section_path: Chemistry and compounds/Solution chemistry
section_name: Solution chemistry
paragraph_text: Solution chemistry
  
  The standard reduction potentials in acidic aqueous solution for some common iron ions are given below:
  
  The red-purple tetrahedral ferrate (VI) anion is such a strong oxidizing agent that it oxidizes ammonia to nitrogen (N) and water to oxygen:
  
  The pale-violet hex aquo complex is an acid such that above pH 0 it is fully hydrolyzed:
  
  As pH rises above 0 the above yellow hydrolyzed species form and as it rises above 2–3, reddish-brown hydrous iron(III) oxide precipitates out of solution. Although Fe has a d configuration, its absorption spectrum is not like that of Mn with its weak, spin-forbidden d–d bands, because Fe has higher positive charge and is more polarizing, lowering the energy of its ligand-to-metal charge transfer absorptions. Thus, all the above complexes are rather strongly colored, with the single exception of the hexaquo ion – and even that has a spectrum dominated by charge transfer in the near ultraviolet region. On the other hand, the pale green iron(II) hexaquo ion does not undergo appreciable hydrolysis. Carbon dioxide is not evolved when carbonate anions are added, which instead results in white iron(II) carbonate being precipitated out. In excess carbon dioxide this forms the slightly soluble bicarbonate, which occurs commonly in groundwater, but it oxidises quickly in air to form iron(III) oxide that accounts for the brown deposits present in a sizeable number of streams.