At least 99.9% of the ion in solution must be removed and incorporated into the solid precipitate that is filtered off, dried, and weighed for a precipitation fraction to be useful as a method for quantitative analysis.
Assume that the ion being analyzed is the Pb2+, and the precipitating agent used is I- ion. In a particular solution [Pb2+] = 0.50 M initially, and enough KI is added to make the [I-] = 0.15 M before any precipitation. Calculate the molar concentration of Pb2+ ion remaining in the solution at equilibrium. Discuss what % remains un-precipitated? Ksp (PbI2) = 8.7 x 10-9