EXP 11

• Group IV Cations: The Alkaline Earth Metals (Ba2+, Sr2+, and Ca2+)
• In a mixture of group cations, Group IV and V will remain in solution after Groups I, II and III have been removed because their chlorides, sulfides and hydroxides are still soluble enough after the other group precipitation conditions.
• Group IV cations form insoluble carbonates and this principle is used to separate them from Group V cations which form soluble carbonates.
• The cations in Group IV are those that precipitate as carbonates upon addition of the group IV reagent, (NH4)2CO3 in NH3/NH4Cl buffer (basic solution).
• Precipitation of Ba2+, Sr2+, and Ca2+ occurs because, under Group IV conditions, the ion product (IP) exceeds the Ksp value only for the carbonates of these three ions. The cations which remain after precipitation of Groups I, II, III and IV comprise Group V.
• Cations of the fourth group react neither with HCl, H2S nor (NH4)2S, but

  ammonium carbonate (in the presence of moderate amounts of ammonia or ammonium ions) forms white precipitates.
• The test has to be carried out in neutral or alkaline solutions.
• In the absence of ammonia or ammonium ions, magnesium will also be precipitated.
• The white precipitates formed with the group reagents are: Barium carbonate (BaCO3), Strontium carbonate (SrCO3), and Calcium carbonate (CaCO3).
• The three alkaline earth metals decompose water at different rates,

  forming hydroxides and hydrogen gas.
• Their hydroxides are strong bases, although with different solubilities; Barium hydroxide is the most soluble, while calcium hydroxide is the least soluble among them.
• Alkaline earth chlorides and nitrates are very soluble; the carbonates, sulfates, phosphates and oxalates are insoluble.
• The sulfides can be prepared only in the dry method; they all hydrolyze in water, forming hydrogen sulfides and hydroxides.
• Unless the anion is colored, the group IV salts form colorless solutions.
• Because the alkaline earth ions behave similarly to each other in aqueous solutions, it is very difficult to distinguish them and specially to separate them.
• There are however differences in the solubilities of some of their salts in non-aqueous media. These differences can be utilized for separations.