sodium citrate

from 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]