Test for chromium ion | Cr metal reactions

Chromium (Cr) is a transition element. Chromium has different oxidation states +2, +3 , +6. The oxidation state +6 has strong oxidizing ability. Chromium hydroxide shows amphoteric characteristics. Chromate (CrO42-) and dichromate Cr2O72-) ions are strong oxidizing agents. Chromium is used to make alloys and in electroplating.

In this tutorial, we study about followings.

  • Some reactions and characteristics of chromium - we study these things before testing Cr3+ ion because they are important in understanding Cr3+ testing experiments.
  • Test for chromium ion

Electrons configuration of chromium

1s2 2s2 2p6 3s2 3p6 3d5 4s1

Major oxidation states of chromium

  • +2 : [Cr(H2O)6]2+(aq)
  • +3 : Cr(OH)3(s)
  • +6 : CrO3(s), Na2Cr2O7

Properties of chromium metal

  • A silvery blue metal.
  • Much resistance to atmospheric corrosion and decay.
  • Readily dissolve in acids to form +II oxidation state.
  • Compounds of +III state occur with oxygen, sulfur and halogens.
  • Density of Cr : 7.20 g cm-3

Melting and boiling points

  • Melting point : 18900C
  • Boiling point : 24800C

Reactions of chromium and compounds

Chromium and dilute acids reaction

Cr react with dilute HCl or dilute H2SO4 to form [Cr(H2O)6]2+(aq) blue complex.

Cr and dilute HCl reaction

Cr and dilute HCl reaction

Cr and dilute H2SO4

Cr and dilute sulfuric acid reaction

[Cr(H2O)6]2+(aq) complex oxidizing behavior

[Cr(H2O)6]2+(aq) complex easily oxidizes to Cr3+ by O2 under atmospheric conditions.

chromium +2 oxidizing

Chromium +3 oxidation state

  • Most stable oxidation state of Chromium is +3.

Aqueous Chromium(III) ion with alkali reactions

[Cr(H2O)6]3+(aq) changes its colour (blue-violet) to dark green by forming Cr(OH)3(s) precipitate with alkali. With excess alkali, precipitate dissolve and green colour aqueous [Cr(OH)6]3-(aq) complex is given.

Aqueous Chromium(III) ion with alkali reactions

Cr(OH)3, this hydroxide is an amphoteric compound.

Cr3+ ion and zinc amalgam

Cr3+ can be reduced to Cr2+ by zinc amalgam. Zinc amalgam is used in clemmensen reduction to reduce aldehyde and ketone to alkanes.

Cr3+ ion and zinc amalgam

Chromium +6 oxidation state

Aqueous Cr3+ ions can be oxidized to chromate(+6) oxidation state by H2O2 in the presence of alkali.

chromium +3 to +6 by hydrogen peroxide

Equilibrium of chromate ions and dichromate ions

Chromate(+6) ions undergo following equilibrium in acidic medium to form dichromate ( Cr2O72- ).

Equilibrium of chromate ions and dichromate ions

Reactions summary of chromium and compounds

reactions of chromium

Test for chromium ion

Here we discuss some experiments to test Cr3+ ion under qualitative analysis. Chromium forms colourful precipitates and solutions with other elements. Therefore we use those colours, precipitates to identify chromium ion.

Aqueous ammonium sulfide and Cr3+ solution reaction

Addition of aqueous ammonium sulfide ( (NH4)2S(aq) ) to Cr3+(aq) solution forms a Cr2S3 precipitate. It quickly hydrolyzes to Cr(OH)3(s) which is a green precipitate.

Addition of aqueous ammonium sulfide to Cr<sup>3+</sup> solution

Ammonia and salts of weak acids such as Na2CO3 , (NH4)2S also precipitates Cr3+ ions as Cr(OH)3. This precipitate somewhat soluble in excess ammonia.

NaOH, Na2O2 and Cr3+ solution reaction

Addition of NaOH, Na2O2 to Cr3+ solution forms Na2CrO4, a yellow solution. Further addition of acetic acid ( CH3COOH ) and BaCl2, a yellow precipitate BaCrO4 is obtained. BaCrO4 is insoluble in acetic acid, but soluble in HNO3 acid.

Ammonium buffer mixture with Cr3+

Cr3+ is completely precipitated as Cr(OH)3 by ammonium buffer solution ( NH4OH / NH4Cl ). Cr(OH)3 is a green precipitate.

Formation of sparingly soluble salts of CrO42-

The anion CrO42- forms sparingly soluble precipitates with Pb2+ , Ag+ , Ba2+. This can be used to detect and separate chromates.

  • PbCrO4: yellow
  • BaCrO4: yellow
  • Ag2CrO4: Brick red

Uses of chromium and its compounds

Chromium(IV) oxide ( CrO2 )

In magnetic recording tapes for better resolution and high frequency response.

Chromium(III) oxide (Cr2O3)

In abrasives, refractory materials, and semiconductors, as a green pigment, especially for colouring glass when chemical and heat resistance are required

Chromium(VI) oxide (CrO3)

Chromium plating, Copper stripping as a corrosion inhibitor in photography

Sodium dichromate (Na2Cr2O7)

Leather tanning, textile manufacture, metal corrosion inhibitor

More to learn about Cr3+ ion

  • CrPO4 is a greenish precipitate.
  • Cr3+ ions can be oxidized by KMnO4, (NH4)2S2O8 in acidic solutions.

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