The solubility of very insoluble salts was first determined by Hevesy (as discussed in Section 8.1). The method was based on the specific activity of the salt, which was identical in both the solid and solution phases, a/m = constant. At first, a pre­cipitate of known specific activity (e. g., lead sulfide) was produced:

Pb(NO3)2 1 210Pb2+ 1 S2- ! PbS (9.31)

The specific activity of the precipitate is measured. Then the precipitate is dis­solved, and the activity of the solution is measured. From the activity of the solu­tion and the specific activity, the concentration of the solution can be calculated. The solubility product of lead sulfide, for example, is LPbS = 10-33 mol2 dm-6. At this small value of solubility product, the solubility determination needs a very sen­sitive analytical method.