JEE Mains Chemistry · The d- and f-Block Elements

Qualitative Analysis of Ions

Cations are separated into groups by a sequence of group reagents and then confirmed by a coloured precipitate or complex, while borax beads, the brown ring and a few named anion tests cover the rest of the salt-analysis scheme.

All The d- and f-Block Elements notes JEE Mains Chemistry strategy

Why this matters

Thirty-five PYQs, the largest page, twenty-nine of them multiple choice, and one from 2026. Nine place a cation in its analytical group by the group reagent; sixteen are confirmatory tests and the colour of each product, led by potassium ferrocyanide, dimethylglyoxime and Nessler's reagent; ten are borax beads, anion tests such as the brown ring, and the preparation of Mohr's salt. It is almost all recall: the three tables below are the page.

Concept 1 of 3: Cation groups and their group reagents

The scheme precipitates cations a group at a time, from the least soluble compounds to the most. Hydrogen sulphide in acid gives only a trace of sulphide ion, enough to bring down only the very insoluble group II sulphides. In ammonia the sulphide ion concentration is far higher, and the group IV sulphides come down too. Ammonium chloride is the brake that keeps each group clean.

Definition

Group I: dilute HCl precipitates $\mathrm{PbCl_2}$ .
Group II: $\mathrm{H_2S}$ in dilute HCl. IIA (copper group): $\mathrm{Pb^{2+}}$ , $\mathrm{Cu^{2+}}$ , $\mathrm{Cd^{2+}}$ , $\mathrm{Hg^{2+}}$ , $\mathrm{Bi^{3+}}$ . IIB (arsenic group): $\mathrm{As^{3+}}$ , $\mathrm{Sb^{3+}}$ , $\mathrm{Sn^{2+}}$ .
Group III: $\mathrm{NH_4OH}$ with $\mathrm{NH_4Cl}$ precipitates hydroxides of $\mathrm{Fe^{3+}}$ , $\mathrm{Al^{3+}}$ , $\mathrm{Cr^{3+}}$ . $\mathrm{NH_4Cl}$ suppresses the $\mathrm{OH^{-}}$ concentration by the common-ion effect, so group IV and V hydroxides stay in solution.
Group IV: $\mathrm{H_2S}$ in $\mathrm{NH_4OH}$ precipitates the sulphides of $\mathrm{Zn^{2+}}$ , $\mathrm{Mn^{2+}}$ , $\mathrm{Co^{2+}}$ , $\mathrm{Ni^{2+}}$ .
Group V: $\mathrm{(NH_4)_2CO_3}$ in $\mathrm{NH_4OH}$ precipitates the carbonates of $\mathrm{Ba^{2+}}$ , $\mathrm{Sr^{2+}}$ , $\mathrm{Ca^{2+}}$ .
Group VI: $\mathrm{Mg^{2+}}$ , tested with ammonium or disodium phosphate as white $\mathrm{MgNH_4PO_4}$ .
$\mathrm{NH_4^{+}}$ (group zero) is tested on the original salt.

Group	Cations	Group reagent	Precipitated as
Zero	$\mathrm{NH_4^{+}}$	No group reagent; heat with NaOH	Ammonia gas, confirmed with Nessler's reagent
I	$\mathrm{Pb^{2+}}$	Dilute HCl	White $\mathrm{PbCl_2}$
II	$\mathrm{Pb^{2+}}$ , $\mathrm{Cu^{2+}}$ , $\mathrm{Cd^{2+}}$ , $\mathrm{As^{3+}}$	$\mathrm{H_2S}$ in dilute HCl	Sulphides: PbS and CuS black, CdS and $\mathrm{As_2S_3}$ yellow Pb²⁺ shows up in group I and again in group II, because PbCl₂ is partly soluble.
III	$\mathrm{Fe^{3+}}$ , $\mathrm{Al^{3+}}$ , $\mathrm{Cr^{3+}}$	$\mathrm{NH_4OH}$ with $\mathrm{NH_4Cl}$	Hydroxides: $\mathrm{Fe(OH)_3}$ reddish-brown, $\mathrm{Al(OH)_3}$ white, $\mathrm{Cr(OH)_3}$ green
IV	$\mathrm{Zn^{2+}}$ , $\mathrm{Mn^{2+}}$ , $\mathrm{Co^{2+}}$ , $\mathrm{Ni^{2+}}$	$\mathrm{H_2S}$ in $\mathrm{NH_4OH}$	Sulphides: ZnS white, MnS buff, CoS and NiS black
V	$\mathrm{Ba^{2+}}$ , $\mathrm{Sr^{2+}}$ , $\mathrm{Ca^{2+}}$	$\mathrm{(NH_4)_2CO_3}$ in $\mathrm{NH_4OH}$	White carbonates
VI	$\mathrm{Mg^{2+}}$	No group reagent; ammonium phosphate	White $\mathrm{MgNH_4PO_4}$

Acidic H₂S catches only group II; alkaline H₂S catches group IV as well, which is why group II must be removed first.

Practice this conceptself-check · 5 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 4 Apr 2026 Shift 2 · Q44Moderate

Example 1 · The d- and f-Block Elements · Qualitative Analysis of Ions

Among

Fe^{3 +},Pb^{2 +},Cu^{2 +}

and

Mn^{2 +}

, identify the one that gets precipitated out while passing

H_{2}\text{ }S

in presence of

NH_{4}OH

as group reagent. The highest possible oxidation state of the corresponding metal is

Mn²⁺ is group IV, Fe³⁺ is group III

Both are d⁵ ions, but

\mathrm{Fe(OH)_3}

precipitates in group III while

\mathrm{Mn^{2+}}

waits for alkaline

\mathrm{H_2S}

as MnS. Do not group transition metal ions by their configuration.

Acid decides which sulphides come down

In dilute HCl the sulphide ion concentration is tiny, so only group II sulphides precipitate. Adding a group IV cation to that step catches nothing; it needs the ammonia step.

Drill 8 more on cation groups and their group reagents

Concept 2 of 3: Confirmatory tests and the colours they give

Once a group is isolated, each cation is proved by one reagent that gives it a unique colour. Most of these products are coordination compounds of d-block ions, which is why they sit in this chapter. Learn each test as a triple: ion, reagent, colour.

Definition

Potassium ferrocyanide, $\mathrm{K_4[Fe(CN)_6]}$ : $\mathrm{Cu^{2+}}$ chocolate-brown $\mathrm{Cu_2[Fe(CN)_6]}$ ; $\mathrm{Fe^{3+}}$ Prussian blue $\mathrm{Fe_4[Fe(CN)_6]_3}$ ; $\mathrm{Zn^{2+}}$ white or bluish-white zinc ferrocyanide.
With EXCESS ferrocyanide, $\mathrm{Fe^{3+}}$ gives the soluble, colloidal Prussian blue $\mathrm{KFe[Fe(CN)_6]}$ .
Dimethylglyoxime in ammonia: $\mathrm{Ni^{2+}}$ gives a brilliant red precipitate, $\mathrm{[Ni(dmg)_2]}$ , with two five-membered chelate rings.
Potassium nitrite in acetic acid: $\mathrm{Co^{2+}}$ gives yellow $\mathrm{K_3[Co(NO_2)_6]}$ ; cobalt is +3, low spin, 0 BM.
Thiocyanate: $\mathrm{Fe^{3+}}$ gives a blood-red $\mathrm{[Fe(SCN)]^{2+}}$ .
Nessler's reagent $\mathrm{K_2[HgI_4]}$ in KOH: $\mathrm{NH_4^{+}}$ gives a brown precipitate. The reagent contains K, Hg, I, O and H, but no N.
Ammonium molybdate in nitric acid: phosphate gives canary-yellow $\mathrm{(NH_4)_3PO_4 \cdot 12MoO_3}$ .
Sodium nitroprusside: sulphide gives a violet $\mathrm{[Fe(CN)_5NOS]^{4-}}$ .

Ion	Reagent	Observation	Product
$\mathrm{Cu^{2+}}$	$\mathrm{K_4[Fe(CN)_6]}$ in acetic acid	Chocolate-brown precipitate	$\mathrm{Cu_2[Fe(CN)_6]}$
$\mathrm{Fe^{3+}}$	$\mathrm{K_4[Fe(CN)_6]}$	Prussian blue precipitate	$\mathrm{Fe_4[Fe(CN)_6]_3}$
$\mathrm{Fe^{3+}}$	KSCN	Blood-red colour	$\mathrm{[Fe(SCN)]^{2+}}$
$\mathrm{Zn^{2+}}$	$\mathrm{K_4[Fe(CN)_6]}$ , after neutralising	White or bluish-white precipitate	$\mathrm{K_2Zn_3[Fe(CN)_6]_2}$
$\mathrm{Ni^{2+}}$	Dimethylglyoxime in $\mathrm{NH_4OH}$	Brilliant red precipitate	$\mathrm{[Ni(dmg)_2]}$ , five-membered chelate rings
$\mathrm{Co^{2+}}$	$\mathrm{KNO_2}$ in acetic acid	Yellow precipitate	$\mathrm{K_3[Co(NO_2)_6]}$
$\mathrm{Mn^{2+}}$	NaOH, then left in air	White precipitate turning brown	$\mathrm{MnO(OH)_2}$
$\mathrm{Mg^{2+}}$	Ammonium phosphate in $\mathrm{NH_4OH}$	White crystalline precipitate	$\mathrm{MgNH_4PO_4}$
$\mathrm{NH_4^{+}}$	Nessler's reagent, $\mathrm{K_2[HgI_4]}$ in KOH	Brown precipitate	Iodide of Millon's base
$\mathrm{PO_4^{3-}}$	Ammonium molybdate in $\mathrm{HNO_3}$	Canary-yellow precipitate	$\mathrm{(NH_4)_3PO_4 \cdot 12MoO_3}$
$\mathrm{S^{2-}}$	Sodium nitroprusside	Violet colour	$\mathrm{Na_4[Fe(CN)_5NOS]}$

Ferrocyanide alone confirms three cations: brown for copper, blue for iron(III), white for zinc.

Practice this conceptself-check · 5 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 2 Apr 2025 · Q41Moderate

Example 2 · The d- and f-Block Elements · Qualitative Analysis of Ions

Choose the correct tests with respective observations. (A)

{CuSO}_{4}

(acidified with acetic acid) +

K_{4}\left\lbrack Fe(CN)_{6} \right\rbrack \rightarrow

Chocolate brown precipitate. (B)

{FeCl}_{3} + K_{4}\left\lbrack Fe(CN)_{6} \right\rbrack \rightarrow

Prussian blue precipitate. (C)

{ZnCl}_{2} + K_{4}\left\lbrack Fe(CN)_{6} \right\rbrack

, neutralised with

{NH}_{4}OH

\rightarrow

White or bluish white precipitate. (D)

{MgCl}_{2} + K_{4}\left\lbrack Fe(CN)_{6} \right\rbrack \rightarrow

Blue precipitate. (E)

{BaCl}_{2} + K_{4}\left\lbrack Fe(CN)_{6} \right\rbrack

, neutralised with NaOH

\rightarrow

White precipitate. Choose the correct answer from the options given below :

The dimethylglyoxime rings are five-membered

Each glyoxime binds nickel through two nitrogens, closing a five-membered Ni–N–C–C–N ring. A statement calling it a six-membered chelate is false.

Nessler's reagent has no nitrogen

The reagent is

\mathrm{K_2[HgI_4]}

in KOH: potassium, mercury, iodine, and oxygen and hydrogen from the alkali. It detects nitrogen as

\mathrm{NH_4^{+}}

; it does not contain any.

Drill 15 more on confirmatory tests and the colours they give

Concept 3 of 3: Borax beads, anion tests and Mohr's salt

A borax bead is a glassy bead of sodium metaborate and boric anhydride. Fused with a trace of a coloured salt, it forms that metal's metaborate, whose colour depends on the metal and on whether the oxidising (outer) or reducing (inner) part of the flame was used. The anion tests work the same way: one reagent, one colour, and a reason behind it.

Definition

Borax bead: $\mathrm{Na_2B_4O_7 \cdot 10H_2O \rightarrow Na_2B_4O_7 \rightarrow 2NaBO_2 + B_2O_3}$ ; $\mathrm{B_2O_3}$ plus the metal oxide gives a coloured metaborate.
Brown ring (nitrate): add freshly prepared $\mathrm{FeSO_4}$ , then pour concentrated $\mathrm{H_2SO_4}$ down the side. $\mathrm{NO_3^{-}}$ is reduced to NO, which forms $\mathrm{[Fe(H_2O)_5(NO)]^{2+}}$ (nitrosoferrous sulphate). NO bonds as $\mathrm{NO^{+}}$ , so iron is +1.
Acetate: neutral $\mathrm{FeCl_3}$ gives a deep red colour; on boiling a brown-red basic ferric acetate precipitates.
Chloride: $\mathrm{AgNO_3}$ in dilute $\mathrm{HNO_3}$ gives curdy white AgCl, which dissolves in $\mathrm{NH_4OH}$ as $\mathrm{[Ag(NH_3)_2]Cl}$ .
Mohr's salt, $\mathrm{FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O}$ : dilute $\mathrm{H_2SO_4}$ is added to stop $\mathrm{FeSO_4}$ hydrolysing, and prolonged heating is avoided so that $\mathrm{Fe^{2+}}$ is not oxidised to $\mathrm{Fe^{3+}}$ .

Test	Conditions	Observation	Reason
Borax bead: Cu	Oxidising flame	Green when hot, blue when cold	Copper metaborate; red and opaque in the reducing flame
Borax bead: Fe	Oxidising and reducing flame	Yellowish-brown hot, yellow cold (oxidising); green (reducing)	Iron(III) metaborate; iron(II) in the reducing flame
Borax bead: Ni	Oxidising flame	Violet when hot, reddish-brown when cold	Nickel metaborate
Borax bead: Mn	Oxidising flame	Violet (amethyst), hot and cold	Manganese metaborate; colourless in the reducing flame
Borax bead: Co	Either flame	Blue, hot and cold	Cobalt metaborate
Borax bead: Cr	Either flame	Green, hot and cold	Chromium metaborate
Brown ring ( $\mathrm{NO_3^{-}}$ )	Fresh $\mathrm{FeSO_4}$ , then conc. $\mathrm{H_2SO_4}$ down the side	Brown ring where the layers meet	$\mathrm{[Fe(H_2O)_5(NO)]^{2+}}$ , Fe +1 The complex is nitrosoferrous sulphate.
Acetate ( $\mathrm{CH_3COO^{-}}$ )	Neutral $\mathrm{FeCl_3}$ , then boil	Deep red colour, then a brown-red precipitate	Basic ferric acetate, Fe +3
Chloride ( $\mathrm{Cl^{-}}$ )	$\mathrm{AgNO_3}$ in dilute $\mathrm{HNO_3}$ , then $\mathrm{NH_4OH}$	Curdy white precipitate that dissolves	AgCl, then $\mathrm{[Ag(NH_3)_2]Cl}$
Mohr's salt preparation	Dilute $\mathrm{H_2SO_4}$ added; no prolonged heating	Pale green crystals	Acid stops hydrolysis; heating would oxidise $\mathrm{Fe^{2+}}$

The bead colour depends on the metal AND on the part of the flame used.

Practice this conceptself-check · 5 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 11 April 2023 · Q33Moderate

Example 3 · The d- and f-Block Elements · Qualitative Analysis of Ions

When a solution of mixture having two inorganic salts was treated with freshly prepared ferrous sulphate in acidic medium, a dark brown ring was formed whereas on treatment with neutral

FeCl_{3}

, it gave deep red colour which disappeared on boiling and a brown red ppt was formed. The mixture contains

Iron in the brown ring is +1

The ring is

\mathrm{[Fe(H_2O)_5(NO)]^{2+}}

. NO bonds as

\mathrm{NO^{+}}

, so

x + 1 = +2

and

x = +1

. Treating NO as neutral gives +2, which is the common wrong option.

The acetate test needs NEUTRAL ferric chloride

Acid destroys the red iron acetate complex. The red colour, and the brown-red precipitate on boiling, appear only with neutral

\mathrm{FeCl_3}

Drill 9 more on borax beads, anion tests and mohr's salt

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (3)

Cation groups and their group reagents7 rows

Group	Cations	Group reagent	Precipitated as
Zero	$\mathrm{NH_4^{+}}$	No group reagent; heat with NaOH	Ammonia gas, confirmed with Nessler's reagent
I	$\mathrm{Pb^{2+}}$	Dilute HCl	White $\mathrm{PbCl_2}$
II	$\mathrm{Pb^{2+}}$ , $\mathrm{Cu^{2+}}$ , $\mathrm{Cd^{2+}}$ , $\mathrm{As^{3+}}$	$\mathrm{H_2S}$ in dilute HCl	Sulphides: PbS and CuS black, CdS and $\mathrm{As_2S_3}$ yellow Pb²⁺ shows up in group I and again in group II, because PbCl₂ is partly soluble.
III	$\mathrm{Fe^{3+}}$ , $\mathrm{Al^{3+}}$ , $\mathrm{Cr^{3+}}$	$\mathrm{NH_4OH}$ with $\mathrm{NH_4Cl}$	Hydroxides: $\mathrm{Fe(OH)_3}$ reddish-brown, $\mathrm{Al(OH)_3}$ white, $\mathrm{Cr(OH)_3}$ green
IV	$\mathrm{Zn^{2+}}$ , $\mathrm{Mn^{2+}}$ , $\mathrm{Co^{2+}}$ , $\mathrm{Ni^{2+}}$	$\mathrm{H_2S}$ in $\mathrm{NH_4OH}$	Sulphides: ZnS white, MnS buff, CoS and NiS black
V	$\mathrm{Ba^{2+}}$ , $\mathrm{Sr^{2+}}$ , $\mathrm{Ca^{2+}}$	$\mathrm{(NH_4)_2CO_3}$ in $\mathrm{NH_4OH}$	White carbonates
VI	$\mathrm{Mg^{2+}}$	No group reagent; ammonium phosphate	White $\mathrm{MgNH_4PO_4}$

Acidic H₂S catches only group II; alkaline H₂S catches group IV as well, which is why group II must be removed first.

Confirmatory tests and the colours they give11 rows

Ion	Reagent	Observation	Product
$\mathrm{Cu^{2+}}$	$\mathrm{K_4[Fe(CN)_6]}$ in acetic acid	Chocolate-brown precipitate	$\mathrm{Cu_2[Fe(CN)_6]}$
$\mathrm{Fe^{3+}}$	$\mathrm{K_4[Fe(CN)_6]}$	Prussian blue precipitate	$\mathrm{Fe_4[Fe(CN)_6]_3}$
$\mathrm{Fe^{3+}}$	KSCN	Blood-red colour	$\mathrm{[Fe(SCN)]^{2+}}$
$\mathrm{Zn^{2+}}$	$\mathrm{K_4[Fe(CN)_6]}$ , after neutralising	White or bluish-white precipitate	$\mathrm{K_2Zn_3[Fe(CN)_6]_2}$
$\mathrm{Ni^{2+}}$	Dimethylglyoxime in $\mathrm{NH_4OH}$	Brilliant red precipitate	$\mathrm{[Ni(dmg)_2]}$ , five-membered chelate rings
$\mathrm{Co^{2+}}$	$\mathrm{KNO_2}$ in acetic acid	Yellow precipitate	$\mathrm{K_3[Co(NO_2)_6]}$
$\mathrm{Mn^{2+}}$	NaOH, then left in air	White precipitate turning brown	$\mathrm{MnO(OH)_2}$
$\mathrm{Mg^{2+}}$	Ammonium phosphate in $\mathrm{NH_4OH}$	White crystalline precipitate	$\mathrm{MgNH_4PO_4}$
$\mathrm{NH_4^{+}}$	Nessler's reagent, $\mathrm{K_2[HgI_4]}$ in KOH	Brown precipitate	Iodide of Millon's base
$\mathrm{PO_4^{3-}}$	Ammonium molybdate in $\mathrm{HNO_3}$	Canary-yellow precipitate	$\mathrm{(NH_4)_3PO_4 \cdot 12MoO_3}$
$\mathrm{S^{2-}}$	Sodium nitroprusside	Violet colour	$\mathrm{Na_4[Fe(CN)_5NOS]}$

Ferrocyanide alone confirms three cations: brown for copper, blue for iron(III), white for zinc.

Borax beads, anion tests and Mohr's salt10 rows

Test	Conditions	Observation	Reason
Borax bead: Cu	Oxidising flame	Green when hot, blue when cold	Copper metaborate; red and opaque in the reducing flame
Borax bead: Fe	Oxidising and reducing flame	Yellowish-brown hot, yellow cold (oxidising); green (reducing)	Iron(III) metaborate; iron(II) in the reducing flame
Borax bead: Ni	Oxidising flame	Violet when hot, reddish-brown when cold	Nickel metaborate
Borax bead: Mn	Oxidising flame	Violet (amethyst), hot and cold	Manganese metaborate; colourless in the reducing flame
Borax bead: Co	Either flame	Blue, hot and cold	Cobalt metaborate
Borax bead: Cr	Either flame	Green, hot and cold	Chromium metaborate
Brown ring ( $\mathrm{NO_3^{-}}$ )	Fresh $\mathrm{FeSO_4}$ , then conc. $\mathrm{H_2SO_4}$ down the side	Brown ring where the layers meet	$\mathrm{[Fe(H_2O)_5(NO)]^{2+}}$ , Fe +1 The complex is nitrosoferrous sulphate.
Acetate ( $\mathrm{CH_3COO^{-}}$ )	Neutral $\mathrm{FeCl_3}$ , then boil	Deep red colour, then a brown-red precipitate	Basic ferric acetate, Fe +3
Chloride ( $\mathrm{Cl^{-}}$ )	$\mathrm{AgNO_3}$ in dilute $\mathrm{HNO_3}$ , then $\mathrm{NH_4OH}$	Curdy white precipitate that dissolves	AgCl, then $\mathrm{[Ag(NH_3)_2]Cl}$
Mohr's salt preparation	Dilute $\mathrm{H_2SO_4}$ added; no prolonged heating	Pale green crystals	Acid stops hydrolysis; heating would oxidise $\mathrm{Fe^{2+}}$

The bead colour depends on the metal AND on the part of the flame used.

Watch out for (6)

Mn²⁺ is group IV, Fe³⁺ is group III
→ Cation groups and their group reagents
Acid decides which sulphides come down
→ Cation groups and their group reagents
The dimethylglyoxime rings are five-membered
→ Confirmatory tests and the colours they give
Nessler's reagent has no nitrogen
→ Confirmatory tests and the colours they give
Iron in the brown ring is +1
→ Borax beads, anion tests and Mohr's salt
The acetate test needs NEUTRAL ferric chloride
→ Borax beads, anion tests and Mohr's salt

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