Editing Qualitative inorganic analysis (section)

=== 4th analytical group of cations ===
The 4th analytical group of cations includes ions that precipitate as sulfides at pH 9. The reagent used is [[ammonium sulfide]] or Na<sub>2</sub>S 0.1 M added to the ammonia/ammonium chloride solution used to detect group 3 cations. 
It includes: [[zinc|Zn<sup>2+</sup>]], [[nickel|Ni<sup>2+</sup>]], [[cobalt|Co<sup>2+</sup>]], and [[manganese|Mn<sup>2+</sup>]]. [[Zinc]] will form a white precipitate, [[nickel]] and [[cobalt]] a black precipitate and [[manganese]] a brick/flesh colored precipitate. [[Dimethylglyoxime]] can be used to confirm nickel presence, while [[ammonium thiocyanate]] in ether will turn blue in the presence of cobalt. This group is sometimes denoted as IIIB since groups III and IV are tested for at the same time, with the addition of sulfide being the only difference.

This includes ions which form sulfides that are insoluble at high concentrations. The reagents used are H<sub>2</sub>S in the presence of NH<sub>4</sub>OH. NH<sub>4</sub>OH is used to increase the concentration of the sulfide ion, by the common ion effect - hydroxide ions from NH<sub>4</sub>OH combine with H<sup>+</sup> ions from H<sub>2</sub>S, which shifts the equilibrium in favor of the ionized form:

:{{chem|H|2|S}} {{eqmR}} 2{{chem|H|+}} + {{chem|S|2-}}

:{{chem|N|H|4|O|H}} {{eqmR}} {{chem|N|H|4|+}} + {{chem|O|H|-}}

:{{chem|O|H-}} + {{chem|H|+}} {{eqmR}} {{chem|H|2|O}}

They contain [[zinc|Zn<sup>2+</sup>]], [[manganese|Mn<sup>2+</sup>]], [[nickel|Ni<sup>2+</sup>]] and [[cobalt|Co<sup>2+</sup>]]