Salt Analysis: Basic Radicals
The architecture of precipitation. Master the Common Ion Effect, $K_{sp}$ limits, and the exact sequence of Group Reagents used to isolate and identify cations.
Module Focus: The Solubility Product ($K_{sp}$)
The systematic analysis of basic radicals (cations) is entirely a game of Ionic Equilibrium. We divide cations into six groups based on their solubility products. A cation will precipitate only when its Ionic Product exceeds its $K_{sp}$. By carefully controlling the concentration of the precipitating reagent using the Common Ion Effect, we can force specific groups to precipitate while leaving others dissolved.
1. The Analytical Cascade (Group Classification)
The analysis must be done in strict sequential order. If you skip a group, the reagents of later groups will incorrectly precipitate the cations of earlier groups.
| Group | Cations | Group Reagent | Precipitate Form |
|---|---|---|---|
| Zero (0) | $NH_4^+$ | NaOH (Heat) or Nessler's | Gas / Brown ppt |
| I | $Pb^{2+}, Ag^+, Hg_2^{2+}$ | Dilute HCl | Chlorides |
| II | $Cu^{2+}, Pb^{2+}, As^{3+}$ | $H_2S$ gas in Dil. HCl | Sulfides |
| III | $Fe^{3+}, Al^{3+}, Cr^{3+}$ | $NH_4OH$ in presence of solid $NH_4Cl$ | Hydroxides |
| IV | $Zn^{2+}, Mn^{2+}, Ni^{2+}, Co^{2+}$ | $H_2S$ gas in presence of $NH_4OH$ | Sulfides |
| V | $Ba^{2+}, Sr^{2+}, Ca^{2+}$ | $(NH_4)_2CO_3$ in $NH_4OH + NH_4Cl$ | Carbonates |
| VI | $Mg^{2+}$ | $Na_2HPO_4$ + $NH_4OH$ | Phosphates |
2. The NEET Logic Traps ($K_{sp}$)
Group II uses $H_2S$ to precipitate sulfides. However, Group IV also precipitates as sulfides. Group II sulfides have a very low $K_{sp}$, while Group IV sulfides have a high $K_{sp}$.
$H_2S \rightleftharpoons 2H^+ + S^{2-}$ (Weak)
The high $[H^+]$ from HCl suppresses the ionization of $H_2S$ (Common Ion Effect). This drops the $[S^{2-}]$ drastically. It becomes just enough to exceed the low $K_{sp}$ of Group II ($CuS, PbS$), but not enough to precipitate Group IV ($ZnS, NiS$).
Group III uses $NH_4OH$ to precipitate hydroxides. However, Magnesium (Group VI) also forms a hydroxide ($Mg(OH)_2$).
$NH_4OH \rightleftharpoons NH_4^+ + OH^-$ (Weak)
Adding solid $NH_4Cl$ creates a massive excess of $NH_4^+$, which suppresses the ionization of $NH_4OH$ (Common Ion Effect). The $[OH^-]$ drops significantly. It is enough to precipitate the low $K_{sp}$ Group III hydroxides ($Fe(OH)_3$), but prevents the precipitation of $Mg(OH)_2$.
3. Iconic Confirmatory Tests
Ammonium salts heated with NaOH release $NH_3$ gas. The gas is passed into Nessler's Reagent ($K_2[HgI_4]$ + KOH).
Iodide of Millon's Base (Brown Precipitate)
Lead chloride ($PbCl_2$) precipitates in Group I. However, it is slightly soluble in cold water and highly soluble in hot water. Because it doesn't precipitate completely in Group I, the remaining $Pb^{2+}$ passes into Group II and precipitates as black $PbS$.
Dissolve the black $CuS$ ppt in $HNO_3$. Add excess $NH_4OH$ to get a deep blue solution ($[Cu(NH_3)_4]^{2+}$). Neutralize with Acetic Acid and add Potassium ferrocyanide.
Cupric ferrocyanide (Chocolate Brown Precipitate)
- Iron: Red-brown ppt of $Fe(OH)_3$. Dissolve in HCl. Add Potassium thiocyanate ($KSCN$) $\rightarrow$ Blood Red color ($[Fe(SCN)]^{2+}$).
- Aluminum: Gelatinous white ppt of $Al(OH)_3$. It is amphoteric, so it dissolves in excess NaOH to form soluble Sodium meta-aluminate ($NaAlO_2$).
Nickel forms a black sulfide. Dissolve in Aqua Regia. Make alkaline with $NH_4OH$ and add Dimethylglyoxime (DMG).
Zinc forms a White sulfide ($ZnS$), while Manganese forms a Buff/Flesh-colored sulfide ($MnS$).
4. Group V: The Flame Tests
Group V cations ($Ba^{2+}, Sr^{2+}, Ca^{2+}$) precipitate as carbonates. They are uniquely identified by their characteristic flame colors when the salt is made into a paste with concentrated HCl and introduced into a non-luminous Bunsen flame.
Apple Green
Crimson Red
Brick Red
NEET Grand Test: Basic Radicals
15 High-Yield Questions testing common ion logic, exact precipitate formulas, and visual color confirmations.
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