Main: EGTA (ethylene glycol tetraacetic acid) is a polyamino carboxylic acid, a chelating agent that is related to the better known EDTA, but with a much higher affinity for calcium than for magnesium ions. It is useful for making buffer solutions that resemble the environment inside living cells where calcium ions are usually at least a thousandfold less concentrated than magnesium. The pKa for binding of calcium ions by tetrabasic EGTA is 11.00, but the protonated forms do not significantly contribute to binding, so at pH 7, the apparent pKa becomes 6.91. See Qin et al. for an example of a pKa calculation. EGTA has also been used experimentally for the treatment of animals with cerium poisoning and for the separation of thorium from the mineral monazite. EGTA is used as a compound in elution buffer in the protein purification technique known as Tandem Affinity Purification (TAP), in which recombinant fusion proteins are bound to calmodulin beads and eluted out by adding EGTA. EGTA is often employed in dentistry and endodontics for the removal of smear layer.