Mada za sehemu hiiQualitative AnalysisMada 3
- The concept of qualitative analysis
- Qualitative Analysis procedures
- Teats for metallic Radicals
In qualitative analysis, we aim to identify the presence of different metal ions (cations) and non-metal ions (anions) in a given sample. This is done through various tests, which involve reactions with different reagents, such as acids, alkalis, and specific reagents for metal and non-metal identification.
Some metal ions can be identified by the color of the flame they produce when subjected to a flame test. The metal ions are heated on a platinum or nichrome wire in a non-luminous flame, and the color of the flame is observed to identify the specific metal ion.
Flame test: characteristic flame colors for metal ions
| Metal Ion | Flame Color |
|---|---|
| Sodium (Na⁺) | Bright yellow |
| Potassium (K⁺) | Lilac |
| Calcium (Ca²⁺) | Brick red |
| Barium (Ba²⁺) | Green |
| Copper(II) (Cu²⁺) | Blue-green |
| Strontium (Sr²⁺) | Crimson red |
When dilute hydrochloric acid (HCl) is added to a solution containing metal ions, it may cause the formation of an insoluble chloride if the metal ion reacts with chloride ions. A precipitate will form for metal ions that form insoluble chlorides.
Action of hydrochloric acid on cations
| Cation | Reaction | Observation |
|---|---|---|
| Lead(II) (Pb²⁺) | Add dilute HCl to the solution | White precipitate of PbCl₂ forms, which dissolves on heating |
| Calcium (Ca²⁺) | Add dilute HCl to the solution | No precipitate forms (Calcium chloride is soluble) |
| Magnesium (Mg²⁺) | Add dilute HCl to the solution | No precipitate forms (Magnesium chloride is soluble) |
Adding dilute sulfuric acid (H₂SO₄) to a solution containing metal ions will often result in the formation of an insoluble sulfate. A precipitate forms for ions that form insoluble sulfates.
Action of sulfuric acid on cations
| Cation | Reaction | Observation |
|---|---|---|
| Lead(II) (Pb²⁺) | Add dilute H₂SO₄ to the solution | White precipitate of PbSO₄ forms |
| Calcium (Ca²⁺) | Add dilute H₂SO₄ to the solution | White precipitate of CaSO₄ forms |
| Magnesium (Mg²⁺) | Add dilute H₂SO₄ to the solution | White precipitate of MgSO₄ forms |
When sodium hydroxide solution is added to a solution of metal ions, it can lead to the formation of hydroxides, some of which are insoluble in water. The nature of the precipitate formed can help identify the metal ion.
Reaction of cations with sodium hydroxide
| Cation | Test | Observation |
|---|---|---|
| Zinc (Zn²⁺) | Add NaOH solution | White precipitate of zinc hydroxide (Zn(OH)₂) forms, dissolves in excess NaOH |
| Aluminum (Al³⁺) | Add NaOH solution | White precipitate of aluminum hydroxide (Al(OH)₃) forms, dissolves in excess NaOH |
| Lead(II) (Pb²⁺) | Add NaOH solution | White precipitate of lead(II) hydroxide (Pb(OH)₂) forms, dissolves in excess NaOH |
| Iron(III) (Fe³⁺) | Add NaOH solution | Red-brown precipitate of ferric hydroxide (Fe(OH)₃) forms |
| Iron(II) (Fe²⁺) | Add NaOH solution | Green precipitate of ferrous hydroxide (Fe(OH)₂) forms |
Aqueous ammonia can react with metal ions, forming precipitates of metal hydroxides. Some metal hydroxides dissolve in excess ammonia to form complex ions.
Reaction of cations with aqueous ammonia
| Cation | Test | Observation |
|---|---|---|
| Copper(II) (Cu²⁺) | Add aqueous ammonia | Blue precipitate of copper(II) hydroxide (Cu(OH)₂) forms, dissolves in excess ammonia to form a deep blue solution |
| Zinc (Zn²⁺) | Add aqueous ammonia | White precipitate of zinc hydroxide (Zn(OH)₂) forms, dissolves in excess ammonia to form a colorless solution |
| Iron(III) (Fe³⁺) | Add aqueous ammonia | Red-brown precipitate of ferric hydroxide (Fe(OH)₃) forms |
| Aluminum (Al³⁺) | Add aqueous ammonia | White precipitate of aluminum hydroxide (Al(OH)₃) forms, dissolves in excess ammonia to form a colorless solution |
After conducting preliminary tests for metal ions, confirmatory tests can be used to further identify the cation present in the sample. These tests help to verify the presence of specific metal ions based on characteristic reactions.
Confirmatory tests for cations
| Cation | Test | Observation |
|---|---|---|
| Iron(III) (Fe³⁺) | Add ammonium thiocyanate solution | A blood-red color forms |
| Iron(II) (Fe²⁺) | Add potassium permanganate solution | The purple color of potassium permanganate is decolorized |
| Copper(II) (Cu²⁺) | Add ammonia solution | A deep blue solution forms |
| Calcium (Ca²⁺) | Add ammonium oxalate solution | A white precipitate of calcium oxalate (CaC₂O₄) forms |
These tests help in confirming the presence of bromide and iodide ions in a sample. Both ions react with silver nitrate solution in the presence of dilute nitric acid to form characteristic precipitates.
Tests for anions
| Test | Observation | Result |
|---|---|---|
| Add dilute HNO₃ followed by AgNO₃ solution | A white precipitate of AgCl forms (for chloride), pale yellow for AgBr (for bromide), yellow for AgI (for iodide) | Chloride, Bromide, or Iodide may be present |
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