Detection of metal ions by flame test
- The sample metallic salt is moistened with concentrated HCl on a Platinum wire. Since no solvent is involved in this reaction, it is also called Dry Test.
- The wire is held in a non-luminous flame of the Bunsen Burner. The following colour changes are observed:
| Salt/ion | Colour in flame |
| Ca-salt (Ca+) | Brick Red |
| Ba-salt (Ba2+) | Apple Green |
| Mg-salt (Mg2+) | none |
| Pb-salt (Pb2+) | Blue/White |
| Ag-salt (Ag+) | none |
| Cu-salt (Cu2+) | Blue-green |
| Fe-salt (Fe2+) | Bright blue or green |
| Fe-salt (Fe3+) | Orange-brown |
Detection of ions by precipitation and complexation reactions
- Ions are detected by their precipitation reactions by a process called Wet Test.
- The first step of every wet test is to prepare stock solutions of the sample.
- The steps for detection of some common ions are as follows:
Test for Ba2+
| Experiment | Observation | Reaction | Inference |
| Aqueous solution of ammonium carbonate was added in the sample | White precipitate was observed. The solution was then boiled in acetic acid and a few drops of potassium chromate was added. Yellow precipitate was seen | Ba2+ + (NH4)2CO3 (aq) → BaCO3 (white precipitate) + 2NH4+ (aq)
BaCO3 (s) + CH3COOH (aq) → Ba(CH3COO)2 + H2O (l) + CO2 (g) Ba2+ (aq) + K2CrO4 (aq) → BaCrO4 (yellow precipitate) + 2K+
|
Ba2+ ion was confirmed |
Test for Ca2+
| Experiment | Observation | Reaction | Inference |
| Ammonium oxalate solution was added to the sample | White precipitate of calcium oxalate was formed | CaCl2 + NH4OOC – COONH4 → Ca(OOC – COO)2 (white precipitate) + 2NH4Cl | Ca2+ ion was confirmed |
Test for Mg2+
| Experiment | Observation | Reaction | Inference |
| Ammonium chloride, ammonium hydroxide and disodium hydrogen phosphate was added to the stock solution | White precipitate was formed | Mg2+ + HPO42- + NH4+ → Mg(NH4)PO4 (white precipitate) + H+ | Mg2+ ion was confirmed |
Test for Pb2+
| Experiment | Observation | Reaction | Inference |
| Dilute hydrochloric acid was added to the stock solution. | White precipitate was formed. On adding potassium iodide, the solution gave a yellow precipitate | Pb2+ (aq) + HCl (aq) → PbCl2 (white precipitate)
Pb2+ (aq) + 2I‑ (aq) → PbI2 (yellow precipitate) |
Pb2+ ion was confirmed |
Test for Ag+
| Experiment | Observation | Reaction | Inference |
| Dilute hydrochloric acid was added to the stock solution. | White precipitate was formed. On adding potassium chromate, the solution gave a brick red precipitate | Ag+ (aq) + HCl (aq) → AgCl (white precipitate)
2AgCl + K2CrO4 (aq) → Ag2CrO4 (brick red precipitate) + 2KCl (aq) |
Ag+ ion was confirmed |
Test for Cu2+
| Experiment | Observation | Reaction | Inference |
| Small amount of ammonium hydroxide was added to the stock solution | Light blue precipitate was formed. On addition of excess ammonium hydroxide, the precipitate turned deep blue | 2CuSO4 (aq) + 2NH4OH → CuSO4.Cu(OH)2 (light blue precipitate) + (NH4)2SO4
CuSO4.Cu(OH)2 + (NH4)2SO4 + NH4OH → 2[Cu(NH3)4]SO4 (deep blue precipitate) + 8H2O |
Cu2+ ion was confirmed |
Test for Fe2+
| Experiment | Observation | Reaction | Inference |
| Small amount of ammonium hydroxide was added to the stock solution | Blue precipitate of ferrous hydroxide was formed | FeSO4 (aq) + NH4OH → Fe(OH)2 (blue precipitate) + (NH4)2SO4 | Fe2+ ion was confirmed |
Test for Fe3+
| Experiment | Observation | Reaction | Inference |
| Small amount of ammonium hydroxide was added to the stock solution | Brown precipitate of ferric hydroxide was formed | FeCl3 (aq) + NH4OH → Fe(OH)3 (brown precipitate) + NH4Cl | Fe3+ ion was confirmed |
Test for Cl–
| Experiment | Observation | Reaction | Inference |
| Few drops of silver nitrate solution was added to the stock solution | White precipitate of silver chloride was formed which was soluble in ammonium hydroxide but insoluble in nitric acid | NaCl + AgNO3 → AgCl (white precipitate) + NaNO3
AgCl + 2NH4OH → [Ag(NH3)2]Cl (soluble) + 2H2O |
Cl– ion was confirmed |
Test for Br–
| Experiment | Observation | Reaction | Inference |
| Few drops dilute nitric acid was added to the stock solution. Afterwards, silver nitrate solution was added | Very pale cream precipitate of silver bromide was formed | NaBr + AgNO3 → NaNO3 + AgBr (very pale cream precipitate) | Br– ion was confirmed |
Test for I–
| Experiment | Observation | Reaction | Inference |
| Few drops dilute nitric acid was added to the stock solution. Afterwards, silver nitrate solution was added | Very pale yellow precipitate of silver iodide was formed | NaI + AgNO3 → NaNO3 + AgI (very pale yellow precipitate) | I– ion was confirmed |
Test for OH–
| Experiment | Observation | Reaction | Inference |
| Red litmus paper was dipped in the stock solution | Red litmus turned blue | OH– ion was confirmed |
Test for CH3COO–
| Experiment | Observation | Reaction | Inference |
| Ethanol was added to the stock solution. Afterwards, a few drops of sulphuric acid was added | A fruity smell was found | CH3COO– + H2SO4 → CH3COOH + HSO4– | CH3COO– ion was confirmed |
Test for SO42-
| Experiment | Observation | Reaction | Inference |
| Few drops of lead acetate solution was added to the stock solution | White precipitate of lead sulphate was formed. | NaSO4 + Pb(CH3COO)2 → PbSO4 (white precipitate) + 2CH3COONa | SO42- ion was confirmed |
Test for CO32-
| Experiment | Observation | Reaction | Inference |
| Few drops barium nitrate solution was added to the stock solution | White precipitate of barium carbonate was formed which on reaction with HCl evolves CO2 gas | ZnCO3 + Ba(NO3)2 → BaCO3 (white precipitate) + Zn(NO3)2
BaCO3 + HCl → BaCl2 + CO2 + H2O |
CO32- ion was confirmed |
Test for PO43-
| Experiment | Observation | Reaction | Inference |
| Concentrated nitric acid was added to the stock solution. Afterwards, ammonium molybdate was added | Yellow precipitate was formed | H3PO4 + (NH4)2MoO4 + H+ → (NH4)3PO4·12MoO3·2H2O (yellow precipitate) + NH4+ + H2O | PO43- ion was confirmed |