8.3 - Identification of Ions

Describe the flame test results.

• Lithium compounds - crimson red flame.

• Sodium compounds - yellow flame.

• Potassium compounds - lilac flame.

• Calcium compounds - orange red flame.

• Copper compounds - green flame.

Describe the sodium hydroxide test results and write the equations.

• Copper (II) forms a blue precipitate:

  • Cu²⁺_(aq) + 2OH^-_(aq) → Cu(OH)_2(s)

• Iron (II) forms a dirty green precipitate:

  • Fe²⁺_(aq) + 2OH^-_(aq) → Fe(OH)_2(s)

• Iron (III) forms a brown precipitate:

  • Fe³⁺_(aq) + 3OH^-_(aq) → Fe(OH)_3(s)

• Al³⁺, Ca²⁺, Mg²⁺ form white precipitates but only the Al(OH)3 dissolves in excess NaOH to form a colorless solution.

  • Ca²⁺_(aq) + 2OH^-_(aq) → Ca(OH)_2(s)

  • Mg²⁺_(aq) + 2OH^-_(aq) → Mg(OH)_2(s)

  • Al³⁺_(aq) + 3OH^-_(aq) → Al(OH)_3(s)

Describe the test for carbonate anions.

• Add dilute acid, e.g. HCl.

• Fizzing observed as CO is released.

• E.g. Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Describe the test for sulfate anions.

• Add a solution containing Ba²⁺ cations, e.g. a solution of BaCl₂.

• White precipitate of BaSO₄ forms.

• E.g. K₂SO₄ + BaCl₂ → 2KCl + BaSO₄

• Can also be thought of a test for barium (II); add sulfates - white precipitate forms.

Describe the silver nitrate/halide anions tests and give the equations.

• Add a solution of AgNO3 (acidified with HNO3)

  • Chlorides - white precipitate, silver chloride; Ag⁺+Cl^- → AgCl

  • Bromides - cream precipitate, silver bromide; Ag⁺+Br^- → AgBr

  • Iodides - yellow precipitate, silver iodide; Ag⁺+I^- → AgI

What are instrumental methods?

They are accurate, sensitive, and rapids methods which are useful when the amount of sample is very small.

Describe the flame emission spectroscopy.

1. Instrumental method used for identifying metal ions in solution or measuring their concentration.

2. Spectroscope measures the exact wavelength of the light emitted by a metal ion.

3. That allows for definite identification - sometimes colors are difficult to distinguish.

4. Concentrations are found by measuring the intensity of light emitted. The more intense light, the greater the concentration of the metal ion in a solution.

5. From the intensity vs. concentration graph, you can read off a relevant concentration value at a given intensity.

What are cations and anions?

Cations are the positive ions; anions are the negative ions.