How to Find Cation and Anion in a Given Salt
In qualitative inorganic analysis, the identification of a salt involves determining two components:
- Cation (Basic radical)
- Anion (Acid radical)
This systematic analysis is commonly followed in CBSE and other board practical chemistry laboratories. The identification is carried out step by step using preliminary tests, dry tests, and wet confirmatory tests.
Step 1: Preliminary Examination (Dry Tests)
(a) Physical Observation
Colour of the salt:
- Blue colour – Copper (Cu2+)
- Green colour – Iron (Fe2+) or Nickel (Ni2+)
- White colour – Zinc (Zn2+), Calcium (Ca2+), Sodium (Na+), Potassium (K+)
Smell of the salt:
- Ammonia smell – Ammonium ion (NH4+)
- Rotten egg smell on heating – Sulphide ion (S2−)
(b) Action of Heat
Heat a small amount of the salt in a dry test tube and observe:
- Crackling sound – Presence of water of crystallization
- Evolution of gas:
- Carbon dioxide – Carbonate (CO32−)
- Sulphur dioxide – Sulphite (SO32−)
- Brown fumes – Nitrate (NO3−)
Step 2: Solubility Test
The salt is tested for solubility in:
- Cold water
- Hot water
- Dilute hydrochloric acid (HCl)
- Dilute nitric acid (HNO3)
This helps in selecting suitable reagents for further analysis.
Step 3: Identification of Anion (Acid Radical)
(A) Test with Dilute Hydrochloric Acid
Add dilute HCl to the salt solution and observe:
| Observation | Inference (Anion) |
|---|---|
| Effervescence; CO2 turns lime water milky | Carbonate (CO32−) |
| Rotten egg smell | Sulphide (S2−) |
| Pungent smelling gas (SO2) | Sulphite (SO32−) |
| No reaction | Proceed to next test |
(B) Test with Concentrated Sulphuric Acid
Add concentrated H2SO4 to solid salt:
| Observation | Anion |
|---|---|
| Brown fumes | Nitrate (NO3−) |
| White fumes with pungent smell | Chloride (Cl−) |
| Red vapours | Bromide (Br−) |
| Violet vapours | Iodide (I−) |
(C) Confirmatory Tests for Anions
- Chloride ion: Add AgNO3 solution → White precipitate soluble in NH4OH
- Sulphate ion: Add BaCl2 solution → White precipitate insoluble in HCl
- Nitrate ion: Brown ring test confirms nitrate
Step 4: Identification of Cation (Basic Radical)
Cations are identified by systematic group analysis using specific reagents.
Group 0: Ammonium Ion
- Warm salt with NaOH → Ammonia gas evolved (Turns red litmus blue)
Group I: Silver, Lead, Mercurous Ions
- Add dilute HCl → White precipitate formed
Group II: Copper, Cadmium, Bismuth etc.
- Pass H2S gas in acidic medium → Coloured precipitate
Group III: Iron, Aluminium, Chromium
- Add NH4OH in presence of NH4Cl → Precipitate formed
Group IV: Zinc, Manganese, Nickel, Cobalt
- Pass H2S gas in basic medium → Precipitate formed
Group V: Calcium, Barium, Strontium
- Add (NH4)2CO3 → White precipitate
Group VI: Sodium, Potassium, Magnesium
- Identified using flame test and special tests
Step 5: Flame Test
| Flame Colour | Cation |
|---|---|
| Golden yellow | Sodium (Na+) |
| Lilac | Potassium (K+) |
| Brick red | Calcium (Ca2+) |
| Apple green | Barium (Ba2+) |
| Blue-green | Copper (Cu2+) |
Final Result
After performing all tests and confirmatory reactions, the salt is reported as:
Cation present: __________
Anion present: __________
Important Practical Tips
- Always identify the anion before the cation
- Write observation and inference clearly
- Confirm results using confirmatory tests
- Flame test is very useful for Group VI cations
This method ensures accurate identification of cations and anions in an unknown salt during qualitative inorganic analysis.
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