Why do we use disodium EDTA?

1 Answer
Apr 27, 2016

Because generally, we don't need all six teeth to bind to a given target in practice, and because EDTA by itself is poorly soluble in water. Disodium EDTA is water-soluble.

EDTA, or ethylenediamminetetraaceticacid, a hexadentate, chelating ligand, looks like this:

https://upload.wikimedia.org/: EDTA binding to a transition metal

So you can see that four "teeth" on EDTA belong to the acetate oxygens, and two of them belong to tertiary amine nitrogens. EDTA has a high complexation formation constant, which means the equilibrium is highly in favor of EDTA binding.

However, EDTA by itself is NOT very water-soluble.

In disodium EDTA, or #"Na"_2"H"_2"Y"# for short, the two nitrogens are protonated because their pKas (#~36# for ammonia) are higher than the pKa of acetic acid (#~4.76#), and thus, they are more easily protonated. In the meantime, the sodium is just an alkali metal counterion, which promotes solubility in water.

This makes #"Na"_2"H"_2"Y"# a water-soluble tetradentate (the two sodiums don't interfere with the binding capabilities of EDTA), not to mention, it works nicely under more typical pH's, i.e. #<= 12#.

In practice, it is generally more useful and convenient to have EDTA partially ionized as disodium EDTA since it is easier to dissolve in water for a complexometric experiment.