Qualitative Analysis procedures

Procedures of qualitative analysis

The procedures of qualitative analysis involve carrying out analytical tests in the appropriate sequence of steps. The common procedures for testing an unknown sample involve making its solution and then testing for different ions present. The steps involved in the systematic analysis of a given sample are as follows:

Preliminary tests

1. Color and smell
2. Flame test
3. Solubility in water
4. Dry heating
5. Action on litmus (for gases evolved)

Tests for acid radicals (anions)

1. Dilute acid test (dilute H₂SO₄)
2. Concentrated acid test (concentrated H₂SO₄)
3. Wet test for acid radicals

Wet tests for basic radicals (cations)

• Using Special Apparatus for Qualitative Analysis
• Use special apparatus for qualitative analysis.

Activity 1: Use special apparatus for qualitative analysis

Carry out preliminary tests on an unknown sample. Preliminary tests include noting the appearance (color, texture, feel, etc.), detecting the smell of gases liberated, observing the action of a substance on litmus, and even tasting the substance under investigation. These tests give a clue about the type of elements present in a sample.

Color and smell of solid substances in qualitative analysis

Substance	Color	Smell
Magnesium oxide (MgO)	White	Odourless
Calcium carbonate (CaCO₃)	White	Odourless
Copper(II) oxide (CuO)	Black	Odourless
Iron(III) oxide (Fe₂O₃)	Reddish-brown	Odourless
Lead(II) sulphate (PbSO₄)	White	Odourless
Calcium sulphate (CaSO₄)	White	Odourless
Zinc oxide (ZnO)	White	Odourless
Silver chloride (AgCl)	White	Odourless
Potassium chloride (KCl)	White	Odourless
Ammonium chloride (NH₄Cl)	White	Pungent (smell of ammonia)
Calcium phosphate (Ca₃(PO₄)₂)	White	Odourless
Sodium chloride (NaCl)	White	Odourless
Sodium carbonate (Na₂CO₃)	White	Odourless
Copper(I) chloride (CuCl)	White	Odourless
Iron(II) sulphate (FeSO₄)	Greenish-white	Odourless

Identification of gas

Gas	Color	Smell	Action on Litmus	Test
CH₃COOH	Colorless	Vinegar-like	Acidic	Liberated as dense white fumes
N₂	Colorless	Odourless	Neutral	No chemical test
Water Vapor	Colorless	Odourless	Neutral	Turns white CuSO₄ blue
NO₂	Reddish-brown	Pungent	Acidic	Not as red as Br₂ vapor, does not condense on the sides of the tube
NH₃	Colorless	Pungent	Alkaline	Forms thick white fumes with contact with HCl gas
HCl	White fumes	Irritating	Acidic	Forms thick white fumes with contact with NH₃ gas
HBr & Br₂	White fumes & reddish-brown gas	Choking	Acidic	HBr resembles HCl, Br₂ bleaches and condenses to a red liquid
Cl₂	Pale green	Bleaches	Choking	Gives white fumes with NH₄OH
I₂	Violet	Choking	Bleaches	Turns starch iodide paper blue
CO₂	Colorless	Odourless	Slightly acidic	Turns lime water milky
CO	Colorless	Odourless	Neutral	Burns with pale blue flame
H₂	Colorless	Odourless	Neutral	Burns with a "pop" sound
H₂S	Colorless	Rotten eggs	Acidic	Burns with blue flame and blackens lead acetate paper
O₂	Colorless	Odourless	Neutral	Re-ignites a glowing splint
SO₂	Colorless	Irritating smell of burning sulphur	Acidic	Decolourizes KMnO₄ solution, K₂Cr₂O₇ from orange to green
SO₃	Colorless	Pungent	Acidic	Fumes in moist air forming white fumes

Colors of some ions in solution

The appearance of a substance in solid or solution form can help in its identification:

1. If a compound and its solution in water are colorless, it is probable that a transition metal is absent.
2. If its color is black, it is probably an oxide or a sulphide.
3. If the solid and its solution in water are colored, probably a transition metal is present.

Color of some ions in hydrated salts and dilute solution

Ion Symbol	Color in Hydrated Salt	Color in Dilute Solution
Fe²⁺	Light green	Light green
Fe³⁺	Yellow to brown	Yellow to brown
Cu²⁺	Blue	Blue
Cr³⁺	Green	Green
Co²⁺	Pink	Pink
Ni²⁺	Green	Green
Mn²⁺	Pink to pale brown	Pale pink
K⁺, Ca²⁺, Na⁺, Ag⁺, NH₄⁺	Colorless	Colorless
Cu²⁺ (Cupric)	Blue	Blue
CrO₄²⁻ (Chromate)	Yellow	Yellow
Cr₂O₇²⁻ (Dichromate)	Orange	Orange
MnO₄⁻ (Permanganate)	Purple	Purple

Colours of common insoluble substances

Carbonates

1. Calcium carbonate (CaCO₃) – White
2. Barium carbonate (BaCO₃) – White
3. Magnesium carbonate (MgCO₃) – White
4. Iron(III) carbonate (Fe₂(CO₃)₃) – Reddish-brown

Chlorides

1. Silver chloride (AgCl) – White
2. Lead chloride (PbCl₂) – White
3. Mercury(I) chloride (Hg₂Cl₂) – White

Nitrates

• Lead(II) nitrate (Pb(NO₃)₂) – White

Oxides

1. Iron(III) oxide (Fe₂O₃) – Red-brown
2. Copper(II) oxide (CuO) – Black
3. Magnesium oxide (MgO) – White

Sulfates

1. Barium sulfate (BaSO₄) – White
2. Calcium sulfate (CaSO₄) – White
3. Copper(II) sulfate (CuSO₄) – Blue

Phosphates

1. Calcium phosphate (Ca₃(PO₄)₂) – White
2. Iron(III) phosphate (FePO₄) – Yellow

Chromates

1. Lead(II) chromate (PbCrO₄) – Yellow
2. Chromium(III) hydroxide (Cr(OH)₃) – Gray-green

Hydroxides

1. Iron(III) hydroxide (Fe(OH)₃) – Brown
2. Copper(II) hydroxide (Cu(OH)₂) – Blue

Tests for specific anions

Chloride (Cl⁻)

1. Add dilute nitric acid (HNO₃) to the solution of the sample.
2. Add silver nitrate (AgNO₃) solution.
3. A white precipitate of silver chloride (AgCl) indicates the presence of chloride ions.
4. If the precipitate is soluble in ammonia solution, it confirms the presence of chloride ions.

Sulfate (SO₄²⁻)

1. Add dilute hydrochloric acid (HCl) to the solution.
2. Add barium chloride (BaCl₂) solution.
3. A white precipitate of barium sulfate (BaSO₄) indicates the presence of sulfate ions.

Nitrate (NO₃⁻)

1. Add sodium hydroxide (NaOH) solution and then aluminum (Al) powder to the solution.
2. Warm the mixture gently.
3. A colorless gas with a characteristic smell of ammonia (NH₃) is evolved, indicating the presence of nitrate ions.

Carbonate (CO₃²⁻)

1. Add dilute hydrochloric acid (HCl) to the solution.
2. If bubbles of gas are produced, the gas should be tested by passing it through lime water (Ca(OH)₂).
3. A milky white precipitate of calcium carbonate (CaCO₃) forms, indicating the presence of carbonate ions.

Phosphate (PO₄³⁻)

1. Add dilute nitric acid (HNO₃) to the sample.
2. Then, add ammonium molybdate ((NH₄)₆Mo₇O₂₄) solution and heat.
3. A yellow precipitate of ammonium phosphomolybdate confirms the presence of phosphate ions.

Acetate (CH₃COO⁻)

1. Add dilute hydrochloric acid (HCl) to the solution and warm gently.
2. A vinegar-like odor (acetic acid) is produced, confirming the presence of acetate ions.

Tests for cations (basic radicals)

Calcium (Ca²⁺)

1. Add dilute sodium hydroxide (NaOH) to the solution.
2. A white precipitate of calcium hydroxide (Ca(OH)₂) forms, which is insoluble in excess sodium hydroxide.

Magnesium (Mg²⁺)

1. Add sodium hydroxide (NaOH) solution to the sample.
2. A white precipitate of magnesium hydroxide (Mg(OH)₂) forms, which is insoluble in excess sodium hydroxide.

Iron (II) (Fe²⁺)

1. Add sodium hydroxide (NaOH) to the solution.
2. A green precipitate of iron (II) hydroxide (Fe(OH)₂) forms, which is soluble in excess sodium hydroxide, forming a green solution of [Fe(OH)₄]²⁻.

Iron (III) (Fe³⁺)

1. Add sodium hydroxide (NaOH) solution to the solution.
2. A brown precipitate of iron (III) hydroxide (Fe(OH)₃) forms, which is insoluble in excess sodium hydroxide.

Copper (II) (Cu²⁺)

1. Add dilute sodium hydroxide (NaOH) to the solution.
2. A blue precipitate of copper (II) hydroxide (Cu(OH)₂) forms, which is insoluble in excess sodium hydroxide.

Zinc (Zn²⁺)

1. Add sodium hydroxide (NaOH) to the solution.
2. A white precipitate of zinc hydroxide (Zn(OH)₂) forms, which dissolves in excess sodium hydroxide to give a colorless solution of [Zn(OH)₄]²⁻.

Ammonium (NH₄⁺)

1. Add sodium hydroxide (NaOH) to the solution and heat gently.
2. Ammonia gas (NH₃) is evolved, which has a pungent smell and turns red litmus paper blue.

Lead (II) (Pb²⁺)

1. Add sodium hydroxide (NaOH) to the solution.
2. A white precipitate of lead (II) hydroxide (Pb(OH)₂) forms, which dissolves in excess sodium hydroxide, forming a colorless solution of [Pb(OH)₄]²⁻.

Copper (I) (Cu⁺)

1. Add sodium hydroxide (NaOH) to the solution.
2. A white precipitate of copper (I) hydroxide (Cu₂O) forms, which turns red upon heating.

Nickel (Ni²⁺)

1. Add sodium hydroxide (NaOH) solution.
2. A green precipitate of nickel hydroxide (Ni(OH)₂) forms, which is insoluble in excess sodium hydroxide.

Cobalt (Co²⁺)

1. Add sodium hydroxide (NaOH) solution.
2. A blue precipitate of cobalt (II) hydroxide (Co(OH)₂) forms, which turns pink upon exposure to air.

Final steps in qualitative analysis

Once all the tests for ions have been completed, it is important to verify the results. The final confirmation involves using a sequence of systematic chemical reactions to rule out the presence of different ions. Additionally, instrumental methods like spectrometry or chromatography can be used to confirm the analysis for unknown samples.