Introduction to Gravimetric Analysis
Gravimetric analysis is a quantitative method in which the substance being tested is converted into an insoluble solid (precipitate), separated, dried, and weighed. The weight of this final product helps calculate the amount of the desired chemical present in the original sample. Since it depends on mass measurement, it is one of the most accurate analytical techniques.
Principles of Gravimetric Analysis
Gravimetric analysis is governed by three major scientific principles that ensure proper formation and separation of the precipitate.
Law of Mass Action and Reversible Reactions
This law states that the rate of a chemical reaction is proportional to the concentration of the reacting substances. During precipitation, the reaction continues until both forward and backward reactions reach equilibrium. By adjusting concentrations, the reaction can be pushed toward complete formation of the precipitate.
Principle of Solubility Product
A slightly soluble salt dissolves only until the product of its ion concentrations reaches a constant value called the Solubility Product (Ksp). When the ionic product becomes higher than Ksp, precipitation takes place. This principle helps predict when a substance will form a solid from solution.
Common Ion Effect
The solubility of a weakly soluble salt decreases when excess of one of its ions is added. This helps force more precipitate to form by adding a salt containing a common ion, making the method more accurate.
Types of Gravimetric Analysis
Gravimetric methods are classified according to how the analyte is measured.
- Physical Gravimetry: Used to separate suspended particles based on size or volatility; widely used in environmental analysis.
- Thermogravimetry: Records weight changes when a substance is heated; useful in polymer, food, and pharmaceutical studies.
- Electrodeposition: Metal ions from a solution are deposited on a cathode using electric current and weighed before and after deposition.
- Precipitative Gravimetry: A chemical reagent is added to form an insoluble precipitate that can be filtered and weighed.
Methods Used in Gravimetric Analysis
Accurate results depend on proper preparation, separation, and weighing of the precipitate.
- Sampling: The sample must represent the whole material being tested.
- Precipitation: A reagent is added to form a stable, low-solubility, filterable precipitate without contaminants.
- Purity of Precipitate: Undesired substances may get trapped inside or on the surface of the precipitate. This contamination is known as co-precipitation. It can occur due to adsorption or occlusion of other substances during crystal growth.
- Digestion: Allowing the precipitate to stand in the solution helps small crystals dissolve and large crystals form, improving purity.
- Aging: Further stabilizes the crystals and removes impurities stuck to the surface.
- Filtration: Separates the precipitate from the liquid using filter paper or filtering crucibles. The liquid that passes through is the filtrate; the solid left behind is the residue.
- Washing: Removes impurities from the surface of the precipitate. Sometimes a common ion is added to prevent dissolution of the solid during washing.
- Drying and Ignition: Heat removes moisture and converts the precipitate into a stable, weighable form. Drying is used up to 250°C, while ignition applies higher temperature.
- Weighing and Calculations: The dried material is weighed, and the amount of analyte is calculated using the gravimetric factor, which relates mass of precipitate to mass of the analyte present.
Applications of Gravimetric Analysis
Gravimetric analysis is widely used where high precision is required, especially in pharmaceutical and laboratory standards.
- Analysis of Chloride as Silver Chloride: Chloride ions are precipitated using silver nitrate and dried as AgCl for weighing.
- Estimation of Lead as Lead Chromate: Lead forms insoluble lead chromate and is measured after drying.
- Estimation of Nickel as Nickel–Dimethylglyoxime Complex: Nickel forms a red-colored complex with dimethylglyoxime in alkaline medium; the pure solid is filtered, dried, and weighed.
