Identifying Cations using Flame Test, Precipitation and Complexation

 

This is part of the HSC Chemistry course under the topic 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) (Fe3+)

Identifying Cations Using Flame Test, Precipitation and Complexation

This video explores a range of qualitative tests that can be conducted in a school laboratory to identify metal cations including:

  • Barium
  • Calcium
  • Magnesium
  • Lead
  • Silver
  • Copper(II)
  • Iron(II) and Iron(III)

 

Common Tests for Metal Cations

Precipitation Tests

  • Distinguishes cations based on the aqueous solubility of ionic compound formed when an anion is added.
  • 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.
  • 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.

 

Flame Tests

  • 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.

Complexation Tests

  • A metal complex refers to a centrally position metal ion surrounded by molecules, bound by coordinate bonds.
    • 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.

 

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.

 

 

 

Pale green flame of Ba2+

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

Orange red flame of Ca2+.

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.

 

PbI2

Identifying 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 [Fe3+] 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+ 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

 

Cross-solubility table between cations & anions outlined by the HSC syllabus

 

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 ammonia containing ionic compounds are aqueous.