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Chemistry · Salt Analysis

Cation Group IV — Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺ (H₂S in Ammoniacal Medium)

The fourth analytical group of the NCERT systematic scheme (Lab Manual, Unit 7) collects the four cations that slip past Groups II and III — cobalt, nickel, manganese and zinc. Their group reagent is hydrogen sulphide, but this time it is passed in an ammoniacal medium, and that single change in conditions is the conceptual heart of the group. NEET examiners test Group IV almost every cycle through matching and ordering questions, so the precise sulphide colours and the four confirmatory tests deserve careful study.

What Group IV Is

In the systematic separation of cations, the original solution is treated with a sequence of group reagents, each precipitating a defined set of ions before the filtrate passes on. Group I (Pb²⁺) falls as chloride with dilute HCl, Group II (Pb²⁺, Cu²⁺, As³⁺) as sulphides with H₂S in acidic medium, and Group III (Fe³⁺, Al³⁺) as hydroxides with NH₄OH in the presence of NH₄Cl. Group IV is the next net cast, and it catches four cations: cobalt (Co²⁺), nickel (Ni²⁺), manganese (Mn²⁺) and zinc (Zn²⁺).

These four are precipitated as their sulphides, exactly like Group II — yet they are a separate group. The distinction is not the precipitating species (sulphide in both cases) but the medium. Group II uses H₂S in dilute acid; Group IV uses H₂S after the solution has been made ammoniacal. Understanding why the same gas, under two different conditions, sorts ions into two groups is the single most examined idea in this topic.

GroupCations (in syllabus)Group ReagentPrecipitated As
IPb²⁺Dilute HClChloride
IIPb²⁺, Cu²⁺, As³⁺H₂S in presence of dilute HClSulphides
IIIFe³⁺, Al³⁺NH₄OH in presence of NH₄ClHydroxides
IVCo²⁺, Ni²⁺, Mn²⁺, Zn²⁺H₂S in presence of NH₄OHSulphides
VBa²⁺, Sr²⁺, Ca²⁺(NH₄)₂CO₃ in presence of NH₄OHCarbonates
VIMg²⁺None—

The Group Reagent: H₂S in Ammoniacal Medium

The Group IV reagent is hydrogen sulphide gas passed through the filtrate after it has been made alkaline with ammonium hydroxide in the presence of ammonium chloride carried over from Group III. Operationally, once Group III gives no precipitate, H₂S is passed for a few minutes through the ammoniacal solution. The appearance of a precipitate — white, black or flesh-coloured — signals the presence of Group IV cations.

The precipitating anion is the sulphide ion, supplied by the ionisation of the weak diprotic acid H₂S. The relevant equilibria are written compactly in mhchem:

$$\ce{H2S <=> 2H+ + S^{2-}}$$

For each metal, the precipitation is simply the metal ion meeting that sulphide ion:

$$\ce{Co^{2+} + S^{2-} -> CoS v}\qquad \ce{Ni^{2+} + S^{2-} -> NiS v}$$

$$\ce{Mn^{2+} + S^{2-} -> MnS v}\qquad \ce{Zn^{2+} + S^{2-} -> ZnS v}$$

Whether these arrows actually run forward depends entirely on how much S²⁻ the solution holds — which is set by the medium. That is the bridge to the next section.

Why the Basic Medium Now Works

A precipitate forms only when the ionic product of a sparingly soluble salt exceeds its solubility product (Ksp). For a metal sulphide MS, that condition is $[\ce{M^{2+}}][\ce{S^{2-}}] > K_{sp}$. The metal-ion concentration is roughly fixed by the sample, so the lever the chemist controls is the sulphide-ion concentration — and H₂S ionisation is governed by the common-ion effect.

In Group II, H₂S is passed in dilute acid. The high concentration of H⁺ pushes the equilibrium $\ce{H2S <=> 2H+ + S^{2-}}$ sharply to the left, so $[\ce{S^{2-}}]$ is kept very small. Only the sulphides with the smallest solubility products — CuS, PbS, As₂S₃ — can reach their Ksp at that tiny sulphide concentration and precipitate. The Group IV sulphides, which have higher solubility products, do not; they remain dissolved and pass on.

In Group IV, ammonium hydroxide is added first. The hydroxide ions consume H⁺, $\ce{H+ + OH- -> H2O}$, removing the species that was suppressing ionisation. With H⁺ withdrawn, the H₂S equilibrium shifts to the right and $[\ce{S^{2-}}]$ rises sharply. Now the ionic product of the higher-Ksp sulphides CoS, NiS, MnS and ZnS finally exceeds their Ksp, and they precipitate. The ammonium chloride present moderates the medium so that the rise in S²⁻ is controlled — enough to throw down Group IV but not so violent as to disturb the orderly scheme.

Figure 1 — Why the medium matters Group II — acidic (dil. HCl) high [H⁺] → low [S²⁻] H₂S ⇌ 2H⁺ + S²⁻ (pushed left) Only low-Kₛₚ sulphides ppt: CuS, PbS, As₂S₃ Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺ stay dissolved +NH₄OH Group IV — ammoniacal H⁺ removed → high [S²⁻] H₂S ⇌ 2H⁺ + S²⁻ (pushed right) Higher-Kₛₚ sulphides now ppt: CoS, NiS, MnS, ZnS ionic product now > Kₛₚ

The only variable that changes between the two groups is the sulphide-ion concentration, controlled by the common-ion effect of H⁺.

NEET Trap

Same reagent, different group — do not confuse it

Both Group II and Group IV use H₂S, so a careless reader writes "sulphide group" and stops. The examiner's point is the medium: acidic for Group II, ammoniacal (basic) for Group IV. Zinc sulphide in particular is precipitated only in Group IV — never in Group II — because its high solubility product needs the higher S²⁻ concentration that only the basic medium provides.

If asked "group reagent for Co²⁺/Ni²⁺/Mn²⁺/Zn²⁺" → H₂S in presence of NH₄OH.

Precipitation Flow and Sulphide Colours

When H₂S is passed into the ammoniacal solution, the colour of the sulphide precipitate is itself the first diagnostic clue. NCERT records four colours, and they map onto the cations as follows: a white precipitate points to Zn²⁺, a buff or flesh-coloured precipitate to Mn²⁺, and a black precipitate to either Ni²⁺ or Co²⁺. Because the two black sulphides cannot be told apart by colour, they are separated later by their confirmatory tests.

CationSulphideColour of PrecipitateInitial Inference
Zn²⁺ZnSWhiteZinc indicated
Mn²⁺MnSBuff / flesh-colouredManganese indicated
Ni²⁺NiSBlackNickel or cobalt indicated
Co²⁺CoSBlackNickel or cobalt indicated
Figure 2 — Group IV precipitation and confirmation flow Ammoniacal solution pass H₂S (NH₄OH medium) Sulphide precipitate — note colour White ZnS → Zn²⁺ Buff / flesh MnS → Mn²⁺ Black NiS / CoS → Ni²⁺ or Co²⁺ Zn²⁺ confirm NaOH → white, excess soluble; K₄[Fe(CN)₆] bluish-white Mn²⁺ confirm NaOH → white ppt Mn(OH)₂ turns brown in air Ni²⁺ confirm aqua regia, then DMG + NH₄OH → rosy-red ppt Co²⁺ confirm aqua regia, then KNO₂ + CH₃COOH → yellow ppt

The colour of the sulphide narrows the field; the confirmatory test on the right pins down the individual cation.

Confirmatory Test for Zn²⁺

A white sulphide indicates zinc. To confirm it, the precipitate is dissolved in dilute hydrochloric acid by boiling, which converts ZnS to soluble zinc chloride with the loss of H₂S:

$$\ce{ZnS + 2HCl -> ZnCl2 + H2S ^}$$

The solution is divided into two parts for two complementary checks.

(a) Sodium hydroxide test. Adding NaOH gives a white precipitate of zinc hydroxide that dissolves in excess NaOH on heating, because Zn(OH)₂ is amphoteric and forms soluble sodium zincate:

$$\ce{ZnCl2 + 2NaOH -> Zn(OH)2 v + 2NaCl}$$

$$\ce{Zn(OH)2 + 2NaOH -> Na2ZnO2 + 2H2O}$$

(b) Potassium ferrocyanide test. The second portion is neutralised with NH₄OH and treated with potassium ferrocyanide solution; a white to bluish-white precipitate of zinc ferrocyanide confirms zinc:

$$\ce{2ZnCl2 + K4[Fe(CN)6] -> Zn2[Fe(CN)6] v + 4KCl}$$

A further dry-test confirmation is the cobalt-nitrate test discussed below, which produces the green mass known as Rinmann's green.

Rinmann's green (dry confirmation for Zn²⁺)

In the charcoal-cavity test, a zinc salt leaves a residue that is yellow when hot and white when cold. When this white ZnO residue is moistened with cobalt nitrate solution and heated strongly in the non-luminous flame, the cobalt nitrate decomposes to cobalt(II) oxide, which combines with zinc oxide to give a green mass — Rinmann's green, CoO·ZnO:

$$\ce{2Co(NO3)2 ->[\Delta] 2CoO + 4NO2 ^ + O2 ^}$$

$$\ce{CoO + ZnO -> CoO.ZnO}\ \text{(green)}$$

The same cobalt-nitrate test gives a pink mass (CoO·MgO) with magnesium, which is why the colour, not just the appearance of a coloured oxide, is the decisive observation.

↺
Build the contrast

The acidic-medium counterpart of this group is where CuS and PbS fall first. Revisit Cation Group II to see exactly why the low-Ksp sulphides precipitate before Group IV ever begins.

Confirmatory Test for Mn²⁺

A buff or flesh-coloured sulphide indicates manganese. The MnS precipitate is dissolved in dilute HCl on boiling to give manganese chloride:

$$\ce{MnS + 2HCl -> MnCl2 + H2S ^}$$

Excess sodium hydroxide then precipitates white manganese(II) hydroxide, which is the diagnostic species — and crucially, it does not stay white:

$$\ce{MnCl2 + 2NaOH -> Mn(OH)2 v + 2NaCl}$$

On standing in air, the white Mn(OH)₂ is oxidised by atmospheric oxygen to brown hydrated manganese dioxide. The white-to-brown colour change on keeping is what confirms manganese:

$$\ce{Mn(OH)2 + [O] -> MnO(OH)2}\ \text{(brown)}$$

Manganese(II) is also the classic example of an ion that can be oxidised right up to purple permanganate (Mn in the +7 state) or to brown MnO₂, which is why "white precipitate that browns on exposure to air" is the phrasing examiners reward.

Confirmatory Test for Ni²⁺

A black sulphide may be either NiS or CoS, so the two are separated by their confirmatory reactions. For nickel, the black NiS is dissolved in aqua regia (a 1:3 mixture of conc. HNO₃ and conc. HCl), which oxidises the sulphide and yields soluble nickel chloride:

$$\ce{3NiS + 2HNO3 + 6HCl -> 3NiCl2 + 2NO ^ + 3S v + 4H2O}$$

The solution is evaporated to dryness to expel excess acid, the residue dissolved in water, and the solution made alkaline with NH₄OH. Addition of dimethylglyoxime (DMG) then gives a brilliant rosy-red precipitate of nickel dimethylglyoximate — the most distinctive single test in the whole group:

$$\ce{Ni^{2+} + 2C4H8N2O2 ->[\text{NH}_4\text{OH}] [Ni(C4H7N2O2)2] v + 2H+}$$

The bright rosy-red colour is unique to nickel; no other Group IV cation gives it, which makes the DMG test both the confirmation of Ni²⁺ and the practical means of distinguishing it from cobalt.

NEET Trap

DMG is for nickel, KNO₂ is for cobalt

Because Ni²⁺ and Co²⁺ both give black sulphides and both dissolve in aqua regia, students swap their confirmatory tests. Fix it by colour of the final precipitate: nickel + DMG → rosy-red; cobalt + potassium nitrite → yellow. The cobalt complex K₃[Co(NO₂)₆] is sometimes also called "potassium cobaltinitrite".

Rosy-red precipitate with DMG ⇒ Ni²⁺, never Co²⁺.

Confirmatory Test for Co²⁺

For cobalt, the black CoS is likewise dissolved in aqua regia, giving cobalt chloride with the evolution of nitrosyl chloride and sulphur:

$$\ce{CoS + HNO3 + 3HCl -> CoCl2 + NOCl ^ + S v + 2H2O}$$

The solution is taken to dryness, the residue dissolved in water, neutralised with NH₄OH and acidified with dilute acetic acid. Addition of solid potassium nitrite (KNO₂) then gives a yellow precipitate of potassium hexanitritocobaltate(III):

$$\ce{CoCl2 + 7KNO2 + 2CH3COOH -> K3[Co(NO2)6] v + 2KCl + 2CH3COOK + NO ^ + H2O}$$

Cobalt also gives the familiar blue borax bead in the dry tests, distinct from nickel's reddish-brown (oxidising flame) to grey (reducing flame) bead. The yellow KNO₂ precipitate, however, is the wet confirmatory test of record in the NCERT scheme.

Worked Confirmation and Observation Table

The following walkthrough shows how an examiner expects a Group IV identification to read from the precipitate to the named cation, citing only the NCERT tests.

Worked Confirmation

A salt solution gives no precipitate in Groups I, II and III. On passing H₂S after adding NH₄OH, a black precipitate forms. The black solid dissolves in aqua regia; the residue, dissolved in water and made alkaline with NH₄OH, gives a rosy-red precipitate with dimethylglyoxime. Identify the cation.

Step 1. The cation precipitates only in the ammoniacal H₂S step → it belongs to Group IV.

Step 2. A black sulphide narrows the choice to Ni²⁺ or Co²⁺ (both NiS and CoS are black).

Step 3. Aqua regia gives soluble chloride: $\ce{3NiS + 2HNO3 + 6HCl -> 3NiCl2 + 2NO + 3S + 4H2O}$.

Step 4. DMG in ammoniacal medium gives the rosy-red nickel dimethylglyoximate — a colour given by no other Group IV cation. The cation is Ni²⁺. (Had the confirmation instead been a yellow precipitate with KNO₂, the answer would have been Co²⁺.)

ObservationInference (cation)
Precipitate appears only on passing H₂S after adding NH₄OHGroup IV present (Co²⁺/Ni²⁺/Mn²⁺/Zn²⁺)
White sulphide; dissolved in HCl, NaOH gives white ppt soluble in excess; K₄[Fe(CN)₆] gives bluish-white pptZn²⁺
White residue + cobalt nitrate, heated → green mass (CoO·ZnO)Zn²⁺ (Rinmann's green)
Buff/flesh sulphide; NaOH gives white ppt that turns brown on keepingMn²⁺
Black sulphide; aqua regia residue + DMG (alkaline) → rosy-red pptNi²⁺
Black sulphide; aqua regia residue + KNO₂ (acetic acid) → yellow pptCo²⁺
Borax bead: blue (hot and cold)Co²⁺
Quick Recap

Group IV in one screen

  • Cations: Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺ — precipitated as sulphides.
  • Group reagent: H₂S in the presence of NH₄OH (ammoniacal / basic medium).
  • Why basic now works: removing H⁺ raises [S²⁻], so the higher-Ksp sulphides finally exceed their Ksp and precipitate.
  • Colours: ZnS white, MnS buff/flesh, NiS and CoS black.
  • Zn²⁺: Zn(OH)₂ soluble in excess NaOH (zincate); K₄[Fe(CN)₆] bluish-white; cobalt-nitrate test → green Rinmann's green.
  • Mn²⁺: white Mn(OH)₂ that browns in air to MnO(OH)₂.
  • Ni²⁺: DMG in NH₄OH → rosy-red. Co²⁺: KNO₂ in acetic acid → yellow K₃[Co(NO₂)₆]; blue borax bead.

NEET PYQ Snapshot — Cation Group IV

Group-placement questions dominate; know which cation sits in Group IV cold.

NEET 2025 · Q.67

Match List I (Ion) with List II (Group Number in Cation Analysis): A. Co²⁺, B. Mg²⁺, C. Pb²⁺, D. Al³⁺ with I. Group-I, II. Group-III, III. Group-IV, IV. Group-VI. Choose the correct answer.

  • (1) A-III, B-II, C-I, D-IV
  • (2) A-III, B-IV, C-II, D-I
  • (3) A-III, B-IV, C-I, D-II
  • (4) A-III, B-II, C-IV, D-I
Answer: (3)

Co²⁺ is a Group IV cation (matches III). Mg²⁺ is Group VI (IV), Pb²⁺ is Group I (I), and Al³⁺ is Group III (II). This gives A-III, B-IV, C-I, D-II.

NEET 2024 · Q.97

Using inorganic qualitative analysis, arrange the cations in increasing group number (0 to VI): A. Al³⁺, B. Cu²⁺, C. Ba²⁺, D. Co²⁺, E. Mg²⁺.

  • (1) B, A, D, C, E
  • (2) B, C, A, D, E
  • (3) E, C, D, B, A
  • (4) E, A, B, C, D
Answer: (1)

Group order: Cu²⁺ (II) < Al³⁺ (III) < Co²⁺ (IV) < Ba²⁺ (V) < Mg²⁺ (VI). Co²⁺ sitting in Group IV is the pivot of the sequence, giving B, A, D, C, E.

Concept · Group IV reagent

Which group reagent precipitates Zn²⁺ in systematic cation analysis, and why is Zn²⁺ not thrown down in Group II?

Key idea

Zn²⁺ is precipitated in Group IV by H₂S in the presence of NH₄OH. ZnS has a high Ksp; in Group II's acidic medium the H⁺ suppresses H₂S ionisation (common-ion effect), keeping [S²⁻] too low to exceed the Ksp of ZnS. The ammoniacal medium of Group IV raises [S²⁻] enough to precipitate it.

FAQs — Cation Group IV

The medium, the colours and the confirmatory tests examiners ask about.

What is the group reagent for Group IV cations?

The group reagent for Group IV is hydrogen sulphide (H2S) passed in the presence of ammonium hydroxide, that is in an ammoniacal (basic) medium. Under these conditions Co2+, Ni2+, Mn2+ and Zn2+ are precipitated as their sulphides — CoS and NiS (black), MnS (buff or flesh) and ZnS (white).

Why is H2S passed in basic medium for Group IV but in acidic medium for Group II?

H2S is a weak diprotic acid. In the acidic medium used for Group II, the high H+ concentration suppresses its ionisation by the common-ion effect, so the sulphide-ion concentration is very low and only the low-Ksp Group II sulphides precipitate. Group IV sulphides have higher solubility products, so they need a higher S2- concentration. Adding NH4OH neutralises H+, the ionisation of H2S increases, S2- rises sharply, and the ionic product of CoS, NiS, MnS and ZnS now exceeds their Ksp, so they precipitate.

What are the colours of the four Group IV sulphide precipitates?

CoS is black, NiS is black, MnS is buff or flesh-coloured, and ZnS is white. A black precipitate therefore indicates Co2+ or Ni2+, a flesh-coloured one indicates Mn2+, and a white one indicates Zn2+.

How is Ni2+ distinguished from Co2+ in Group IV?

Both give black sulphides that dissolve in aqua regia. Nickel is confirmed by adding dimethylglyoxime (DMG) to the ammoniacal solution, giving a brilliant rosy-red precipitate of nickel dimethylglyoximate. Cobalt is confirmed by adding solid potassium nitrite to the acetic-acid-acidified solution, giving a yellow precipitate of potassium hexanitritocobaltate(III), K3[Co(NO2)6].

What is Rinmann's green and how does it confirm zinc?

When the white residue (ZnO) from the charcoal cavity is moistened with cobalt nitrate solution and heated strongly in a non-luminous flame, cobalt nitrate decomposes to CoO which combines with ZnO to form a green mass, CoO·ZnO. This green colour is called Rinmann's green and confirms the presence of zinc.

Why is zinc sulphide not precipitated in Group II?

ZnS has a relatively high solubility product. In the acidic medium of Group II, the H+ ions suppress the ionisation of H2S (common-ion effect), keeping the S2- concentration so low that the ionic product of ZnS does not exceed its Ksp. Hence ZnS escapes Group II and is precipitated only in Group IV, where the basic medium raises the S2- concentration.

Related Topics
Salt Analysis — Chapter Hub All cation and anion groups in the systematic scheme. Cation Group II H₂S in acidic medium — the low-Ksp sulphides. Cation Group III Fe³⁺ and Al³⁺ as hydroxides with NH₄OH/NH₄Cl. Cation Group V Ba²⁺, Sr²⁺, Ca²⁺ precipitated as carbonates. Cation Group VI Mg²⁺ — the cation with no group reagent. Dry Heating Test Borax bead and charcoal cavity clues for Co, Ni, Zn.