Chromic Acid - Molecular Chromic Acid

Molecular Chromic Acid

Molecular chromic acid, H₂CrO₄, has much in common with sulfuric acid, H₂SO₄. Both are classified as strong acids, though only the first proton is lost easily.

H₂CrO₄ − + H+

The pK_a 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 zero, that is, with an acid concentration of about 1 mol dm−3. A further complication is that the ion − has a marked tendency to dimerize, with the loss of a water molecule, to form the dichromate ion, 2−:

2 − 2− + H₂O, log K_D = 2.05.

Furthermore, the dichromate can be protonated:

− 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 − 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 charge transfer 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).