Is it ensure, insure, or assure?
1610s, from Latin precipitantem, present participle of praecipitare (see precipitate (v.)). As a noun in chemistry from 1680s. The adjective senses now are taken by precipitate (adj.).
precipitant pre·cip·i·tant (prĭ-sĭp'ĭ-tənt)n. A substance that causes a precipitate to form when it is added to a solution.