Gravimetric analysis is a quantitative analytical method that determines the concentration of an analyte by converting it into a solid of known composition and measuring its mass. It is one of the most accurate classical techniques in analytical chemistry.
Principle of Gravimetric Analysis
In gravimetric analysis, the analyte is selectively precipitated from solution as a sparingly soluble compound with a known stoichiometry. The precipitate is filtered, washed to remove impurities, dried or ignited to constant mass, and weighed. The mass of the analyte is calculated from the mass of the precipitate using the gravimetric factor: mass of analyte = mass of precipitate × (molar mass of analyte / molar mass of precipitate).
Precipitation Methods
In precipitation gravimetry, the analyte is converted to a solid precipitate by adding a precipitating agent, and the precipitate must have very low solubility (Ksp < 10^-10) to minimize losses during filtration. Volatilization gravimetry converts the analyte to a volatile species that is collected and weighed, or the mass loss of the sample after heating is measured; this is used for determining water content and carbonate analysis. Electrogravimetry involves electroplating the analyte onto an electrode and measuring the mass increase.
Requirements for a Good Precipitate
A good precipitate must have low solubility, meaning it is sufficiently insoluble that no significant loss occurs during filtration, typically less than 0.1 mg. It must have high purity and should not co-precipitate impurities; digestion, which involves heating the precipitate in the mother liquor, improves purity by allowing crystalline reorganization. The precipitate must have a definite stoichiometry after drying or ignition, known as known composition. It must also be easy to filter — large, crystalline precipitates such as BaSO4 and AgCl are easier to filter than gelatinous ones like Fe(OH)3.
Steps in Gravimetric Analysis
- Sample Preparation: Dissolve the sample in an appropriate solvent and adjust pH and temperature.
- Precipitation: Add the precipitating agent slowly with constant stirring to minimize supersaturation and promote large crystal growth.
- Digestion: Heat the solution to allow crystals to grow and expel impurities.
- Filtration: Use a filter paper (ashless grade) or a sintered-glass crucible of appropriate pore size.
- Washing: Remove adsorbed impurities using a wash solution that does not redissolve the precipitate.
- Drying or Ignition: Heat to constant mass to remove water and convert to a stable weighing form.
- Weighing: Cool in a desiccator and weigh to constant mass on an analytical balance.
Applications
Gravimetric analysis is used for determination of chloride as AgCl in water and food samples; analysis of sulfate as BaSO4 in environmental and industrial samples; determination of nickel as nickel dimethylglyoximate in alloys; measurement of silicon in ores and minerals by volatilization as SiF4; and ash content determination in coal, polymers, and food products.
