coprecipitation
/ koh-prih-sip-ih-TAY-shun /
Picture a crowd rushing through a turnstile so fast that a few people who never bought tickets get swept through alongside the ticket-holders. Coprecipitation is that unwanted stowaway effect: as your wanted solid forms, it drags down other substances that should have stayed dissolved.
Coprecipitation is the carrying-down of normally soluble impurities together with a precipitate as it forms. The impurity may sit on the crystal surface, be trapped inside the growing crystal, or substitute into the crystal lattice — different routes, all leaving the solid heavier and dirtier than it should be.
It matters because in gravimetry your answer is a mass, so any tag-along impurity makes the result too high. Understanding coprecipitation tells you how to fight it: precipitate slowly from dilute solution, digest the solid, and sometimes dissolve and re-precipitate to shed the stowaways.
When barium sulfate is precipitated from a solution also containing nitrate, some nitrate is occluded inside the crystals; on ignition it can leave the result slightly too high or low depending on what it becomes.
Soluble impurities ride down with the precipitate they should not join.
Coprecipitation is different from postprecipitation, where a second solid slowly forms on top of the first after it has settled. Coprecipitation happens during formation; postprecipitation happens afterward and worsens with standing time.