Ions which precipitate out of solution in the presence of an acidic solution containing sulfur are called "Acid insoluble sulfides." When a compound such as H2S is placed in an acidic solution, its dissasociation reaction is driven towards the left because of the excess amount of H+ ions in the solution. Therefore, there is little S2- in the solution. Any ion which still precipitates out of solution with this small amount of sulfur ion left, has a very large affinity for the sulfur ion.

Examples of such ions are Cu2+ and Hg22+