Detection of Elements — Revision Notes

⚡ 30-Second Revision

Lassaigne's Test Principle: — Convert covalent N, S, X to ionic $NaCN$, $Na_2S$, $NaX$.
Nitrogen: — Lassaigne's extract + $FeSO_4$ (fresh) + $FeCl_3$ + dil. $HCl \rightarrow$ Prussian Blue ($Fe_4[Fe(CN)_6]_3$).
Sulfur:

- Lassaigne's extract + dil. $CH_3COOH$ + $(CH_3COO)_2Pb \rightarrow$ Black $PbS$. - Lassaigne's extract + Sodium Nitroprusside ($Na_2[Fe(CN)_5NO]$) $\rightarrow$ Violet color ($[Fe(CN)_5NOS]^{4-}$).

Halogens: — Lassaigne's extract + dil. $HNO_3$ (boil) + $AgNO_3$.

- $Cl$: White ppt ($AgCl$), soluble in dil. $NH_4OH$. - $Br$: Pale yellow ppt ($AgBr$), sparingly soluble in conc. $NH_4OH$. - $I$: Yellow ppt ($AgI$), insoluble in $NH_4OH$.

N & S (together): — Lassaigne's extract + $FeCl_3 \rightarrow$ Blood-red color ($[Fe(SCN)]^{2+}$).
Phosphorus: — Compound + Oxidizing agent ($conc. HNO_3$) $\rightarrow PO_4^{3-}$. Then $PO_4^{3-}$ + Ammonium Molybdate $\rightarrow$ Canary Yellow ppt ($(NH_4)_3PO_4 \cdot 12MoO_3$).

2-Minute Revision

Qualitative detection of elements in organic compounds primarily focuses on identifying nitrogen, sulfur, halogens, and phosphorus. The cornerstone for N, S, and halogens is Lassaigne's test, where the organic compound is fused with sodium metal. This converts the covalently bonded heteroatoms into water-soluble ionic sodium salts (NaCN, Na₂S, NaX). The resulting aqueous extract is then used for specific tests.

For nitrogen, the extract reacts with ferrous sulfate and ferric chloride in acidic medium to form a characteristic Prussian blue precipitate ($Fe_4[Fe(CN)_6]_3$). Sulfur is detected by either a black precipitate of lead sulfide ($PbS$) with lead acetate or a violet coloration with sodium nitroprusside.

Halogens are identified after acidifying the extract with nitric acid (to remove interfering cyanide and sulfide ions) and adding silver nitrate. Chlorine gives a white precipitate ($AgCl$) soluble in ammonium hydroxide, bromine a pale yellow ($AgBr$) sparingly soluble, and iodine a yellow ($AgI$) insoluble precipitate.

If both nitrogen and sulfur are present, they form sodium thiocyanate ($NaSCN$), which gives a blood-red color with ferric chloride. Phosphorus detection is separate, involving oxidation to phosphate ions, followed by reaction with ammonium molybdate to yield a canary yellow precipitate.

5-Minute Revision

The detection of elements in organic compounds is a critical step in their characterization. Organic compounds primarily contain carbon and hydrogen, but often include heteroatoms like nitrogen, sulfur, halogens (Cl, Br, I), and phosphorus. Since these are covalently bonded, they must first be converted into ionic forms for detection.

Lassaigne's Test (Sodium Fusion): This is the universal method for N, S, and halogens. The organic compound is heated with sodium metal, converting the elements into ionic sodium salts:

Nitrogen $\rightarrow$ Sodium cyanide ($NaCN$)
Sulfur $\rightarrow$ Sodium sulfide ($Na_2S$)
Halogens $\rightarrow$ Sodium halides ($NaX$)
If N and S both present $\rightarrow$ Sodium thiocyanate ($NaSCN$)

The fused mass is then plunged into water, boiled, and filtered to obtain Lassaigne's extract.

1. Detection of Nitrogen:

* Reagents: Lassaigne's extract, freshly prepared $FeSO_4$, $FeCl_3$, dilute $HCl$. * Reactions: $CN^- + Fe^{2+} \rightarrow [Fe(CN)_6]^{4-}$; $3[Fe(CN)_6]^{4-} + 4Fe^{3+} \rightarrow Fe_4[Fe(CN)_6]_3 \downarrow$ * Observation: Deep blue precipitate (Prussian Blue).

2. Detection of Sulfur:

* Lead Acetate Test: Extract + $CH_3COOH$ + $(CH_3COO)_2Pb \rightarrow$ Black precipitate of $PbS$. * Sodium Nitroprusside Test: Extract + $Na_2[Fe(CN)_5NO] \rightarrow$ Deep violet coloration ($[Fe(CN)_5NOS]^{4-}$).

3. Detection of Halogens (Cl, Br, I):

* Procedure: Extract + dilute $HNO_3$ (boil to remove $CN^-, S^{2-}$ interference) + $AgNO_3$. * Observations: * Chlorine: White precipitate ($AgCl$), soluble in dilute $NH_4OH$. * Bromine: Pale yellow precipitate ($AgBr$), sparingly soluble in concentrated $NH_4OH$. * Iodine: Yellow precipitate ($AgI$), insoluble in $NH_4OH$.

4. Detection of Nitrogen and Sulfur (together):

* Reagents: Lassaigne's extract, $FeCl_3$. * Reaction: $SCN^- + Fe^{3+} \rightarrow [Fe(SCN)]^{2+}$ * Observation: Blood-red coloration.

5. Detection of Phosphorus:

* Procedure: Organic compound + Oxidizing agent (e.g., $conc. HNO_3$) $\rightarrow$ Phosphate ions ($PO_4^{3-}$). Then $PO_4^{3-}$ + Ammonium Molybdate solution. * Reaction: $PO_4^{3-} + 12(NH_4)_2MoO_4 + 21HNO_3 \rightarrow (NH_4)_3PO_4 \cdot 12MoO_3 \downarrow + 21NH_4NO_3 + 12H_2O$ * Observation: Canary yellow precipitate (Ammonium Phosphomolybdate).

Remember the specific reagents, the characteristic colors/precipitates, and the purpose of each step, especially interference removal.

Prelims Revision Notes

1
Lassaigne's Test Principle: — Converts covalently bonded N, S, Halogens into ionic forms ($NaCN$, $Na_2S$, $NaX$) by fusion with sodium metal. This allows for detection using standard inorganic tests.
2
Nitrogen Detection (Prussian Blue Test):

* Reagents: Lassaigne's extract, freshly prepared $FeSO_4$, $FeCl_3$, dilute $HCl$. * Intermediate: Sodium ferrocyanide ($Na_4[Fe(CN)_6]$). * Final Product: Ferric ferrocyanide ($Fe_4[Fe(CN)_6]_3$). * Observation: Deep blue precipitate (Prussian Blue).

1
Sulfur Detection:

* Lead Acetate Test: Extract + acetic acid + lead acetate solution $\rightarrow$ Black precipitate of $PbS$. * Sodium Nitroprusside Test: Extract + sodium nitroprusside solution $\rightarrow$ Deep violet coloration ($[Fe(CN)_5NOS]^{4-}$).

1
Halogen Detection (Silver Nitrate Test):

* Interference Removal: Boil Lassaigne's extract with dilute $HNO_3$ to decompose $NaCN$ and $Na_2S$ (as $HCN \uparrow$ and $H_2S \uparrow$). This prevents false positives. * Reagent: $AgNO_3$ solution.

* Chlorine (Cl): White precipitate of $AgCl$, soluble in dilute $NH_4OH$. * Bromine (Br): Pale yellow precipitate of $AgBr$, sparingly soluble in concentrated $NH_4OH$. * Iodine (I): Yellow precipitate of $AgI$, insoluble in $NH_4OH$.

* Fluorine (F): Not detected by this method as $AgF$ is soluble.

1
Simultaneous N and S Detection:

* Formation: $NaSCN$ (sodium thiocyanate) during fusion. * Reagent: $FeCl_3$. * Observation: Blood-red coloration ($[Fe(SCN)]^{2+}$ complex).

1
Phosphorus Detection:

* Method: Oxidize organic compound to phosphate ions ($PO_4^{3-}$), typically with $conc. HNO_3$ or $Na_2O_2$. * Reagent: Ammonium molybdate solution. * Observation: Canary yellow precipitate of ammonium phosphomolybdate ($(NH_4)_3PO_4 \cdot 12MoO_3$).

1
Safety: — Sodium metal is highly reactive with water; handle with care. Fusion involves high temperatures.

Vyyuha Quick Recall

Nice Salty Halogens Please!

Nitrogen: Prussian Blue (Prussian Blue)
Sulfur: Black Lead (Black PbS) or Very Nice (Violet Nitroprusside)
Halogens: White, Pale, Yellow (AgCl, AgBr, AgI) - Soluble, Sparingly, Insoluble (in $NH_4OH$)
Phosphorus: Canary Yellow Molybdate (Ammonium Phosphomolybdate)