Uranyl - Aqueous Chemistry

Aqueous Chemistry

The uranyl ion can be viewed as the end result of extensive hydrolysis the highly charged, hypothetical, U6+ cation.

6+ → 2+ + 4H+ + n-4 H₂O

The driving force for this hypothetical reaction is the reduction in charge density on the uranium atom. The number of water molecules attached to the uranyl ion in aqueous solution is mostly five. Further hydrolysis occurs, with a further reduction in charge density when one or more equatorial water molecules is replaced by an hydroxide ion. In fact the aqueous uranyl ion is a weak acid.

2+ + + H+; pK_a = ca. 4.2

As pH increases polymeric species with stoichiometry 2+ and + are formed before the hydroxide UO₂(OH)₂ precipitates. The hydroxide dissolves in strongly alkaline solution to give hydroxo complexes of the uranyl ion.

The uranyl ion can be reduced by mild reducing agents, such as zinc metal, to the oxidation state +4. Reduction to uranium(3+) can be done using a Jones reductor.