Advances on the Determination of the Astatine Pourbaix Diagram: Predomination of AtO(OH)2- over At- in Basic Conditions

It is generally assumed that astatide (At-) is the predominant astatine species in basic aqueous media. This assumption is questioned in non-complexing and non-reductive aqueous solutions by means of high-pressure anion-exchange chromatography. Contrary to what is usually believed, astatide is found to be a minor species at pH=11. A different species, which also bears a single negative charge, becomes predominant when the pH is increased beyond 7. Using competition experiments, an equilibrium constant value of 10(-6.9) has been determined for the formation of this species from AtO(OH) with the exchange of one proton. The identification of this species, AtO(OH)(2)(-), is achieved through relativistic quantum mechanical calculations, which rule out the significant formation of the AtO2- species, while leading to a hydrolysis constant of AtO(OH) in excellent agreement with experiment when the AtO(OH)(2)(-) species is considered. Beyond the completion of the Pourbaix diagram of astatine, this new information is of interest for the development of At-211 radiolabeling protocols.