4.8 : Precipitation and Co-precipitation

Precipitation and coprecipitation methods can be used to separate a mixture of ions in a solution. In qualitative inorganic analysis, ions that form sparingly soluble precipitates with the same reagent are separated based on the differences in solubility products. For example, consider the separation of Cu(II) and Fe(II) ions by precipitation as insoluble sulfides. First, copper(II) sulfide is precipitated by the addition of acidic H₂S, where the dissociation of H₂S is suppressed. Adding H₂S increases the product of sulfide ion and copper ion concentrations. When this product exceeds the solubility product of copper(II) sulfide, precipitation occurs. On the other hand, H₂S alone cannot be used to precipitate iron(II) sulfide because its solubility product is higher than that of copper(II) sulfide. Successful precipitation of FeS requires the addition of aqueous ammonia.

Coprecipitation is a method used to remove otherwise soluble contaminants by using a substance that can coprecipitate with the contaminants. This technique can be used to isolate trace components from a solution. For instance, the tendency of arsenic to coprecipitate with ferric hydroxide is exploited in a process called gathering. During this process, Fe(II) or solid iron particles are added to water contaminated by arsenic. The iron particles are then oxidized for several hours to obtain ferric hydroxide precipitates, which now contain arsenic. Finally, the arsenic-bearing precipitate is removed by repetitive filtration to produce safe drinking water.