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.

Questions



What is the chromium and halogens reaction

Chromium metal reacts with the halogens to form chromium(III) halides. As an example, chromium reacts with chlorine gas to give chromium(III) chloride.



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