temperature
• Filtration media that can be used

  • Filter papers
  • Filter pulps
  • Filter mats
  • Crucible fitted with porous plate (sintered glass filters)
  • Crucible to be used at high temperature
• Washing


  Removal of co-precipitated impurities, and there is a need to add electrolyte to wash liquid because some precipitate cannot be washed with pure water
• Drying on Ignition


  Removes solvent and washes electrolytes, done by heating at 110-120 degree C for 1-2 hours, converts hydroscopic compounds to non-hydroscopic compounds
• Weighing


  Use properly calibrated analytical balance, good weighing technique must be observed
• Precipitation


  The most common method used in gravimetric analysis
• Precipitation reaction


  The most important step in gravimetric analysis, it involves both physical and chemical processes
• 2 types of nucleation

  1. Supersaturation

  2. Induced
• Supersaturation


  The solution phase that contains more dissolved salt than equilibrium, the driving force is for the system to approach equilibrium
• Induced


  If you need to add something in order to proceed to nucleation, or if you need to add something to speed up the nucleation process, you are inducing nucleation
• Crystal growth


  The enlargement process of particles, the nucleus will grow by the possession of particles precipitate in to the nucleus and they will form a crystal of a specific geometric shape
• Crystal Growth Steps


  1. Diffusion of ion

  2. Deposition on surface
• Different Properties of a Precipitate

  • It should be insoluble enough so that no significant loss during filtration in washing will happen
  • Easily separated by filtration
  • Stable to atmospheric condition
  • Convertible to pure compound of definite composition, either by ignition or by simple chemical operations such as evaporations
  • Must have large crystals
  • Should be free of contaminants
• Factors Affecting Solubility of Precipitate

  • Temperature
  • Solvent Composition
  • Diverse ion effect
• Temperature


  Heat increases the solubility of most solids, therefore the solubility of precipitate will increase with increasing temperature
• Solvent Composition


  The nature of the solvent influences the solubility, especially in organic substances
• Diverse ion effect


  Slightly soluble salts show an increase in solubility in the presence of decreased concentration of certain salts
• Gravitation/Gravimetric Factor


  The analyte (solute) is converted to a precipitate in order to identify its weight