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.
1s2 2s2 2p6 3s2 3p6 3d5 4s1
Cr react with dilute HCl or dilute H2SO4 to form [Cr(H2O)6]2+(aq) blue complex.
[Cr(H2O)6]2+(aq) complex easily oxidizes to Cr3+ by O2 under atmospheric conditions.
[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.
Cr(OH)3, this hydroxide is an amphoteric compound.
Cr3+ can be reduced to Cr2+ by zinc amalgam. Zinc amalgam is used in clemmensen reduction to reduce aldehyde and ketone to alkanes.
Aqueous Cr3+ ions can be oxidized to chromate(+6) oxidation state by H2O2 in the presence of alkali.
Chromate(+6) ions undergo following equilibrium in acidic medium to form dichromate ( Cr2O72- ).
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.
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.
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.
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.
Cr3+ is completely precipitated as Cr(OH)3 by ammonium buffer solution ( NH4OH / NH4Cl ). Cr(OH)3 is a green precipitate.
The anion CrO42- forms sparingly soluble precipitates with Pb2+ , Ag+ , Ba2+. This can be used to detect and separate chromates.
In magnetic recording tapes for better resolution and high frequency response.
In abrasives, refractory materials, and semiconductors, as a green pigment, especially for colouring glass when chemical and heat resistance are required
Chromium plating, Copper stripping as a corrosion inhibitor in photography
Leather tanning, textile manufacture, metal corrosion inhibitor
More to learn about Cr3+ ion
Qualitative Analysis for anions and cations