The 4th analytical group of cations includes ions which form sulfides that are insoluble at high concentrations. The reagents used are HS in the presence of NHOH. NHOH is used to increase the concentration of the sulfide ion, by the common ion effect - hydroxide ions from NHOH combine with H ions from HS, which shifts the equilibrium in favor of the ionized form:
They contain Zn, Mn, Ni and Co