Systematic analysis, in which metal ions in an unknown sample are identified by precipitating them sequentially, is a comprehensive exercise in inorganic chemistry and is frequently tested in entrance examinations.
Basic Knowledge
The standard high-school-level procedure for systematic analysis (representative metal ions in a mixture):
① Add dilute HCl → Group 1 (Ag+, Pb2+) precipitates as AgCl and PbCl2 white precipitates. PbCl2 dissolves in hot water, while AgCl (white) forms a complex and dissolves in aqueous NH3.
② Add H2S under acidic conditions → Group 2 (Cu2+, Cd2+, Pb2+, Hg2+, etc.) precipitates as CuS (black), CdS (yellow), PbS (black), and so on.
③ Add aqueous NH3/an ammonium chloride buffer → Group 3 (Fe3+, Al3+, Cr3+) precipitates as Fe(OH)3 (reddish brown), Al(OH)3 (white), and Cr(OH)3 (gray-green).
④ Add H2S under basic conditions → Group 4 (Zn2+, Mn2+, Ni2+, Co2+, etc.) precipitates as ZnS (white), MnS (pale pink), NiS (black), and CoS (black).
⑤ Add (NH4)2CO3 → Group 5 (Ca2+, Sr2+, Ba2+) precipitates as CaCO3 and similar compounds.
⑥ The remaining Group 6 ions: Na+, K+, Mg2+, and NH4+ are identified by flame tests and other methods.
White precipitates (AgCl, PbCl2, BaSO4, Al(OH)3, Zn(OH)2, ZnS, CaCO3)
Black precipitates (CuS, PbS, HgS, FeS, Ag2S)
Yellow precipitates (CdS, AgI, PbI2, K2CrO4 compounds)
Reddish-brown precipitates (Fe(OH)3, Fe2O3·nH2O)
Blue precipitates (Cu(OH)2 light blue, Turnbull’s blue/Prussian blue)
Green precipitates (Cr(OH)3, Ni(OH)2)
Further Study (Principles and Applications)
The principles of systematic analysis are the differences in solubility products Ksp and selective dissolution caused by pH and ligands. In acidic H2S, the concentration of S2- is low, so only metal sulfides with small Ksp values (= difficult to dissolve) precipitate (Group 2). Under basic conditions, the concentration of S2- increases, so more soluble ZnS, MnS, and similar compounds also precipitate (Group 4).
In modern analytical chemistry, systematic analysis has been replaced by ICP emission spectroscopy, atomic absorption spectroscopy, and ICP-MS, which can quantify down to the ppt (10-12) level. Nevertheless, systematic analysis remains educationally valuable for understanding the principles.
In practice, knowledge of these precipitation reactions and complex formation remains essential as pretreatment in water-quality testing, soil-contamination surveys, ore analysis, and metal recycling.
- Group 1 = AgCl, PbCl2 (precipitate with dilute HCl)
- Group 2 = CuS, CdS, PbS, HgS (acidic H2S)
- Group 3 = Fe(OH)3, Al(OH)3, Cr(OH)3 (aqueous NH3)
- Group 4 = ZnS, MnS, NiS, CoS (basic H2S)
- Group 5 = CaCO3 and similar compounds (ammonium carbonate)
- Group 6 = Na+, K+, and similar ions (flame tests)
- Principle = selectivity based on Ksp, pH, and ligands
Points to Note (Common Confusions and Frequent Errors)
① Pb2+ appears in both Groups 1 and 2 (PbCl2 is slightly soluble in cold water and may be missed, then precipitates as PbS in Group 2). ② Al(OH)3 and Zn(OH)2 are amphoteric and dissolve in excess NaOH; therefore, Group 3 uses aqueous NH3, not NaOH. ③ NH4+ does not give a flame test, so it must be detected separately using Nessler’s reagent or another method. ④ Behavior of copper ions with excess aqueous NH3: first a light-blue Cu(OH)2 precipitate forms, then it dissolves as the deep-blue [Cu(NH3)4]2+ complex.
Practice
- Write the procedure for separating Ag+, Cu2+, Fe3+, Zn2+, and Ba2+ from a mixture by systematic analysis.
- Al(OH)3 and Zn(OH)2 are both white precipitates. State a method for distinguishing them.
- Explain why using NaOH instead of aqueous NH3 in systematic analysis is inconvenient.