In general, the effectiveness of coagulants increases with the charge on the metal ion. The sodium (Na+) in sodium chloride (NaCl) is not a very effective coagulant. The calcium (Ca2+) in gypsum
(CaSO4) is more effective because it carries a +2 charge. The aluminum (Al3+) in alum and
the ferric-iron (Fe3+) in ferric sulfate are more effective yet because they carry a +3 charge. Some companies now manufacture various synthetic “polyelectrolytes,” which are large, long-chained molecules with even more charge than the metal salt coagulants listed here.