Science Serving Maryland's Coasts

Isabel Christy, Whitman College

Class Year: 

Project Title: 

Determination of stability constants for metal complexes with DTPA and salicylaldoxime at seawater ionic strength

The concentration of Gd-DTPA, an MRI contrasting agent, has been increasing in urban waterways over the past three decades. While Gd-DTPA is so stable in freshwater that toxic Gd3+ is unlikely to be released, little is known about the stability of this complex in marine environments. At high salinity, and thus high concentrations of Mg and Ca, Mg and Ca may compete with Gd sufficiently to release Gd3+ into the environment. In this study, the stability constants for complexes of DTPA with Ca and Mg were measured at seawater ionic strength by potentiometric titration to better understand the behavior of Gd-DTPA in marine environments. From these constants, the side-reaction coefficient for DTPA in seawater was calculated to understand the extent to which Mg and Ca compete for DTPA with trace metals and to determine conditional DTPA stability constants for seawater. This study concluded that Gd-DTPA largely dissociates in seawater, releasing toxic Gd3+.