If two cations are in a solution, it may be possible to separate them by taking advantage of the differing solubility products of their salts with a particular anion. Differential precipitation is done by carefully adding a measured amount of the anion to the solution so that its final concentration is high enough to precipitate practically all of one salt, yet simultaneously low enough that the other salt does not precipitate at all. Ores rich in silver often contain large amounts of lead. The rock is crushed, mixed with concentrated HNO3, and filtered. Ag+ and Pb2+ can then be separated by differential precipitation with I- by adding solid KI to the solution.
Supposed in a particular solution [Ag+] = 3x10-6M and [Pb2+] =4x 10-3 Minitially. Enough KI is added to make [I-] = 2 x10-4M before any precipitation. Show that AgI will precipitate, while all the Pb2+ ions remain in solution. Ksp(AgI) = 1.5 x 10-16 and Ksp(PbI2) = 8.7 x 10-9.