Precipitation and Co-precipitation

Ions form sparingly soluble salts when their ion concentration product exceeds the solubility product. Ions with lower solubility products precipitate first. For example, copper sulfide precipitates in acidic hydrogen sulfide solution because the copper and sulfide ion concentration product exceeds copper sulfide's solubility product, while ferrous ions remain dissolved.

Decreasing solution acidity increases sulfide ion concentration. In acidic conditions, copper sulfide precipitates while iron remains in solution. When acidity decreases, the elevated sulfide concentration causes iron sulfide to precipitate. This pH-dependent separation exploits differences in solubility products between copper sulfide and iron sulfide.

Coprecipitation occurs when soluble contaminants are trapped within or on a precipitate's surface. In water treatment, iron particles are oxidized to form ferric hydroxide precipitate, which coprecipitates arsenic. The arsenic-bearing precipitate is then removed by filtration, producing safe drinking water from contaminated sources.

The gathering process adds iron particles or iron(II) to arsenic-contaminated water. Iron oxidizes over several hours to form ferric hydroxide precipitate. Arsenic coprecipitates with the ferric hydroxide due to their chemical affinity. Repetitive filtration removes the arsenic-bearing precipitate, yielding non-toxic drinking water.

Iron(II) sulfide has a higher solubility product than copper(II) sulfide. Acidic hydrogen sulfide alone does not generate sufficient sulfide ion concentration to exceed iron(II) sulfide's solubility product. Aqueous ammonia must be added to increase sulfide ion concentration enough to trigger iron(II) sulfide precipitation.

Solubility product differences allow selective separation of ions in qualitative analysis. Ions forming sparingly soluble salts with the same reagent are separated based on their solubility product values. Lower solubility product ions precipitate first, enabling systematic identification and isolation of specific ions in a mixture.

Simple precipitation removes only the target ion by exceeding its solubility product. Coprecipitation intentionally uses a precipitate to capture otherwise soluble contaminants, isolating trace components from solution. Both techniques separate mixtures, but coprecipitation specifically exploits the tendency of contaminants to associate with the precipitate surface or structure.