Salt Analysis: Acidic Radicals
Secure your Practical Chemistry marks. Master the classification of anions, the iconic Chromyl Chloride test, the Brown Ring complex, and specific precipitate colors.
Module Focus: The Volatility Hierarchy
In inorganic qualitative analysis, acidic radicals (anions) are detected based on their volatility when treated with acids. We categorize them into three strict groups: those decomposed by Dilute Acids (evolving gases), those decomposed only by Concentrated Acids (requiring stronger oxidation/displacement), and a Special Group that doesn't evolve gases with acids and must be tested via precipitation.
1. Classification of Anions
React with Dilute $HCl$ or $H_2SO_4$ to evolve characteristic gases.
- $CO_3^{2-}$ (Carbonate)
- $S^{2-}$ (Sulfide)
- $SO_3^{2-}$ (Sulfite)
- $NO_2^-$ (Nitrite)
- $CH_3COO^-$ (Acetate)*
React only with Conc. $H_2SO_4$ (often requiring heat).
- $Cl^-$ (Chloride)
- $Br^-$ (Bromide)
- $I^-$ (Iodide)
- $NO_3^-$ (Nitrate)
- $C_2O_4^{2-}$ (Oxalate)
Do not react with acids. Identified strictly by precipitation reactions.
- $SO_4^{2-}$ (Sulfate)
- $PO_4^{3-}$ (Phosphate)
2. Group I: Dilute Acid Group Tests
Adding dil. $H_2SO_4$ causes brisk effervescence of a colorless, odorless gas ($CO_2$).
- Initially turns Milky: Formation of insoluble $CaCO_3\downarrow$.
- Excess $CO_2$: Milkiness disappears! Forms soluble Calcium bicarbonate $Ca(HCO_3)_2$.
Evolves $H_2S$ gas, which has the distinct smell of rotten eggs.
- Lead Acetate Paper: Turns Black ($PbS\downarrow$).
- Sodium Nitroprusside: Turns solution Deep Purple/Violet ($Na_4[Fe(CN)_5NOS]$).
Evolves reddish-brown fumes of $NO_2$ gas immediately with dilute acid.
Trap: It gives the Brown Ring test directly with Dilute Acid (unlike Nitrate which requires Conc. Acid).
3. Group II: Conc. Acid Group Tests
These require heating the solid salt with concentrated $H_2SO_4$.
A. The Chromyl Chloride Test (For Chloride, $Cl^-$)
This is the definitive test for solid chlorides. It involves a beautiful three-step color change sequence.
Not all chlorides respond to this test. Covalent or highly insoluble chlorides will NOT give red vapors of $CrO_2Cl_2$.
Note: Bromides and Iodides also do not give this test (they release $Br_2$ or $I_2$ instead of forming chromyl compounds).
B. The Brown Ring Test (For Nitrate, $NO_3^-$)
To an aqueous solution of the nitrate salt, add freshly prepared $FeSO_4$ solution. Then, carefully pour Concentrated $H_2SO_4$ down the side of the test tube.
Nitric acid oxidizes $Fe^{2+}$ to $Fe^{3+}$, and is itself reduced to Nitric Oxide ($NO$). The $NO$ gas then coordinates with the remaining $[Fe(H_2O)_6]^{2+}$ to form the brown ring complex at the junction of the two liquids.
(Pentaaquanitrosyliron(I) ion)
In the brown ring complex, the NO ligand is formally attached as the Nitrosonium ion ($NO^+$).
To balance the overall +2 charge of the complex, the Iron atom must be in the highly unusual +1 oxidation state ($Fe^+$).
C. The Layer Test (For $Br^-$ and $I^-$)
If conc. $H_2SO_4$ is added to bromides/iodides, they release $Br_2$ (reddish-brown) and $I_2$ (violet) gases, but it can be hard to distinguish. The Layer Test is much cleaner.
Add a few drops of Chlorine water ($Cl_2$) to the salt solution, followed by an organic solvent like $CHCl_3$ or $CS_2$. Shake well.
- Chlorine displaces $Br_2$ or $I_2$ because it is more reactive ($Cl_2 + 2Br^- \rightarrow 2Cl^- + Br_2$).
- The halogens are non-polar and dissolve preferentially in the organic layer (which sinks to the bottom).
- Orange/Brown Layer: Confirms Bromide ($Br_2$).
- Violet Layer: Confirms Iodide ($I_2$).
4. Group III: Independent Group ($SO_4^{2-}$, $PO_4^{3-}$)
These anions do not yield gases with acids. They are detected by specific precipitation reactions.
Add Barium Chloride ($BaCl_2$) solution.
Distinction Trap:
Sulfite ($SO_3^{2-}$) also gives a white ppt with $BaCl_2$. However, $BaSO_4$ is insoluble in Conc. HCl, whereas $BaSO_3$ dissolves and releases $SO_2$ gas.
Boil the extract with concentrated $HNO_3$ and add Ammonium Molybdate solution.
Ammonium Phosphomolybdate:
$(NH_4)_3PO_4 \cdot 12MoO_3$
NEET Grand Test: Acidic Radicals
15 High-Yield Questions testing precipitation colors, Brown Ring logic, and Chromyl Chloride exceptions.
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