Selective precipitation exploits differences in Ksp. When a reagent is gradually added to a solution containing two ions that form insoluble salts, the salt with the lower IP/Ksp requirement precipitates first. Example: A solution contains 0.01 M Cl- and 0.01 M I-. Adding AgNO3 gradually: AgI precipitates first (Ksp = 8.5 x 10^-17 << Ksp of AgCl = 1.8 x 10^-10). AgI starts precipitating when [Ag+] = Ksp(AgI)/[I-] = 8.5 x 10^-15 M. AgCl starts precipitating when [Ag+] = Ksp(AgCl)/[Cl-] = 1.8 x 10^-8 M. So AgI precipitates completely before AgCl begins. This principle underlies qualitative analysis of cations in Groups I-V.