Identification of Cations

Cations (excluding NH₄⁺ ion belonging to group zero) have been divided into six groups depending upon the differences in solubility of chlorides, sulphides, hydroxides and carbonates. Of these ions, the cations are precipitated by adding some reagents called, group reagents.

Group O

Cations: NH₄⁺

Group Reagent: Conc. NaOH

No precipitate is obtained. A colourless gas ammonia with pungent smell is evolved on warming.

Group I

Cations: Pb²⁺

Group Reagent: dil. HCI

Precipitate: Chloride

Group II

Cations: Cu²⁺ , As³⁺

Group Reagent: H₂S (g) in the presence of dil HCl

Precipitate: Sulphide

Group III

Cations: Fe³⁺, Al³⁺

Group Reagent: excess of NH₄OH in presence of excess of NH₄Cl (s)

Precipitate: Hydroxide

Group IV

Cations: Co²⁺, Zn²⁺, Mn²⁺, Ni²⁺

Group Reagent: H₂S (g) in the presence of NH₄Cl (s) and NH₄OH excess

Precipitate: Sulphide 

Group V

Cations: Ba²⁺, Sr²⁺, Ca²⁺

Group Reagent: (NH₄)₂CO₃ solution in the presence of NH₄Cl and NH₄OH

Precipitate: Carbonate 

Group VI

Cations: Mg²⁺

Group Reagent: NH₄H₂PO₄

Precipitate: Phosphate 

Original Salt Solution

Before proceeding to detect the cations the solution of the salt has to be prepared.

Take a pinch of salt in a clean test tube. Add 2-3 mL of distilled water, shake well. If the solid is not soluble in cold water then heat the contents. If the salt is still not soluble in hot water add few drops of conc. HCl.

Precautions

• Do not add excess of conc. HCl. This might create a problem during cation analysis.
• If on addition of conc. HCl a gas is evolved, wait till the effervescence ceases and then add 2-3 drops more of conc. HCl.
• Use only distilled water to prepare original solution.

General Rules for Analysis of Cations

• The group reagent should be added in the systematic order.
• Test for the higher group radicals are performed only when the radicals of a lower group are found absent.
• Slight excess of the group reagent should be added to ensure complete precipitation of group radicals.
• When a precipitate is obtained for a particular group, it is filtered and the residue is used for the analysis of the radicals of that group. Before carrying out the analysis, wash the residue two times with distilled water.
• To find out the presence of a cation in a particular group, a small portion of the solution is used which is known as the test portion.