Precipitants

A precipitant is a reagent that causes a dissolved substance to form a solid, or precipitate, from solution. In solution chemistry, a precipitant either lowers the solubility of a species or reacts with ions to form an insoluble product. Precipitation occurs when the product of the reacting ion concentrations exceeds the solubility product constant (Ksp), or when solvent conditions change, such as by adding a non-solvent, altering pH, or increasing ionic strength.

Common inorganic precipitants include sulfate, carbonate, hydroxide, and halide ions. For example, adding sulfate to a solution containing Ba2+ yields barium sulfate, a sparingly soluble solid. Silver nitrate with chloride gives silver chloride. Precipitation is widely used to separate ions, remove contaminants, or synthesize solid materials.

In analytical chemistry and environmental processing, precipitants remove specific ions or impurities from solutions, enabling qualitative or quantitative analysis and purification. In biochemistry, precipitants such as ammonium sulfate, polyethylene glycol (PEG), and ethanol are used to “salt out” proteins, aiding purification, while DNA precipitation is often achieved with ethanol or isopropanol.

In crystallography and materials science, precipitants are used to control crystallization, driving the formation of solid crystals from solution by adjusting solubility conditions, solvent composition, or temperature. The choice of precipitant depends on the desired selectivity, the target compound’s solubility behavior, and compatibility with other components.

Note: the precipitant is the reagent that drives precipitation, whereas the solid formed is the precipitate.