In link-ul despre carbonatul de sodiu:
http://en.wikipedia.org/wiki/Sodium_carbonate
la capitolul "Sodium carbonate test" spune cam asa:
Carbonatul de sodiu este folosit in distingerea intre diferiti ioni ai metalelor obisnuite, care se precipita sub forma carbonatilor respectivului metal. Testul poate distinge intre Cu, Fe si Ca/Zn/Pb.
In sarea metalului se adauga solutie de carbonat de calciu. Un precipitat albastru indica ionul Cu2+. Un precipitat verde murdar indica ionul Fe2+. Un precipitat galben maroniu indica ionul Fe3+. Un precipitat alb indica ionii Ca2+,
Zn2+ sau Pb2+.
"Sodium carbonate test[edit]
The sodium carbonate test (not to be confused with sodium carbonate extract test) is used to distinguish between some common metal ions, which are precipitated as their respective carbonates. The test can distinguish between Cu, Fe and Ca/Zn/Pb. Sodium carbonate solution is added to the salt of the metal. A blue precipitate indicates Cu2+ ion. A dirty green precipitate indicates Fe2+ ion. A yellow-brown precipitate indicates Fe3+ ion. A white precipitate indicates Ca2+, Zn2+ or Pb2+ ion. The compounds formed are, respectively, copper(II) carbonate, iron(II) carbonate, iron(III) oxide, calcium carbonate, zinc carbonate and lead(II) carbonate. This test is used to precipitate the ion present as almost all carbonates are insoluble. While this test is useful for telling these cations apart, it fails if other ions are present, because most metal carbonates are insoluble and will precipitate. In addition, calcium, zinc and lead ions all produce white precipitates with carbonate, making it difficult to distinguish between them. Instead of sodium carbonate, sodium hydroxide may be added, this gives nearly the same colours, except that lead and zinc hydroxides are soluble in excess alkali, and can hence be distinguished from calcium. For the complete sequence of tests used for qualitative cation analysis, see qualitative inorganic analysis."