Question

Why is HCI added on a drop-by-drop basis to ensure complete precipitation of Group 1 chlorides initially, rather than just added all at once? HCI forms a mixture of AgCI, PbCI2, Hg2CI2

Answer 1

As each precipates, what ions not precipated remain in solution. Drop by drop allows for catching the last ions, without putting more HCl into solution.

Answer 2

You want to precipitate all of the metals; therefore, you must have an excess of HCl. But you don't want to add too much because HCl forms complex ions with the metals (AgCl2^-, AgCl3^-2, etc, PbCl3^- and PbCl4^-2) which means that not all of the metal would be in form of the solid. In the end, then, some of the metals that should have been pptd in group I remain in solution for the remaining groups and may interfere down the line.