Chromium

• Chromium, abbreviated Cr, commonly exhibits two charges, also known as "oxidation states": +6 and +3. Cr(+6)--hexavalent chromium--is most commonly observed in the polyatomic anions chromate, CrO4(2-), and dichromate, Cr2O7(2-).
  
  

Chromium Reduction

• During reduction, Cr(+6) reduces to Cr(+3):
  
  Cr2O7(2-) + 14 H(+) + 6 e ---> 2 Cr(+3) + 7 H2O
  
  CrO4(2-) + 4 H2O + 3 e ---> Cr(+3) + 5 OH(-)
  
  

Potentials

• The conversion of dichromate ion to Cr(+3) exhibits a potential of +1.33 V and the conversion of chromate ion exhibits a potential of -0.13 V. The positive value of the dichromate reduction indicates that dichromate ion is a strong oxidizing agent, whereas chromate ion is not.