EXP 6

• The Group II cations are the metals precipitated by hydrogen sulphide, H2S, in the presence of hydrochloric acid, HCl, and whose sulfides are insoluble in ammonium sulphide, (NH4)2S.
• The cations of the second group are traditionally divided into two sub-groups; the copper sub-group and the arsenic sub-group.
• The basis of this division is the solubility of the sulfide precipitates in

  ammonium polysulfide reagents.
• While sulfides of copper sub-group are insoluble in this reagent,

  those of the arsenic sub-group do dissolve under the formation of thiosalts.
• the metals of group III are not precipitated by the group reagents for Groups I and Groups II, but are all precipitated, in the presence of ammonium chloride, by hydrogen sulfide from their solutions made alkaline with ammonia solution.
• The metals, with the exception of aluminum and chromium which are precipitated as the hydroxides owing to the complete hydrolysis of sulfides in aqueous solutions, are precipitated as the sulfides.
• Iron, aluminum and chromium (often accompanied by a little manganese) are also precipitated as hydroxides by ammonia solution

  in the presence of ammonium chloride, while the other metals of the group remain in solution and may be precipitated as sulfides by hydrogen sulfide.
• Iron – aluminum group (Iron, Aluminum, and Chromium) or Group III-A
• Zinc group (Nickel, Cobalt, Manganese and Zinc) or Group III-B