Qualitative Tests for Cations: Precipitation, Complexation and Flame Tests

 

This is part of the HSC Chemistry course under Module 8 Section 1: Analysis of Inorganic Compounds. 

HSC Chemistry Syllabus

Conduct qualitative investigations – using flame tests, precipitation and complexation reactions as appropriate – to test for the presence in aqueous solution of the following

  • cations: barium (Ba2+), calcium (Ca2+), magnesium (Mg2+), lead(II) (Pb2+), silver ion (Ag+), copper(II) (Cu2+), iron(II) (Fe2+), iron(III) (3+)

Identifying Cations Using Flame Test, Precipitation and Complexation

This video explores qualitative tests that can be conducted in a school laboratory to identify metal cations outlined in the HSC Chemistry syllabus.

     

    Qualitative Investigations for Metal Cations

    How do Precipitation Tests Work?

    • Precipitation tests work based on the water solubility of the salt formed when an anion is added to the metal cation.
    • In general, large divalent cations are more likely to produce precipitate e.g. Ba2+, Pb2+
    • Most effective when used to distinguish cations that produce compounds with vastly different solubility products (Ksp) with a given anion.
    • Qualitatively identifies cation by: colour of precipitation and/or whether a precipitate forms.
    • For example, adding SO42– to distinguish between Ba2+ and Cu2+ BaSO4(s) is a white precipitate whereas CuSO4(aq) is a light blue aqueous solution.

     

    How do Flame Tests Work?

    • Conducting of the flame test is a qualitative application of spectroscopy which involves the interaction between matter e.g. metals and electromagnetic radiation e.g. light.
    • Spectroscopy: when atomic electrons absorb heat from the flame, they are excited to a higher orbit (excited state).
      • Electrons only absorb discrete amounts of energy that equal to the difference in energy levels between orbits.
      • Excited electrons can return to their normal orbits (ground state) and release energy in form of electromagnetic waves. In flame tests, the electromagnetic waves have wavelengths in the visible light spectrum.

     

    • The colour of emitted light depends on its wavelength (l) and thus, the discrete amount of energy originally absorbed during electronic excitation.
      • Electron orbits of metal ions have different energy levels due to their different ionic structure. This causes different amounts of energy to be absorbed when they are exposed to a Bunsen burner flame. Consequently, when excited electrons return to their normal orbits, each metal ion produces a different wavelength of visible light, and thus shows a different colour.
      • Flame test is only useful for identifying certain metal ions, specifically metal ions that emit visible light.

     

    Barium (Ba2+)

    • Precipitation test: adding sulfate ions (SO42–) will produce a white precipitate (BaSO4).
    • 🔥Flame test – pale green; cider/apple green

     

    Calcium (Ca2+)

    • Precipitation test: adding sulfate ions (SO42–) will produce a white precipitate (CaSO4).
    • 🔥Flame test – orange red; colour of the sun. This is the best test to differentiate between Ba2+ and Ca2+ ions

     

    Magnesium (Mg2+)

    • Precipitation tests for Mg2+ ions are not specific so therefore they should not be used to differentiate Mg2+ from other ions.
    • 🔥Flame test – no colour
    • Mg2+ ions are usually best identified through a process of elimination.

     

     

    Lead(II) (Pb2+)

    • Precipitation tests
      • Adding a strong base e.g. NaOH will produce a white precipitate (Pb(OH)2).
      • Adding iodide ions e.g. NaI will produce a yellow precipitate (Pbl2)
      • Adding chloride ions e.g. NaCl will produce a white precipitate (PbCl2)
    • ☠ Solutions containing Pb2+ should NOT be tested with flame test as vaporised Pb2+ is a health hazard.

    How do Complexation Tests Work?

    • A metal complex refers to a central metal ion surrounded by molecules (ligands), bound by coordinate bonds.
      • Metal complexes usually have distinct colours which depend on the identity of the metal centre and ligands.
      • For example, Co2+ ion forms a pink complex with six water molecules and a blue complex with four chloride ions.

     

        

     

    • Fe3+ ion forms a yellow complex with six water molecules and a blood red complex with a thiocyanate ion (SCN–).

     

     

    • Complexation is only useful for identifying transition metal ions as non-transition metals cannot form metal complexes.
    • Transition metal complexes have the unique ability to absorb visible light waves.
      • Perceived colour of a solution is the colour of visible light that it does not For example, a Co2+ ion solution is pink because it does not absorb pink visible light.

     

    How to Identify Transition Metals

    Silver (Ag+)

    • Precipitation tests
      • Adding chloride ions e.g. NaCl or HCl will produce a white precipitate (AgCl)
      • Adding bromide ions e.g. NaBr or HBr will produce a cream precipitate (AgBr)
      • Adding iodide ions e.g. NaI or HI will produce a cream precipitate (AgI)

     

    • Complexation – silver chloride [AgCl(s)] dissolves in ammonia as it forms a diamminesilver ion complex. In contrast, PbCl2 precipitate remains undissolved in ammonia. This is the best test to differentiate between Ag+and Pb2+ ions.

    $$AgCl(s) + 2NH_3(aq) \rightarrow [Ag(NH_3)_2]^+(aq) + Cl^-(aq)$$

    • Adding hydroxide ions e.g. NaOH will produce AgOH which quickly decomposes to form brown Ag2O. Ag2O is soluble in acid and alkaline solutions.

     

    $$Ag^+(aq) + OH^-(aq) \rightarrow AgOH(s)$$ 

    $$2AgOH(s) \rightarrow Ag_2O(aq) + H_2O(l)$$

     

    AgCl, AgBr, AgI

     

     

     

     

     

     

    Brown appearance of silver oxide Ag2O

     

    Copper(II) (Cu2+)

    • Complexation
      • Cu2+ in aqueous solution is light blue due to the formation of copper hydrate complex (Cu(H2O)62+). For example, copper(II) sulfate (CuSO4).
      • Adding ammonia (NH3) will produce a deep blue complex

    • Precipitation test
      • Adding a strong base e.g. NaOH will produce a blue precipitate (Cu(OH)2). Formation of the precipitate deepends the light blue colour of aqueous Cu2+ ions.

          

     

    • 🔥Flame test – green/blue

    Light blue appearance of CuSO4 (aq) solution

     

     

     

    Blue/green flame of Cu2+ions.        

     

    Iron(II) (Fe2+)

    • Precipitation test: adding a strong base e.g. NaOH will produce a green precipitate Fe(OH)2(s).

     

    • Oxidation test: adding acidified permanganate (MnO4–) will oxidise Fe2+ to Fe3+, causing the purple permanganate solution to decolourise.

     

     

    Purple permanganate decolorises when Fe2+ ions (pale green) are present.

     

    • Complexation: Iron(II) hexahydrate Fe(H2O)62+ has a pale green colour. This may be easy to identify if [Fe2+] is high enough. For example, iron(II) chloride (see image on right)

     

     

     

    Precipitation of Fe(OH)2(s)

     

    Pale green appearance of Fe(H2O)62+

     

     

    Iron(III) (Fe3+)

    • Precipitation test – adding a strong base e.g. NaOH will produce a brown precipitate Fe(OH)3(s).

     

    • Complexation
      • Iron(III) hexahydrate Fe(H2O)63+ has a pale yellow colour. This may be easy to identify if [Fe3+] is high enough. For example, iron(III) chloride (see image below)
      • Adding thiocyanate ion (SCN–) will produce a blood-red solution.

     

    Precipitation of Fe(OH)3(s)

     

     

       

     

    Concentrated solution of iron(III) chloride. The yellow appearance is due to iron(III) hexahydrate complex

     

    Iron(III) thiocyanate complex

     

    Sequence of Tests for Identifying Cations

     

    • Pb2+ and Ag+ are identified with Cl–. Ag+ containing precipitates are usually white/cream in appearance. PbI2 is distinctively bright yellow. The precipitation tests for these two ions should be conducted first within a sequence of tests.
    • Ca2+ and Ba2+ are identified with SO42– When both ions are present, a flame test should be conducted. Ca2+ and Ba2+ions produce red and pale-green coloured flames respectively.
    • Cu2+, Fe2+ and Fe3+ are identified with OH– Cu(OH)2 is a blue, Fe(OH)2 is green and Fe(OH)3 is brown.
    • Mg2+ ions are best identified last within a sequence of test. It should not produce a colour in the flame test and precipitate with Cl– ions in the first step. However, it does produce a white precipitate with SO42– and OH– 

     

    Flame test

    Test using NaCl(aq)

    Test using Na2SO4(aq)

    Test using NaOH(aq)

    Lead Pb2+

    X

    White precipitate

    White precipitate

    White precipitate

    Silver Ag+

    X

    White precipitate

    White precipitate

    Brown precipitate (Ag2O)

    Barium Ba2+

     

    X

    White precipitate

    X

    Calcium Ca2+

    X

    White precipitate

    X

    Magnesium Mg2+

    X

    X

    White precipitate

    White precipitate

    Copper(II)

    Cu2+

    X

    X

    Blue precipitate

    Iron(II) Fe2+

    X

    X

    X

    Green precipitate

    Iron(III) Fe3+

    X

    X

    X

    Brown precipitate

     

    HSC Solubility Table Between Cations and Anions

     

    Chloride Cl–

    Bromide Br–

    Iodide I–

    Sulfate SO42–

    Phosphate PO43–

    Carbonate CO32–

    Hydroxide OH–

    Acetate CH3COO–

    Lead Pb2+

    White

    White

    Yellow

    White

    White

    White

    White

    Sparingly soluble (white)

    Silver Ag+

    White

    Cream

    Yellow

    White

    Yellow

    Yellow

    Brown

    Sparingly soluble (white)

    Barium Ba2+

    ✓

    ✓

    ✓

    White

    White

    white

    ✓

    ✓

    Calcium Ca2+

    ✓

    ✓

    ✓

    White

    White

    White

    Sparingly soluble (cloudy white)

    ✓

    Magnesium Mg2+

    ✓

    ✓

    ✓

    ✓

    White

    White

    White

    ✓

    Copper(II)

    Cu2+

    ✓

    ✓

    White

    ✓

    Blue-green

    White

    Blue

    ✓

    Iron(II) Fe2+

    ✓

    ✓

    ✓

    ✓

    Brown

    White

    Green

    ✓

    Iron(III) Fe3+

    ✓

    ✓

    ✓

    ✓

    Brown

    Orange-red

    Brown

     

    Note: all nitrates, sodium, potassium and ammonium ionic compounds are water soluble.