Qualitative Inorganic Analysis
The detective work of chemistry — a systematic scheme of colours, gases and precipitates that names the ions hidden inside an unknown salt
- What acid radicals (anions) and basic radicals (cations) are, and the logic of a systematic scheme.
- The preliminary dry tests — flame, borax bead, charcoal cavity.
- How anions split into the dilute-acid and concentrated-acid groups, plus the brown ring test.
- The separation of cations into analytical Groups I–VI and their group reagents.
- Why ammonium chloride is added before Group III, via the common-ion effect.
- The confirmatory tests for the common cations and anions.
What Qualitative Analysis Does
Qualitative analysis answers a simple question — which ions are present? — without measuring how much. An inorganic salt splits into an acid radical (the anion, e.g. \(\ce{Cl-},\ \ce{SO4^2-}\)) and a basic radical (the cation, e.g. \(\ce{Na+},\ \ce{Cu^2+}\)). The art is a systematic sequence: tests are applied in an order that removes one possibility at a time, so the result is unambiguous.
Preliminary Dry Tests
Before any wet chemistry, a few quick observations on the dry salt narrow the field. Colour hints at the cation; heating may release a tell-tale gas; and the flame, borax bead and charcoal cavity tests give colours specific to certain metals.
| Observation | Likely ion |
|---|---|
| Blue / blue-green salt | \(\ce{Cu^2+}\) |
| Light green | \(\ce{Fe^2+}\) |
| Pink | \(\ce{Co^2+},\ \ce{Mn^2+}\) |
| Flame: golden yellow | \(\ce{Na+}\) |
| Flame: brick red | \(\ce{Ca^2+}\) |
| Flame: apple green | \(\ce{Ba^2+}\) |
| Borax bead: blue (hot & cold) | \(\ce{Co^2+}\) |
| Borax bead: green (hot) | \(\ce{Cr^3+}\) |
Detecting Acid Radicals (Anions)
Anions are sorted by their reaction with acids. The dilute-acid group evolves a gas with dilute \(\ce{H2SO4}\) or \(\ce{HCl}\); the concentrated-acid group needs concentrated \(\ce{H2SO4}\); a few anions are found by separate precipitation tests.
| Group | Anions | Reagent | Clue |
|---|---|---|---|
| Dilute acid | \(\ce{CO3^2-},\ \ce{SO3^2-},\ \ce{S^2-},\ \ce{NO2-}\) | dilute \(\ce{H2SO4}\) | gas evolved |
| Conc. acid | \(\ce{Cl-},\ \ce{Br-},\ \ce{I-},\ \ce{NO3-}\) | conc. \(\ce{H2SO4}\) | gas / vapour |
| Independent | \(\ce{SO4^2-},\ \ce{PO4^3-}\) | precipitation tests | specific precipitate |
Confirmatory Anion Tests
Once an anion's group is known, a specific test confirms it. The most celebrated is the brown ring test for nitrate.
Add fresh \(\ce{FeSO4}\) to the nitrate solution, then pour concentrated \(\ce{H2SO4}\) gently down the side. \(\ce{NO3-}\) is reduced to \(\ce{NO}\), which binds \(\ce{Fe^2+}\) to give the brown ring at the interface.
| Anion | Confirmatory test | Result |
|---|---|---|
| \(\ce{CO3^2-}\) | pass gas through lime water | milky (\(\ce{CaCO3}\)) |
| \(\ce{S^2-}\) | lead acetate paper | black (\(\ce{PbS}\)) |
| \(\ce{SO4^2-}\) | \(\ce{BaCl2}\) + dil. \(\ce{HCl}\) | white, acid-insoluble (\(\ce{BaSO4}\)) |
| \(\ce{Cl-}\) | \(\ce{AgNO3}\) | white, \(\ce{NH3}\)-soluble (\(\ce{AgCl}\)) |
| \(\ce{Br-}\) | \(\ce{AgNO3}\) | pale yellow, sparingly \(\ce{NH3}\)-soluble |
| \(\ce{I-}\) | \(\ce{AgNO3}\) / starch | yellow \(\ce{AgI}\) / blue with starch |
The Cation Group Scheme
Cations are separated into six analytical groups, each precipitated by a common group reagent. The order is fixed and must be followed exactly — each group reagent only precipitates its own ions because the earlier groups have already been removed.
Group Reagents in Detail
Each group is defined by the reagent that precipitates it. The precipitate's nature — chloride, sulphide, hydroxide or carbonate — follows from the solubility products of the ions involved.
| Group | Reagent | Precipitate type | Cations |
|---|---|---|---|
| Zero | \(\ce{NaOH}\) (heat) | \(\ce{NH3}\) gas | \(\ce{NH4+}\) |
| I | dilute \(\ce{HCl}\) | chlorides | \(\ce{Pb^2+},\ \ce{Ag+},\ \ce{Hg2^2+}\) |
| II | \(\ce{H2S}\) / dil. \(\ce{HCl}\) | sulphides | \(\ce{Cu^2+},\ \ce{Cd^2+},\ \ce{Bi^3+},\ \ce{Sn^2+}...\) |
| III | \(\ce{NH4Cl}\) + \(\ce{NH4OH}\) | hydroxides | \(\ce{Fe^3+},\ \ce{Al^3+},\ \ce{Cr^3+}\) |
| IV | \(\ce{H2S}\) / \(\ce{NH4OH}\) | sulphides | \(\ce{Co^2+},\ \ce{Ni^2+},\ \ce{Mn^2+},\ \ce{Zn^2+}\) |
| V | \(\ce{(NH4)2CO3}\) | carbonates | \(\ce{Ba^2+},\ \ce{Sr^2+},\ \ce{Ca^2+}\) |
| VI | flame / special | — | \(\ce{Mg^2+},\ \ce{Na+},\ \ce{K+}\) |
The Role of NH₄Cl — A Common-Ion Story
Two of the cleverest steps in the scheme rely on the common-ion effect to control the concentration of a precipitating ion — so that only the intended group falls out.
With \([\ce{OH-}]\) kept low, only the very insoluble Group III hydroxides (\(\ce{Fe(OH)3},\ \ce{Al(OH)3},\ \ce{Cr(OH)3}\)) precipitate. The more soluble Group IV hydroxides stay dissolved, ready for the next step. The same control of \([\ce{S^2-}]\) by acidity separates Group II from Group IV.
Confirmatory Cation Tests
Group separation only tells you which group an ion belongs to. A confirmatory test — usually a distinctive colour or precipitate — clinches the identity.
| Cation | Test | Result |
|---|---|---|
| \(\ce{NH4+}\) | Nessler's reagent | brown precipitate |
| \(\ce{Pb^2+}\) | \(\ce{KI}\) | yellow \(\ce{PbI2}\) |
| \(\ce{Cu^2+}\) | excess \(\ce{NH4OH}\) | deep blue \(\ce{[Cu(NH3)4]^2+}\) |
| \(\ce{Fe^3+}\) | \(\ce{KSCN}\) | blood-red colour |
| \(\ce{Al^3+}\) | \(\ce{NH4OH}\) | white gelatinous \(\ce{Al(OH)3}\) |
| \(\ce{Ni^2+}\) | dimethylglyoxime (DMG) | rosy-red precipitate |
| \(\ce{Ba^2+}\) | \(\ce{K2CrO4}\) | yellow \(\ce{BaCrO4}\) |
Putting It to Work
Problem. A salt with dilute \(\ce{H2SO4}\) gives a colourless gas that turns lime water milky. Name the anion.
Solution. A lime-water-milky gas is \(\ce{CO2}\), from a carbonate:
Problem. What ion forms the coloured species in the brown ring test, and what is that species?
Solution. \(\ce{NO3-}\) is reduced to \(\ce{NO}\), which binds \(\ce{Fe^2+}\):
Problem. A solution gives a white precipitate with dilute \(\ce{HCl}\). To which group does the cation belong, and name three possibilities.
Solution. Insoluble chloride with dilute HCl is the Group I reagent:
Problem. Explain why \(\ce{NH4Cl}\) is added before \(\ce{NH4OH}\) in Group III.
Solution. Common-ion effect suppresses \(\ce{OH-}\), so only Group III hydroxides drop:
Problem. Which test confirms \(\ce{Fe^3+}\), and what is the observation?
Solution. Potassium thiocyanate gives a deeply coloured complex:
Problem. How does \(\ce{AgNO3}\) distinguish \(\ce{Cl-},\ \ce{Br-}\) and \(\ce{I-}\)?
Solution. Colour of the silver halide and its solubility in \(\ce{NH3}\) differ:
Chapter Summary
Identify acid radicals (anions) and basic radicals (cations) by a systematic scheme.
Colour, heating, flame, borax bead and charcoal cavity narrow the field first.
Dilute-acid vs concentrated-acid groups; the brown ring test confirms nitrate.
I (dil HCl) → II (H₂S/acid) → III (NH₄Cl+NH₄OH) → IV (H₂S/base) → V (carbonate) → VI.
Common-ion effect lowers \([\ce{OH-}]\), so only Group III hydroxides precipitate.
Specific colours/precipitates clinch each ion — KSCN (Fe³⁺), DMG (Ni²⁺), etc.
Problems
For each item, first decide whether it tests an anion, a cation group, or the underlying solubility logic — then apply the relevant rule. Difficulty rises down the list.
- Define acid radical and basic radical, and explain why analysis follows a fixed scheme.
- Give the flame colours used to detect \(\ce{Na+},\ \ce{Ca^2+}\) and \(\ce{Ba^2+}\).
- How are anions classified into the dilute-acid and concentrated-acid groups?
- Describe the brown ring test and name the coloured species formed.
- How would you confirm a sulphate ion in solution?
- List the six cation groups with their group reagents in order.
- Why must the group reagents be applied in a fixed sequence?
- Explain, using the common-ion effect, why \(\ce{NH4Cl}\) is added before \(\ce{NH4OH}\) in Group III.
- Why does \(\ce{H2S}\) precipitate Group II in acidic but Group IV in ammoniacal medium?
- Give the confirmatory tests for \(\ce{Cu^2+}\) and \(\ce{Fe^3+}\).
- How does silver nitrate distinguish chloride, bromide and iodide?
- Name the reagent used to confirm \(\ce{Ni^2+}\) and state the observation.