Chemical of the Week 19 -EDTA



At the minute I love that nine o clock feeling, when the marking is done, lunches are made and it’s time to sit down to the box set. So over the last week we have been immersed in the Netflix series ‘Making a Murderer’. Well, surely this must be taking me away from all thoughts of chemistry, but just like Homeland up springs a molecule which plays a leading role in the story. This time ethylenediamine tetraacetic acid which is more commonly referred to as EDTA. It’s IUPAC name is 2,2′,2”,2”’-(Ethane-1,2-diyldinitrilo)tetraacetic acid so EDTA is definitely less of a mouthful. It pops up at the end of the A2 course when we are looking at multi dentate chelating agents.


So what is it doing in a real life court room drama – well there is debate over whether a blood sample was planted at a crime scene and one way to establish did the blood come from a storage vial is to test for the presence of EDTA. EDTA is added to blood as it acts as an anti coagulating agent which means it stops the blood from clotting and is the best chemical for the preservation of cellular components and morphology of blood cells. How does EDTA work ? Well if you look at the structure of the molecule you will see that it has four carboxylic acid groups and two amine groups.  The acid groups can be deprotonated meaning that there are six pair of electrons that can act a ligands around a central metal ion, in the case above being the calcium ions in the blood. In the complex [Ca(EDTA)]2–, chelation involves the two nitrogen atoms and the four deprotonated (-COO–) groups (the overall charge of the complex takes into account these 6 coordinate bonds). EDTA is a very hard working chemical and has uses in the food industry, detergents, water treatment and is even used to treat lead poisoning.

Well back to my tv show – it turns out the test carried out in FBI labs didn’t detect any EDTA but an analytical chemistry expert for the defence offered the observation that as the level of detection for the test was not reported she could not conclude that there had not been any EDTA in the blood sample. I read an interesting post from a chemist who suggested that a control sample should be run. This could be achieved by putting an EDTA blood sample on the car surface and then collecting swabs in the same way as the evidence swabs. As always science does not lie but it is important that it is presented in its entirety.