page_title: Iron
section_title: Solution chemistry
hierarchical_section_title: Iron / Chemistry and compounds / Solution chemistry
caption_attribution_description: English: The left solution is sodium ferrate. The right solution is a permanganate.
context_page_description: Iron is a chemical element with symbol Fe and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is by mass the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.
  In its metallic state, iron is rare in the Earth's crust, limited mainly to deposition by meteorites. Iron ores, by contrast, are among the most abundant in the Earth's crust, although extracting usable metal from them requires kilns or furnaces capable of reaching 1,500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to master that process in Eurasia only about 2000 BCE, and the use of iron tools and weapons began to displace copper alloys, in some regions, only around 1200 BCE. That event is considered the transition from the Bronze Age to the Iron Age. In the modern world, iron alloys, such as steel, inox, cast iron and special steels are by far the most common industrial metals, because of their mechanical properties and low cost.
  Pristine and smooth pure iron surfaces are mirror-like silvery-gray.
context_section_description: 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 nitrogen and ammonia at room temperature, and even water itself in acidic or neutral solutions:
  4 FeO²⁻
  ₄ + 10 H
  ₂O → 4 Fe³⁺
   + 20 OH⁻
   + 3 O₂
  The Fe³⁺ ion has a large simple cationic chemistry, although the pale-violet hexaquo ion [Fe(H₂O)₆]³⁺ is very readily hydrolyzed when pH increases above 0 as follows:
  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 Fe³⁺ has an d⁵ configuration, its absorption spectrum is not like that of Mn²⁺ with its weak, spin-forbidden d–d bands, because Fe³⁺ 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 [Fe(H₂O)₆]²⁺ 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.
caption: Comparison of colors of solutions of ferrate (left) and permanganate (right)
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