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
2 answers
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.
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.