The Collaborative International Dictionary of English v.0.48:

chelating agent \che"lat*ing a`gent\, n. (Chem.)
   a compound which forms a chelate with a metal ion.

   Note: Commonly used chelating agents in laboratory use are
         EDTA and sodium citrate. Citrate is used as an
         additive to blood collected in blood banks, to chelate
         the calcium ions and retard clotting.
         [PJC]