The cations with no group reagent
The systematic analysis of cations works by adding a sequence of group reagents to the original solution, each of which precipitates a defined set of ions. Group I falls as chloride with dilute HCl; Group II as sulphides in acidic H₂S; Group III as hydroxides with NH₄Cl and NH₄OH; Group IV as sulphides in alkaline H₂S; Group V as carbonates with NH₄OH and (NH₄)₂CO₃. After all five filtrations, the solution that remains carries the cations that none of those reagents could precipitate.
Four ions sit in this residual category. Magnesium is placed in Group VI, the last group, and the alkali metals sodium and potassium together with the ammonium ion form the zero group. The NCERT flow chart states this explicitly: when no precipitate forms in Group V, one takes the original solution to "test Group VI, Mg²⁺". Because there is no precipitating reagent, every one of these ions is confirmed by a direct reaction — a flame colour, a characteristic precipitate, or an evolved gas.
| Group | Cations | Group reagent | How detected |
|---|---|---|---|
| Zero group | NH₄⁺ | None | Warm with NaOH → NH₃ gas (tested first) |
| Group VI | Mg²⁺ | None | Na₂HPO₄ + NH₄OH → white ppt |
| Zero group | Na⁺ | None | Golden-yellow flame |
| Zero group | K⁺ | None | Lilac flame (through blue glass) |
A useful way to picture the whole scheme is as a funnel: each group reagent strips out one layer, and what trickles through the last filter is Group VI plus the zero-group ions. The schematic below traces that descent and shows where the final tests attach.
The five group reagents peel off Groups I–V; Group VI (Mg²⁺) and the zero-group alkali ions are confirmed in what is left. NH₄⁺, also zero group, is checked at the very start.
Why NH₄⁺ is tested first
The ammonium ion presents a logical problem. Several group reagents — NH₄Cl, NH₄OH, (NH₄)₂CO₃ — themselves contain ammonium. The moment the analyst begins precipitating Group III, ammonium is being poured into the mixture. Any test for NH₄⁺ run after that point would be meaningless, since the reagents would manufacture a false positive. The NCERT manual therefore places the zero-group ammonium test at the start, performed on the original salt (or its aqueous solution), before any group reagent is introduced.
The test itself is simple. A little of the salt is warmed with sodium hydroxide solution. If ammonium is present, ammonia gas is liberated, recognised first by its sharp smell and then by two confirmations described below. Because the alkali displaces the weak base from its salt, the reaction is general to all ammonium salts:
$$\ce{(NH4)2SO4 + 2NaOH ->[\Delta] Na2SO4 + 2NH3 ^ + 2H2O}$$
"Zero group" is not "Group zero precipitate"
The zero group has no group reagent and produces no precipitate at a group stage. NH₄⁺ is detected only by the gas test on the original solution; Na⁺ and K⁺ only by flame and confirmatory micro-precipitates. Do not look for a "zero-group precipitate" — there is none.
Order to remember: NH₄⁺ first (on original salt) → Groups I–V by reagents → Mg²⁺ (Group VI) and Na⁺/K⁺ last.
Confirming the ammonium ion
Once the smell of ammonia is detected, two independent confirmations follow. First, a glass rod dipped in concentrated hydrochloric acid is brought near the mouth of the test tube. Dense white fumes of ammonium chloride appear as the ammonia and hydrogen chloride vapours meet:
$$\ce{NH3 + HCl -> NH4Cl}$$
The same evolved gas also turns moist red litmus blue, since ammonia dissolves to give an alkaline solution. Second — and most sensitive — the gas is passed into Nessler's reagent, an alkaline solution of potassium tetraiodomercurate(II), K₂[HgI₄]. A brown colouration or precipitate of basic mercury(II) amido-iodide forms:
$$\ce{2K2[HgI4] + NH3 + 3KOH -> HgO\cdot Hg(NH2)I v + 7KI + 2H2O}$$
The brown product (often written as the iodide of Millon's base) is diagnostic; even faint traces of ammonium give a yellow-to-brown tint. Together, the litmus, white-fume and Nessler responses make NH₄⁺ one of the most secure identifications in the whole scheme.
The carbonate-precipitation logic that lets Mg²⁺ slip past Group V is covered in Cation Group V (Ba²⁺, Sr²⁺, Ca²⁺).
Magnesium — Group VI
Magnesium is the sole occupant of Group VI. It evades Group V because that group is precipitated as carbonates in the presence of ammonium salts. Ammonium ions suppress the carbonate-ion concentration through the equilibrium below, so the solubility product of magnesium carbonate is never reached and Mg²⁺ stays in solution:
$$\ce{NH4+ + CO3^2- <=> NH3 + HCO3-}$$
Magnesium is therefore detected on the solution remaining after Group V. The NCERT confirmatory test adds disodium hydrogen phosphate (Na₂HPO₄) in the presence of ammonium hydroxide, after which the inner walls of the test tube are scratched with a glass rod. A white crystalline precipitate of magnesium ammonium phosphate forms:
$$\ce{Mg^2+ + Na2HPO4 + NH4OH -> Mg(NH4)PO4 v + 2Na+ + H2O}$$
Two supporting tests reinforce the identification. Adding ammonium hydroxide directly to a magnesium solution gives a white gelatinous precipitate of magnesium hydroxide, sparingly soluble and best seen in the absence of excess ammonium salt:
$$\ce{Mg^2+ + 2NH4OH -> Mg(OH)2 v + 2NH4+}$$
And in the dry way, a white residue moistened with a drop of cobalt nitrate and strongly ignited gives a pale pink mass — magnesium gives the pink cobalt response, distinct from the blue (Al³⁺) and green (Zn²⁺) products, which is why the colour seen in the cobalt-nitrate test discriminates these white-residue cations.
Mg²⁺ is Group VI, not Group V
Examiners repeatedly pair Mg²⁺ with Ba²⁺/Ca²⁺/Sr²⁺ to bait the wrong group. The alkaline earths Ba²⁺, Sr²⁺, Ca²⁺ are Group V (precipitated as carbonates); Mg²⁺ is Group VI (no group reagent). The white precipitate that confirms Mg²⁺ is Mg(NH₄)PO₄, not a carbonate.
Cobalt nitrate colours: pink = Mg²⁺ · blue = Al³⁺ · green = Zn²⁺.
Sodium — golden-yellow flame
Sodium is identified primarily by its intense golden-yellow flame. Volatile sodium chloride formed on the platinum wire is excited in the non-luminous flame; the de-excitation of electrons emits the familiar yellow sodium-D light. The colour is persistent and so strong that even a trace of sodium impurity colours the flame — a property that becomes a complication when looking for potassium.
A wet confirmatory test is available where the flame is ambiguous. Adding zinc uranyl acetate reagent to a neutral or acetic-acid solution of a sodium salt slowly throws down a pale-yellow crystalline precipitate of sodium zinc uranyl acetate, $\ce{NaZn(UO2)3(CH3COO)9}$, on standing and scratching. The flame colour, however, remains the standard NEET marker for Na⁺.
Potassium — lilac through blue glass
Potassium gives a lilac (violet) flame, but the colour is faint and fleeting. The trouble is sodium: its golden-yellow emission, present even as an impurity, completely masks the delicate potassium colour to the naked eye. The remedy is optical — the flame is viewed through blue cobalt glass, which absorbs the yellow sodium light and transmits the crimson-violet of potassium. Through the glass, a sodium flame looks colourless while potassium shows as a clear pink-violet.
Cobalt-blue glass absorbs the yellow sodium light, so a masked potassium flame becomes visible as lilac. This is why K⁺ is always read "through blue glass".
Wet confirmation of potassium uses one of two reagents. With sodium cobaltinitrite, sodium hexanitritocobaltate(III), a yellow precipitate of potassium cobaltinitrite forms; alternatively, with chloroplatinic acid a yellow precipitate of potassium chloroplatinate, $\ce{K2[PtCl6]}$, separates:
$$\ce{2KCl + H2[PtCl6] -> K2[PtCl6] v + 2HCl}$$
Both yellow precipitates corroborate the flame observation, but for NEET the lilac-through-blue-glass flame remains the headline identifier of K⁺.
Observation → inference table
The four ions of this stage collapse neatly into a single observation-inference grid. Every entry is a direct test; none depends on a group precipitate.
| Cation | Test | Observation | Reaction product |
|---|---|---|---|
| NH₄⁺ (zero) | Warm with NaOH (on original salt) | NH₃ smell; moist red litmus → blue; white fumes with HCl rod | NH3, NH4Cl |
| NH₄⁺ (zero) | Gas into Nessler's reagent | Brown colouration / precipitate | HgO·Hg(NH2)I |
| Mg²⁺ (VI) | Na₂HPO₄ + NH₄OH, scratch tube | White crystalline precipitate | Mg(NH4)PO4 |
| Mg²⁺ (VI) | NH₄OH added | White gelatinous precipitate | Mg(OH)2 |
| Mg²⁺ (VI) | Cobalt nitrate + ignition | Pale pink mass | pink residue |
| Na⁺ (zero) | Flame test | Persistent golden-yellow | — |
| Na⁺ (zero) | Zinc uranyl acetate | Pale-yellow crystalline precipitate | NaZn(UO2)3(CH3COO)9 |
| K⁺ (zero) | Flame through blue glass | Lilac / crimson-violet | — |
| K⁺ (zero) | Sodium cobaltinitrite / H₂[PtCl₆] | Yellow precipitate | K2[PtCl6] |
Worked confirmation
A white salt is soluble in water. On warming with NaOH it gives a pungent gas that turns moist red litmus blue and a brown precipitate with Nessler's reagent. The original solution gives no precipitate through Groups I–V; with Na₂HPO₄ and NH₄OH it gives a white crystalline precipitate. Identify the ions present.
Step 1 — Zero group: The pungent gas, litmus change and brown Nessler product all point to ammonia, so the cation NH₄⁺ is present: $\ce{NH4+ ->[NaOH][\Delta] NH3}$, and $\ce{2K2[HgI4] + NH3 + 3KOH -> HgO\cdot Hg(NH2)I + 7KI + 2H2O}$.
Step 2 — Groups I–V absent: No precipitate with any group reagent rules out Pb²⁺, the sulphide groups, the hydroxide group and the carbonate group.
Step 3 — Group VI: The white crystalline precipitate with disodium hydrogen phosphate confirms Mg²⁺: $\ce{Mg^2+ + Na2HPO4 + NH4OH -> Mg(NH4)PO4 v + 2Na+ + H2O}$.
Conclusion: the salt contains NH₄⁺ (zero group) and Mg²⁺ (Group VI) — for example, a magnesium ammonium salt. Both cations belong to the no-group-reagent stage, which is exactly why a structured order of testing was essential.
Group VI & the zero group at a glance
- Four cations have no group reagent: Mg²⁺ (Group VI) and Na⁺, K⁺, NH₄⁺ (zero group).
- NH₄⁺ is tested first, on the original salt, because group reagents add ammonium and would give false positives later.
- NH₄⁺: warm with NaOH → NH₃ (smell, blue litmus, white fumes with HCl), brown precipitate with Nessler's reagent.
- Mg²⁺: white Mg(OH)₂ with NH₄OH; white crystalline Mg(NH₄)PO₄ with Na₂HPO₄; pink cobalt-nitrate residue.
- Na⁺: golden-yellow flame; yellow precipitate with zinc uranyl acetate.
- K⁺: lilac flame seen through blue cobalt glass; yellow K₂[PtCl₆] or cobaltinitrite precipitate.