page_title: Chromic acid
section_title: Molecular chromic acid
hierarchical_section_title: Chromic acid / Molecular chromic acid
caption_attribution_description: English: Predominance diagram for chromate. Español: Diagrama de predominancia para cromatos.
context_page_description: The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the molecular species, H₂CrO₄ of which the trioxide is the anhydride. Chromic acid features chromium in an oxidation state of +6. It is a strong and corrosive oxidising agent.
context_section_description: Molecular chromic acid, H₂CrO₄, has much in common with sulfuric acid, H₂SO₄. Only sulfuric acid can be classified as part of the 7 strong acids list. Due to the laws pertinent to the concept of "first order ionization energy", the first proton is lost most easily. It behaves extremely similar to sulfuric acid deprotonation. Since the process of polyvalent acid-base titrations have more than one proton (especially when the acid is starting substance and the base is the titrant), protons can only leave an acid one at a time. Hence the first step is as follows: 
  H₂CrO₄ ⇌ [HCrO₄]⁻ + H⁺
  The pKₐ for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6. The value at zero ionic strength is difficult to determine because half dissociation only occurs in very acidic solution, at about pH 0, that is, with an acid concentration of about 1 mol dm⁻³. A further complication is that the ion [HCrO₄]⁻ has a marked tendency to dimerize, with the loss of a water molecule, to form the dichromate ion, [Cr₂O₇]²⁻:
  2 [HCrO₄]⁻ ⇌ [Cr₂O₇]²⁻ + H₂O      log KD = 2.05.
  Furthermore, the dichromate can be protonated:
  [HCr₂O₇]⁻ ⇌ [Cr₂O₇]²⁻ + H⁺      pK = 1.8
  The pK value for this reaction shows that it can be ignored at pH > 4.
  Loss of the second proton occurs in the pH range 4–8, making the ion [HCrO₄]⁻ a weak acid.
  Molecular chromic acid could in principle be made by adding chromium trioxide to water (cf. manufacture of sulfuric acid).
  CrO₃ + H₂O ⇌ H₂CrO₄
  but in practice the reverse reaction occurs when molecular chromic acid is dehydrated. This is what happens when concentrated sulfuric acid is added to a dichromate solution. At first the colour changes from orange (dichromate) to red (chromic acid) and then deep red crystals of chromium trioxide precipitate from the mixture, without further colour change. The colours are due to LMCT transitions.
  Chromium trioxide is the anhydride of molecular chromic acid. It is a Lewis acid and can react with a Lewis base, such as pyridine in a non-aqueous medium such as dichloromethane (Collins reagent).
caption: Partial predominance diagram for chromate
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