🔍 Qualitative Inorganic Salt Analysis

Qualitative analysis provides the forensic experimental foundation of inorganic chemistry. High-yield exam problems center on group reagent solubility product thresholds ($K_{sp}$ vs $Q_{sp}$), confirmatory color complexes, interfering anions (Oxalate, Phosphate, Borate, Fluoride), and amphoteric hydroxide redissolution.

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1. 🗂️ Systematic Cation Group Separation Scheme

Group	Cations Present	Group Reagent	Precipitated Form	Chemical & $K_{sp}$ Principle
0	$N{H}_4^{+}$	$\text{NaOH solution, }\mathrm{\Delta }$	$N{H}_3\uparrow$ gas	Tested with Nessler's reagent $\to$ Brown ppt of Millon's base ($HgO\cdot Hg(N{H}_2)I$)
I	$P{b}^{2+},A{g}^{+},H{g}_2^{2+}$	Dilute $HCl$	Chlorides ($PbC{l}_2,AgCl,H{g}_{2}C{l}_2$)	Low $K_{sp}$ of chlorides; $PbC{l}_2$ dissolves in hot water
II (A & B)	IIA: $P{b}^{2+},C{u}^{2+},B{i}^{3+},C{d}^{2+},H{g}^{2+}$ IIB: $A{s}^{3+},S{b}^{3+},S{n}^{2+},S{n}^{4+}$	$H_{2}S$ gas in presence of dilute $HCl$	Sulfides ($CuS,CdS,A{s}_2{S}_3,\dots$)	Common Ion Effect of $H^{+}$ from $HCl$ suppresses $[S^{2-}]$, precipitating only very low $K_{sp}$ sulfides
III	$F{e}^{3+},A{l}^{3+},C{r}^{3+}$	$N{H}_{4}OH$ in presence of solid $N{H}_{4}Cl$	Hydroxides ($Fe(OH{)}_3$ red-brown, $Al(OH{)}_3$ gelatinous white, $Cr(OH{)}_3$ green)	Common Ion Effect of $N{H}_4^{+}$ suppresses $[O{H}^{-}]$, preventing precipitation of higher $K_{sp}$ Group IV/V hydroxides
IV	$Z{n}^{2+},M{n}^{2+},C{o}^{2+},N{i}^{2+}$	$H_{2}S$ gas in presence of $N{H}_{4}OH+N{H}_{4}Cl$	Sulfides ($ZnS$ white, $MnS$ flesh/buff, $CoS$ black, $NiS$ black)	Basic medium increases $[S^{2-}]$ sufficiently to exceed higher $K_{sp}$ of Group IV sulfides
V	$B{a}^{2+},S{r}^{2+},C{a}^{2+}$	$(N{H}_4{)}_{2}C{O}_3$ in presence of $N{H}_{4}OH+N{H}_{4}Cl$	Carbonates ($BaC{O}_3,SrC{O}_3,CaC{O}_3$)	White precipitates; separated using acetic acid & potassium chromate
VI	$M{g}^{2+}$	$N{a}_{2}HP{O}_4$ in presence of $N{H}_{4}OH$	White crystalline $Mg(N{H}_4)P{O}_4$	High solubility prevents earlier precipitation

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2. 🧪 Classic Confirmatory Color Reactions Master Matrix

2.1 Cations Confirmatory Tests

1. $C{u}^{2+}$ (Cupric Ion):
   • With excess $N{H}_{4}OH$: Deep intense Prussian/Azure blue solution of $[Cu(N{H}_3{)}_4{]}^{2+}$.
   • With Potassium Ferrocyanide $K_4[Fe(CN{)}_6]$: Chocolate brown precipitate of $C{u}_2[Fe(CN{)}_6]$.
2. $F{e}^{3+}$ (Ferric Ion):
   • With Potassium Ferrocyanide $K_4[Fe(CN{)}_6]$: Prussian Blue precipitate of $F{e}_4[Fe(CN{)}_6{]}_3$.
   • With Potassium Thiocyanate $KSCN$: Blood-red solution of $[Fe(SCN)(H_{2}O{)}_5{]}^{2+}$.
3. $F{e}^{2+}$ (Ferrous Ion):
   • With Potassium Ferricyanide $K_3[Fe(CN{)}_6]$: Turnbull's Blue precipitate of $F{e}_3[Fe(CN{)}_6{]}_2$.
4. $N{i}^{2+}$ (Nickel Ion):
   • With Dimethylglyoxime (DMG) in ammoniacal medium: Rosy-red precipitate of $[Ni(DMG{)}_2]$ (Square planar, stabilized by intramolecular H-bonding).
5. $C{o}^{2+}$ (Cobalt Ion):
   • With $\text{solid }KN{O}_2+C{H}_{3}COOH$: Yellow precipitate (Fischer's Salt) $K_3[Co(N{O}_2{)}_6]$.
6. $P{b}^{2+}$ (Lead Ion):
   • With $KI$ solution: Golden yellow precipitate of $Pb{I}_2$ (dissolves on boiling, reappears as glittering spangles on cooling — "Golden Spangles Test").

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3. 🧪 Anions Classification & Detection

3.1 Class A: Dilute $H_{2}S{O}_4$ Group (Volatile Gases)

• Carbonate ($C{O}_3^{2-}$): Effervescence of colorless, odorless $C{O}_2$ gas $ o$ turns lime water milky.
• Sulfite ($S{O}_3^{2-}$): Choking smell of burning sulfur ($S{O}_2$ gas) $ o$ turns acidified $K_{2}C{r}_2{O}_7$ filter paper from orange to green ($C{r}^{3+}$).
• Sulfide ($S^{2-}$): Rotten egg smell ($H_{2}S$ gas) $ o$ turns lead acetate paper shiny black ($PbS$), gives purple color with sodium nitroprusside ($[Fe(CN{)}_{5}NOS{]}^{4-}$).
• Nitrite ($N{O}_2^{-}$): Pungent reddish-brown fumes ($N{O}_2$ gas) $ o$ turns starch-iodide paper blue.

3.2 Class B: Concentrated $H_{2}S{O}_4$ Group

• Chloride ($C{l}^{-}$): Pungent white fumes of $HCl$ (dense white fumes with $N{H}_{4}OH$) $ o$ confirmed by Chromyl Chloride Test ($Cr{O}_{2}C{l}_2$).
• Bromide ($B{r}^{-}$): Reddish-brown vapors ($B{r}_2$) $ o$ dissolves in $C{S}_2/CC{l}_4$ to give orange-red layer.
• Iodide ($I^{-}$): Violet vapors ($I_2$) $ o$ dissolves in $C{S}_2/CC{l}_4$ to give violet layer.
• Nitrate ($N{O}_3^{-}$): Brown Ring Test:$$\text{Aqueous nitrate solution}+\text{freshly prepared }FeS{O}_4+\text{conc. }{H}_{2}S{O}_4\text{ along test-tube wall}\to \text{Brown Ring of }\mathbf{[}\mathbf{Fe}\mathbf{(}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}{\mathbf{)}}_{\mathbf{5}}\mathbf{(}\mathbf{NO}\mathbf{)}\mathbf{]}{\mathbf{SO}}_{\mathbf{4}}$$(In the brown ring complex $[Fe(H_{2}O{)}_5(NO){]}^{2+}$, Iron is in $+1$ oxidation state and $NO$ is $N{O}^{+}$ with 3 unpaired electrons $impliesmu=3.87extBM$).