Salt Analysis Virtual Lab

Aim

To identify one anion (acid radical) and one cation (basic radical) in the given inorganic salt by systematic qualitative analysis.

Apparatus and reagents

• Test tubes, test-tube holder and stand, boiling tube, Bunsen burner, glass rod, filter funnel and paper, platinum wire, delivery tube
• Acids: dilute and concentrated H₂SO₄, HCl and HNO₃; acetic acid
• Group reagents: NaOH, dil. HCl, H₂S gas (or thioacetamide), solid NH₄Cl, NH₄OH, (NH₄)₂CO₃, Na₂HPO₄
• Special reagents: lime water, lead acetate paper, BaCl₂, AgNO₃, FeSO₄, ammonium molybdate, sodium nitroprusside, FeCl₃, KI, K₂CrO₄, K₄[Fe(CN)₆], KSCN, dimethylglyoxime, Nessler's reagent, magneson, ammonium oxalate

The simulator includes 16 salts that are common in school laboratories: lead acetate, lead nitrate, ammonium chloride, ammonium bromide, ammonium phosphate, copper sulphate, ferric chloride, aluminium sulphate, zinc sulphate, zinc sulphide, manganese chloride, nickel sulphate, barium chloride, strontium nitrate, calcium carbonate, magnesium sulphate.

Theory

Finding the anion

Anions are grouped by the acid that displaces them. Dilute H₂SO₄ releases gases only from salts of weak or unstable acids: carbonate (CO₂), sulphide (H₂S), sulphite (SO₂), nitrite (brown NO₂) and acetate (vinegar smell). If nothing happens, concentrated H₂SO₄ with heat displaces the more volatile acids: chloride gives HCl fumes, bromide and iodide are oxidised to coloured Br₂ and I₂ vapours, and nitrate gives brown NO₂ when copper turnings are added. Sulphate and phosphate react with neither, so they have independent tests with BaCl₂ and ammonium molybdate.

Finding the cation: group separation

Cations are separated into groups by adding reagents in a fixed order. Each reagent precipitates only the ions whose compound has a very low solubility product (K_sp) under those conditions, and the filtrate goes on to the next group. The trick is controlling the concentration of the precipitating ion with the common-ion effect: H⁺ keeps S²⁻ low in Group II, NH₄⁺ keeps OH⁻ low in Group III, and removing H⁺ with NH₄OH raises S²⁻ for Group IV. That is why the order matters: if you skip a group, an earlier cation is still in solution and can precipitate later and fool you. The simulator models this, so try it.

Procedure

1. Preliminary examination. Note colour, smell and solubility. Blue suggests Cu²⁺, green Ni²⁺, light pink Mn²⁺ and yellowish brown Fe³⁺. Do a flame test.
2. Dilute H₂SO₄ test. Warm a pinch of salt with dil. H₂SO₄ and test any gas with lime water, lead acetate paper or acidified dichromate paper.
3. Concentrated H₂SO₄ test. If the dilute test is negative, heat the salt with conc. H₂SO₄; add copper turnings if brown fumes are faint.
4. Independent tests. Test the sodium carbonate extract for sulphate (dil. HCl + BaCl₂) and the salt solution for phosphate (conc. HNO₃ + ammonium molybdate).
5. Confirm the anion with its specific test (AgNO₃, chromyl chloride, layer test, brown ring, nitroprusside, FeCl₃).
6. Original solution. Dissolve the salt in water; if insoluble, in dil. HCl.
7. Group 0. Heat the solid salt with NaOH and test for NH₃.
8. Groups I to VI. Add each group reagent in order. Stop at the first group that gives a precipitate.
9. Confirm the cation with the confirmatory test for that group, then write the result.

Observation table (sample: lead nitrate)

Result: anion NO₃⁻, cation Pb²⁺; the salt is lead nitrate, Pb(NO₃)₂.

Precautions

• Use a small quantity of salt and reagent; a large excess hides colour changes.
• Add conc. H₂SO₄ slowly down the side of the tube in the brown ring test and do not shake.
• Pass H₂S in a fume cupboard; it is poisonous.
• Never skip a group. Always test the filtrate of one group with the reagent of the next.
• Add NH₄Cl before NH₄OH in Group III and boil off H₂S before Group III.
• Prepare the FeSO₄ solution fresh; old solution has been oxidised to Fe³⁺.

Viva questions with answers

1. What is the difference between a group reagent and a confirmatory reagent?

A group reagent precipitates every cation of a group together; a confirmatory reagent gives a result specific to a single ion.

2. Why does PbCl₂ appear in both Group I and Group II?

PbCl₂ is fairly soluble in cold water, so precipitation in Group I is incomplete. The remaining Pb²⁺ is removed as black PbS in Group II.

3. Why can Zn²⁺ not precipitate in Group II?

ZnS has a much larger K_sp than CuS. In acidic solution the S²⁻ concentration is too low for the ionic product of ZnS to exceed its K_sp.

4. Why is Mg²⁺ not precipitated in Group V?

NH₄Cl keeps the carbonate ion concentration low (and forms soluble complexes), so MgCO₃'s solubility product is not exceeded.

5. What is the brown ring made of?

The nitrosyl iron complex [Fe(H₂O)₅(NO)]²⁺, formed when NO from the nitrate reduction combines with excess Fe²⁺.

6. Why is the chromyl chloride test not given by bromides and iodides?

Chromyl bromide and chromyl iodide do not form. Bromide and iodide are oxidised to Br₂ and I₂ instead, so NaOH stays colourless and no yellow lead chromate appears.

7. What is a lake?

A coloured adsorption compound formed when a dye such as litmus is taken up by a gelatinous precipitate such as Al(OH)₃.

Ions covered

Anions: Carbonate (CO₃²⁻), Sulphide (S²⁻), Sulphite (SO₃²⁻), Nitrite (NO₂⁻), Acetate (CH₃COO⁻), Chloride (Cl⁻), Bromide (Br⁻), Iodide (I⁻), Nitrate (NO₃⁻), Sulphate (SO₄²⁻), Phosphate (PO₄³⁻).
Cations: Ammonium (NH₄⁺), Lead (Pb²⁺), Copper(II) (Cu²⁺), Iron(III) (Fe³⁺), Aluminium (Al³⁺), Zinc (Zn²⁺), Manganese(II) (Mn²⁺), Nickel (Ni²⁺), Barium (Ba²⁺), Strontium (Sr²⁺), Calcium (Ca²⁺), Magnesium (Mg²⁺).

Salt analysis is one of the highest-scoring experiments in the CBSE Class 11 and 12 chemistry practical exam, and the logic behind it (K_sp, common-ion effect, coloured complexes) is examined in JEE Main and NEET under ionic equilibrium, the d-block and coordination compounds. Before analysing salts, warm up with the flame test lab, try the titration lab, and practise previous-year JEE Main questions or the full question bank.

Frequently asked questions