Intermediate Topic 706 of 1102 Inorganic Chemistry
Qualitative Analysis of Cations
Systematic precipitation and tests identify metal ions in solution.
In-depth explanation of Qualitative Analysis of Cations
Qualitative analysis identifies the metal ions in an unknown solution using a systematic scheme of precipitation and confirmatory tests. In the classical scheme, cations are grouped by the solubility of their chlorides, sulfides, hydroxides and carbonates. Group I ions, Ag+, Pb2+ and Hg2 2+, precipitate as chlorides when dilute hydrochloric acid is added. Group II ions form sulfides in acid, while Group III and Group IV ions precipitate as hydroxides, sulfides or carbonates in basic solution. Each separated group is then tested with reagents to confirm the ion present.
Going deeper: Qualitative Analysis of Cations
Key ideas: Systematic group separation: Group I (Ag+, Pb2+, Hg2 2+) precipitate with dilute HCl as chlorides; Group II (Cu2+, Cd2+, Bi3+, Sn2+) with H2S in acid; Group III (Fe3+, Al3+, Cr3+) with NH4OH and NH4Cl; Group IV (Zn2+, Ni2+, Co2+, Mn2+) with H2S in basic medium; Group V (Ba2+, Sr2+, Ca2+) with ammonium carbonate; Group VI (Mg2+, NH4+, Na+, K+) soluble. Control of [S2-] through pH uses Ksp, and NH4Cl suppresses OH- to avoid precipitating group IV and V.
Common mistakes and connections: Students mix the order of reagents: adding H2S in basic medium early would precipitate groups II and IV together. Without removing H2S before group III, Fe2+ may precipitate incorrectly. Confirmatory tests: Cu2+ with excess NH3 deep blue; Fe3+ with SCN-; Ni2+ with dimethylglyoxime red. Links to solubility product, common ion effect and complexation.
Visualization of Qualitative Analysis of Cations
Flame and precipitate colours identify cations; the flame colours of the metal ions here help confirm which metal is in the sample. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Adding dilute HCl to a solution containing Pb2+ gives a white precipitate of lead chloride, which places lead in Group I.
Frequently asked questions about Qualitative Analysis of Cations
What is Qualitative Analysis of Cations?
Systematic precipitation and tests identify metal ions in solution.
Is Qualitative Analysis of Cations a beginner, intermediate or advanced topic?
Qualitative Analysis of Cations is a intermediate topic in Inorganic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Qualitative Analysis of Cations?
Study these earlier topics in Inorganic Chemistry first: Hemoglobin, Chlorophyll, Cisplatin.
What comes after Qualitative Analysis of Cations?
The next topics in Inorganic Chemistry are Qualitative Analysis of Anions and Pigments and Dyes.
Can I practise Qualitative Analysis of Cations with quizzes?
Yes. The interactive lesson includes a self-check question on Qualitative Analysis of Cations, and the Inorganic Chemistry quiz draws on every topic in this area.
Where Qualitative Analysis of Cations fits in Inorganic Chemistry
Qualitative Analysis of Cations is topic 91 of 113 in Inorganic Chemistry.